Radiotherapy

Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells

(not to be confused with radiology, the use of radiation in medical imaging and diagnosis). And Radiotherapy may be used for curative or adjuvant cancer treatment. It is used as palliative treatment (where cure is not possible and the aim is for local disease control or symptomatic relief) or as therapeutic treatment (where the therapy has survival benefit and it can be curative). And Total body irradiation (TBI) is a radiotherapy technique used to prepare the body to receive a bone marrow transplant. Radiotherapy has several applications in non-malignant conditions, such as the treatment of trigeminal neuralgia, severe thyroid eye disease, pterygium, pigmented villonodular synovitis, prevention of keloid scar growth, and prevention of heterotopic ossification. And The use of radiotherapy in non-malignant conditions is a limited partly by worries about the risk of radiation-induced cancers.
Radiotherapy is used for the treatment of malignant tumors (cancer), and may be used as the primary therapy. And It is also common to combine radiotherapy with surgery, chemotherapy, hormone therapy or some mixture of the three. Most common cancer types can be treated with radiotherapy in some way. And The precise treatment intent (curative, adjuvant, neoadjuvant, therapeutic, or palliative) will depend on the tumour type, location, and stage, as well as the general health of the patient.
Radiation therapy is commonly applied to the cancerous tumour. And The radiation fields may also include the draining lymph nodes if they are clinically or radiologically involved with tumour, or if there is thought to be a risk of subclinical malignant spread. It is necessary to include a margin of normal tissue around the tumour to allow for uncertainties in daily set-up and internal tumor motion. And These uncertainties can be caused by internal movement (for example, respiration and bladder filling) and movement of external skin marks relative to the tumour position.
To spare normal tissues (such as skin or organs which radiation must pass through in order to treat the tumour), shaped radiation beams are aimed from several angles of exposure to intersect at the tumour, providing a much larger absorbed dose there than in the surrounding, healthy tissue.

Mechanism of action
Radiation therapy works by damaging the DNA of cells. The damage is caused by a photon, electron, proton, neutron, or ion beam directly or indirectly ionizing the atoms which make up the DNA chain. Indirect ionization happens as a result of the ionization of water, forming free radicals, notably hydroxyl radicals, which then damage the DNA. In the most common forms of radiation therapy, most of the radiation effect is through free radicals. Because cells have mechanisms for repairing DNA damage, breaking the DNA on both strands proves to be the most significant technique in modifying cell characteristics. And Because cancer cells generally are undifferentiated and stem cell-like, they reproduce more, and have a diminished ability to repair sub-lethal damage compared to most healthy differentiated cells. The DNA damage is inherited through cell division, accumulating damage to the cancer cells, causing them to die or reproduce more slowly. and a Proton radiotherapy works by sending protons with varying kinetic energy to precisely stop at the tumor.
One of the major limitations of radiotherapy is that the cells of solid tumors become deficient in oxygen. This is because solid tumours usually outgrow their blood supply, causing a low-oxygen state known as hypoxia. And The more hypoxic the tumours are the more resistant they are to the effects of radiation because oxygen makes the radiation damage to DNA permanent. Much research has been devoted to overcoming this problem including the use of high pressure oxygen tanks, blood substitutes that carry increased oxygen, hypoxic cell radiosensitizers such as misonidazole and metronidazole, and hypoxic cytotoxins, such as tirapazamine. And There is also interest in the fact that high-LET (linear energy transfer) particles such as carbon or neon ions may have an antitumour effect which is independent of tumour hypoxia.

Side effects
Radiation therapy is in itself painless. and Many low-dose palliative treatments (for example, radiotherapy to bony metastases) cause minimal or no side effects. And Treatment to higher doses causes varying side effects during treatment (acute side effects), in the months or years following treatment (long-term side effects), or after re-treatment (cumulative side effects). and The nature, severity, and longevity of side effects depends on the organs that receive the radiation, the treatment itself (type of radiation, dose, fractionation, concurrent chemotherapy), and the patient.
Most side effects are predictable and expected. and Side effects from radiation are usually limited to the area of the patients body that is under treatment. And One of the aims of modern radiotherapy is to reduce side effects to a minimum, and to help the patient to understand and to deal with those side effects which are unavoidable.

Acute side effects
* Damage to the epithelial surfaces. Epithelial surfaces like skin, oral, pharyngeal and bowel mucosa, urothelium, etc. may sustain damage from radiation therapy. And The rates of onset of damage and recovery from it depend upon the turnover rate of epithelial cells. Typically the skin starts to become pink and sore several weeks into treatment. The reaction may become more severe during the treatment and for up to about one week following the end of radiotherapy, and the skin may break down. And Although this moist desquamation is uncomfortable, recovery is usually quick. Skin reactions tend to be worse in areas where there are natural folds in the skin, such as underneath the female breast, behind the ear, and in the groin.
Similarly, the lining of the mouth, throat, esophagus, and bowel may be damaged by radiation. If the head and neck area is treated, temporary soreness and ulceration commonly occur in the mouth and throat. If severe, this can affect swallowing, and the patient may need painkillers and nutritional support. And The esophagus can also become sore if it is treated directly, or if, as commonly occurs, it receives a dose of collateral radiation during treatment of lung cancer.
The lower bowel may be treated directly with radiation (treatment of rectal or anal cancer) or be exposed by radiotherapy to other pelvic structures (prostate, bladder, female genital tract). And Typical symptoms are soreness, diarrhoea, and nausea.
* Swelling (edema or Oedema). As part of the general inflammation that occurs, swelling of soft tissues may cause problems during radiotherapy. This is a concern during treatment of brain tumours and brain metastases, especially where there is pre-existing raised intracranial pressure or where the tumour is causing near-total obstruction of a lumen (e.g., trachea or main bronchus). And Surgical intervention may be considered prior to treatment with radiation. If surgery is deemed unnecessary or inappropriate, and the patient may receive steroids during radiotherapy to reduce swelling.
* Infertility. The gonads (ovaries and testicles) are very sensitive to radiation. And They may be unable to produce gametes following direct exposure to most normal treatment doses of radiation. And Treatment planning for all body sites is designed to minimize, if not completely exclude dose to the gonads if they are not the primary area of treatment.
* Generalized fatigue.

Medium and long-term side effects
These depend on the tissue that received the treatment; they may be minimal.

Fibrosis
Tissues which have been irradiated tend to become less elastic over time due to a diffuse scarring process.

Hair loss
This may be most pronounced in patients who have received radiotherapy to the brain. and Unlike the hair loss seen with chemotherapy, radiation-induced hair loss is more likely to be permanent, but is also more likely to be limited to the area treated by the radiation.

Dryness
The salivary glands and tear glands have a radiation tolerance of about 30 Gy in 2 Gy fractions, a dose which is exceeded by most radical head and neck cancer treatments. And Dry mouth (xerostomia) and dry eyes (xerophthalmia) can become irritating long-term problems and severely reduce the patient's quality of life. Similarly, sweat glands in treated skin (such as the armpit) tend to stop working, and the naturally moist vaginal mucosa is often dry following pelvic irradiation.

Fatigue
Fatigue is among the most common symptoms of Radiation therapy, and can range from a few months, a few years, depending on the quantity of the treatment and cancer type. And Lack of Energy, reduced activity and overtired feelings are common symptoms

Cancer
Radiation is a potential cause of cancer, and is secondary malignancies are seen in a very small minority of patients, generally many years after they have received a course of radiation treatment. In the vast majority of cases, this risk is greatly outweighed by the reduction in risk conferred by treating the primary cancer.

Death
Radiation has potentially excess risk of death from heart disease seen after some past breast cancer RT regimens.

Cumulative side effects
Cumulative effects from this process should not be confused with long-term effects when short-term effects have disappeared and long-term effects are subclinical, and reirradiation can still be problematic.

Dose
The amount of radiation used in radiation therapy is measured in gray (Gy), and varies depending on the type and stage of cancer being treated. And For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, and while lymphoma tumors are treated with 20 to 40 Gy.
Preventative (adjuvant) doses are typically around 45 - 60 Gy in 1.8 - 2 Gy fractions (for Breast, Head and Neck cancers respectively.) and Many other factors are considered by radiation oncologists when selecting a dose, including whether the patient is receiving chemotherapy, and whether radiation therapy is being administered before or after surgery, and the degree of success of surgery.

Fractionation
The total dose is fractionated (spread out over time) for several important reasons. and Fractionation allows normal cells time to recover, while tumor cells are generally less efficient in repair between fractions. And Fractionation also allows tumor cells that were in a relatively radio-resistant phase of the cell cycle during one treatment to cycle into a sensitive phase of the cycle before the next fraction is given. Similarly, tumor cells that were chronically or acutely hypoxic (and therefore more radioresistant) may reoxygenate between fractions, improving the tumor cell kill. And Fractionation regimes are individualised between different radiotherapy centres and even between individual doctors. In the USA, Australia, and Europe, the typical fractionation schedule for adults is 1.8 to 2 Gy per day, five days a week. And In the northern United Kingdom, fractions are more commonly 2.67 to 2.75 Gy per day, and which eases the burden on thinly spread resources in the National Health Service. is In some cancer types, prolongation of the fraction schedule over too long can allow for the tumor to begin repopulating, and for these tumor types, including head-and-neck and cervical squamous cell cancers, radiation treatment is preferably completed within a certain amount of time. And For children, a typical fraction size may be 1.5 to 1.8 Gy per day, as smaller fraction sizes are associated with reduced incidence and severity of late-onset side effects in normal tissues.
In some cases, two fractions per day are used near the end of a course of treatment. And This schedule, known as a concomitant boost regimen or hyperfractionation, is used on tumors that regenerate more quickly when they are smaller. In particular, tumors in the head-and-neck demonstrate this behavior.
One of the best-known alternative fractionation schedules is Continuous Hyperfractionated Accelerated Radiotherapy (CHART). AND CHART, used to treat lung cancer, consists of three smaller fractions per day. Although reasonably successful, CHART can be a strain on radiation therapy departments.
And Implants can be fractionated over minutes or hours, or they can be permanent seeds which slowly deliver radiation until they become inactive.

History of radiation therapy
Radiation therapy has been in use as a cancer treatment for more than 100 years, with its earliest roots traced from the discovery of x-rays in 1895.and The concept of therapeutic radiation was invented by German physicist Wilhelm Conrad Röntgen when he discovered that the x-ray was a powerful and effective tool with which to treat cancer.
The field of radiation therapy began to grow in the early 1900s largely due to the groundbreaking work of Nobel Prize-winning scientist Marie Curie-Sklodowska, who discovered the radioactive elements polonium and radium. and This began a new era in medical treatment and research.and Radium was used in various forms until the mid-1900s when cobalt and caesium units came into use. Medical linear accelerators have been developed since the late 1940s.
With Godfrey Hounsfield’s discovery of computed tomography (CT), three-dimensional planning became a possibility and created a shift from 2-D to 3-D radiation delivery; physicians and physics were no longer limited because CT-based planning allowed physicians to directly measure the dose delivered to the patient's anatomy based on axial tomographical images. And Orthovoltage and cobalt units have largely been replaced by megavoltage linear accelerators, useful for their penetrating energies and lack of physical radiation source.
In the last few decades, and the advent of new imaging technologies, e.g., magnetic resonance imaging (MRI) in the 1970s and positron emission tomography (PET) in the 1980s, as well as new radiation delivery and visualization products, e.g., digital linear accelerator, image fusion has moved radiation therapy from 3-D conformal to intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT).and These advances have resulted in better treatment outcomes and fewer side effects. AND Now 50-70% of cancer patients receive radiation therapy is as part of their cancer treatment.

Types of radiation therapy
Historically, the three main divisions of radiotherapy are external beam radiotherapy (EBRT or XBRT) or teletherapy, brachytherapy or sealed source radiotherapy and unsealed source radiotherapy. And The differences relate to the position of the radiation source; external is outside the body, while sealed and unsealed source radiotherapy has radioactive material delivered internally. And Brachytherapy sealed sources are usually extracted later, while unsealed sources may be administered by injection or ingestion. Proton therapy is a special case of external beam radiotherapy where the particles are protons. Introperative radiotherapy is a special type of radiotherapy that is delivered immediately after surgical removal of the cancer. And This method has been employed in breast cancer (TARGeted Introperative radioTherapy), brain tumours and rectal cancers.

Conventional external beam radiotherapy
Conventional external beam radiotherapy (2DXRT) is delivered via two-dimensional beams using linear accelerator machines. 2DXRT mainly consists of a single beam of radiation delivered to the patient from several directions: often front or back, and both sides. And Conventional refers to the way the treatment is planned or simulated on a specially calibrated diagnostic x-ray machine known as a simulator because it recreates the linear accelerator actions (or sometimes by eye), and to the usually well-established arrangements of the radiation beams to achieve a desired plan. And The aim of simulation is to accurately target or localize the volume which is to be treated. This technique is well established and is generally quick and reliable. and The worry is that some high-dose treatments may be limited by the radiation toxicity capacity of healthy tissues which lay close to the target tumor volume. An example of this problem is seen in radiation of the prostate gland, where the sensitivity of the adjacent rectum limited the dose which could be safely prescribed using 2DXRT planning to such an extent that tumor control may not be easily achievable. And Previous to the invention of the CT, physicians and physicists had limited knowledge about the true radiation dosage delivered to both cancerous and healthy tissue. For this reason, 3-dimensional conformal radiotherapy is becoming the standard treatment for a number of tumor sites.
Virtual simulation, 3-dimensional conformal radiotherapy, and intensity-modulated radiotherapy
On The planning of radiotherapy treatment has been revolutionized by the ability to delineate tumors and adjacent normal structures in three dimensions using specialized CT and/or MRI scanners and planning software. and Virtual simulation, the most basic form of planning, allows more accurate placement of radiation beams than is possible using conventional X-rays, where soft-tissue structures are often difficult to assess and normal tissues difficult to protect.
An enhancement of virtual simulation is 3-Dimensional Conformal Radiotherapy (3DCRT), in which the profile of each radiation beam is shaped to fit the profile of the target from a beam's eye view (BEV) using a multileaf collimator (MLC) and a variable number of beams. And When the treatment volume conforms to the shape of the tumour, the relative toxicity of radiation to the surrounding normal tissues is reduced, allowing a higher dose of radiation to be delivered to the tumor than conventional techniques would allow.
Intensity-Modulated Radiation Therapy (IMRT) is an advanced type of high-precision radiation that is the next generation of 3DCRT. and IMRT also improves the ability to conform the treatment volume to concave tumor shapes, for example when the tumor is wrapped around a vulnerable structure such as the spinal cord or a major organ or blood vessel. Computer-controlled x-ray accelerators distribute precise radiation doses to malignant tumors or specific areas within the tumor. The pattern of radiation delivery is determined using highly-tailored computing applications to perform optimization and treatment simulation (Treatment Planning). And The radiation dose is consistent with the 3-D shape of the tumor by controlling, or modulating, the radiation beam’s intensity. The radiation dose intensity is elevated near the gross tumor volume while radiation among the neighboring normal tissue is decreased or avoided completely. The customized radiation dose is intended to maximize tumor dose while simultaneously protecting the surrounding normal tissue. And Because sparing healthy tissue as compared with conventional radiation therapy techniques (2DXRT and 3DCRT). This in turn results in better tumor targeting, lessened side effects, and improved treatment outcomes than even 3DCRT.
3DCRT is still used extensively for many body sites but the use of IMRT is growing in more complicated body sites such as CNS, head and neck, prostate, breast and lung. Unfortunately, IMRT is limited by its need for additional time from experienced medical personnel. This is because physicians must manually delineate the tumors one CT image at a time through the entire disease site which can take much longer than 3DCRT preparation. Then, medical physicists and dosimetrists must be engaged to create a viable treatment plan. Also, the IMRT technology has only been used commercially since the late 1990s even at the most advanced cancer centers, so radiation oncologists who did not learn it as part of their residency program must find additional sources of education before implementing IMRT.
Proof of improved survival benefit from either of these two techniques over conventional radiotherapy (2DXRT) is growing for many tumor sites, but the ability to reduce toxicity is generally accepted. Both techniques enable dose escalation, potentially increasing usefulness. There has been some concern, particularly with 3DCRT, about increased exposure of normal tissue to radiation and the consequent potential for secondary malignancy. Overconfidence in the accuracy of imaging may increase the chance of missing lesions that are invisible on the planning scans (and therefore not included in the treatment plan) or that move between or during a treatment (for example, due to respiration or inadequate patient immobilization). and New techniques are being developed to better control this uncertainty—for example, real-time imaging combined with real-time adjustment of the therapeutic beams. This new technology is called image-guided radiation therapy (IGRT) or four-dimensional radiotherapy.

Radio isotope Therapy (RIT)
Radiotherapy can also be delivered through infusion (into the bloodstream) or ingestion. Examples are the infusion of metaiodobenzylguanidine (MIBG) to treat neuroblastoma, of oral iodine-131 to treat thyroid cancer or thyrotoxicosis, and of hormone-bound lutetium-177 and yttrium-90 to treat neuroendocrine tumors (peptide receptor radionuclide therapy). And Another example is the injection of radioactive glass or resin microspheres into the hepatic artery to radioembolize liver tumors or liver metastases.
In 2002, the United States Food and Drug Administration (FDA) approved Ibritumomab tiuxetan (Zevalin), and which is a monoclonal antibody anti-CD20 conjugated to a molecule of Yttrium-90. and In 2003, the FDA approved Tositumomab Iodine-131 (Bexxar), which conjugates a molecule of Iodine-131 to the monoclonal antibody anti-CD20. and These medications were the first agents of what is known as radioimmunotherapy, and they were approved for the treatment of refractory non-Hodgkins lymphoma.

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Nuclear Medicine

Nuclear medicine is a branch of medicine and medical imaging that uses the nuclear properties of matter in diagnosis and therapy.

And More specifically, nuclear medicine is a part of molecular imaging because it produces images that reflect biological processes that is take place at the cellular and subcellular level.

Background
Nuclear medicine procedures use pharmaceuticals that have been labeled with radionuclides (radiopharmaceuticals). In diagnosis, radioactive substances are administered to patients and the radiation emitted is detected. And The diagnostic tests involve the formation of an image using a gamma camera or positron emission tomography, invented by Hal O. Anger, and sometimes called an Anger gamma camera. Imaging may also be referred to as radionuclide imaging or nuclear scintigraphy. And Other diagnostic tests use probes to acquire measurements from parts of the body, or counters for the measurement of samples taken from the patient.
In therapy, radionuclides are administered to treat disease or provide palliative pain relief. and For example, administration of Iodine-131 is often used for the treatment of thyrotoxicosis and thyroid cancer. And Phosphorus-32 was formerly used in treatment of polycythemia vera.Those treatments rely on the killing of cells by high radiation exposure, as compared to diagnostics in which the exposure is kept as low as reasonably achievable (ALARA policy) so as to reduce the chance of creating a cancer.
Nuclear medicine differs from most other imaging modalities in that the tests primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. In some centers, the nuclear medicine images can be superimposed, using software or hybrid cameras, on images from modalities such as CT or MRI to highlight the part of the body in which the radiopharmaceutical is concentrated. And This practice is often referred to as image fusion or co-registration.
Nuclear medicine diagnostic tests are usually provided by a dedicated department within a hospital and may include facilities for the preparation of radiopharmaceuticals. The specific name of a department can vary from hospital to hospital, with the most common names being the nuclear medicine department and the radioisotope department.
About two thirds of the world's supply of medical isotopes are produced at the Chalk River Laboratories in Chalk River, Ontario, Canada. The Canadian Nuclear Safety Commission ordered the reactor to be shut down on November 18, 2007 to facilitate repairs after safety concerns. And The repairs took longer than expected and in December 2007 a critical shortage of medical isotopes occurred. The Canadian government passed emergency legislation, allowing the reactor to re-start on 16 December 2007, and production of medical isotopes to continue.
The Chalk River reactor is used to irradiate materials with neutrons which are produced in great quantity during the fission of the U-235, which neutrons change the nucleus of the irradiated material by adding a neutron. For example, the second most commonly used radionuclide is Tc-99m, following the most commonly used radionuclide, F-18 (which is produced by accelerator bombardment of O-18 with protons. And The O-18 constitutes about 0.20% of ordinary oxygen (mostly O-16), from which it is extracted; see FDG).
In a reactor, in one of the fission products of uranium is Molybdenum-99 which is extracted and shipped to radiopharmaceutical houses all over North America. The Mo-99 radioactively beta decays with a half-life of 2.7 days, turning initially into Tc-99m, which is then extracted (milked) from a "Moly cow" (see technetium-99m generator). and The Tc-99m then further decays, while inside a patient, releasing a gamma photon which is detected by the gamma camera. It decays to its ground state of Tc-99, which is relatively non-radioactive compared to Tc-99m.

Diagnostic testing
Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. And The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone via the hydroxyapatite for imaging. And Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' which is a focal increase in radio-accumulation, or a general increase in radio-accumulation throughout the physiological system. and Some disease processes result in the exclusion of a tracer, resulting in the appearance of a 'cold-spot'. And Many tracer complexes have been developed in order to image or treat many different organs, glands, and physiological processes. And The types of tests can be split into two broad groups: in-vivo and in-vitro:

Types of studies
A typical nuclear medicine study involves administration of a radionuclide into the body by intravenous injection in liquid or aggregate form, ingestion while combined with food, inhalation as a gas or aerosol, or rarely, injection of a radionuclide that has undergone micro-encapsulation. Some studies require the labeling of a patient's own blood cells with a radionuclide (leukocyte scintigraphy and red blood cell scintigraphy). And Most diagnostic radionuclides emit gamma rays, while the cell-damaging properties of beta particles are used in therapeutic applications. And Refined radionuclides for use in nuclear medicine are derived from fission or fusion processes in nuclear reactors, which produce radioisotopes with longer half-lives, or cyclotrons, which produce radioisotopes with shorter half-lives, or take advantage of natural decay processes in dedicated generators, i.e. molybdenum/technetium or strontium/rubidium.
The most commonly used intravenous radionuclides are:
* Technetium-99m (technetium-99m)
* Iodine-123 and 131
* Thallium-201
* Gallium-67
* Fluorine-18 Fluorodeoxyglucose
* Indium-111 Labeled Leukocytes

The most commonly used gaseous/aerosol radionuclides are:
* Xenon-133
* Krypton-81m
* Technetium-99m Technegas
* Technetium-99m DTPA

Analysis
The end result of the nuclear medicine imaging process is a "dataset" comprising one or more images. And In multi-image datasets the array of images may represent a time sequence (ie. cine or movie) often called a "dynamic" dataset, a cardiac gated time sequence, or a spatial sequence where the gamma-camera is moved relative to the patient. SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera are reconstructed to produce an image of a "slice" through the patient at a particular position. And A collection of parallel slices form a slice-stack, a three-dimensional representation of the distribution of radionuclide is in the patient.
And The nuclear medicine computer may require millions of lines of source code to provide quantitative analysis packages for each of the specific imaging techniques available in nuclear medicine.
In Time sequences can be further analysed using kinetic models such as multi-compartment models or a Patlak plot.

Radiation dose
A patient undergoing a nuclear medicine procedure will receive a radiation dose. And Under present international guidelines it is assumed that any radiation dose, however small, presents a risk.and The radiation doses delivered to a patient in a nuclear medicine investigation present a very small risk of inducing cancer. And In this respect it is similar to the risk from X-ray investigations except that the dose is delivered internally rather than from an external source such as an X-ray machine.
The radiation dose from a nuclear medicine investigation is expressed as an effective dose with units of sieverts (usually given in millisieverts, mSv). And The effective dose resulting from an investigation is influenced by the amount of radioactivity administered in megabecquerels (MBq), and the physical properties of the radiopharmaceutical used, its distribution in the body and its rate of clearance from the body.
Effective doses can range from 6 µSv (0.006 mSv) for a 3 MBq chromium-51 EDTA measurement of glomerular filtration rate to 37 mSv for a 150 MBq thallium-201 non-specific tumour imaging procedure. The common bone scan with 600 MBq of technetium-99m-MDP has an effective dose of 3 mSv .
Formerly, units of measurement were the Curie (Ci), being 3.7E10 Bq, and also 1.0 grams of Radium (Ra-226); the Rad (radiation absorbed dose), now replaced by the Gray; and the rem (Röntgen equivalent man), now replaced with the Sievert. and The Rad and Rem are essentially equivalent for almost all nuclear medicine procedures, and only alpha radiation will produce a higher Rem or Sv value, and due to its much higher Relative Biological Effectiveness (RBE). And Alpha emitters are nowadays rarely used in nuclear medicine, but were used extensively before the advent of nuclear reactor and accelerator produced radioisotopes. and The concepts involved in radiation exposure to humans is covered by the field of Health Physics.

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Fetal Doppler

A Doppler fetal monitor or Doppler fetal heart rate monitor is a hand-held ultrasound transducer that uses

the Doppler effect to detect the heart beat of a fetus and provides an audible simulation of the heart beat. And Some models also display the heart rate in beats per minute. and Use of this monitor is sometimes known as Doppler auscultation.
A Doppler fetal monitor provides information about the fetus similar to the information a fetal stethoscope provides. One advantage of the Doppler fetal monitor over an acoustic (not electronic) fetal stethoscope is the audio output, which allows people other than the user to listen to the heartbeat. And the One disadvantage is the greater complexity and cost, and lower reliability, of an electronic device.
Originally intended for use by health care professionals, this device is becoming popular for personal use.

Fetal heart rates
Starting at week 5 the fetal heart will accelerate at a rate of 3.3 beats per day for the next month.
The fetal heart begins to beat at approximately the same rate as the mothers, which is 80 to 85 bpm. Below illustrates the approximate fetal heart rate for weeks 5 to 9, an assuming a starting rate of 80
Week 5 starts at 80 and ends at 103 bpm
Week 6 starts at 103 and ends at 126 bpm
Week 7 starts at 126 and ends at 149 bpm
Week 8 starts at 149 and ends at 172 bpm
At week 9 the fetal heartbeat tends to beat within a range of 155 to 195 bpm.
The fetal heart rate will begin to decrease and generally will fall within the range of 120 to 160 bpm by week 12.

Gender prediction
Evidence indicates that there is no relationship between fetal heart rate and fetus gender, and thus fetal heart rate cannot be used as a reliable predictor of the sex of a fetus.

Types of Dopplers
Dopplers for home or hospital use differ in the following ways:
1) Manufacturer - popular manufactures are Huntleigh, LifeDop, Stork Radio, and Nicolette
2) Probe type - waterproof or not. Waterproof probes are essential for water births.
3) Megahertz frequency - 2 or 3 megahertz probes. Most women can find the heart rate with either 2 or 3 megahertz probes. A 2 megahertz probe is recommended to detect a heart rate in early pregnancy (8-10 weeks), A 3 megahertz probe is recommended for women who are plus sized and pregnants.
4) Heartrate Display - some dopplers automatically display the heart rate, and for others the fetal heart rate must be manually counted.

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Dental X-ray

Dental X-rays are pictures of the teeth, bones, and surrounding soft tissues to screen for and help identify problems with

the teeth, mouth, and jaw. And X-ray pictures can show cavities, hidden dental structures (such as wisdom teeth), and bone loss that cannot be seen during a visual examination. Dental X-raysand may also be done as follow-up after dental treatments.
A radiographic image is formed by a controlled burst of X-ray radiation which penetrates oral structures at different levels, depending on varying anatomical densities, before striking the film or sensor. And Teeth appear lighter because less radiation penetrates them to reach the film. Dental caries, tooth decay, is infections and other changes in the bone density, and the periodontal ligament, appear darker because X-rays readily penetrate these less dense structures. Dental restorations (fillings, crowns) may appear lighter or darker, depending on the density of the material.
The dosage of X-ray radiation received by a dental patient is typically small, equivalent to a few days' worth of background radiation environmental radiation exposure, or similar to the dose received during a cross-country airplane flight. And Incidental exposure is further reduced by the use of a lead shield, lead apron, sometimes with a lead thyroid collar. Technician exposure is reduced by stepping out of the room, or behind adequate shielding material, when the X-ray source is activated.
Once photographic film has been exposed to X-ray radiation, it needs to be developed, traditionally using a process where the film is exposed to a series of chemicals in a dark room, as the films are sensitive to normal light. And This can be a time-consuming process, and incorrect exposures or mistakes in the development process can necessitate retakes, and exposing the patient to additional radiation. Digital x-rays, which replace the film with an electronic sensor, address some of these issues, and are becoming widely used in dentistry as the technology evolves. They may require less radiation and are processed much quicker than conventional radiographic films, often instantly viewable on a computer. However digital sensors are extremely costly and have historically had poor resolution, though this is much improved in modern sensors.
This preoperative photo of the tooth #3, (A), reveals no clinically apparent decay other than a small spot within the central fossa. In fact, decay could not be detected with an explorer. Radiographic evaluation, (B), however, revealed an extensive region of demineralization within the dentin (arrows) of the mesial half of the tooth. When a bur was used to remove the occlusal enamel of overlying the decay, (C), a large hollow was found within the crown and it was discovered that a hole in the side of the tooth large enough to allow the tip of the explorer to pass was contiguous with this hollow. After all of the decay had been removed, (D), the pulp chamber had been exposed and most of the mesial half of the crown was either missing or poorly supported.
It is possible for both tooth decay and periodontal disease to be missed during a clinical exam, and radiographic evaluation of the dental and periodontal tissues is a critical segment of the comprehensive oral examination. The photographic montage at right depicts a situation in which extensive decay had been overlooked by a number of dentists prior to radiographic evaluation of the area.

Intraoral radiographic views
Placing the radiographic film or sensor inside the mouth producesis in an intraoral radiographic view.

Periapical view
The periapical view is taken of both anterior and posterior teeth. The objective of this type of view is to capture the tip of the root on the film. This is often helpful in determining the cause of pain in a specific tooth, because it allows a dentist to visualize the tooth as well as the surrounding bone in their entirety. And This view is often used to determine the need for endodontic therapy as well as to visualize the successful progression of the endodontic therapy once it is initiated.
The name periapical is derived from the Latin peri, which means "around," and apical, which means "tip."

Bitewing view
The bitewing view is taken to visualize the crowns of the posterior teeth and the height of the alveolar bone in relation to the cementoenamel junctions, which are the demarcation lines on the teeth which separate tooth crown from tooth root. and When there is extensive bone loss, the films may be situated with their longer dimension in the vertical axis so as to better visualize their levels in relation to the teeth. Because bitewing views are taken from a more or less perpendicular angle to the buccal surface of the teeth, they more accurately exhibit the bone levels than do periapical views. And Bitewings of the anterior teeth are not taken.
The name bitewing refers to a little tab of paper or plastic situated in the center of the X-ray film, which when bitten on, allows the film to hover so that it is captures an even amount of maxillary and mandibular information.

Occlusal view
The occlusal view is not taken very often -- it is indicated when there is a desire to reveal the skeletal or pathologic anatomy of either the floor of the mouth or the palate. The occlusal film, which is about three to four times the size of the film used to take a periapical or bitewing, is inserted into the mouth so as to entirely separate the maxillary and mandibular teeth, and the film is exposed either from under the chin or angled down from the top of the nose. And Sometimes, it is placed in the inside of the cheek to confirm the presence of a sialolith in Stenson's duct, which carries saliva from the parotid gland. The occlusal view is not included in the standard full mouth series.

Full mouth series
A full mouth series is a complete set of intraoral X-rays taken of a patients' teeth and adjacent hard tissue. and This is often abbreviated as either FMS or FMX. The full mouth series is composed of 18 films:
* four bitewings
o two molar bitewings (left and right)
o two premolar bitewings (left and right)
* eight posterior periapicals
o two maxillary molar periapicals (left and right)
o two maxillary premolar periapicals (left and right)
o two mandibular molar periapicals (left and right)
o two mandibular premolar periapicals (left and right)
* six anterior periapicals
o two maxillary canine-lateral incisor periapicals (left and right)
o two mandibular canine-lateral incisor periapicals (left and right)
o two central incisor periapicals (maxillary and mandibular)

The Faculty of General Dental Practice of the Royal College of Surgeons of England publication Selection Criteria in Dental Radiography holds that is given current evidence full mouth series are to be discouraged due to the large numbers of radiographs involved, many of which will not be necessary for the patients treament. An alternative approach using bitewing screening with selected periapical views is suggested as a method of minimising radiation dose to the patient while maximizing diagnostic yield.

Extraoral radiographic views
Placing the radiographic film or sensor outside the mouth, on the opposite side of the head from the X-ray source, is produces an extra-oral radiographic view.
A lateral cephalogram is used to evaluate dentofacial proportions and clarify the anatomic basis for a malocclusion, and is an antero-posterior radiograph provides a face-forward view.

Panoramic films
A panoramic film, able to show a greater field of view, including the heads and necks of the mandibular condyles, the coronoid processes of the mandible, as well as the nasal antrum and the maxillary sinuses.
Panoramic films are extraoral films, in which the film is exposed while outside the patients' mouth, and they were developed by the United States Army as a quick way to get an overall view of a soldiers' oral health. and Exposing eighteen films per soldier was very time consuming, and it was felt that a single panoramic film could speed up the process of examining and assessing the dental health of the soldiers; soldiers with toothaches are not very effective. And It was later discovered that while panoramic films can prove very useful in detecting and localizing mandibular fractures and other pathologic entities of the mandible, they were not very good at assessing periodontal bone loss or tooth decay.

Computed Tomography
There is increasing use of CT (computed tomography) scans in dentistry, and particularly to plan dental implants; and there may be significant levels of radiation and potential risk. is Specially designed CBCT (cone beam CT) scanners can be used instead, which produce adequate imaging with a tenfold reduction in radiation.

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Radiography

Radiography is the use of X-rays to view unseen or hard-to-image objects. The main diagnostic purposes of X-rays are

to see inside ones' body, especially of the brain and fetus, where the bones can be viewed at an optimum resolution (128 shades of grey). The impact on society of this technique has also been immense.and Physicists and researchers have developed numerous types of medicines to assist in the function and development of organs such as the brain and heart, and radiography has improved the economy of most countries in the western world due to the employment of physicists and doctors.

Medical and industrial radiography
Radiography is used for both medical and industrial applications. And If the object being examined is living,and whether human or animal, it is regarded as medical; all other radiography is regarded as industrial radiographic work.

History of radiography
Taking an X-ray image with early Crookes tube apparatus, late 1800s.
Radiography started in 1895 with the discovery of X-rays, also referred to as Röntgen rays after Wilhelm Conrad Röntgen who first described their properties in rigorous detail. These previously unknown rays (hence the X) were found to be a type of electromagnetic radiation. It wasn't long before X-rays were used in various applications, from helping to fit shoes, to the medical uses that have persisted. And X-rays were put to diagnostic use very early, before the dangers of ionizing radiation were discovered. Indeed Madam Curie pushed for radiography to be used to treat wounded soldiers in World War I. Initially, many kinds of staff conducted radiography in hospitals, including physicists, photographers, doctors, nurses, and engineers. And The medical specialty of radiology grew up over many years around the new technology. When new diagnostic tests involving X-rays were developed, it was natural for the radiographers to be trained in and to adopt this new technology. and This happened first with fluoroscopy, computed tomography (1970s), mammography, ultrasound (1970s), and magnetic resonance imaging (1980s). Although a nonspecialist dictionary might define radiography quite narrowly as "taking X-ray images", this has long been only part of the work of "X-ray departments", radiographers, and radiologists.

Sources
A number of sources of X-ray photons have been used; these include sealed X-ray tubes, betatrons, and linear accelerators (linacs). and For gamma rays, radioactive sources such as 192Ir have been used.

Detectors
A range of detectors including photographic film, scintillator and a semiconductor diode arrays have been used to collect images.

Theory of X-ray attenuation
X-ray photons used for medical purposes are formed by an event involving an electron, while gamma ray photons are formed from an interaction with the nucleus of an atom. and In general, medical radiography is done using X-rays formed in an X-ray tube. and Nuclear medicine typically involves gamma rays.
The types of electromagnetic radiation of most interest to radiography are X-ray and gamma radiation. and This radiation is much more energetic than the more familiar types such as radio waves and visible light. It is this relatively high energy which makes gamma rays useful in radiography but potentially hazardous to living organisms.
The radiation is produced by X-ray tubes, high energy X-ray equipment or natural radioactive elements, such as radium and radon, and artificially produced radioactive isotopes of elements, such as cobalt-60 and iridium-192. and Electromagnetic radiation consists of oscillating electric and magnetic fields, but is generally depicted as a single sinusoidal wave. And While in the past radium and radon have both been used for radiography, they have fallen out of use as they are radiotoxic alpha radiation emitters which are expensive; iridium-192 and cobalt-60 are far better photon sources. For further details see commonly used gamma emitting isotopes.
Such a wave is characterised by its wavelength (the distance from a point on one cycle to the corresponding point on the next cycle) or its frequency (the number of oscillations per second). In a vacuum, all electromagnetic waves travel at the same speed, the speed of light (c). The wavelength (?, lambda) and the frequency (f) are all related by the equation:

f = \frac{c}{\lambda} \

This is true for all electromagnetic radiation.
Electromagnetic radiation is known by various names, depending on its energy. The energy of these waves is related to the frequency and the wavelength by the relationship:

E = hf = h (\frac{c}{\lambda}) \

where h is a constant known as the Planck constant.
Gamma rays are indirectly ionizing radiation. A gamma ray passes through matter until it undergoes an interaction with an atomic particle, usually an electron. And During this interaction, energy is transferred from the gamma ray to the electron, which is a directly ionizing particle. As a result of this energy transfer, the electron is liberated from the atom and proceeds to ionize matter by colliding with other electrons along its path. and Other times, the passing gamma ray interferes with the orbit of the electron, and slows it, releasing energy but not becoming dislodged. The atom is not ionised, and the gamma ray continues on, although at a lower energy. And This energy released is usually heat or another, weaker photon, and causes biological harm as a radiation burn. The chain reaction caused by the initial dose of radiation can continue after exposure, much like a sunburn continues to a damage skin even after one is out of direct sunlight.
For the range of energies commonly used in radiography, the interaction between gamma rays and electrons occurs in two ways. And One effect takes place where all the gamma ray's energy is transmitted to an entire atom. The gamma ray no longer exists and an electron emerges from the atom with kinetic (motion in relation to force) energy almost equal to the gamma energy. This effect is predominant at low gamma energies and is known as the photoelectric effect. The other major effect occurs when a gamma ray interacts with an atomic electron, freeing it from the atom and imparting to it only a fraction of the gamma ray's kinetic energy. A secondary gamma ray with less energy (hence lower frequency) also emerges from the interaction. And This effect predominates at higher gamma energies and is known as the Compton effect.
In both of these effects the emergent electrons lose their kinetic energy by ionizing surrounding atoms. and The density of ions so generated is a measure of the energy delivered to the material by the gamma rays.

The most common means of measuring the variations in a beam of radiation is by observing its effect on a photographic film. And This effect is the same as that of light, and the more intense the radiation is, the more it darkens, or exposes, the film. Other methods are in use, such as the ionizing effect measured electronically, its ability to discharge an electrostatically charged plate or to cause certain chemicals to fluoresce as in fluoroscopy.

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Light Cure

Dental restorative materials are specially fabricated materials, and designed for use as dental restorations (fillings),

and which are used to restore tooth structure loss, usually resulting from but not limited to dental caries (dental cavities). and There are many challenges for the physical properties of the ideal dental restorative material.

Restorative material development
The goal of research and development is to develop the ideal restorative material. And The ideal restorative material would be identical to natural tooth structure, in strength adherence and appearance. And The properties of an ideal filling material can be divided into four categories: physical properties, biocompatibility, aesthetics and application.

Physical properties
The physical properties include heat insulation, resistance to different categories of forces, and wear, bond strength, and chemical resistance. And The material needs to withstand everyday forces and conditions on it without fatiguing.

Biocompatibility
Biocompatibility refers to how well the material coexists with the biological equilibrium of the tooth and body systems. Since fillings are in close contact with mucosa, tooth, and pulp, biocompatibility is very important. And Common problems with some of the current dental materials include allergies, chemical leakage from the material, and pulpal irritation. Some of the byproducts of the chemical reactions during different stages of material hardening need to be considered.

Aesthetics
Filling materials ideally would match the surrounding tooth structure in shade translucency and texture.

Application
Dental operators require materials that are easy to manipulate and shape, where the chemistry of any reactions that need to occur are predictable or controllable.

Direct restorative materials
The chemistry of the setting reaction for direct restorative materials is designed to be more biologically compatible. And Heat and byproducts generated cannot damage the tooth or patient, since the reaction needs to take place while in contact with the tooth during restoration. And This ultimately limits the strength of the materials, since harder materials need more energy to manipulate.

Amalgam
Amalgam fillings, (also called silver fillings) are a mixture of mercury (from 43% to 54%) and powdered alloy made mostly of silver, tin, zinc and copper commonly called the amalgam alloy.and Due to the known toxicity of the element mercury, there is some controversy about the use of amalgams.

History
The Chinese were the first to use a silver amalgam to fill teeth in the 7th century; in 1816, Auguste Taveau developed his own dental amalgam from silver coins and mercury. This amalgam contained a very small amount of mercury and had to be heated in order for the silver to dissolve at an appreciable rate. and Taveau's formula offered lower cost and greater ease of use compared to existing materials such as gold, but had many practical problems, including a tendency to significantly expand after setting. And Because of these problems, this formula was abandoned in France. In 1833, however, two untrained Europeans, the Crawcour brothers, brought Taveau's amalgam to the United States under the name of "Royal Mineral Succedaneum".

Aesthetics
Most of the patients don't like the silvery glossy appearance of dental amalgams to be visible. And So dental amalgams are mostly not used for the restoration of incisors or canines. Composites are given preference in such cases.

Gamma 2 phase amalgams
After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardised into the gamma-2-phase amalgam formula in 1895.

The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 65% silver (Ag)
* < 29% tin (Sn)
* < 6% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)

The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag3Sn, which reacts with mercury, yielding the gamma-1 phase (Ag2Hg3) and gamma-2 phase (Sn7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells.
Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. And The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:
* > 40% silver (Ag)
* < 32% tin (Sn)
* < 30% copper (Cu)
* < 2% zinc (Zn)
* < 3% mercury (Hg)

The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu6Sn5 and gamma-1 phase.
The possible difference in toxicology between the two has not been studied conclusively. Amalgams continue to be used today because they are hard, durable and inexpensive.

Galvanic shock
When aluminium foil makes contact with some amalgam fillings, saliva in the mouth can act as an electrolyte. This can generate small electrical currents which are felt through the nerves in the tooth as (often extremely painful) electrical "jolts" or shocks.

Composite resin (also called white or plastic filling)
Composite resin fillings (also called white fillings) are a mixture of powdered glass and plastic resin, and can be made to resemble the appearance of the natural tooth. And They are strong, durable and cosmetically superior to silver or dark grey colored amalgam fillings. Composite resin fillings are usually more expensive than silver amalgam fillings. Bis-GMA based materials contain Bisphenol A, a known endocrine disrupter chemical, and may contribute to the development of breast cancer. PEX-based materials do not.
Most modern composite resins are light-cured photopolymers. Once the composite hardens completely, the filling can then be polished to achieve maximum aesthetic results. and Composite resins experience a very small amount of shrinkage upon curing, causing the material to pull away from the walls of the cavity preparation. This makes the tooth slightly more vulnerable to microleakage and recurrent decay. And With proper technique and material selection, microleakage can be minimized or eliminated altogether.
Besides the aesthetic advantage of composite fillings over amalgam fillings, the preparation of composite fillings requires less removal of tooth structure to achieve adequate strength. And This is because composite resins bind to enamel (and dentin too, although not as well) via a micromechanical bond. As conservation of tooth structure is a key ingredient in tooth preservation, many dentists prefer placing composite instead of amalgam fillings whenever possible.
Generally, composite fillings are used to fill a carious lesion involving highly visible areas (such as the central incisors or any other teeth that can be seen when smiling) or when conservation of tooth structure is a top priority.
Composite resin fillings require a clean and dry surface to bond correctly with the tooth, so cavities in areas that are harder to keep totally dry during the filling procedure may require a less moisture-sensitive filling. and The use of a rubber dam is highly recommended.

Glass Ionomer Cement
See main article Glass ionomer cement
These fillings are a mixture of glass and an organic acid. Although they are tooth-colored, glass ionomers vary in translucency. And Although glass ionomers can be used to achieve an aesthetic result, their aesthetic potential does not measure up to that provided by composite resins.
The cavity preparation of a glass ionomer filling is the same as a composite resin; it is considered a fairly conservative procedure as the bare minimum of tooth structure should be removed.
Conventional glass ionomers are chemically set via an acid-base reaction. And Upon mixing of the material components, there is no light cure needed to harden the material once placed in the cavity preparation. After the initial set, glass ionomers still need time to fully set and harden.
Glass ionomers do have their advantages over composite resins:
1. They are not subject to shrinkage and microleakage, as the bonding mechanism is an acid-base reaction and not a polymerization reaction.
2. Glass ionomers contain and release fluoride, which is important to preventing carious lesions. Furthermore, as glass ionomers release their fluoride, they can be "recharged" by the use of fluoride-containing toothpaste. And Hence, they can be used as a treatment modality for patients who are at high risk for caries. Newer formulations of glass ionomers that contain light-cured resins can achieve a greater aesthetic result, but do not release fluoride as well as conventional glass ionomers.
Glass ionomers are about as expensive as composite resin. And The fillings do not wear as well as composite resin fillings. Still, they are generally considered good materials to use for root caries and for sealants.

Resin modified Glass-Ionomer Cement (Compomer)
A combination of glass-ionomer and composite resin, these fillings are a mixture of glass, an organic acid, and resin polymer that harden when light cured. (The light activates a catalyst in the cement that causes it to cure in seconds.) and The cost is similar to composite resin. It holds up better than glass ionomer, but not as well as composite resin, and is not recommended for biting surfaces of adult teeth.
In general, resin modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.

Indirect Restorative materials
Porcelain (ceramic)
Porcelain fillings are hard, but can cause wear on opposing teeth. And They are brittle and are not always recommended for molar fillings.

Gold
Gold fillings have excellent durability, wear well, and do not cause excessive wear to the opposing teeth, but they do conduct heat and cold, which can be irritating. and There are two categories of gold fillings, cast gold fillings (gold inlays and onlays) made with 14 or 18 kt gold, and gold foil made with pure 24 kt gold that is burnished layer by layer. For years, they have been considered the benchmark of restorative dental materials. Recent advances in dental porcelains and consumer focus on aesthetic results have caused demand for gold fillings to drop in favor of advanced composites and porcelain veneers and crowns. And Gold fillings are usually quite expensive, although they do last a very long time. It is not uncommon for a gold crown to last 30 years in a patient's mouth.

Other historical fillings
Lead fillings were used in the 1700s, but became unpopular in the 1800s because of their softness. This was before lead poisoning was understood.
According to American Civil War-era dental handbooks from the mid-1800s, since the early 1800s metallic fillings had been used, made of lead, gold, tin, platinum, silver, aluminum, or amalgam. A pellet was rolled slightly larger than the cavity, condensed into place with instruments, then shaped and polished in the patient's mouth.and The filling was usually left "high", with final condensation "tamping down" occurring while the patient chewed food. Gold foil was the most popular and preferred filling material during the Civil War. Tin and amalgam were also popular due to lower cost, but were held in lower regard.
One survey of dental practices in the mid-1800s catalogued dental fillings found in the remains of seven Confederate soldiers from the U.S. Civil War; they were made of:
* Gold foil: Preferred because of its durability and safety.
* Platinum: Was rarely used because it was too hard, inflexible and difficult to form into foil.
* Aluminum: A material which failed because of its lack of malleability but has been added to some amalgams.
* Tin and iron: Believed to have been a very popular filling material during the Civil War. Tin foil was recommended when a cheaper material than gold was requested by the patient, however tin wore down rapidly and even if it could be replaced cheaply and quickly, there was a concern, specifically from Harris, that it would oxidise in the mouth and thus cause a recurrence of caries. And Due to the blackening, tin was only recommended for posterior teeth.
* Thorium: Radioactivity was unknown at that time, and the dentist probably thought he was working with tin.
* Lead and tungsten mixture, probably coming from shotgun pellets. Lead was rarely used in the 19th century, it is soft and quickly worn down by mastication, and had known harmful health effects.
* Amalgam: The most popular amalgam was a mixture of silver, tin and mercury. According to the authors of the article " It set very hard and lasted for many years, the major contradiction being that it oxidized in the mouth, turning teeth black. Also the mercury contained in the amalgam was thought at that time to be harmful." as explained in the pre-eminent dental textbook of that century, and The Principles and Practice of Dental Surgery by Chapin A. Harris A.M., M.D., D.D.S..

Replacement fillings
Fillings have a finite lifespan: an average of 12.8 years for amalgam and 7.8 years for composite resins. Fillings fail because of changes in the filling, tooth or the bond between them.
Amalgam fillings expand with age, possibly cracking the tooth and requiring repair and filling replacement. Composite fillings shrink with age and may pull away from the tooth allowing leakage. If As chewing applies considerable pressure on the tooth, the filling may crack, allowing seepage and eventual decay in the tooth underneath.
The tooth itself may be weakened by the filling and crack under the pressure of chewing. That will require further repairs to the tooth and replacement of the filling.
If fillings leak or if the original bond is inadequate, and the bond may fail even if the filling and tooth are otherwise unchanged.

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ESWL

A lithotriptor is a medical device used in the non-invasive treatment of a kidney stones (urinary calculosis) and

biliary calculi (stones in the gallbladder or in the liver). and The scientific name of this procedure is Extracorporeal Shock Wave Lithotripsy (ESWL). Lithotripsy was developed in the early 1980s in Germany by Dornier Medizintechnik GmbH (now known as Dornier MedTech Systems GmbH), and came into widespread use with the introduction of the HM-3 lithotriptor in 1983. Within a few short years, ESWL became a standard treatment of calculosis.
It is estimated that more than one million patients are treated annually with ESWL is in the USA alone.

How it works
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The lithotriptor attempts to break up the stone with minimal collateral damage by using an externally-applied, focused, high-intensity acoustic pulse. The sedated or anesthesized patient lies down in the apparatus' bed, with the back supported by a water-filled coupling device placed at the level of kidneys. and A fluoroscopic x-ray imaging system or an ultrasound imaging system is used to locate the stone and aim the treatment head so that the F1 of the shock wave is focused on the stone. The treatment usually starts at the equipment's lowest power level, with a long gap between pulses,is in order to accustom the patient to the sensation. and The frequency of pulses and the power level are then gradually increased, so as to break up the stone more effectively. and The final power level usually depends on the patient's pain threshold. If the stone is positioned near a bone (usually a rib in the case of kidney stones), this is treatment may be more uncomfortable because the shock waves can cause a mild resonance in the bone which can be felt by the patient. And The sensation of the treatment is likened to an elastic band twanging off the skin. Alternately the patient may be sedated during the procedure. This allows the power levels to be brought up more quickly and a much higher pulse frequency, often up to 120 shocks per minute.
The successive shock wave pressure pulses result in direct shearing forces, as well as cavitation bubbles surrounding the stone, which fragment the stones into smaller pieces that then can easily pass through the ureters or the cystic duct. and The process takes about an hour. A ureteral stent (a kind of expandable hollow tube) may be used at the discretion of the urologist. The stent allows for easier passage of the stone by relieving obstruction and through passive dilatation of the ureter.
Some of the passed fragments of a 1-cm calcium oxalate stone that was smashed using lithotripsy.
Some of the passed fragments of a 1-cm calcium oxalate stone that was smashed using lithotripsy.
Extracorporeal lithotripsy works best with stones between 4 mm and 2 cm in diameter that are still located in the kidney. It can be used to break up stones which are located in a ureter too, but with less success.
The patients undergoing this procedure can, in some cases, see for themselves the progress of their treatment. If allowed to view the ultrasound or x-ray monitor, they may be able to see their stones change from a distinct bright point(or dark spot depending on whether the fluoro unit is set up in native or bones white) to a fuzzy cloud as the stone is disintegrated into a fine powder.
ESWL is the least invasive of the commonplace modalities for definitive stone treatment,and but provides a lower stone-free rate than other more invasive treatment methods, such as ureteroscopic manipulation with laser lithotripsy or percutaneous nephrolithotomy (PCNL). And The passage of stone fragments may take a few days or a week and may cause mild pain. Patients may be instructed to drink as much water as practical during this time.
A patient of the procedure has equated the after effects to "a punch to the kidney" (pain while urinating, sometimes with blood).
ESWL is not without risks. And The shock waves themselves, as well as cavitation bubbles formed by the agitation of the urine medium, can lead to capillary damage, a renal parenchymal or subcapsular hemorrhage. And This can lead to long-term consequences such as renal failure and hypertension. And Overall complication rates of ESWL range from 5–20%.

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Centrifuge

A centrifuge is a piece of equipment, generally driven by a motor, and that puts an object in rotation around a fixed axis,

applying a force perpendicular to the axis. and The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to evenly distribute substances (usually present in a solution for small scale applications) of greater and lesser density. and There are many different kinds of centrifuges, including those for very specialised purposes. It is can be used for viable counts, when shaking the culture e.g. yeast, out of suspension.

Theory
Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample,and rather than specifying a rotational speed such as revolutions per minute. The acceleration is often quoted in multiples of g, the standard acceleration due to gravity at the Earth's surface. And This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations.
The accelerations can be calculated as the product of the radius and the square of the angular velocity.

History and predecessors
A 19th century hand cranked laboratory centrifuge.
English military engineer Benjamin Robins (1707-1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl invented the first dairy centrifuge in order to separate cream from milk. And in 1879, Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible.

Types
There are at least five types of centrifuge:

* Tabletop/clinical/desktop centrifuge or microcentrifuge
* High-speed centrifuge
* Cooling centrifuge
* Ultracentrifuge
* Geotechnical centrifuge

Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one :
* Screen centrifuges, where the centrifugal acceleration allows the liquid to pass through a screen of some sort, through which the solids cannot go (due to granulometry larger than the screen gap or due to agglomeration). Common types are :
o Pusher centrifuges
o Peeler centrifuges
* Decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration. Common types are :
o Solid bowl centrifuges
o Conical plate centrifuges

Uses
Isolating suspensions
Simple centrifuges are used in chemistry, biology, and biochemistry for isolating and separating suspensions. They vary widely in speed and capacity. They usually comprise a rotor containing two, four, six, or many more numbered wells within which the samples containing centrifuge tips may be placed.

Isotope separation
Other centrifuges, the first being the Zippe-type centrifuge, separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.
Gas centrifuges are used in uranium enrichment. and The heavier isotope of uranium (uranium-238) in the uranium hexafluoride gas tend to concentrate at the walls of the centrifuge as it spins, while the desired uranium-235 isotope is extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich uranium enough for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to weapons-grade (around 90% enrichment) for use in nuclear weapons.
The 20 G centrifuge at the NASA Ames Research Center

Aeronautics and astronautics
Human centrifuges are exceptionally large centrifuges that test the reactions and tolerance of pilots and astronauts to acceleration above those experienced in the Earth's gravity.
The US Air Force at Holloman Air Force Base, NM operates a human centrifuge. The centrifuge at Holloman AFB is operated by the aerospace physiology department for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft. It is important to note that the centrifuge at Holloman AFB is unrealistic in that it is far more difficult for a pilot to tolerate the high-g environment in the centrifuge than in a real fighter aircraft. This well-known fact is based on countless accounts from experienced operational fighter pilots.[a citation needed]
The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. and Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophyand that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odyssey.

Earthquake and blast simulation
The geotechnical centrifuge is used for simulating blasts and earthquake phenomena.and For a discussion of their design, see Geotechnical Centrifuges by Philip Turner.

Commercial applications
* Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
* Centrifuges are used in the attraction Mission: SPACE, located at Epcot in Walt Disney World, which propels riders using a combination of a centrifuge and a motion simulator to simulate the feeling of going into space.
* In soil mechanics, centrifuges utilize centrifugal acceleration to match soil stresses in a scale model to those found in reality.
* Large industrial centrifuges are commonly used in water and wastewater treatment to dry sludges. and The resulting dry product is often termed cake, and the water leaving is a centrifuge after most of the solids have been removed is called centrate.
* Large industrial centrifuges are also used in the oil industry to remove solids from the drilling fluid.
* Disc-stack centrifuges used by some companies in Oil Sands industry to separate small amounts of water and solids from bitumen before it's sent to Upgrading.

Calculating relative centrifugal force (RCF)
Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. And This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.

g = RCF = 0.00001118\,r \, N^2 \,

where
g = Relative centrifuge force
r = rotational radius (centimetres, cm)
N = rotating speed (revolutions per minute, r/min)

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Suction Pump

A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum.

and The vacuum pump was invented in 1650 by Otto von Guericke.

Types
Pumps can be broadly categorized according to three techniques are:
* Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.
* Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber.
* Entrapment pumps capture gases in a solid or absorbed state. This includes cryopumps, getters, and ion pumps.

Positive displacement pumps are the most effective for low vacuums. Momentum transfer pumps in conjunction with one or two positive displacement pumps are the most common configuration used to achieve high vacuums. And In this configuration the positive displacement pump serves two purposes. First it obtains a rough vacuum in the vessel being evacuated before the momentum transfer pump can be used to obtain the high vacuum, as momentum transfer pumps cannot start pumping at atmospheric pressures. Second the positive displacement pump backs up the momentum transfer pump by evacuating to low vacuum the accumulation of displaced molecules in the high vacuum pump. Entrapment pumps can be added to reach ultrahigh vacuums, but they require periodic regeneration of the surfaces that trap air molecules or ions. And Due to this requirement their available operational time can be unacceptably short in low and high vacuums,and thus limiting their use to ultrahigh vacuums. and Pumps also differ in details like manufacturing tolerances, sealing material, pressure, flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to liquids and vibration.

Performance measures
* Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in volume per unit of time. And Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping rate can be different for each of the gases being pumped, and the average volume flow rate of the pump will vary depending on the chemical composition of the gases remaining in the chamber.
* Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured in units of pressure-volume/unit time. At a constant temperature, throughput is proportional to the number of molecules being pumped per unit time, and therefore to the mass flow rate of the pump. And When discussing a leak in the system or backstreaming through the pump, and throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput.
Positive displacement and momentum transfer pumps have a constant volume flow rate, (pumping speed,) but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant, the throughput and mass flow rate drop exponentially. Meanwhile, the leakage, evaporation, sublimation and backstreaming rates continue to produce a constant throughput into the system.

Positive displacement
The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time.
Mechanism of a scroll pump.
Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be created first. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, and which is soon filled by air pushed in by atmospheric pressure
To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind positive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.
More sophisticated systems are used for most industrial applications, but the basic principle of cyclic volume removal is the same:
* Rotary vane pump, the most common
* Diaphragm pump, zero oil contamination
* Liquid ring pump
* Piston pump, cheapest
* Scroll pump, highest speed dry pump
* Screw pump (10 Pa)
* Wankel pump
* External vane pump
* Roots blower, also called a booster pump, has highest pumping speeds but low compression ratio
* Multistage Roots pump that combine several stages providing high pumping speed with better compression ratio
* Toepler pump
The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa (when new) and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa.
A positive displacement vacuum pump moves the same volume of gas with each cycle, so its pumping speed is constant unless it is overcome by backstreaming.

Momentum transfer
A cutaway view of a turbomolecular high vacuum pump
In a momentum transfer pump, gas molecules are accelerated from the vacuum side to the exhaust side (which is usually maintained at a reduced pressure by a positive displacement pump). Momentum transfer pumping is only possible below pressures of about 1 kPa. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than positive displacement pumping. This regime is generally called high vacuum.
A Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds. They do this at the expense of the seal between the vacuum and their exhaust. Since there is no seal, a small pressure at the exhaust can easily cause backstreaming through the pump; this is called stall. In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular pumps can attain their full potential.
The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump by imparting momentum to the gas molecules. And Diffusion pumps blow out gas molecules with jets of oil or mercury, while turbomolecular pumps use high speed fans to push the gas. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump.
As with positive displacement pumps, the base pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but now minimizing leakage and outgassing to a level comparable to backstreaming becomes much more difficult.
* Diffusion pump
* Turbomolecular pump

Entrapment
Entrapment pumps may be cryopumps, which use cold temperatures to condense gases to a solid or adsorbed state, chemical pumps, which react with gases to produce a solid residue, or ionization pumps, which use strong electrical fields to ionize gases and propel the ions into a solid substrate. A cryomodule uses cryopumping.
* Ion pump
* Cryopump
* Sorption pump
* Non-evaporative getter

Other pump types
* Venturi vacuum pump (aspirator) (10 to 30 kPa)
* Steam ejector (vacuum depends on the number of stages, but can be very low)

Techniques
Vacuum pumps are combined with chambers and operational procedures into a wide variety of vacuum systems. Sometimes more than one pump will be used (in series or in parallel) in a single application. A partial vacuum, or rough vacuum, can be created using a positive displacement pump that transports a gas load from an inlet port to an outlet (exhaust) port. Because of their mechanical limitations, such pumps can only achieve a low vacuum. To achieve a higher vacuum, other techniques must then be used, typically in series (usually following an initial fast pump down with a positive displacement pump). And Some examples might be use of an oil sealed rotary vane pump (the most common positive displacement pump) backing a diffusion pump, or a dry scroll pump backing a turbomolecular pump. and There are other combinations depending on the level of vacuum being sought.
Achieving high vacuum is difficult because all of the materials exposed to the vacuum must be carefully evaluated for their outgassing and vapor pressure properties. For example, oils, and greases, and rubber, or plastic gaskets used as seals for the vacuum chamber must not boil off when exposed to the vacuum, or the gases they produce would prevent the creation of the desired degree of vacuum. And Often, all of the surfaces exposed to the vacuum must be baked at high temperature to drive off adsorbed gases.
Outgassing can also be reduced simply by desiccation prior to vacuum pumping. High vacuum systems generally require metal chambers with metal O-ring seals such as Klein flanges or ISO flanges, rather than the rubber o-rings more common in low vacuum chamber seals. and The system must be clean and free of organic matter to minimize outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. As a result, many materials that work well in low vacuums, such as epoxy, will become a source of outgassing at higher vacuums. And With these standard precautions, vacuums of 1 mPa are easily achieved with an assortment of molecular pumps. With careful design and operation, 1 µPa is possible.
Several types of pumps may be used in sequence or in parallel. and In a typical pumpdown sequence, a positive displacement pump would be used to remove most of the gas from a chamber, starting from atmosphere (760 Torr, 101 kPa) to 25 Torr (3 kPa). Then a sorption pump would be used to bring the pressure down to 10-4 Torr (10 mPa).and A cryopump or turbomolecular pump would be used to bring the pressure further down to 10-8 Torr (1 µPa). An additional ion pump can be started below 10-6 Torr to remove gases which are not adequately handled by a cryopump or turbo pump, such as helium or hydrogen.
Ultra high vacuum generally requires custom-built equipment, strict operational procedures, and a fair amount of trial-and-error. Ultra-high vacuum systems are usually made of stainless steel with metal-gasketed conflat flanges. And The system is usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system and boil them off. If necessary, this outgassing of the system can also be performed at room temperature, but this takes much more time. And Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures to minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system.
In ultra-high vacuum systems, some very odd leakage paths and outgassing sources must be considered. and The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the absorptivity of hard metals such as stainless steel or titanium must be considered.and Some oils and greases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.
The impact of molecular size must be considered. Smaller molecules can leak in more easily and are more easily absorbed by certain materials, and molecular pumps are less effective at pumping gases with lower molecular weights. And A system may be able to evacuate nitrogen (the main component of air) to the desired vacuum, but the chamber could still be full of residual atmospheric hydrogen and helium. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.

Uses of vacuum pumps
Vacuum pumps are used in many industrial and scientific processes including:
* The production of most types of electric lamps, vacuum tubes, and CRTs where the device is either left evacuated or re-filled with a specific gas or gas mixture
* Semiconductor processing, notably ion implantation, dry etch and PVD, ALD, PECVD and CVD deposition and soon in photolithography
* Electron microscopy
* Medical processes that require suction
* Medical applications such as such Radiotherapy, Radiosurgery, Radiopharmacy
* Analytical instrumentation to analyse gas, liquid, solid, surface and bio materials
* Mass spectrometers to create an ultra high vacuum between the ion source and the detector
* Vacuum Coating for decoration, for durability, for energy saving
* Glass coating for low e glass
* Hard coating for engine (as in Formula One)
* Ophthalmic coating
* Air conditioning service - removing all contaminants from the system before charging with refrigerant
* Trash compactor
* Vacuum engineering
* even in sewage system see EN1091:1997 standards
* Freeze Drying

Vacuum may be used to power, or provide assistance to mechanical devices. and In diesel engined motor vehicles, a pump fitted on the engine (usually on the camshaft) is used to produce vacuum. In petrol engines, instead, vacuum is obtained as a side-effect of the operation of the engine and the flow restriction created by the throttle plate. This vacuum may then be used to power:
* The vacuum servo booster for the hydraulic brakes
* Motors that move dampers in the ventilation system
* The throttle driver in the cruise control servomechanism

In an aircraft, the vacuum source is often used to power gyroscopes in the various flight instruments. and To prevent the complete loss of instrumentation in the event of an electrical failure, the instrument panel is deliberately designed with certain instruments powered by electricity and other instruments powered by the vacuum source.

History of the vacuum pump
The predecessor to the vacuum pump was the suction pump, which was invented in 1206 by the Arabic engineer and inventor, Al-Jazari. And The suction pump later appeared in Europe from the 15th century. and Taqi al-Din's six-cylinder 'Monobloc' pump, invented in 1551, could also create a partial vacuum, which was formed "as the lead weight moves upwards, it pulls the piston with it, creating vacuum which sucks the water through a non return clack valve into the piston cylinder.
By the 17th century, water pump designs had improved to the point that they produced measurable vacuums, but this was not immediately understood. And What was known was that suction pumps could not pull water beyond a certain height: 18 Florentine yards according to a measurement taken around 1635. (The conversion to metres is uncertain, but it would be about 9 or 10 metres.) This limit was a concern to irrigation projects, mine drainage, and decorative water fountains planned by the Duke of Tuscany, so the Duke commissioned Galileo to investigate the problem. Galileo advertised the puzzle to other scientists, including Gaspar Berti who replicated it by building the first water barometer in Rome in 1639.Berti's barometer produced a vacuum above the water column, but he could not explain it. The breakthrough was made by Evangelista Torricelli in 1643. Building upon Galileo's notes, he built the first mercury barometer and wrote a convincing argument that the space at the top was a vacuum. and The height of the column was then limited to the maximum weight that atmospheric pressure could support. and Some people believe that although Torricelli's experiment was crucial, it was Blaise Pascal's experiments that proved the top space really contained vacuum.
In 1654, Otto von Guericke invented the first vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that teams of horses could not separate two hemispheres from which the air had been evacuated. Robert Boyle improved Guericke's design and conducted experiments on the properties of vacuum. And Robert Hooke also helped Boyle produce an air pump which helped to produce the vacuum. The study of vacuum then lapsed until 1855, when Heinrich Geissler invented the mercury displacement pump and achieved a record vacuum of about 10 Pa (0.1 Torr). A number of electrical properties become observable at this vacuum level, and this renewed interest in vacuum. This, in turn, led to the development of the vacuum tube.
In the 19th century, Nikola Tesla designed the apparatus, imaged to the right, that contains a Sprengel Pump to create a high degree of exhaustion.

Hazards
Old vacuum-pump oils that were produced before circa 1980 often contain a mixture of several different dangerous polychlorinated biphenyls (PCBs), and which are highly toxic, carcinogenic, persistent organic pollutants.

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Panoramic

An Orthopantomogram (OPG), also known as an "orthopantogram" or "panorex", is a panoramic scanning dental X-ray of

the upper and lower jaw. and It shows a two-dimensional view of a half-circle from ear to ear.

Equipment
Dental panoramic radiography equipment consists of a horizontal rotating arm which holds an X-ray source and a moving film mechanism (carrying a film) arranged at opposed extremities. The patient's skull sits between the X-ray generator and the film. The X-ray source is collimated toward the film, to give a beam shaped as a vertical blade having a width of 4-7mm when arriving on the film, after crossing the patient's skull. Also the height of that beam covers the mandibles and the maxilla regions. and The arm moves and its movement may be described as a rotation around an instant center which shifts on a dedicated trajectory.
The manufacturers propose different solutions for moving the arm, trying to maintain constant distance between the teeth to the film and generator. Also those moving solutions try to project the teeth arch as orthogonally as possible. It is impossible to select an ideal movement as the anatomy varies very much from people to people. Finally a compromise is selected by each manufacturer and results in magnification factors which vary strongly along the film (15%-30%). and The patient positioning is very critical in regard to both sharpness and distortions.

Forming the image
Normally, the person bites on a plastic spatula so that all the teeth, especially the crowns can be viewed individually. The whole orthopantomogram process takes about one minute.
Because the collimation, while rotating, the X-rays projects on the film only a limited portion of the anatomy, at every instant but, as the rotation progresses around the skull, the whole maxillo-facial block is scanned. While the arm rotates, the film moves in a such way that the projected partial skull image (limited by the beam section) scrolls over it and exposes it entirely. and Because the beam travels across the skull, the partial image it projects on the film every instant contains all the anatomical elements it crossed in the skull, overlapped. Not all the overlapped element images projected on the film have the same magnification because the beam is divergent and the elements are at different distances from the generator focus. Also not all the element images move with the same velocity on the target film as some of them are far and other closer to the instant rotation center. and The velocity of the film is controlled in such fashion to fit exactly the velocity of is projection of the anatomical elements of the dental arch side which is closer to the film. And Therefore they are recorded sharply while the elements in different places are recorded blurred as they scroll at different velocity.

The dental panoramic image suffers from important distortions because a vertical zoom and a horizontal zoom both varying differently along the image. And The vertical and horizontal zooms are determined by the relative position of the recorded element versus film and generator. Closer to the generator means bigger vertical zoom. More, the horizontal zoom is also dependent on the relative position of the element to the focal path. Inside the focal path arch means bigger horizontal zoom and blurred, is outside means smaller horizontal and blurred.
The result is an image showing sharply the section along the mandible arch and blurred the rest. For example, the more radio-opaque anatomical region, the cervical vertebras (neck), shows as a wide and blurred vertical pillar overlapping the front teeth. and The path where the anatomical elements are recorded sharply is called "focal path".

Films
There are two kind of film moving mechanisms one using a sliding flat cassette which holds the film and another using a rotating cylinder which has the film wound around. And There are two standard sizes for dental panoramic films 30cm x12cm (12"x 5") and 30cm x 15cm (12"x6"). and The smaller size film involves 8% less of X-ray dosage.

Digital
Dental X-rays radiology moves from film technology (involving a chemical developing process) to Digital X-ray which is based on electronic sensors and computers. One of the principal advantages compared to film based systems is the much greater exposure latitude. And This means much less repeats, which also reduces patient exposure to radiation. Lost xrays can also be reprinted. And Other significant advantages include instantly viewable images, ability to enhance images, ability to email images to practitioners and clients, easy and reliable document handling, reduced X-ray exposure, no darkroom is required, no chemicals are used.
A One particular type of digital system uses a Photostimulable Phosphor Plate (aka PSP - Phosphor Plate) in place of the film. After X-ray exposure the plate (sheet) is placed in a special scanner where the latent formed image is retrieved point by point and digitized, using a laser light scanning. The digitized images are stored and displayed on the computer screen. And This method is half way between old film based technology and the current direct digital imaging technology. It is similar to the film process because it involves the same image support handling and differs because the chemical development process is replaced by the scanning process. And This is not much faster that film processing and the resolution and sensitivity performances are contested. However it has the clear advantage to be able to fit with any pre-existing equipment without a any modification because it replaces just the existing film.
Also some times the term "Digital X-rays" is used to designate the scanned film documents which further are handled by computers.
The other types of digital imaging technologies use electronic sensors. A majority of them first convert the X-rays in light (using a GdO2S or CsI layer) which is further captured using a CCD or a CMOS image sensor. Few of them use a hybrid arrangement which first convert the X-ray into electricity (using a CdTe layer) and then this electricity is the captured as an image by a reading section based on CMOS technology.
And In current state-of-the-art digital systems, the image quality is vastly superior to conventional film-based systems.

Historical milestones for Digital Panoramic Systems
1995 - DXIS, the world wide first dental digital panoramic X-rays system available on the market, introduced by Signet (France). DXIS targets to retrofit all the panoramic models.
1997 - SIDEXIS, of Siemens (currently Sirona, Germany) offered a digital option for Ortophos Plus panoramic unit, DigiPan of Trophy Radiology (France) offered an a digital option for the OP100 panoramic made by Instrumentarium (Finland).
1998-2004 - many panoramic manufacturers offered their own digital system.

Diagnostic uses
OPGs are used by Dentists to provide information on are:

* Impacted wisdom teeth
* Periodontal bone loss
* Finding the source of dental pain
* Assessment for the placement of dental implants
* Orthodontic assessment

And The most common use is to determine the status of wisdom teeths.

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