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Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography. The roentgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air. The rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram.
The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is numerically equal to the gray (Gy). X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.
X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the dose. To generate an image of the cardiovascular system, including the arteries and veins (angiography) an initial image is taken of the anatomical region of interest. A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators. The most commonly known methods are photographic plates, photographic film in cassettes, and rare earth screens. Before the advent of the digital computer and before invention of digital imaging, photographic plates were used to produce most radiographic images. Since photographic plates are sensitive to X-rays, they provide a means of recording the image, but they also required much X-ray exposure (to the patient), hence intensifying screens were devised.
Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue.
Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. For many applications, counters are not sealed but are constantly fed with purified gas, thus reducing problems of contamination or gas aging.
Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier.
This process produces an emission spectrum of X-ray frequencies, sometimes referred to as the spectral lines. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube. A second image is then taken of the same region after iodinated contrast material has been injected into the blood vessels within this area. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen.
In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. The spectral lines generated depend on the target (anode) element used and thus are called characteristic lines. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. Photographic film largely replaced these plates, and it was used in X-ray laboratories to produce medical images.
In the pursuit of a non-toxic contrast material, many types of high atomic number elements were evaluated.
After the plate is X-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory.
The radiologist or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel. In more recent years, computerized and digital radiography has been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g. For example, the first time the forefathers used contrast it was chalk, and was used on a cadaver's vessels.
This process, known as a Townsend avalanche, is detected as a sudden current, called a "count" or "event". When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film.
Photographic plates are mostly things of history, and their replacement, the "intensifying screen", is also fading into history. The metal silver (formerly necessary to the radiographic & photographic industries) is a non-renewable resource. Thus it is beneficial that this is now being replaced by digital (DR) and computed (CR) technology. Where photographic films required wet processing facilities, these new technologies do not.
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