To obtain an image with any type of image detector the part of the patient to be X-rayed is placed between the X-ray source and the image receptor to produce a shadow of the internal structure of that particular part of the body. X-rays are partially blocked ("attenuated") by dense tissues such as bone, and pass more easily through soft tissues. 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. 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 the pursuit of nontoxic contrast materials, many types of high atomic number elements were evaluated. Some elements chosen proved to be harmful – for example,
thorium was once used as a contrast medium (
Thorotrast) – which turned out to be toxic, causing a very high incidence of cancer decades after use. Modern contrast material has improved and, while there is no way to determine who may have a sensitivity to the contrast, the incidence of serious allergic reactions is low.
X-ray film Mechanism Typical x-ray film contains
silver halide crystal "grains", typically primarily
silver bromide. Grain size and composition can be adjusted to affect the film properties, for example to improve
resolution in the developed image. When the film is exposed to radiation the halide is
ionised and free
electrons are trapped in
crystal defects (forming a
latent image). Silver ions are attracted to these defects and
reduced, creating clusters of
transparent silver
atoms. In the developing process these are converted to
opaque silver atoms which form the viewable image, darkest where the most radiation was detected. Further developing steps stabilise the sensitised grains and remove unsensitised grains to prevent further exposure (e.g. from
visible light).
Replacement s as film x-rays. The first radiographs (X-ray images) were made by the action of X-rays on sensitized glass photographic plates. X-ray film (photographic film) soon replaced the glass plates, and film has been used for decades to acquire (and display) medical and industrial images. Gradually, digital
computers gained the ability to store and display enough data to make digital imaging possible. Since the 1990s, computerized radiography and digital radiography have been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g. to inspect welded seams). The metal
silver (formerly necessary to the radiographic & photographic industries) is a
non-renewable resource although silver can easily be reclaimed from spent X-ray film. Where X-ray films required wet processing facilities, newer digital technologies do not. Digital archiving of images also saves physical storage space.
Photostimulable phosphors Phosphor plate radiography is a method of recording X-rays using
photostimulated luminescence (PSL), pioneered by
Fuji in the 1980s. A photostimulable phosphor plate (PSP) is used in place of the photographic plate. 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. The
light given off during laser stimulation is collected by a
photomultiplier tube, and the resulting signal is converted into a digital image by computer technology. The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. The technique may also be known as computed radiography (CR).
Image intensifiers taken during
cholecystectomy X-rays are also used in "real-time" procedures such as
angiography or contrast studies of the hollow organs (e.g.
barium enema of the small or large intestine) using
fluoroscopy.
Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
Semiconductor detectors Solid state detectors use
semiconductors to detect x-rays. Direct digital detectors are so-called because they directly convert x-ray photons to electrical charge and thus a digital image. Indirect systems may have intervening steps for example first converting x-ray photons to
visible light, and then an electronic signal. Both systems typically use
thin film transistors to read out and convert the electronic signal to a digital image. Unlike film or CR no manual scanning or development step is required to obtain a digital image, and so in this sense both systems are "direct". Both types of system have considerably higher
quantum efficiency than CR. X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by
Peltier effect or even cooler
liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called
energy-dispersive X-ray spectroscopy (EDX or EDS); it is often used in small
X-ray fluorescence spectrometers.
Silicon drift detectors (SDDs), produced by conventional
semiconductor fabrication, provide a cost-effective and high resolving power radiation measurement. Unlike conventional X-ray detectors, such as Si(Li), they do not need to be cooled with liquid nitrogen. These detectors are rarely used for imaging and are only efficient at low energies. Practical application in
medical imaging started in the early 2000s. Amorphous
selenium is used in commercial large area flat panel X-ray detectors for
mammography and general
radiography due to its high spatial resolution and x-ray absorbing properties. However Selenium's low atomic number means a thick layer is required to achieve sufficient sensitivity.
Cadmium telluride (
CdTe), and its alloy with
zinc,
cadmium zinc telluride, is considered one of the most promising semiconductor materials for x-ray detection due to its wide
band-gap and high quantum number resulting in room temperature operation with high efficiency. Current applications include
bone densitometry and
SPECT but flat panel detectors suitable for radiographic imaging are not yet in production. Current research and development is focused around energy resolving
pixel detectors, such as
CERN's
Medipix detector and
Science and Technology Facilities Council's
HEXITEC detector. Common
semiconductor diodes, such as
PIN photodiodes or a
1N4007, will produce a small amount of current in
photovoltaic mode when placed in an X-ray beam.
Indirect detectors Indirect detectors are made up of a
scintillator to convert x-rays to visible light, which is read by a TFT array. This can provide sensitivity advantages over current (amorphous selenium) direct detectors, albeit with a potential trade-off in resolution. Indirect
flat panel detectors (FPDs) are in widespread use today in medical, dental, veterinary, and industrial applications. The TFT array consists of a sheet of glass covered with a thin layer of silicon that is in an amorphous or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these
thin-film transistors (TFTs) is attached to a light-absorbing photodiode making up an individual
pixel (picture element).
Photons striking the photodiode are converted into two
carriers of electrical charge, called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, X-rays first impinge upon
scintillators made from such materials as
gadolinium oxysulfide or
caesium iodide. The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array. ==Dose measurement==