X-ray detectors are devices used to measure the flux, spatial distribution, spectrum, and/or other properties of X-rays. Detectors can be divided into two major categories: imaging detectors (such as photographic plates and X-ray film (photographic film), now mostly replaced by various digitizing devices like image plates or flat panel detectors) and dose measurement devices (such as ionization chambers, Geiger counters, and dosimeters used to measure the local radiation exposure, dose, and/or dose rate, for example, for verifying that radiation protection equipment and procedures are effective on an ongoing basis).
X-ray imaging
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
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
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.
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