X-ray detector

X-ray detectors are devices used to measure the

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.^[1]

Typical x-ray film contains

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.^[7] 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.^[8] Where X-ray films required wet processing facilities, newer digital technologies do not. Digital archiving of images also saves physical storage space.^[9]

Phosphor plate radiography^[10] is a method of recording X-rays using photostimulated luminescence (PSL), pioneered by Fuji in the 1980s.^[11] 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.^[12] 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).^[13]

X-rays are also used in "real-time" procedures such as

Solid state detectors use

Practical application in medical imaging started in the early 2000s.^[18] 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.^[19] However Selenium's low atomic number means a thick layer is required to achieve sufficient sensitivity.^[20]

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.

X-rays going through a

Gas detectors are usually single pixel detectors measuring only the average dose rate over the gas volume or the number of interacting photons as explained above, but they can be made spatially resolving by having many crossed wires in a wire chamber.

It was demonstrated in the 1960s that silicon PN

Self-developing radiochromic film can provide very high resolution measurements, for dosimetry and profiling purposes, particularly in radiotherapy physics.^[32]