X-ray detector

Source: Wikipedia, the free encyclopedia.
X-ray generator
and an imaging detector.

X-ray detectors are devices used to measure the

spectrum, and/or other properties of X-rays
.

Detectors can be divided into two major categories: imaging detectors (such as

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

Fish bone pierced in the upper esophagus. Right image without contrast medium, left image during swallowing with contrast medium.

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]

X-ray film

Mechanism

See also: Photographic film

Typical x-ray film contains

visible light).[5]: 159 [6]

Replacement

A video discussing a study that showed that digital x-rays were equally effective in identifying occupational lung diseases 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.[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]

Photostimulable phosphors

Main article: Photostimulated luminescence
A piece of photostimulable phosphor plate

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]

Image intensifiers

Main article: X-ray image intensifier
Radiograph taken during cholecystectomy

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

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

See also: Digital radiography

Solid state detectors 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".[14] Both types of system have considerably higher quantum efficiency than CR.[14]

Direct detectors

Since the 1970s,

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.[16]

Practical application in medical imaging started in the early 2000s.[17] 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.[18] However Selenium's low atomic number means a thick layer is required to achieve sufficient sensitivity.[19]

HEXITEC detector.[23][24]

Common

1N4007, will produce a small amount of current in photovoltaic mode when placed in an X-ray beam.[25][26]

Indirect detectors

Indirect detectors are made up of a

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

Gas detectors

Main article:
Gaseous ionization detectors
Plot of ion current as function of applied voltage for a wire cylinder gaseous radiation detector.

X-rays going through a

electrons. An incoming photon will create a number of such ion pairs proportional to its energy. If there is an electric field in the gas chamber ions and electrons will move in different directions and thereby cause a detectable current. The behaviour of the gas will depend on the applied voltage
and the geometry of the chamber. This gives rise to a few different types of gas detectors described below.

dose rate the gas is exposed to.[7] Ion chambers are widely used as hand held radiation survey meters
to check radiation dose levels.

avalanche effect
greatly increasing the output signal. Since every electron cause an avalanche of approximately the same size the collected charge is proportional to the number of ion pairs created by the absorbed x-ray. This makes it possible to measure the energy of each incoming photon.

Geiger–Müller counters use an even higher electric field so that UV-photons are created.[28] These start new avalanches, eventually resulting in a total ionization of the gas around the anode wire. This makes the signal very strong, but causes a dead time after each event and makes it impossible to measure the X-ray energies.[29]

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.

Silicon PN solar cells

It was demonstrated in the 1960s that silicon PN

extreme UV, soft X-rays, and hard X-rays. This form of detection operates via photoionization, a process where ionizing radiation strikes an atom and releases a free electron.[30] This type of broadband ionizing radiation sensor requires a solar cell, an ammeter
, and a visible light filter on top of the solar cell that allows the ionizing radiation to hit the solar cell while blocking unwanted wavelengths.

Radiochromic film

Main article: Radiochromic film

Self-developing radiochromic film can provide very high resolution measurements, for dosimetry and profiling purposes, particularly in radiotherapy physics.[31]

References

  1. ^ "Contrast Medium Reactions: Overview, Types of Iodinated Contrast Media, Adverse Reactions to ICM". Medscape. 2 June 2016. Retrieved 17 December 2016.
  2. ^ "Radiographic Film". NDT Resource Center. Retrieved 16 December 2016.
  3. ISBN 978-1-4613-0383-1
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  4. ISBN 9783527406111
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  5. ISBN 978-92-0-131010-1
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  6. ^ "Developing Film". NDT Resource Centre. Archived from the original on 7 February 2020. Retrieved 16 December 2016.
  7. ^
    S2CID 4393848
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  8. ^ Masebinu, Samson O.; Muzenda, Edison (2014). Review of Silver Recovery Techniques from Radiographic Effluent and X-ray Film Waste (PDF). Proceedings of the World Congress on Engineering and Computer Science. Vol. II. San Francisco.
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  18. ISBN 978-1-4614-5067-2
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  19. ^ a b Ristić, S G (18–19 October 2013). "The digital flat-panel X-Ray detectors" (PDF). Proceedings of the Third Conference on Medical Physicsand Biomedical Engineering. Skopje: IAEA. pp. 65–71.
  20. S2CID 5829568
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  21. PMID 22412323
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  23. doi:10.1088/1748-0221/10/04/C04015
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  24. doi:10.1016/j.nima.2009.01.046
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  25. ^ Gonzalez, G, J. (October 2016). "Desarrollo de un detector de rayos X usando fotodiodos" [Development of an X-ray detector using photodiodes]. INIS (in Spanish). 48 (7): 13.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  26. ^ "1n4007 diode as an X-Ray detector". YouTube. Retrieved 4 December 2019.
  27. ^ Albert C. Thompson. X-Ray Data Booklet, Section 4-5: X-ray detectors (PDF).
  28. ISBN 978-1-4614-4012-3
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  29. ISBN 9780080569642
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  30. ^ Photovoltaic Effect Produced in Silicon Solar Cells by x-ray and Gamma-Rays, Karl Scharf, January 25, 1960, Journal of Research of the National Bureau of Standards
  31. ISSN 0094-243X
    .
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