Flat-panel detector

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A portable aSi flat-panel detector is used to visualise the movement of liquids in sand cores under high pressure.

Flat-panel detectors are a class of

image sensors used in digital photography and video. They are used in both projectional radiography and as an alternative to x-ray image intensifiers (IIs) in fluoroscopy
equipment.

Principles

Light spreading in the scintillator material leads to loss of resolution in indirect detectors which direct detectors do not experience

X-rays
pass through the subject being imaged and strike one of two types of detectors.

Indirect detectors

Indirect detectors contain a layer of

pixels each containing a thin-film transistor form a grid patterned in amorphous silicon on the glass substrate.[1] Unlike an LCD, but similar to a digital camera's image sensor chip, each pixel also contains a photodiode which generates an electrical signal in proportion to the light produced by the portion of scintillator layer in front of the pixel. The signals from the photodiodes are amplified and encoded by additional electronics positioned at the edges or behind the sensor array in order to produce an accurate and sensitive digital representation of the x-ray image.[2]

Direct FPDs

Direct conversion imagers utilize

photoconductors, such as amorphous selenium (a-Se), to capture and convert incident x-ray photons directly into electric charge.[3] X-ray photons incident upon a layer of a-Se generate electron-hole pairs via the internal photoelectric effect. A bias voltage applied to the depth of the selenium layer draw the electrons and holes to corresponding electrodes; the generated current is thus proportional to the intensity of the irradiation. Signal is then read out using underlying readout electronics, typically by a thin-film transistor (TFT) array.[4][5]

By eliminating the optical conversion step inherent to indirect conversion detectors, lateral spread of optical photons is eliminated, thus reducing blur in the resulting signal profile in direct conversion detectors. Coupled with the small pixel sizes achievable with TFT technology, a-Se direct conversion detectors can thus provide high spatial resolution. This high spatial resolution, coupled with a-Se's relative high quantum detection efficiency for low energy photons (< 30 keV), motivate the use of this detector configuration for mammography, in which high resolution is desirable to identify microcalcifications.[6]

Advantages and disadvantages

Flat-panel detector used in digital radiography

Flat-panel detectors are more sensitive and faster than

film. Their sensitivity allows a lower dose of radiation for a given picture quality than film. For fluoroscopy, they are lighter, far more durable, smaller in volume, more accurate, and have much less image distortion than x-ray image intensifiers and can also be produced with larger areas.[7] Disadvantages compared to IIs can include defective image elements, higher costs and lower spatial resolution.[8]

In general radiography, there are time and cost savings to be made over computed radiography and (especially) film systems.[9][10] In the United States, digital radiography is on course to surpass use of computed radiography and film.[11][12]

In mammography, direct conversion FPDs have been shown to outperform film and indirect technologies in terms of resolution[citation needed], signal-to-noise ratio, and quantum efficiency.[13] Digital mammography is commonly recommended as the minimum standard for breast screening programmes.[14][15]

See also

  • X-ray detectors

References

  1. doi:10.1016/S0222-0776(99)80006-6
    .
  2. S2CID 16677678
    .
  3. ^ Direct vs. Indirect Conversion Archived January 2, 2010, at the Wayback Machine
  4. PMID 9434965
    .
  5. doi:10.1016/j.nima.2005.04.053
    .
  6. ^ M.J. Yaffe, “Detectors for Digital Mammography,” in Digital Mammography, edited by U. Bick and F. Diekmann (2010).
  7. PMID 16862412
    .
  8. PMID 21415199
    .
  9. PMID 12532253
    .
  10. ^ "CR versus DR -- what are the options?". AuntMinnie.com. 31 July 2003. Retrieved 23 July 2017.
  11. ^ "Medicare to cut analog x-ray payments starting in 2017". AuntMinnie.com. 7 February 2016. Retrieved 23 July 2017.
  12. ^ "Digital Radiology: Global Transition of the X-ray Image Capture Process". Imaging Technology News. 8 February 2013. Retrieved 23 July 2017.
  13. ISBN 9781439875469
    .
  14. ^ NHS Breast Screening Programme (2016). Clinical guidelines for breast cancer screening assessment (4 ed.). Public Health England.
  15. S2CID 31652981
    .

External links
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