Flat-panel detector

Flat-panel detectors are a class of solid-state x-ray digital radiography devices similar in principle to the 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.

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

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 scintillator material, typically either gadolinium oxysulfide or cesium iodide, which converts the x-rays into light. Directly behind the scintillator layer is an amorphous silicon detector array manufactured using a process very similar to that used to make LCD televisions and computer monitors. Like a TFT-LCD display, millions of roughly 0.2 mm 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, 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

References
  1. Kump, K; Grantors, P; Pla, F; Gobert, P (December 1998). "Digital X-ray detector technology". RBM-News. 20 (9): 221–226. doi:10.1016/S0222-0776(99)80006-6.
  2. Kotter, E.; Langer, M. (19 March 2002). "Digital radiography with large-area flat-panel detectors". European Radiology. 12 (10): 2562–2570. doi:10.1007/s00330-002-1350-1. PMID 12271399. S2CID 16677678.
  3. Direct vs. Indirect Conversion Archived January 2, 2010, at the Wayback Machine
  4. Zhao, W.; Rowlands, J.A. (1995). "Digital radiology using active matrix readout of amorphous selenium: theoretical analysis of detective quantum efficiency". Medical Physics. 24 (12): 1819–33. doi:10.1118/1.598097. PMID 9434965.
  5. Zhao, Wei; Hunt, D.C.; Tanioka, Kenkichi; Rowlands, J.A. (September 2005). "Amorphous selenium flat panel detectors for medical applications". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 549 (1–3): 205–209. Bibcode:2005NIMPA.549..205Z. 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. Seibert, J. Anthony (22 July 2006). "Flat-panel detectors: how much better are they?". Pediatric Radiology. 36 (S2): 173–181. doi:10.1007/s00247-006-0208-0. PMC 2663651. PMID 16862412.
  8. Nickoloff, Edward Lee (March 2011). "AAPM/RSNA Physics Tutorial for Residents: Physics of Flat-Panel Fluoroscopy Systems". RadioGraphics. 31 (2): 591–602. doi:10.1148/rg.312105185. PMID 21415199.
  9. Andriole, Katherine P. (1 September 2002). "Productivity and Cost Assessment of Computed Radiography, Digital Radiography, and Screen-Film for Outpatient Chest Examinations". Journal of Digital Imaging. 15 (3): 161–169. doi:10.1007/s10278-002-0026-3. PMC 3613258. 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. 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. Markey, Mia K. (2012). Physics of Mammographic Imaging. Taylor & Francis. p. 9. ISBN 9781439875469.
  14. NHS Breast Screening Programme (2016). Clinical guidelines for breast cancer screening assessment (4 ed.). Public Health England.
  15. Lee, Carol H.; Dershaw, D. David; Kopans, Daniel; Evans, Phil; Monsees, Barbara; Monticciolo, Debra; Brenner, R. James; Bassett, Lawrence; Berg, Wendie; Feig, Stephen; Hendrick, Edward; Mendelson, Ellen; D'Orsi, Carl; Sickles, Edward; Burhenne, Linda Warren (January 2010). "Breast Cancer Screening With Imaging: Recommendations From the Society of Breast Imaging and the ACR on the Use of Mammography, Breast MRI, Breast Ultrasound, and Other Technologies for the Detection of Clinically Occult Breast Cancer". Journal of the American College of Radiology. 7 (1): 18–27. doi:10.1016/j.jacr.2009.09.022. PMID 20129267. S2CID 31652981.
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