Flat-panel detector
Flat-panel detectorsare a class ofsolid-statex-raydigital radiographydevices similar in principle to theimage sensorsused in digital photography and video. They are used in bothprojectional radiographyand as an alternative tox-ray image intensifiers(IIs) influoroscopyequipment.
Principles
[edit]
X-rayspass through the subject being imaged and strike one of two types of detectors.
Indirect detectors
[edit]Indirect detectors contain a layer ofscintillatormaterial, typically eithergadolinium oxysulfideorcesium iodide,which converts the x-rays into light. Directly behind the scintillator layer is anamorphous silicondetector array manufactured using a process very similar to that used to makeLCDtelevisions and computer monitors. Like aTFT-LCDdisplay, millions of roughly 0.2 mmpixelseach containing athin-film transistorform 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 aphotodiodewhich 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 thesensor arrayin order to produce an accurate and sensitive digital representation of the x-ray image.[2]
Direct FPDs
[edit]Direct conversion imagers utilizephotoconductors,such as amorphousselenium(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. Abiasvoltage 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 athin-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 formammography,in which high resolution is desirable to identifymicrocalcifications.[6]
Advantages and disadvantages
[edit]
Flat-panel detectors are more sensitive and faster thanfilm.Their sensitivity allows a lower dose of radiation for a given picture quality than film. Forfluoroscopy,they are lighter, far more durable, smaller in volume, more accurate, and have much less image distortion thanx-ray image intensifiersand can also be produced with larger areas.[7]Disadvantages compared to IIs can include defective image elements, higher costs and lower spatial resolution.[8]
Ingeneral radiography,there are time and cost savings to be made overcomputed radiographyand (especially) film systems.[9][10]In theUnited States,digital radiographyis on course to surpass use of computed radiography and film.[11][12]
Inmammography,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 forbreast screening programmes.[14][15]
See also
[edit]References
[edit]- ^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.
- ^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.PMID12271399.S2CID16677678.
- ^Direct vs. Indirect ConversionArchivedJanuary 2, 2010, at theWayback Machine
- ^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.PMID9434965.
- ^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.
- ^M.J. Yaffe, “Detectors for Digital Mammography,” in Digital Mammography, edited by U. Bick and F. Diekmann (2010).
- ^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.PMC2663651.PMID16862412.
- ^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.PMID21415199.
- ^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.PMC3613258.PMID12532253.
- ^"CR versus DR -- what are the options?".AuntMinnie.com.31 July 2003.Retrieved23 July2017.
- ^"Medicare to cut analog x-ray payments starting in 2017".AuntMinnie.com.7 February 2016.Retrieved23 July2017.
- ^"Digital Radiology: Global Transition of the X-ray Image Capture Process".Imaging Technology News.8 February 2013.Retrieved23 July2017.
- ^Markey, Mia K. (2012).Physics of Mammographic Imaging.Taylor & Francis. p. 9.ISBN9781439875469.
- ^NHS Breast Screening Programme (2016).Clinical guidelines for breast cancer screening assessment(4 ed.). Public Health England.
- ^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.PMID20129267.S2CID31652981.
