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DOI: 10.1148/rg.273065075
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Advances in Digital Radiography: Physical Principles and System Overview1

Markus Körner, MD, Christof H. Weber, MD, Stefan Wirth, MD, Klaus-Jürgen Pfeifer, MD, Maximilian F. Reiser, MD, and Marcus Treitl, MD

1 From the Department of Clinical Radiology, University Hospital Munich, Nussbaumstr 20, 80336 Munich, Germany. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received April 21, 2006; revision requested August 15 and received September 18; accepted September 18. All authors have no financial relationships to disclose.

Figure 1
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  		Figure 1.  Chart illustrates a digital radiography system. After image exposure, the imaging data are digitally processed and stored in a digital archive. A centralized image management system is used for further distribution of the images to viewing stations, information systems, and electronic patient records.

 

Figure 2
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  		Figure 2.  Chart provides a systematic overview of various types of digital detectors. CCD = charge-coupled device, FPD = flat-panel detector, TFT = thin-film transistor.

 

Figure 3
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  		Figure 3.  Drawing illustrates a CR system based on storage-phosphor image plates. Image generation is separated into two steps. First, the image plate (IP) is exposed to x-ray energy, part of which is stored within the detective layer of the plate. Second, the image plate is scanned with a laser beam, so that the stored energy is set free and light is emitted. An array of photomultipliers collects the light, which is converted into electrical charges by an analog-to-digital (A/D) converter.

 

Figure 4A
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  		Figure 4a.  Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.

 

Figure 4B
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  		Figure 4b.  Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.

 

Figure 5A
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  		Figure 5a.  CCD-based indirect conversion DR system. (a) Drawing illustrates a lens-coupled CCD-based system. The incident x-ray energy is converted into light by a scintillator. The emitted light has to be bundled by an optical lens to fit the size of the CCD chip, which subsequently converts the light energy into electrical charges. (b) Drawing illustrates a slot-scan CCD-based system. The patient is scanned with a fan-shaped beam of x-rays. A simultaneously moving CCD detector of the same size collects the emitted light and converts the light energy into electrical charges.

 

Figure 5B
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  		Figure 5b.  CCD-based indirect conversion DR system. (a) Drawing illustrates a lens-coupled CCD-based system. The incident x-ray energy is converted into light by a scintillator. The emitted light has to be bundled by an optical lens to fit the size of the CCD chip, which subsequently converts the light energy into electrical charges. (b) Drawing illustrates a slot-scan CCD-based system. The patient is scanned with a fan-shaped beam of x-rays. A simultaneously moving CCD detector of the same size collects the emitted light and converts the light energy into electrical charges.

 

Figure 6
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  		Figure 6.  Drawing illustrates an amorphous silicon–based indirect conversion DR system. X-ray energy is converted into visible light in a scintillator layer. The emitted light is then converted into electrical charges by an array of silicon-based photodiodes and read out by a TFT array.

 

Figure 7
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  		Figure 7.  Image postprocessing. The image on the far left represents the initially acquired raw data without any processing. The other three images have been digitally processed in different ways to illustrate the influence of various software tools on image appearance. Contrast enhancement (second image from left) makes anatomic structures more visible and distinguishable, contrast reduction (second image from right) results in smoothing of the structures, and edge enhancement (image on far right) provides sharper delineation of the fine structures of bones.

 

Figure 8
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  		Figure 8.  Graph illustrates the dynamic range of screen-film combinations and digital detectors. Screen-film systems have only a limited tolerance for radiation exposure, resulting in a steep and tight curve, whereas the curve for digital detectors is less steep and covers a wider range. As a result, an optimal signal response will occur over a wider exposure range with digital detectors than with screen-film combinations.

 

Figure 9
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  		Figure 9.  Graph illustrates the DQE curves for four digital detectors. CR 1 = needle-structured storage phosphor and line scanner (MD5.0/DX-S; Agfa-Ge-vaert, Mortsel, Belgium), CR 2 = unstructured storage phosphor and flying-spot scanner (MD40/ADC Compact, Agfa-Gevaert), Indirect FPD = CsI-based flat-panel detector (Pixium 4600; Trixell, Moirans, France), Direct FPD = selenium-based flat-panel detector (DR 9000; Kodak, Rochester, NY).

 




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