In a flat-panel digital radiography detector, the absorber is coupled to what element to form the image?

• A. Thin-film transistor
• B. Photomultiplier tube
• C. Scintillating crystal
• D. Photodiode array

Answer: (A)

In flat-panel digital radiography detectors, the method of turning X-ray energy into a measurable signal hinges on how the absorber interacts with the readout. In direct-conversion designs, the absorber material (such as amorphous selenium) is placed directly on top of a thin-film transistor (TFT) array. When an X-ray photon is absorbed, it creates charge carriers right in the absorber, and these charges are collected by the adjacent TFT electrodes at each pixel. The TFTs then read out and transfer the signal to form the digital image. This direct coupling means the image is produced without an intermediate light stage, giving fast readout and good spatial resolution.

Photomultiplier tubes are not used in modern flat-panel detectors; they belong to older image-intensifier setups. A scintillating crystal is used in indirect detectors to first convert X-rays to light, which is then captured by a photodiode array and ultimately read out, rather than being coupled directly to the readout transistor at each pixel.