In direct capture flat-panel detectors, which material directly converts x-ray energy into electrical charge?

• A. Photodiode
• B. Scintillation layer
• C. Amorphous selenium (A-Se) detector
• D. TFT array

Answer: (C)

Direct capture means the X-ray photon is converted straight into an electrical signal in the detector layer. Amorphous selenium acts as a photoconductor, absorbing X-rays and directly generating electron-hole pairs. An applied electric field then collects these charges, producing a measurable electrical signal without any intermediate light conversion.

In contrast, a scintillation layer (such as CsI) first converts X-ray energy into visible light, which is then detected by a photodiode and read out by the TFT array—this is indirect conversion. The photodiode and TFT array themselves don’t directly convert X-rays to charge; they handle the light-to-electrical conversion and the signal readout, respectively.