Which part of the detector element detects radiation?

• A. Capture area
• B. Storage capacitor
• C. Thin-film transistor (TFT) gate
• D. Scintillation layer

Answer: (A)

The part that detects radiation is the capture area—the photosensitive region of the detector element where X-ray energy is converted into electrical charge. In indirect detectors, X-rays first interact in the scintillation layer to produce light, and that light is then absorbed by the capture area (the photodiode region) to generate charge. In direct detectors, X-rays interact directly in the capture area’s photosensitive material to create charge. The charge then goes to the storage capacitor for holding and is later read out through the TFT. The scintillation layer’s job is to convert X-rays to light (a transduction step), the storage capacitor holds the signal, and the TFT gate handles addressing and readout, but the actual conversion of incident radiation into a measurable electrical signal happens in the capture area.