Which component of a detector element stores the charge collected from radiation?

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

Answer: (A)

The signal from radiation is held in a tiny capacitor inside each detector pixel. This storage capacitor collects and retains the charge generated by the radiation (in indirect detectors, light from the scintillator is converted to charge by the photodiode and then stored). The charge sits on that capacitor until the pixel is read out, which lets the electronics measure the signal accurately and can even allow integration over the exposure time.

The thin-film transistor gate mainly acts as a switch to access the stored charge and route it to the readout circuit; it doesn’t store the charge itself. The scintillation layer’s job is to convert x-rays into light, not to hold electrical charge. The idea of a capture area isn’t about storing the signal, so it isn’t the component that preserves the charge for readout.