Which device acts as the switch within a flat-panel detector to release the collected charge?

• A. Thin-film transistor (TFT)
• B. Scintillation layer
• C. Photodiode
• D. Charge-coupled device (CCD)

Answer: (A)

The switch in a flat-panel detector is the thin-film transistor. After the scintillator converts x-rays to light and the photodiode collects that light as electrical charge, each pixel stores the charge on a small capacitor. The TFT at that pixel acts as a switch: when activated, it connects the storage capacitor to the readout electronics so the charge can be released and digitized; when off, the charge remains stored with minimal leakage. The scintillation layer and the photodiode perform the energy conversion (x-ray to light, then light to charge), not switching. A CCD is a different detector approach that uses charge transfer across a device rather than per-pixel switching in a flat-panel array.