Which device may be used in place of the photomultiplier tube (PMT) within a computed radiography (CR) image reader system?

• A. Thin-film transistor (TFT)
• B. Amorphous selenium (a-Se)
• C. Charge-coupled device (CCD)
• D. Analog-to-digital converter (ADC)

Answer: (C)

In a CR image reader, the stored image on the phosphor plate is read out by stimulating it with a laser and collecting the light that is emitted as the stored energy is released. That emitted light must be converted into an electrical signal that can be processed and digitized. A photomultiplier tube is a classic detector for this light, but a charge-coupled device can perform the same role by directly converting the incoming light into an electrical signal that can be read out and digitized with high sensitivity and good spatial resolution. This makes the CCD a viable alternative to the PMT in many CR reader designs.

The other options don’t fit as the primary light-detecting element in this context. A thin-film transistor array is typically involved in direct digital detectors for display or readout but is not used as the light detector in the CR readout path. Amorphous selenium is used as a detector material in some digital radiography systems, not as the photodetector in a CR reader’s laser-scanned readout. An analog-to-digital converter is part of the signal chain that digitizes the electrical signal, not the device that converts light to an electrical signal.