The first conversion step in the scintillator-based chain described is to convert X-rays into light.

• A. X-rays to light
• B. Light to X-rays
• C. Charge to light
• D. Light to charge

Answer: (A)

In this imaging chain, the scintillator material absorbs the incoming X-ray photons and then relaxes the excited states by emitting photons in the visible (or near-UV) range. This process is the first conversion step, turning X-ray energy into light. The other options describe processes that either reverse the direction (light producing X-rays), or involve electrical charge producing light (or light producing charge) in contexts that aren’t the initial step in this chain. After the scintillator emits light, a photodetector would convert that light into an electrical signal.