In digital radiography, which statement correctly distinguishes direct-conversion detectors from indirect-conversion detectors?

• A. Direct-conversion uses a photoconductor such as amorphous selenium to directly convert X-ray energy to an electrical signal; indirect uses a scintillator such as CsI or gadolinium oxysulfide to convert X-rays to light, then to electricity.
• B. Direct-conversion uses a scintillator to convert X-rays to light and CsI; indirect uses a photoconductor like amorphous selenium.
• C. Direct-conversion is slower than indirect and uses gadolinium oxysulfide; indirect uses amorphous selenium.
• D. Direct-conversion uses photodiode array alone; indirect uses a photomultiplier.

Answer: (A)

In digital radiography, the way X-ray energy becomes an electrical signal is the key difference. Direct-conversion detectors use a photoconductor, such as amorphous selenium, to turn X-ray photons directly into electrical charge with no intermediate light step. The charge produced in the selenium is collected by electrodes and read out as the image signal. Indirect-conversion detectors, on the other hand, first use a scintillator like CsI or gadolinium oxysulfide to convert X-rays into visible light, and then a photosensor (often a silicon-based array) detects that light and converts it to an electrical signal. This two-step process is why indirect systems involve light diffusion before signal generation.

That’s why the statement identifying direct-conversion with amorphous selenium and indirect-conversion with a scintillator such as CsI or gadolinium oxysulfide is correct. The other options mix up the roles (indirect would not use a photoconductor for the primary X-ray-to-charge step), reference materials in ways that don’t fit how these detectors are built, or imply components (like a photomultiplier) not typical of modern indirect detectors.