Which of the following are primary factors that limit spatial resolution in a digital radiography system?

• A. Focal length, film speed, patient thickness, exposure time.
• B. Detector material, light spread, and screen efficiency.
• C. Pixel size, focal spot size, geometric unsharpness, sampling frequency, and intrinsic MTF.
• D. Window level, window width, and grayscale bit depth.

Answer: (C)

Spatial resolution in digital radiography is determined by how finely the image is sampled and how sharply detail is preserved by the imaging chain, from the x-ray interaction to the detector’s response. The factors that most directly limit this are the detector’s pixel size (the sampling grid), the finite focal spot size (which causes geometric blur), the geometric unsharpness that arises from object-to-detector geometry, the sampling frequency (Nyquist limit set by pixel pitch), and the detector’s intrinsic ability to transfer contrast at different spatial frequencies (intrinsic MTF).

Pixel size sets the smallest detail you can sample; smaller pixels allow finer detail, but they also require more signal to maintain image quality. The focal spot size contributes to blur because a larger focal spot projects more geometric spread of the x-ray beam, which blurs edges. Geometric unsharpness, quantified by how focal spot size interacts with object-to-image distances (for example, unsharpness grows with larger OID and smaller SID), directly degrades sharpness. Sampling frequency dictates the maximum spatial detail that can be captured by the digital grid; if the sampling is too coarse, high-frequency detail is lost (Nyquist limit). Intrinsic MTF describes how well the detector itself preserves contrast at each spatial frequency; a detector with low MTF will blur fine structures even if sampling is fine.

The other options don’t capture this combination of sampling, geometry, and detector response. They either reference display settings or non-critical factors for resolution (like exposure time or patient thickness) or mix in screen/film concepts that aren’t primary limits in digital systems.