Which metric is used to evaluate the spatial resolution performance of a digital radiography system?

• A. Noise Power Spectrum
• B. Contrast-detail analysis
• C. Dynamic Range
• D. Modulation Transfer Function

Answer: (D)

The main idea here is how well a digital radiography system preserves image detail as the level of detail (spatial frequency) increases. The Modulation Transfer Function (MTF) is the tool that quantifies this directly. It describes how the system transfers contrast from the object to the image for patterns of different spatial frequencies. In other words, it tells you how much of the original contrast of fine details is preserved when those details are represented at various sizes in the image.

MTF is obtained from the system’s impulse response (often described by the point-spread function or line-spread function) and shows how the image modulation (contrast) diminishes as you move to higher frequencies, where finer details reside. A higher MTF at a given frequency means the system preserves contrast for that level of detail better, indicating superior spatial resolution. As frequency increases, the MTF typically decreases, reflecting the limits of the imaging chain to reproduce very fine structures.

The other options relate to image quality in different ways but do not directly measure how well the system resolves fine detail across spatial frequencies. Noise Power Spectrum describes how noise is distributed across frequencies, not how detail is transferred. Dynamic range concerns the span between the largest and smallest detectable signals. Contrast-detail analysis evaluates how visible details are across a range of contrasts and sizes, which is more about perceptual detectability than the fundamental frequency-based transfer of detail.