Which of the following units is used to express the resolution of a diagnostic image?

The resolution of a diagnostic image is primarily expressed in line pairs per millimeter (lp/mm). This unit measures the ability of a system to distinguish between two closely spaced objects. Higher values of line pairs indicate better resolution, meaning that fine details in the image can be distinguished more clearly. This metric is crucial in fields like radiology, where distinguishing small differences in tissue densities or structures can significantly impact diagnosis and treatment.

In contrast, Hounsfield units are used for quantifying the density of tissues in computed tomography (CT) imaging, which is not a measure of resolution. Decibels are primarily a measure of sound intensity and are irrelevant in the context of image resolution. Pixel count refers to the total number of pixels in an image, which can contribute to overall image quality, but does not directly assess the system's ability to resolve fine details in the context of diagnostic imaging. Therefore, line pairs per millimeter stands out as the most relevant and accurate unit for expressing image resolution in diagnostic images.