Lange Mammography Practice Exam

If the focal spot size of a mammography unit is 0.3 mm, what does this indicate about the effective focal spot?

The actual focal spot is 0.3 mm The beam size on the receptor is 0.3 mm The distance to the receptor is 0.3 mm The effective focal spot size is 0.3 mm

Correct Answer:

The effective focal spot size is 0.3 mm

Explanation:

In mammography, the focal spot size you see listed is the effective focal spot—the size of the focal spot projected onto the image receptor, which directly influences geometric unsharpness and image sharpness. Because the x-ray beam diverges from the focal spot, the actual focal spot (the physical size at the anode) can be smaller, but the geometry of the setup magnifies its appearance on the receptor. A stated focal spot size of 0.3 mm therefore indicates that the effective focal spot at the receptor is 0.3 mm, the dimension that governs resolution in the image. The actual focal spot could be smaller, and the beam footprint on the receptor depends on magnification, not just the raw focal spot size.

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