The projected focal spot size measured along the central axis of the x-ray tube at the image receptor is known as the:

• A. Nominal focal spot
• B. Actual focal spot
• C. Measured focal spot
• D. Effective focal spot

Answer: (C)

The key idea is how the focal spot size appears at the image receptor due to the line-focus principle. The actual focal spot is the physical size on the anode, but the angle of the anode causes the spot to be projected differently toward the image receptor. The size that is seen along the central axis at the image receptor is the effective focal spot—that is, the projected focal spot size. This is the figure that governs image sharpness, because it’s the size that photons effectively originate from when forming the image. The nominal size is simply the rated value, and the actual focal spot is the true physical dimension; terms like measured focal spot aren’t standard descriptors for this projection.