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

• A. nominal focal spot
• B. measured focal spot
• C. effective focal spot
• D. actual focal spot

Answer: (B)

The main idea is that the size seen on the image receptor is the effective focal spot. This is the focal spot as projected through the geometry of the imaging setup onto the image receptor, so it reflects how large the focal spot appears after magnification and how it contributes to geometric unsharpness. The effective focal spot depends on the actual focal spot size and the magnification factor (SID divided by SOD). The manufacturer’s nominal focal spot is just the stated size, and the actual focal spot is the physical size on the anode. “Measured focal spot” isn’t the standard term for the projection onto the receptor, whereas the accepted term for the size seen at the image receptor is the effective focal spot.