Which of the following occurs when the anode angle decreases?

• A. Effective focal spot size decreases and heel effect increases
• B. Effective focal spot size increases and heel effect decreases
• C. Both increase
• D. Both decrease

Answer: (A)

When the anode angle is reduced, two geometry-based effects come into play: the line-focus principle and the heel effect. The line-focus principle tells us that the effective focal spot on the image is the projection of the actual focal spot through the angled anode surface. A smaller (shallower) anode angle makes this projection larger, so the effective focal spot size increases. At the same time, the heel effect becomes more pronounced with a smaller angle because photons exiting toward the cathode side experience less attenuation than those on the anode side, and the differences in path length through the anode material are greater, leading to a stronger intensity variation across the beam.

So, decreasing the anode angle leads to an increased heel effect and an increased effective focal spot size. The option that mentions the heel effect increasing aligns with this, while the part asserting that the effective focal spot size decreases is opposite to the geometry involved.