What is the approximate difference in exposure intensity between the two edges of a 14x17 film due to heel effect?

• A. 100%
• B. 5%
• C. 30%
• D. 50%

Answer: (C)

Heel effect is the variation in X-ray beam intensity across the field caused by the angle of the anode target and self-absorption within the target and filters. Photons that head toward the anode side pass through more material, so fewer photons exit that edge, while the cathode side encounters less attenuation and is more intense. For a large field like 14x17, this difference is noticeable and is typically about 30% between the edges, with the cathode edge being the brighter by roughly that amount. So the best estimate for the exposure difference is about 30%. The other options would imply far larger or far smaller differences than what is commonly observed for this field size.