When using a 14 x 17 inch exposure field, the intensity of the central ray measures 100 mGy. What is the approximate intensity of the beam within the cathode side of the x-ray field?

When using a 14 x 17 inch exposure field, the intensity of the central ray measures 100 mGy. What is the approximate intensity of the beam within the cathode side of the x-ray field?

• A. 120 mGy
• B. 80 mGy
• C. 100 mGy
• D. 140 mGy

Answer: (A)

The key idea here is the heel effect: because the anode is tilted and there is absorption in the anode heel, the beam is not uniform across the field. The cathode side ends up brighter than the center (and certainly brighter than the far edge on the anode side). For a large field like 14 x 17 inches, this difference is noticeable, and a common, practical approximation is around a 20% increase in intensity on the cathode side compared with the central ray. So if the central ray is 100 mGy, the cathode side would be about 20% higher, roughly 120 mGy. That makes the 120 mGy option the best match. The other values would imply no heel effect (100 mGy) or an unrealistically large difference (like 140 mGy) for this field size.

The key idea here is the heel effect: because the anode is tilted and there is absorption in the anode heel, the beam is not uniform across the field. The cathode side ends up brighter than the center (and certainly brighter than the far edge on the anode side). For a large field like 14 x 17 inches, this difference is noticeable, and a common, practical approximation is around a 20% increase in intensity on the cathode side compared with the central ray.

So if the central ray is 100 mGy, the cathode side would be about 20% higher, roughly 120 mGy. That makes the 120 mGy option the best match.

The other values would imply no heel effect (100 mGy) or an unrealistically large difference (like 140 mGy) for this field size.