With a source-to-image distance (SID) of 80 cm, the light field should match the exposure field within which range?

With a source-to-image distance (SID) of 80 cm, the light field should match the exposure field within which range?

• A. +/- 0.8 cm (0.3 inches)
• B. +/- 4.0 cm (1.6 inches)
• C. +/- 1.6 cm (0.6 inches)
• D. +/- 8.0 cm (3.1 inches)

Answer: (C)

The test is about how closely the light field needs to align with the exposure field. In radiography, the light field is used to visualize and plan the actual X-ray beam, so the two fields should match within a small tolerance to ensure the anatomy is fully covered and unnecessary tissue isn’t irradiated. For an SID of 80 cm, the acceptable mismatch is about 2% of the distance. 2% of 80 cm equals 1.6 cm, which is roughly 0.6 inches. So the light field should match the exposure field within ±1.6 cm (about ±0.6 inches). This keeps the collimation accurate across typical distances and reduces the risk of cutting off anatomy or exposing skin outside the area of interest.

The test is about how closely the light field needs to align with the exposure field. In radiography, the light field is used to visualize and plan the actual X-ray beam, so the two fields should match within a small tolerance to ensure the anatomy is fully covered and unnecessary tissue isn’t irradiated.

For an SID of 80 cm, the acceptable mismatch is about 2% of the distance. 2% of 80 cm equals 1.6 cm, which is roughly 0.6 inches. So the light field should match the exposure field within ±1.6 cm (about ±0.6 inches).

This keeps the collimation accurate across typical distances and reduces the risk of cutting off anatomy or exposing skin outside the area of interest.