Within what percentage of SID must the collimator light and the actual irradiated area be accurate?

Within what percentage of SID must the collimator light and the actual irradiated area be accurate?

• A. 2%
• B. 5%
• C. 10%
• D. 15%

Answer: (A)

For safe and consistent radiography, the light field that you see on the collimator must match the actual X-ray beam within a very small error, expressed as a percentage of the source-to-image-distance (SID). This alignment ensures the right anatomy is exposed, protects patient tissues outside the intended area, and helps keep patient dose predictable. Two percent is the standard tolerance because it keeps misalignment very small in practical terms. For example, at a SID of 40 inches, the allowed difference between the light field and the irradiated field is 0.8 inches. At 72 inches, the allowance would be about 1.44 inches. Larger tolerances would let substantial portions of tissue receive unintended exposure or lead to cropping of anatomy, increasing the need for repeats and patient dose. So, the best answer reflects the tight 2% tolerance to maintain accurate field size and patient safety; larger percentages would permit clinically unacceptable misalignment.

For safe and consistent radiography, the light field that you see on the collimator must match the actual X-ray beam within a very small error, expressed as a percentage of the source-to-image-distance (SID). This alignment ensures the right anatomy is exposed, protects patient tissues outside the intended area, and helps keep patient dose predictable.

Two percent is the standard tolerance because it keeps misalignment very small in practical terms. For example, at a SID of 40 inches, the allowed difference between the light field and the irradiated field is 0.8 inches. At 72 inches, the allowance would be about 1.44 inches. Larger tolerances would let substantial portions of tissue receive unintended exposure or lead to cropping of anatomy, increasing the need for repeats and patient dose.

So, the best answer reflects the tight 2% tolerance to maintain accurate field size and patient safety; larger percentages would permit clinically unacceptable misalignment.