Mosby Digital Image Acquisition Practice Test

A small change in kVp can significantly affect receptor exposure. The 15% rule is a practical guideline used in radiography to predict how exposure changes with kVp. When you drop kVp by about 15%, the photons have lower energy and are more likely to be absorbed by the patient rather than reaching the image receptor, so the number of photons that reach the receptor drops roughly by half if the mAs stays the same. That’s why receptor exposure may be halved with a 15% decrease in kVp.

The other statements stray from how kVp and mAs relate: kVp isn’t a fixed percentage of mAs, increasing kVp by 15% would not halve exposure (it would increase exposure unless mAs is adjusted), and a visible change in exposure isn’t tied to a universal 15% change in mAs—the rule describes how to balance kVp and mAs to maintain or predict exposure.