The 15% kVp rule: how to trade kVp and mAs in radiography

Question

This ventrodorsal radiograph of a cat was taken at 75 kVp and 5 mAs (data at the foot of the image) and the exposure was correct. The next patient is a cat of the same size that will not keep still, and you want to halve the exposure time to avoid blurring. Reference ventrodorsal radiograph:

Ventrodorsal radiograph of a cat's caudal abdomen, pelvis and pelvic limbs, with the exposure factors printed at the foot — Image: Ferndale Veterinary Clinic · Public domain, Wikimedia Commons

Which adjustment keeps a similar overall exposure?

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A. Keep 75 kVp and double the exposure time to compensate

Doubling the time increases mAs and exposure, but worsens exactly what you wanted to avoid: motion blur.

B. Reduce kVp by 15% and halve the mAs

Lowering kVp by 15% is roughly equivalent to halving exposure; if you also halve the mAs, the image is clearly underexposed (noisy on digital systems).

C. Increase kVp by 15% (about 86 kVp) and halve the mAs (2.5 mAs)

Correct. Increasing kVp by 15% roughly doubles detector exposure, which compensates for halving the mAs. The image will have slightly less contrast (a longer grey scale), an acceptable trade-off for removing motion blur.

D. Double the kVp (150 kVp) and halve the mAs

kVp is not linearly related to exposure: doubling it grossly overexposes, increases scatter and destroys contrast. The practical rule is 15%, not double.

lightbulb Key takeaway

kVp controls energy (penetration and contrast) and mAs the number of photons (exposure and, on digital systems, noise). Rule of thumb: +15% kVp ≈ ×2 mAs. For the thorax use high kVp and low mAs with short times; for bone and abdomen, lower kVp for more contrast.

Which adjustment keeps a similar overall exposure?

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