Extremity technique: kVp, mAs and grids

kVp and mAs

Extremities are thin and need contrast between bone and soft tissue, so they use the lowest kVp range in the department, roughly the 50s to the low 70s for most digital systems, and a low mAs. Thicker parts need more. A useful rule is the 15 percent rule: raising kVp by about 15 percent has about the same effect on receptor exposure as doubling the mAs. Lowering kVp by 15 percent is like halving the mAs. Most extremity techniques are set manually from a technique chart, because the AEC chambers are larger than a finger or wrist.