What does mAs represent and how does it affect exposure?

• A. Milliampere-seconds; a measure of the tube output independent of exposure factors
• B. Milliampere-seconds; the product determines receptor exposure
• C. Milliampere-seconds; determines beam quality
• D. Milliampere-seconds; controls image contrast

Answer: (B)

mAs stands for milliampere-seconds—the product of the tube current (mA) and the exposure time (s). This quantity controls how many X-ray photons are produced. Receptor exposure, or the amount of radiation reaching the image receptor, is determined by mAs when distance, filtration, and kVp are held constant. Beam quality—the energy of the photons—is set by kVp and filtration, not by mAs. Image contrast is influenced mainly by kVp and subject factors, not by the amount of exposure. So increasing mAs raises receptor exposure and reduces image noise, but does not change beam energy or inherent contrast, and it increases patient dose. That’s why the product most directly determines receptor exposure.