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Exam category: Image Production → Image Acquisition & Evaluation
Why it matters: mA, time, kVp, and SID are the four dials a technologist actually turns on every single exposure. The registry tests the relationships between them far more than raw definitions.
mA and time together control how many x-rays are produced (quantity). kVp controls both quantity and how penetrating they are (quality). Distance (SID) controls how spread out that beam is by the time it reaches the patient and receptor. Master how these trade off against each other and most exposure-factor questions become arithmetic, not memorization.
mAs = mA × time (seconds) — this single number represents x-ray quantity, and it's also literally the electrostatic charge (in millicoulombs) delivered to the tube.
mAs reciprocity law: any mA/time combination that produces the same mAs produces the same radiation intensity. 400 mA × 0.1s = 40 mAs, and 200 mA × 0.2s = 40 mAs — same exposure. Registry tolerance for reciprocity variability is ±10%.
kVp controls beam penetrating power (quality) and contributes to quantity. The 15% rule: raising or lowering kVp by 15% has the same effect on receptor exposure as doubling or halving the mAs.
Worked example: 75 kVp × 1.15 = 86 kVp — roughly the same exposure as doubling the mAs at 75 kVp. To keep exposure the same while raising kVp by 15%, cut the mAs in half.
The 15% rule is a mid-range approximation, not a law. Bontrager gives the absolute anchors, and they don't sit at a constant 15%: in the 50–70 kVp range an 8–10 kVp increase doubles the exposure, while in the 80–100 kVp range it takes 12–15 kVp. Run those against the percentages and the rule is comfortably accurate through the middle of the scale and drifts at the ends — so at a low or high kVp the change needed to hold receptor exposure steady can be more or less than 15%. Apply it as the registry's rule of thumb; don't treat it as a fixed power law.
Beam intensity is inversely proportional to the square of the distance from the source. As SID increases, the beam spreads over a larger area, and intensity at the receptor drops — so mAs has to increase to compensate. As SID decreases, mAs must decrease.
Students mix up which factor to blame for a density error. If density is wrong but contrast looks fine, the problem is almost always mA or time (quantity only) — not kVp, which would also shift contrast/scale.
Source: Fauber, Radiographic Imaging and Exposure, 6th ed., Ch. 6 (mAs, the 15% rule, inverse square law) and Ch. 2 (mAs reciprocity — its ±10% variability verified directly against the 6th ed. text); the absolute kVp anchors for the 15% rule (8–10 kVp at 50–70, 12–15 kVp at 80–100) per Bontrager, Textbook of Radiographic Positioning and Related Anatomy, 10th ed., Ch. 1. Revised 2026-08-02: this module previously carried a direct quotation from Fauber 5th ed. qualifying the 15% rule across the kVp range. The 6th ed. states the rule without that qualification, so the quotation and its attribution were removed; the caveat itself is retained because Bontrager's own kVp anchors demonstrate it independently.
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