Primary exposure factors

Primary exposure factors

Exam category: Image Production → Image Acquisition & Evaluation
Why it matters: mA, exposure time, kVp, and distance are the four factors you set before every exposure, and the registry tests the relationships between them far more often than it tests their definitions. Most exposure-factor items are arithmetic once you know which relationship applies.

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Overview

mA and exposure time together control how many x-rays are produced. kVp controls how penetrating those x-rays are, and it affects how many are produced as well. Distance controls how far the beam has spread by the time it reaches the patient and the image receptor. Everything else here is either a formula for trading one factor against another, or a rule for adjusting them when the patient, the pathology, or the equipment changes.

1. mAs: the quantity factor

mAs = mA × exposure time in seconds. The product is a single number representing x-ray quantity, and it is also literally the electrostatic charge delivered to the tube, measured in millicoulombs.

mAs is directly proportional to receptor exposure. Double the mAs and the exposure reaching the receptor doubles; halve it and the exposure halves. There is no square, no percentage, and no conversion factor to apply. This is the one relationship on the whole page that is plain multiplication.

The reciprocity law: any combination of mA and time producing the same mAs produces the same radiation intensity. 400 mA at 0.1 s is 40 mAs, and 200 mA at 0.2 s is also 40 mAs, so both deliver the same exposure. Registry tolerance for reciprocity variability is ±10%.

Reciprocity is why the mA and time controls are worth having separately. When the two are interchangeable for exposure purposes, you pick the pair that solves a clinical problem: the shortest exposure time the machine allows, with the mA raised to hold the mAs constant, because a short time is the control for motion unsharpness.

Questions often hand you an mAs and an mA and ask for the time, sometimes in milliseconds. Rearrange rather than memorizing a second formula: time = mAs ÷ mA. At 20 mAs and 400 mA, the time is 20 ÷ 400 = 0.05 s, which is 50 ms.