# Radiographic Technique Parameters > Radiographic technique parameters in Radiology Technology: kVp, mAs, the 15 per cent rule, inverse square law, exposure maintenance formula and AEC basics. - Canonical URL: https://prepelephant.com/topics/allied/radiology-and-imaging-technology/radiographic-technique-parameters - Exam / course: Allied Health · Subject: Radiology and Imaging Technology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Radiographic Technique Parameters", PrepElephant, https://prepelephant.com/topics/allied/radiology-and-imaging-technology/radiographic-technique-parameters ## Direct answer Four numbers decide every exposure: kilovoltage (beam quality, penetration and contrast), milliamperage and time together as mAs (quantity of photons reaching the receptor), source-to-image distance, and object-to-image distance. The 15 per cent rule says raising kVp by 15 per cent doubles receptor exposure, so it can replace a doubling of mAs when you must shorten exposure time. Distance obeys the inverse square law — doubling the SID quarters the beam intensity — and the exposure maintenance formula mAs₂ = mAs₁ × (D₂ ÷ D₁)² restores the same receptor exposure at a new distance. Automatic exposure control terminates the exposure at a preset radiation dose to the ionisation chamber, not a preset time. ## What you must remember - kVp controls penetration and contrast: raising kVp lowers subject contrast (more Compton scatter reaches the film) but lets you cut mAs and time. - mAs controls receptor exposure only; within limits it changes density, not contrast, on the image. - 15 per cent rule: +15% kVp ≈ ×2 receptor exposure; +30% kVp ≈ ×4. Use it to halve time for a restless patient, accepting a contrast penalty. - Inverse square law: intensity ∝ 1 ÷ distance²; exposure maintenance formula: mAs₂ = mAs₁ × (D₂ ÷ D₁)². - Standard distances: 100 cm focus-to-film distance for most work, 180 cm for chest radiography to reduce magnification and geometric blur. - Variable-kVp charts add 2 kVp per centimetre of part thickness; Sante's rule for the abdomen: kVp = (2 × thickness in cm) + 40 — a classic viva one-liner. - AEC: select chambers over the anatomy of interest, respect the minimum response time, and rely on the backup timer to catch a failed termination. ## Working through an exposure change Take a supine KUB technique of 70 kVp, 40 mAs at 100 cm and change it three ways, as an exam problem would. First, distance: the film must now be taken at 180 cm because the patient cannot be moved from a trolley against the wall stand. Exposure maintenance gives 40 × (180 ÷ 100)² = 40 × 3.24 ≈ 130 mAs — far beyond a breath-hold, which is precisely why abdomen work stays at 100 cm. Second, time: the patient will not hold still for 0.4 s at 100 mA. Applying the 15 per cent rule, 70 → 80 kVp doubles receptor exposure, so 20 mAs at 80 kVp matches 40 mAs at 70 kVp, and 200 mA for 0.1 s achieves it. The price is lower contrast on a thick abdomen; on a chest it is exactly the trade you want anyway. Third, AEC: the same KUB on a Bucky with two central chambers selected. Set 75 kVp and 300 mA, choose the shortest time the generator allows, and the chamber terminates the exposure itself; your mAs setting is only a ceiling. Two practical cautions follow. If the patient's abdomen is thicker than the chamber calibration field expects (a very large or a paediatric patient), termination comes late or never — the backup timer, usually set at about 150 to 250 per cent of expected time, ends the exposure and prevents a burn-out generator run. And if you select a chamber lying outside the anatomy — for example over an air-filled gastric bubble — the chamber "sees" too little radiation and the exposure runs long, overexpressing the film. ## Where students slip The 15 per cent rule gets applied backwards more often than forwards: it is a 15 per cent increase, not a 15 mAs change, and it is not symmetric on the contrast axis — you cannot recover lost contrast by adding mAs. The second slip is distance: candidates halve mAs when going from 200 cm to 100 cm; the correct factor is (100 ÷ 200)² = one quarter, so halving is not enough. Third, AEC is treated as "automatic, therefore I need not think": chamber selection and patient positioning still decide whether the automatic termination happens at the right moment. ## Frequently asked questions ### What does the 15 per cent rule state? Increasing kVp by 15 per cent approximately doubles receptor exposure, allowing mAs to be halved; conversely, reducing kVp by 15 per cent requires mAs to be doubled. ### How is mAs adjusted when the focus-to-film distance changes? By the exposure maintenance formula: mAs₂ = mAs₁ × (D₂ ÷ D₁)², a direct consequence of the inverse square law. ### Which parameter controls radiographic contrast? Kilovoltage primarily, because it sets the energy-dependent balance of photoelectric and Compton attenuation between tissues; mAs alters quantity of exposure, not contrast. ### What is the minimum response time of an AEC system? The shortest exposure the AEC can produce — typically about 1 millisecond — below which the system cannot terminate accurately, relevant for thin body parts and paediatrics where preset short techniques may be preferable. ### Why is chest radiography performed at 180 cm? To reduce magnification of the heart and geometric blurring by increasing source-to-object distance, at the cost of a much larger mAs demand under the inverse square law.