Radiographic Technique Parameters
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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.