Radiological Technology: Exposure Factors, Image Quality, and Dose Control
The two dials that matter most
In projection radiography, kVp sets beam energy and therefore subject contrast, while mAs sets photon quantity and therefore receptor exposure and quantum noise. The classic trap is carrying film-screen logic into digital imaging: in CR/DR, mAs no longer controls image brightness (window/level does) - it controls noise. Underexposure shows as mottle; overexposure shows as a high exposure index (EI) with no visual penalty, which is precisely why EI/DI monitoring is a required competency.
Geometry and dose
Receptor exposure follows the inverse square law (intensity ∝ 1/d²), and the 15% rule states that a 15% kVp increase roughly doubles receptor exposure, so mAs is halved to compensate. Grids improve contrast at the cost of dose; know the four grid errors - off-level, off-center, off-focus (reversed grid), and Moiré from frequency mismatch. Collimation reduces dose, improves contrast by cutting scatter, and must never be used to "fix" an overexposed part.
Traps that appear in questions
- Confusing spatial resolution (detail, line-pairs/mm, limited by pixel size and focal spot) with contrast resolution (low-contrast detectability, limited by noise).
- Forgetting the anode heel effect - thinner patient parts belong toward the anode side.
- Mixing up AEC behaviour: AEC terminates exposure on a density/receptor signal, so a mispositioned part or wrong detector selection causes over/under-exposure, not a corrected image.
- Assuming barium and iodinated contrast have identical indications; barium is contraindicated where perforation is suspected.
Worked example
A part is exposed at 80 kVp, 20 mAs, 100 cm SID. Moving to 150 cm: intensity ratio = (100/150)² = 0.44, so new mAs ≈ 20 ÷ 0.44 ≈ 45 mAs to hold receptor exposure constant.