About half of a patient’s lifetime radiation exposure comes from medical imaging. A standard coronary CT angiogram delivers around 2.1 mSv; a calcium score delivers about 0.7 mSv. Knowing what those numbers mean — and how to make them smaller without sacrificing the diagnostic answer — is the heart of ALARA.
1 Dosimetry metrics
- Absorbed dose — energy deposited per unit mass. Measured in Grays (Gy).
- Dose equivalent & effective dose — absorbed dose adjusted for biological impact and whole-body context. Measured in Sieverts (Sv).
- CTDIvol (CT Dose Index, volumetric) and DLP (Dose-Length Product) — the per-scan values reported on the dose page. They estimate the energy deposited per unit of tissue mass.
2 Biological effects
- Deterministic effects — skin erythema, cataracts. Threshold-based: nothing happens below a certain dose, then severity scales above it.
- Stochastic effects — cancer. No threshold: probability increases linearly with dose. This is the basis for ALARA.
3 Dose-reduction strategies
- Shorten the z-axis scan length — only scan what you need.
- Lower the tube potential (kV) — particularly effective for small patients.
- Automatic Exposure Control (AEC) — modulates tube current (mA) through thinner tissues.
- Higher helical pitch — fewer overlapping rotations per scan.
- Iterative reconstruction — preserves diagnostic quality at lower noise budgets, so kV/mA can drop further.