About CT Radiation Dose
CT accounts for roughly 25% of all medical radiation exposures but contributes ~75% of the collective effective dose from medical imaging in the United States. Understanding and communicating CT dose is essential for clinical decision-making, patient counseling, and quality improvement.
Key Dose Metrics
CTDIvol (mGy) — Volume CT Dose Index. A standardized measure of radiation output for a specific CT protocol. Represents the dose to the phantom (not the patient). Does not account for scan length.
DLP (mGy·cm) — Dose-Length Product. CTDIvol × scan length. Accounts for how long the scan ran. This is the primary input for effective dose estimation.
Effective Dose (mSv) — The weighted sum of organ doses, reflecting whole-body stochastic risk equivalent. Calculated as DLP × k (conversion factor). Useful for risk communication, not individual patient dosimetry.
ICRP Conversion Factors (k-values)
| Region | k (mSv/mGy·cm) | Typical DLP | Typical Eff. Dose |
|---|---|---|---|
| Head | 0.0023 | 900 mGy·cm | ~2.1 mSv |
| Neck | 0.0054 | 300 mGy·cm | ~1.6 mSv |
| Chest | 0.014 | 450 mGy·cm | ~6.3 mSv |
| Abdomen | 0.015 | 500 mGy·cm | ~7.5 mSv |
| Pelvis | 0.015 | 450 mGy·cm | ~6.8 mSv |
| Abdomen + Pelvis | 0.015 | 900 mGy·cm | ~13.5 mSv |
Context: Common Dose Comparisons
Background radiation in the US: ~3 mSv/year. Chest CT: ~6–8 mSv (400–600 years background equivalent). CXR: ~0.02 mSv. Abdomen/pelvis CT: ~10–15 mSv. As low as reasonably achievable (ALARA) principles apply — request CT only when clinically indicated and justify the radiation exposure.
References
ICRP Publication 103. Annals of the ICRP. 2007;37(2-4).
ACR Dose Index Registry (DIR). American College of Radiology. 2024.
ImPACT CT Patient Dosimetry Calculator. Version 1.0.4.