Radiation Protection and Dose Optimization
Dose metrics
CT dose is described by CTDIvol (volume CT dose index, mGy) and DLP (dose-length product, mGy·cm). CTDIvol reflects scanner output for a given protocol, not the patient's actual absorbed dose; SSDE adjusts for patient size. Effective dose (mSv) estimates stochastic risk and is calculated as DLP × k, where k is a region-specific conversion factor.
Optimization strategies
Apply ALARA. Adjust tube current (mA/mAs) with automatic exposure control, lower kVp for small patients or contrast-enhanced studies, use iterative reconstruction to reduce noise at lower dose, limit scan length to the indicated anatomy, and choose pitch appropriately. Overranging (z-overscan) occurs with helical scans, especially at high pitch and wide collimation. Shield superficial tissues such as breast, thyroid, and gonads when it does not interfere with the diagnostic task; bismuth shields placed in the scan field can increase internal dose.
Traps
High pitch generally lowers dose but can degrade image quality; CTDIvol is not patient dose; DLP is not effective dose; increasing collimation can increase overranging. AEC may increase mA in obese patients, which is appropriate.
Worked example
A chest CT reports DLP = 400 mGy·cm. Using k = 0.014 mSv/(mGy·cm) for chest, effective dose ≈ 400 × 0.014 = 5.6 mSv. If scan length is reduced from 40 cm to 30 cm at the same CTDIvol, DLP falls by 25%, to roughly 4.2 mSv.