CTDI
Computed Tomography Dose Index — standardized measurement of CT dose per scan rotation, measured in a CTDI phantom. Reported in mGy. The patient-report number is CTDIvol (volume-averaged), which accounts for pitch on helical acquisitions; CTDIw is the weighted phantom average.
Why it matters to buyers: Core regulatory dose metric. CTDIvol on a patient report enables dose committees, accreditation surveys, and dose-monitoring software to compare protocols across the fleet and against published reference levels. CTDI outliers flag tube / DAS / protocol issues.
Why it matters to engineers: Measured with a 100 mm pencil ionization chamber in a 16 cm (head) or 32 cm (body) PMMA phantom. Annual physicist measurement is required by ACR accreditation. Drift outside spec on the phantom test means the tube output, kV calibration, or filter chain has changed; recalibrate before scanning patients.