CT dose index (CTDI) (measured in mGy) is a standardized measure of radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners. In the past CTDI₁₀₀(measured over a 100 mm long ionization chamber) and CTDI_w(weighted average of dose across a single slice) were used; for helical scanners in current use, the parameter CTDI_vol is the more commonly used index.

CTDI₁₀₀(mGy) is a linear measure of dose distribution over a pencil ionization chamber and hence does not take into consideration the topographical variation of a human body and is therefore not in clinical use.
CTDI_w(mGy)is closer to the human dose profile as compared with the CTDI₁₀₀
- 2/3 CTDI₁₀₀ (periphery) + 1/3 CTDI₁₀₀ (center)
CTDI_vol (mGy)is obtained by dividing CTDI_w by pitch factor.

Another commonly used index is the dose-length product (DLP) which factors in the length of the scan to show overall dose output

DLP (mGy*cm) CTDI_vol x scan length

Practical points

It is important to remember that the CTDI_vol or its derivative the DLP, as seen on consoles and outputted on the DICOM images, do not represent the actual absorbed or effective dose for the patient. They should be taken as an index of radiation output by the system for comparison purposes.

If the AP and lateral dimensions of the patient are available, then the SSDE can be used to estimate the absorbed dose.