CT dose index

CT dose index (CTDI) 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  the most relevant one. 

CTDI₁₀₀ is a linear measure of dose distrubition 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 is closer to the human dose profile as compared with the CTDI₁₀₀ . it is simply:

2/3 CTDI₁₀₀ (periphery)  +   1/3 CTDI₁₀₀ (center)

CTDI _vol is obtained by dividing CTDI_w by pitch factor.

A point to remember is that CTDI_vol or its derivative the  DLP* , as seen on consoles and passed on into the DICOM image do not represent the actual dose administered to the patient; they should be taken as an index of radiation output by the system for comparison purposes.

* DLP (dose length product = CTDI_vol x scan length)