Abstract
PURPOSE: The weighted CT dose index (CTDI(w) ) has been extended for a nominal total collimation width (nT) greater than 40 mm and relies on measurements of CTDIfreeair. The purpose of this work was to compare three methods of measuring CTDIfreeair and subsequent calculations of CTDI(w) to investigate their clinical appropriateness. METHODS: The CTDIfreeair, for multiple nTs up to 160 mm, was calculated from (1) high-resolution air kerma profiles from a step-and-shoot translation of a liquid ionization chamber (LIC) (considered to be a dosimetric reference), (2) pencil ionization chamber (PIC) measurements at multiple contiguous positions, and (3) air kerma profiles obtained through the continuous translation of a solid-state detector. The resulting CTDIfreeair was used to calculate the CTDI(w) , per the extended formalism, and compared. RESULTS: The LIC indicated that a 40 mm nT should not be excluded from the extension of the CTDI(w) formalism. The solid-state detector differed by as much as 8% compared to the LIC. The PIC was the most straightforward method and gave equivalent results to the LIC. CONCLUSIONS: The CTDI(w) calculated with the latest CTDI formalism will differ most for 160 mm nTs (e.g., whole-organ perfusion or coronary CT angiography) compared to the previous CTDI formalism. Inaccuracies in the measurement of CTDIfreeair will subsequently manifest themselves as erroneous calculations of the CTDI(w) , for nTs greater than 40 mm, with the latest CTDI formalism. The PIC was found to be the most clinically feasible method and was validated against the LIC.