Expansion of applicability of multiple scan average dose measurements for CT dosimetry

<i>Objective.</i>The CT dose index (CTDI) can be directly measured only for axial scan protocols; however, these days, helical protocols are applied in most cases, and sometimes axial protocols are not even available. The multiple scan average dose (MSAD) method can address this issue, but it is not widely adopted. This article aims to support the appropriateness of the MSAD method first by showing that it is measurable with the CTDI tools, and second by presenting a simple rule for scan pitch to avoid the main uncertainty of the measurement.<i>Approach.</i>Monte Carlo simulation was combined with its analytical postprocessing to accurately calculate scattering and speed up calculations to investigate several imaging and beam parameter combinations. The calculations were validated by measurements, and the model was used to examine the uncertainty of MSAD caused by an uncontrolled parameter.<i>Main results.</i>The widespread 150 mm long CTDI phantom and the 100 mm long ionization chamber are applicable for the MSAD method and reach80.8±0.2% efficiency in the estimation of CTDI_vol, in contrast to78.0±0.3% efficiency of the CTDI₁₀₀method. By utilizing the periodicity of the MSAD measurement, the uncertainty can be reduced to1.38±0.02% in contrast to26.85±0.16% in the worst case.<i>Significance.</i>The MSAD measurement with the CTDI setup is slightly more efficient than the CTDI₁₀₀measurement protocol, so there is no need for a long phantom or farmer chamber to use MSAD in quality assurance (QA) measurements. The MSAD applies to both helical and axial scans; therefore, we recommend using direct MSAD measurement for QA in the clinical environment.