Purpose: CTDIvol is the current standard measure of CT dosimetry which requires each measurement at a single axial scan. However, modern scanners use helical protocols more than axial protocols, and it is tedious or sometimes problematic to convert helical protocols into equivalent axial protocols. A helical CTDIvol measurement method has been proposed and showed promising results to replace the current standard axial method. However, it was unknown if the method can be utilized for dual energy CT helical acquisitions. In this study, the feasibility of the direct helical acquisition CTDIvol measurement method was evaluated with dual energy protocols.
Methods: 21 Clinical dual energy protocols from two GE CT scanners were evaluated for the helical CTDIvol method. CTDI phantoms of 16 and 32 cm diameters and a CT ion chamber were used for the measurement. The scan range was prescribed as 10 cm to cover the CT ion chamber. Air kerma readings from the ion chamber were used to calculate CTDIvol. To evaluate the performance of the proposed method, the discrepancy and repeatability indices were assessed along with other statistical analysis including linear regression, Paired Student t-test, and correlation analysis.
Results: Paired Student t-test shows the direct helical measurement has no statistically significant difference with the manufacturer reported CTDIvol (p=0.51), exhibiting the absolute discrepancy of -0.12±0.93 (mGy) (mean±standard deviation), discrepancy index of 1.5±3.7 (%) and repeatability index of 98.5±1.5 (%). The linear regression model indicates there is a good linear fitting (y=1.0286x-0.946) between the measured and displayed CTDIvol. No correlation is found between the discrepancy/repeatability indices and pitch, collimation widths, or bowtie filters (p>0.05).
Conclusion: The result suggests that the direct helical CTDIvol measurement method is a good alternative for the axial CTDIvol method in evaluating CTDIvol of dual energy CT scanners during quality assurance testing.