Estimating the CTDI_vol with helical acquisitions: Results from a national generalizability study

Background: While many clinical computed tomography (CT) protocols use helical scanning, the traditional method for measuring the volume CT Dose Index (CTDI_vol) requires modifying the helical protocol to perform a single axial rotation. This modification can present challenges and mismatched settings across various scanner models.

Purpose: This study investigates the generalizability of a helical methodology for estimating CTDI_vol across a diverse range of participants, CT scanner models, and protocol parameters.

Methods: A web-based platform collected axial and helical CTDI_vol measurements from 24 medical physicists who submitted 569 data sets obtained using four CT protocols on scanners from seven CT manufacturers. Various parameters were tested for tube voltage (70-140 kVp), rotation time (0.25-1.50 s), beam width (8-80 mm), and pitch (0.29-3.0) settings. Measurements from the two methodologies were assessed for reproducibility using three repeated exposures and then compared to each other and to the scanner-displayed CTDI_vol. Agreement between the methodologies was assessed using Bland-Altman analysis, linear regression, paired t-tests, and a paired two one-sided tests (TOST) procedure with equivalence margins of 5% of the mean protocol CTDI_vol. The impact of beam width and pitch on measurement accuracy was assessed using linear regression analysis and an independent t-test.

Results: This study demonstrated better measurement reproducibility with the helical method (p < 0.05) and excellent concordance between helical and axial measurements (CCC > 0.99), with an average difference of -0.61 mGy (limits of agreement: -4.54 and 3.32). The TOST analysis confirmed that the measurement methods were statistically equivalent within the defined equivalence margins. The number of measurements that differed from the displayed CTDI_vol by more than ± 20% was 10 for the axial method and 22 for the helical method. We did not identify a linear correlation between measurement accuracy and beam width or pitch (R² < 0.06). However, differences between axial and helical methods were significant for protocols with beam widths up to 40 mm versus those greater than 40 mm, as well as for protocols with pitch factors up to 1.0 compared to those greater than 1.0 (p < 0.001).

Conclusion: Utilizing the same equipment currently used for measuring CTDI_vol and a simple measurement setup, the helical method offers an alternative measurement methodology that can be seamlessly implemented by medical physicists and adopted by regulatory and accrediting bodies for routine quality control of CT scanners.