The helically-acquired CTDI as an alternative to traditional methodology.

Purpose: Most clinical computed tomography (CT) protocols use helical scanning; however, the traditional method for CTDI measurement replaces the helical protocol with an axial scan, which is not easily accomplished on many scanners and may lead to unmatched collimation settings and bowtie filters. This study assesses whether CTDI can be accurately measured with a helical scan and determines the impact of pitch, collimation width, and excess scan length.

Methods: CTDI was measured for 95 helical protocols on 31 CT scanners from all major manufacturers. CTDI was measured axially, then again helically, with the scan range set to the active area of the pencil chamber seen on the localizer image. CTDI measurements using each method were compared to each other and to the scanner-displayed CTDI . To test the impact of scan length, the study was repeated on four scanners, with the scan range set to the phantom borders seen on the localizer.

Results: It was not possible to match the collimation width between the axial and helical modes for 12 of the 95 protocols tested. For helical and axial protocols with matched collimation, the difference between the two methods averaged below 1 mGy with a correlation of R  = 0.99. The difference between the methods was not statistically significant (P = 0.81). The traditional method produced four measurements that differed from the displayed CTDI by >20%; no helical measurements did. The accuracy of the helical CTDI was independent of protocol pitch (R  = 0.0) or collimation (R  = 0.0). Extending the scan range to the phantom borders increased the measured CTDI by 2.1%-9.7%.

Conclusion: There was excellent agreement between the two measurement methods and to the displayed CTDI , without protocol or vendor dependence. The helical CTDI measurement can be accomplished more easily than the axial method on many scanners and is reasonable to use for QC purposes.