Room: Exhibit Hall | Forum 9
Purpose: Most clinical CT protocols use helical scanning; however, the traditional method for CTDIvol measurement replaces the helical protocol with an axial scan, which is not easily accomplished on many scanners. This study assesses whether CTDIvol can be accurately measured with a helical scan and determines the impact of pitch and collimation width.
Methods: CTDIvol was measured for 81 helical protocols on 27 CT scanners from all major manufacturers. CTDIvol was measured using the traditional method, then again using the helical protocol, with the scan range set to the active area of the pencil chamber seen on the topogram. The CTDIvol equation for the helical scan was altered by setting nxT and table increment to 100mm. CTDIvol measurements using each method were compared to each other and to the scanner-displayed CTDIvol.
Results: Seven scanners (26%) were unable to match axial and helical collimation for one or more protocols. For helical and axial protocols with matched collimation, the difference between the two methods averaged 0.6 mGy (range -4.0 to 7.7) with a correlation of RÂ²=0.99. For protocols without matched collimation, the helical CTDIvol was less than the axial CTDIvol by an average of -1.12 mGy (range -13.3 to 2.7). Averaged across all systems, the agreement between the scanner display and the measured CTDIvol did not depend significantly on the measurement method (p=0.57). The traditional method produced 3 measurements that differed from the displayed CTDIvol by >20%; no helical measurements did. The accuracy of the helical CTDIvol was independent of protocol pitch (RÂ²=0.01) or collimation (RÂ²=0.0).
Conclusion: There was excellent agreement between the two measurement methods and to the displayed CTDIvol, without protocol or vendor dependence. The helical CTDIvol measurement can be accomplished more easily than the axial method on many scanners and is reasonable to use for QC purposes.