Room: Exhibit Hall
Purpose: Most clinical CT protocols use helical scanning; however, the standard methodology for CTDI(vol) measurement replaces the helical protocol with an equivalent axial scan, which is not easily accomplished on many scanners. Altering protocols may result in unavoidable changes to bowtie filters and collimation, which impede measurement accuracy. The purpose of this study was to determine whether CTDI(vol) can be accurately measured with a helical scan.
Methods: CTDI(vol) was measured for helical protocols on 15 CT scanners from all major manufacturers. Adult and pediatric head and abdomen protocols were tested. CTDI(vol) was measured using the axial methodology, then again using the helical protocol. For the helical measurements, the scan length was equal to the active area of the pencil chamber, as seen on the topogram, and the CTDI(vol) equation was altered by setting both nxT and table increment to 100mm. CTDI(vol) measurements using each method were compared to each other and to the CTDI(vol) displayed by the scanner.
Results: Excluding systems in which it was impossible to match collimation between the axial and helical protocols, the difference between the axial and helical methods averaged 0.4 mGy (range -2.9 to 3.9). Among all systems, the difference between the axial measurements and the displayed CTDI(vol) averaged -0.9 mGy (range -7.5 to 5.1). The difference between the helical measurements and the displayed CTDI(vol) averaged -1.6 mGy (range â€“9.1 to 2.0). The helical measurements agreed with the displayed CTDI(vol) better than the axial measurements for 70% of protocols tested. Two axial measurements differed from the displayed CTDI(vol) by >20%; no helical measurements did.
Conclusion: There was excellent agreement between the two measurement methods and to the displayed CTDI(vol). The helical measurement method can be accomplished more easily than the axial method on many scanners and is a reasonable testing method for QC purposes.