Purpose: In present day clinical practice the majority of CT exams employ helical scan protocols. Helical protocol CTDIvol is routinely determined by measuring CTDIw for axial scans. On some CT scanners axial scans with the same detector configuration employed on helical protocols are not available, and CTDIw cannot be measured directly with the standard CTDI phantom. Presented is a method to directly measure CTDIw on helical scan protocols.
Methods: CTDIw is measured on helical scans by centering the CTDI phantom in the CT gantry as with axial CTDI measurements. The body CTDI phantom was modified by the addition of brackets to the sides of the phantom. The brackets support the phantom on the CT gantry shroud with the tabletop below. The phantom remains stationary during helical scans, and surface and axis doses measured. The measured doses are divided by the number of gantry rotations to determine values for one rotation, and CTDIw calculated.CTDIw for body and head helical scan protocols were measured and CTDIvol determined on a recently installed Siemens Somatom Drive CT Scanner that did not have axial scan protocols with the same detector configurations. CTDIw adult body measurements were made for helical and axial scans on a GE Discovery CT Scanner employing the same detector configuration and technical factors.
Results: The body and head CTDIvol measured on the Siemens Scanner were 13.45 and 48.93 mGy. The corresponding displayed values were 12.80 and 47.78 mGy; 5% and 2.4% lower. The GE helical scan and axial body CTDIw measurements were 10.90 and 10.94 mGy.
Conclusion: From the good agreement of axial and helical results for the GE scanner, the suspended phantom method is viable and allows one to directly measure CTDIw with helical scans. The results for the Siemens scanner show how the method can be applied.