Direct measurement of CTDI_w on helical CT scans

Medical physics computed tomography (CT) practice involves measurements to determine CTDI_vol on representative clinical CT protocols. In current practice the majority of CT exams employ helical scans. To determine CTDI_vol for a helical scan, one measures CTDI_w with an axial scan, then divides by the pitch. Problems arise in CT units where one is unable to select an axial scan with the same detector configuration and pre-patient (bowtie) filtration that is employed on the helical scan. Presented is a method to measure CTDI_w on helical scans.

The body and head CTDI phantoms were supported on the gantry shroud with brackets attached to the phantom. The phantom is above the tabletop and remains stationary during helical scans as the table moves beneath the phantom. With the phantom stationary, the CTDI_w associated with head and body helical scans was measured. CTDI_w was also measured for head and body axial scans with the same pre-patient filtrations and detector configurations.

For both the head and body CTDI phantom the agreement between the axial and helical CTDI_w measurements was <1.5%.

Body and head CTDI_w and CTDI_vol can be directly measured by employing helical scans with the method in this paper.