Direct measurement of CTDI(w) on helical CT scans

PURPOSE: 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. METHODS: 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. RESULTS: For both the head and body CTDI phantom the agreement between the axial and helical CTDI(w) measurements was <1.5%. CONCLUSIONS: Body and head CTDI(w) and CTDI(vol) can be directly measured by employing helical scans with the method in this paper