For a given beam collimation, the selection of a higher value of reconstructed image slice width increases the normalized effective dose. For helical scans with pitch=1, broader beam collimation is associated with increased z overscanning and consequently higher normalized effective dose value, when other scanning parameters are held constant. ![]() Given that the same kilovoltage and tube load per rotation were used in both axial and helical scans, the above differences may be attributed to z overscanning. The percentage differences in the normalized effective dose between contiguous axial and helical scans with pitch=1, may reach 13.1%, 35.8%, 29.0%, and 21.5%, for head and neck, chest, abdomen and pelvis, and trunk studies, respectively. Results: Scan technique was 120 kV tube voltage, 10 mA current, and table speed of 10 cm/s. Mean patient widths and acquisition techniques were used to calculate the weighted average free-in-air KERMA, which was multiplied by the patient area to estimate KAP. Data were collected for 50 head exams (lateral localizer only), 15 head/neck exams, 50 chest exams, and 50 abdomen/pelvis exams. The beam profile and width were used to calculate a weighted average air KERMA per unit mAs as a function of intercepted x-axis beam width for objects symmetric about the localizer centerline.Patient areas were measured using manually drawn regions and divided by localizer length to determine average width. ![]() ![]() The z-axis beam width was measured as a function of distance from isocenter. Methods: In-plane beam intensity profiles were measured in localizer acquisition mode using OSLs for a 64 slice MDCT scanner (Lightspeed VCT, GE Medical Systems, Waukesha WI). Purpose: To estimate the free-in-air KERMA-Area Product (KAP) incident on patients due to CT localizer scans for common CT exams.
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