The combination of PA and Lateral localizers is most robust against effects of patient off-centering. Conclusion: Patient off-centering substantially impacts radiation dose, which depends on the amount of vertical offcentering and type of the localizer used. Similar behavior was found for different TCM strengths, but effects were more pronounced for very strong compared with very weak modulation strengths. For table positions ranging from lowest to highest, CTDI-vol ranged from −23% to 17% for AP, from −8% to 0.3% for lateral and from −7% to 1% for a combination of PA and Lateral in either order. For an average TCM strength and PA localizer, CTDI-vol ranged from 75% higher to 12% lower than the isocenter value for the lowest more » to highest table positions, respectively. Results: Magnification factors from AP and PA localizers ranged from 0.79–1.11 and 1.35–0.91, respectively from lowest to highest positions (highest with phantom located closest to X-ray tube), with a smaller impact when using Lateral, PA+Lateral, Lateral+PA localizer types, all of which behaved similarly. CTDI-vol was recorded for the 165 combinations of table height, localizer type, and TCM strength, and magnification factors measured from localizer images. At each position, scans were acquired using 5 different localizer types: PA, AP, Lateral, PA+Lateral, and Lateral+PA. Data was acquired at 11 table height positions with phantom center at isocenter and at 10 other positions between 142mm below and 53mm above. Methods: A CTDI body phantom was scanned using an abdominal protocol with three TCM strengths (Siemens Definition AS40, CAREDose4D very weak, average, very strong). Our aim was to investigate the effects of vertical off-centering on CTDI-vol when using different localizer types and TCM strengths.
Purpose: Apparent patient size in CT localizers varies with localizer type and patient positioning, resulting in variation in effective mAs as guided by Tube Current Modulation (TCM).
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