Dependency of prescribed CT dose on table height, patient size, and localizer acquisition for one clinical MDCT.

PURPOSE: The purpose of this study was to quantify the effect that table height, patient size, and localizer acquisition order may have on AEC prescribed dose.

METHOD AND MATERIALS: Three phantoms were used for this study: the Mercury Phantom, acrylic sheets, and an anthropomorphic phantom. A lateral (LAT) and a posterior-anterior (PA) localizer was acquired for each phantom at different table heights on a MDCT scanner (GE Discovery CT750 HD). AEC scan acquisitions were prescribed for each combination of phantom, localizer orientation, and table height ±4 cm with the center position; the displayed CTDIvol was recorded. Based on the institutional dose monitoring program, the relationship between change in CTDIvol and change in table height were studied for LAT and AP localizers for clinical exams.

RESULTS: For all phantom scans based on the PA localizer, the percent change in ranged between -18% and 42% for table heights 4 cm below and above proper centering; while for the LAT localizer, the percent change in CTDIvol from ideal were no greater than 12% different for ±4 cm differences in table height. Change in CTDIvol and change in table height displayed a strong linear relationship for AP localizer exams (P = 0.002), and weak correlation for LAT localizer exams (P = 0.12).

CONCLUSIONS: Since uncertainty in vertical patient positioning is inherently greater than lateral positioning, the LAT localizer should be utilized to precisely and reproducibly deliver the intended amount of radiation prescribed by CT protocols.