Room: AAPM ePoster Library
Purpose: To evaluate CT scanner tube current modulation (TCM) performance using the method recommended by AAPM TG233 and to compare the resulting metrics between scanners of various manufacturers and models.
Methods: 32cm CTDI_vol phantom was scanned using clinical routine abdomen protocols on a GE LightSpeed VCT and a Siemens Definition AS. Both AP and lateral localizers were acquired prior to the image acquisition; the phantom was scanned in both table directions. For each slice, tube current value was extracted from DICOM header using MATLAB, and the effective diameter of phantom was calculated. The slopes (a) and the linear correlation relation coefficients (RmA_eff²) of ln(mAs_eff) vs. phantom effective diameter were determined for each image set.
Results: The resulting average a- and RmA_eff2-values differed significantly between GE scanner (a=0.0057, RmA_eff²=0.887) and Siemens scanner (a=0.0134, RmA_eff²=0.948). A disparity between the scanner TCM response to object size scanning direction was recognized, i.e., when scanning in the direction of decreasing phantom size, a-value increased significantly for Siemens scanner (26.9%), but remained relatively unchanged for GE scanner (-2.95%). The directionality of the table movement also had some effects on scanners’ TCM response. When the table was traveling out of the bore, a-value increased by 7.39% for GE, but decreased by 1.47% for Siemens scanner. On the other hand, when the table was traveling into the bore, a-value decreased by 5.66% for GE scanner, but increased by 13.86% for Siemens scanner.
Conclusion: The resulting a-values suggest that the metric differs drastically between scanners of different manufacturers, while RmA_eff²-values suggest the log-linear model recommended by TG233 better describes the TCM response of Siemens scanner as compare to that of the GE scanner. However, a-value is a viable metrics for characterizing scanner TCM response, and ongoing investigations including evaluation of other CT models will be presented at the meeting.