Technical Note: Evaluating automatic tube current modulation in CT using the standard CTDI dosimetry phantom.

Abstract

To assess the utility of the standard body CTDI phantom in characterizing the operation scheme of tube current modulation (TCM) systems in CT. The body CDTI phantom was used to characterize two TCM systems: TCM1 and TCM2 , implemented in scanners from different vendors. The phantom was aligned at the gantry isocenter in two configurations. In configuration A, the facet planes of the phantom were parallel to the patient table, while in configuration B they were vertical to the patient table and parallel to the patient's long axis. Acquisitions were performed using the routine abdominal examination protocol. mA(z) profiles were recorded from images' DICOM header. The water equivalent diameter (dw ) and oval ratio (OR) were calculated as a function of z-axis location. Image noise was defined as the standard deviation (SD) of the mean Hounsfield unit value measured in a region of interest at the center of the phantom's image. Regression analysis was performed to modulated mA and SD vs dw and OR. The spatial concordance between the change in phantom size and change in mA (SCmA ) was calculated as the percent difference in the slope of mA(z) change between the 1st and 2nd half of the phantom. The corresponding spatial concordance between the change in phantom size and change in image noise (SCnoise ) was calculated. Modulated mA(z) along the z-axis did not substantially differentiate between configurations A and B. Correlation between ln(mA) and OR was found to be higher compared to correlation between ln(mA) and dw . SCmA was 48% for TCM1 and 33% for TCM2 . The corresponding SCnoise was 29% for TCM1 and 16% for TCM2 . Apart from routine CT dosimetry evaluations, the standard CTDI phantom positioned in configuration A or B may additionally be used by medical physicists to evaluate the performance of TCM operational characteristics.