Differences in behavior of tube current modulation techniques for thoracic CT examinations between male and female anthropomorphic phantoms.

Our objective was to investigate the differences in behavior of tube current modulation (TCM) techniques for thoracic CT examinations between male and female anthropomorphic phantoms. The phantoms were scanned with an automatic exposure control system in the longitudinal (z-) and angular-longitudinal (xyz-) TCM, in addition to the fixed-mA which was used as a reference. Axial dose distributions were measured at the levels of the breasts and the diaphragm, and longitudinal dose distributions were measured from the thoracic-inlet level to the diaphragm level at the center and periphery of the phantoms by use of eight solid-state detectors. Image noise was quantitatively measured continuously from the top to the bottom images of the phantoms. With the male phantom, the percentage of average absorbed dose with the xyz-TCM mode compared to the z-TCM mode was 90.2 % at the level of the nipples. This value was significantly smaller than that for the female phantom (95.6 %, P < 0.0001). With either phantom, the percentage of absorbed doses in the longitudinal direction with the xyz-TCM mode compared to the z-TCM mode at the center of the phantom was almost the same as the percent ratio at the periphery of the phantom. Therefore, the effect of xyz-TCM was less pronounced with the female phantom, especially on the reduction of the breast dose. The increase of image noise at the level of the supraclavicular fossa (in the male phantom) and at the level of the diaphragm (both phantoms) could not be avoided with the use of TCM techniques.