SU-C-12A-01: Primary Vs. Scatter Contribution to Body CTDI: Experiment Results.
PURPOSE
To quantify experimentally scatter contribution to CTDI in center and peripheral holes of the body (32-cm) phantom.


METHODS
All experiments were performed in Service Mode of 750CT-Discovery with both tube and table were in stationary position, and with large filter and 40- mm collimation. First, transmission through acrylic was measured using acrylic rods of various lengths (1-cm to 33-cm), at 80, 120, and 140 kVp. These data were utilized to obtain dose from primary beam at different depths of acrylic. Thus, scatter-to-primary ratio (SPR) was determined for center-hole by measuring exposure with a pencil chamber. For peripheral hole SPR, dose was measured at eight different tube angles from 0° to 180°. Change in primary dose with tube angle due to both shape of bowtie filter and distance from tube was measured in air; hence, dose from primary from each of the 8 phantom measurements was determined and SPR was calculated by integrating the fitted models for primary and scatter distributions as a function of distance from tube. Also, scatter contribution from different segments of phantom to center-hole dose was measured using a custom-built acrylic phantom, which had similar SPR at the center-hole to that of standard body phantom.


RESULTS
After correcting for stray radiation (off-focus, scatter from collimators, etc.) for primary measurements, preliminary results of SPRs in center-hole dose of the body phantom for 80, 120, and 140 kVp were 6.8, 6.2, and 6.0, respectively, and for peripheral hole were 1.45, 1.44, and 1.39. Forward-scatter and backscatter contributions to center dose, at any point during tube rotation, were respectively, 62% and 38% with 80 kVp, and 60% and 40% with 120 and 140 kVp.


CONCLUSION
SPR in both center and peripheral holes of body CTDI phantom was determined experimentally. Also, distribution of contributing scatter to center hole was estimated.