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.