TH-E-110-10: Doses in Volume-Of-Interest Cone-Beam Computed Tomography (CBCT)- a Monte Carlo Simulation Study

Abstract

Purpose: Volume‐of‐interest (VOI) cone‐beam CT(CBCT) is a technique to image a pre‐selected VOI with enhanced image quality while maintaining an acceptable average breast dose. This technique requires a high exposure VOI scan with a low exposure full‐field scan. In this study, Monte Carlo(MC) simulation was used to estimate the doses of the full‐field scan, VOI scan and various combinations of the two for breast imaging. Methods: For MC simulation, BEAMnrc and DOSXYZnrc were used to simulate CBCT breast imaging and estimate the average doses. For validation, air kermas at the VOI and iso‐center were measured and simulated for comparison. For simulation study, a 13‐cm‐diam, 9‐cm‐high Lucite cylinder was scanned by an x‐ray source located at 88 cm away. The VOI was defined to be a 2.5‐ cm‐diam, 2.5‐cm‐high cylindrical volume located at various radial positions in the middle plane of the phantom. Two scanning techniques were simulated: (1) the breast centered scan, (2) the VOI centered scan. The former requires a moving collimator to be used during the VOI scan while the latter requires only a stationary collimator to be used. 400 million incident photons were used to simulate 300 projections over 360 degrees to achieve 3D dose maps. Results: It was found that the average VOI doses decreased by 37% in the VOI scans as compared to those in the full‐field scans. The additional dose incurred by a VOI scan (VOI centered) was 6% of the dose from a full‐field scan for the same x‐ray techniques used. Conclusions: Our results indicated that it is feasible to employ a high exposure VOI scan in conjunction with a low exposure full‐field scan to enhance the image quality within the VOI while maintaining the overall average dose to be equal to or even lower than that of a regular full‐field scan. This work was supported in part by grants CA104759, CA124585 and CA13852 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.