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Monte Carlo Dosimetry for a Dedicated Cone-Beam CT Head Scanner

A Sisniega

A Sisniega1*, W Zbijewski1 , J Xu1 , H Dang1 , J W Stayman1 , N Aygun1 , V E Koliatsos1 , X Wang2 , D H Foos2 , J H Siewerdsen1 , (1) Johns Hopkins University, Balitmore, MD, (2) Carestream Health, Rochester, NY


WE-EF-207-5 (Wednesday, July 15, 2015) 1:45 PM - 3:45 PM Room: 207

Purpose: Cone-Beam CT (CBCT) is an attractive platform for point-of-care imaging of traumatic brain injury and intracranial hemorrhage. This work implements and evaluates a fast Monte-Carlo (MC) dose estimation engine for development of a dedicated head CBCT scanner, optimization of acquisition protocols, geometry, bowtie filter designs, and patient-specific dosimetry.
Methods: Dose scoring with a GPU-based MC CBCT simulator was validated on an imaging bench using a modified 16 cm CTDI phantom with 7 ion chamber shafts along the central ray for 80 - 100 kVp (+2 mm Al, +0.2 mm Cu). Dose distributions were computed in a segmented CBCT reconstruction of an anthropomorphic head phantom with 4x10⁵ tracked photons per scan (5 min runtime). Circular orbits with angular span ranging from short scan (180° + fan angle) to full rotation (360°) were considered for fixed total mAs per scan. Two aluminum filters were investigated: aggressive bowtie, and moderate bowtie (matched to 16 cm and 32 cm water cylinder, respectively).
Results: MC dose estimates showed strong agreement with measurements (RMSE<0.001 mGy/mAs). A moderate (aggressive) bowtie reduced the dose, per total mAs, by 20% (30%) at the center of the head, by 40% (50%) at the eye lens, and by 70% (80%) at the posterior skin entrance. For the no bowtie configuration, a short scan reduced the eye lens dose by 62% (from 0.08 mGy/mAs to 0.03 mGy/mAs) compared to full scan, although the dose to spinal bone marrow increased by 40%. For both bowties, the short scan resulted in a similar 40% increase in bone marrow dose, but the reduction in the eye lens was more pronounced: 70% (90%) for the moderate (aggressive) bowtie.
Conclusions: Dose maps obtained with validated MC simulation demonstrated dose reduction in sensitive structures (eye lens and bone marrow) through combination of short-scan trajectories and bowtie filters.

Funding Support, Disclosures, and Conflict of Interest: Xiaohui Wang and David Foos are employees of Carestream Health

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