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Simultaneous Reduction of Radiation Dose and Scatters for CBCT by Using Collimators

T Li

T Li*, X Li, Y Yang, Y Zhang, D Heron, M Huq, University of Pittsburgh Medical Center, Pittsburgh, PA

WE-G-141-8 Wednesday 4:30PM - 6:00PM Room: 141

Purpose:On-board CBCT has been widely available in radiotherapy clinic for target localization. However, the extra radiation dose is always a concern for its frequent use. Additionally, the relatively large scatters in CBCT often impair the image quality. By using collimators/grids, some of the X-rays are stopped from reaching the patient and the detectors, hence both the scatters and the patient dose are simultaneously reduced. We show in this work that the data can still be reconstructed without any noticeable artifacts. We further investigate the relationship between the CBCT image quality and the amount of beams being blocked by collimators.

Methods:A CBCT system with collimators was simulated following the typical geometry used in clinic. Different collimator designs were tested by varying the size and the number of the collimator holes, and at the same time, the ratio between transmitted beams to blocked beams was varied from 10 to 1/10, resulting in hundreds of different simulation scenarios. The simulated projections for lung and pelvis phantoms were reconstructed using the total variation technique. The image quality was examined by root mean square errors (RMSEs) and compared with the conventional CBCT image.

Results:The CBCT image quality increases as the amount of beams passing through the collimators increases, and decreases as the size of the collimator holes increases while keeping the beam transmission rate constant. With ultra-high resolution collimators, the RMSEs were comparable to the conventional CBCT image until the beam transmission rate is reduced below 20%.

Conclusion:Both the scatters and radiation dose can be dramatically reduced for CBCT by using collimators, while the image quality remains similar to the conventional CBCT. With high resolution collimators, the patient dose reduction can easily reach 1/4 of the conventional CBCT.

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