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External beam ablation therapy has the potential to treat cardiac arrhythmias non-invasively by targeting arrhythmogenic myocardial tissue; however, a challenge of treating cardiac tissue with beam ablation therapy is cardiac motion. Currently, cardiac motion is typically compensated by expansion of the target volume which can potentially lead to collateral damage of surrounding healthy tissue. This collateral damage could be minimized by gating the beam delivery to a portion of the cardiac cycle. In prior work, we evaluated cardiac motion using anatomic landmarks in multi-phase cardiac computed tomography volumes of swine hearts across the left atria and ventricles. Other work evaluated left atrial motion using implanted fiducial clips. In the current work, we extend this prior work by quantifying cardiac motion using gold standard implanted fiducial clips across all four chambers of the heart. Cardiac motion varied by chamber, ranging from 2.1 to 7.2 mm in the x direction, 7.2 to 8.1 mm in the y direction, and 3.1 to 9.7 mm in the z direction. In addition, we quantify the reduction in motion if delivery were gated to phases 40% to 90% of the cardiac cycle, which corresponds to treating across 50% of the cardiac cycle. Cardiac motion across 50% of the cardiac cycle ranged from 1.1 to 5.3 mm in the x direction, 4.5 to 5.2 mm in the y direction, and 1.2 to 7.8 mm in the z direction. Percentage reduction in motion for treating during 50% of the cardiac cycle ranged from 18% to 47% in the x direction, 31% to 43% in the y direction, and 11% to 61% in the z direction. These results demonstrate that a substantial improvement in target localization could be achieved by gating the beam to 50% of the cardiac cycle.
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