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Cardiac motion remains a challenge in the treatment of ventricular tachycardia with external beam ablation therapy. Current techniques involve expansion of the treatment area which can lead to unwanted collateral damage. Surrounding healthy tissue could be spared by gating the delivery of the beam to the cardiac cycle. In prior work, we assessed cardiac motion using in vivo fiducial markers and demonstrated that motion would be reduced if treatment were gated to half of the cardiac cycle, approximately corresponding to diastole. In the current work, we extend our prior analysis by quantitatively assessing the optimal gating window for motion reduction in the left ventricle. Motion was assessed in five porcine models with two fiducial clips per animal for a total of ten clips. The minimal cardiac motion occurred when the gating window started at 70% of the cardiac cycle. Without gating, three-dimensional cardiac motion was 7.0 ± 3.9 in x (left/right), 5.3 ± 2.5 in y (anterior/posterior), and 5.6 ± 2.3 in z (superior/inferior) mm. Using an optimal gating window, cardiac motion was 3.1 ± 1.8 in x (left/right), 2.5 ± 1.2 in y (anterior/posterior), and 3.1 ± 1.7 in z (superior/inferior) mm. The percentage reduction in motion with optimal gating was 51 ± 23 in x (left/right), 49 ± 21 in y (anterior/posterior), and 45 ± 24 % in z (superior/inferior). This work demonstrates that gating shows significant promise for reducing the effects of left ventricular motion when treating ventricular tachycardia with external beam ablation therapy.
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