No abstract

Stereotactic arrhythmia radioablation (STAR) is an emerging treatment option for atrial fibrillation (AF). However, it faces possibly the most challenging motion compensation scenario: both respiratory and cardiac motion. Multi-leaf collimator (MLC) tracking is clinically used for lung cancer treatments but its capabilities with intracardiac targets is unknown. We report the first experimental results of MLC tracking for intracardiac targets. Five AF STAR plans of varying complexity were created. All delivered 5  ×  10 Gy to both pulmonary vein antra. Three healthy human target motion trajectories were acquired with ultrasound and programmed into a motion platform. Plans were delivered with a linac to a dosimeter placed on the motion platform. For each motion trace, each plan was delivered with no MLC tracking and with MLC tracking with and without motion prediction. Dosimetric accuracy was assessed with γ-tests and dose metrics. MLC tracking improved the dosimetric accuracy in all measurements compared to non-tracking experiments. The average 2%/2 mm γ-failure rate was improved from 13.1% with no MLC tracking to 5.9% with MLC tracking (p < 0.001) and 7.2% with MLC tracking and no motion prediction (p < 0.001). MLC tracking significantly improved the consistency between planned and delivered target dose coverage. The 95% target coverage with the prescription dose (V100) was improved from 60% of deliveries with no MLC tracking to 80% of deliveries with MLC tracking (p  =  0.03). MLC tracking was successfully implemented for the first time for intracardiac motion compensation. MLC tracking provided significant dosimetric accuracy improvements in AF STAR experiments, even with challenging cardiac and respiratory-induced target motion and complex treatment plans. These results warrant further investigation and optimisation of MLC tracking for intracardiac target motion compensation.

Purpose The safe delivery of cardiac radioablation (CR) for atrial fibrillation (AF) is challenged by multi‐direction target motion, cardiac rate variability, target proximity to critical structures, and the importance of complete target dose coverage for therapeutic benefit. Careful selection of appropriate treatment procedures is therefore essential. This work characterizes AF cardiac radioablation target motion and target proximity to surrounding structures in both healthy and AF participants to guide optimal treatment technique and technology choice. Methods Ten healthy participants and five participants with AF underwent MRI acquisition. Multi‐slice, cardiac‐gated, breath‐hold cines were acquired and interpolated to create three‐dimensional images for 18–30 cardiac phases. Treatment targets at the left and right pulmonary vein ostia (CTVLeft and CTVRight respectively) and adjacent cardiac structures were contoured and their displacements throughout the cardiac cycle were assessed. Target proximity to surrounding structures were measured. Free‐breathing real‐time two‐dimensional cine images were also acquired at 4 Hz frequency for between 1‐ and 2‐min duration. The motion of easily identifiable points within the target, diaphragm and sternum was measured to assess respiratory motion. Results Target motion due to cardiac contraction was most prominent in the medial‐lateral direction and of 4–5 mm magnitude. CTVRight displacements were smaller in participants with AF than healthy participants in normal sinus rhythm. Nearby cardiac structures often moved with different magnitudes and motion trajectories. CTVLeft and/or CTVRight were in direct contact with the esophagus in 73% of participants. Target motion due to respiration was most prominent in the superior–inferior direction and of 13–14 mm magnitude in both healthy and AF participants. Conclusion AF CR target motion and relative displacement was characterized. The combination of target motion magnitude and relative displacement to critical structures highlights the importance of personalizing motion compensation techniques for effective AF CR treatments.

Sparsely sampled cardiac cine accelerated acquisitions show promise for faster evaluation of left-ventricular function. Myocardial strain estimation using image feature tracking methods is also becoming widespread. However, it is not known whether highly accelerated acquisitions also provide reliable feature tracking strain estimates. Twenty patients and twenty healthy volunteers were imaged with conventional 14-beat/slice cine acquisition (STD), 4× accelerated 4-beat/slice acquisition with iterative reconstruction (R4), and a 9.2× accelerated 2-beat/slice real-time acquisition with sparse sampling and iterative reconstruction (R9.2). Radial and circumferential strains were calculated using non-rigid registration in the mid-ventricle short-axis slice and inter-observer errors were evaluated. Consistency was assessed using intra-class correlation coefficients (ICC) and bias with Bland-Altman analysis. Peak circumferential strain magnitude was highly consistent between STD and R4 and R9.2 (ICC = 0.876 and 0.884, respectively). Average bias was -1.7 ± 2.0 %, p < 0.001, for R4 and -2.7 ± 1.9 %, p < 0.001 for R9.2. Peak radial strain was also highly consistent (ICC = 0.829 and 0.785, respectively), with average bias -11.2 ± 18.4 %, p < 0.001, for R4 and -15.0 ± 21.2 %, p < 0.001 for R9.2. STD circumferential strain could be predicted by linear regression from R9.2 with an R of 0.82 and a root mean squared error of 1.8 %. Similarly, radial strain could be predicted with an R of 0.67 and a root mean squared error of 21.3 %. Inter-observer errors were not significantly different between methods, except for peak circumferential strain R9.2 (1.1 ± 1.9 %) versus STD (0.3 ± 1.0 %), p = 0.011. Although small systematic differences were observed in strain, these were highly consistent with standard acquisitions, suggesting that accelerated myocardial strain is feasible and reliable in patients who require short acquisition durations.