Investigating the feasibility of magnetic resonance imaging guided cardiac radioablation for atrial fibrillation

Over 37 million people suffer from atrial fibrillation (AF). Catheter ablation is the main curative non-pharmacological treatment, but is invasive, technical, resource intensive, and success rates are moderate. Cardiac radioablation (CR) has been proposed as a non-invasive treatment alternative but further investigation is required before CR can be confidently and routinely used clinically. This thesis investigates the feasibility of magnetic resonance imaging (MRI) guided AF CR. Specifically, this thesis focuses on motion management within four main studies by: evaluating the feasibility of real-time radiation steering to AF CR targets. evaluating the feasibility of determining the real-time target location non-invasively with MRI. The first study demonstrated the proof-of-concept of real-time radiation steering to AF CR targets using multileaf collimator (MLC) tracking in a phantom study. MLC tracking provided significant dosimetric accuracy improvements compared to deliveries without MLC tracking despite complex target motion. The second study answered an important question regarding the overall feasibility of MRI guided AF CR by illustrating that AF CR targets were visible on MRI, indicating that MRI guided AF CR target tracking may be plausible. The third study characterized AF CR target motion, target displacement to surrounding structures, and target motion surrogacy. The findings highlighted the complexity of AF CR motion management requirements. The fourth study utilized the gained knowledge in earlier studies to develop near real-time non-invasive tracking methods for AF CR cardiac-induced target motion. Small retrospective tracking errors on MRI datasets illustrated the feasibility of non-invasive AF CR target tracking and implied its potential for improving AF CR treatment efficacy. This body of work has collectively illustrated the promising proof-of-concept of a non-invasive MRI guided AF CR treatment, warranting further development of this technique

MRI-guided cardiac-induced target motion tracking for atrial fibrillation cardiac radioablation.

Background and purpose: Atrial fibrillation (AF) cardiac radioablation (CR) challenges radiotherapy tracking: multiple small targets close to organs-at-risk undergo rapid differential cardiac contraction and respiratory motion. MR-guidance offers a real-time target tracking solution. This work develops and investigates MRI-guided tracking of AF CR targets with cardiac-induced motion. Materials and methods: A direct tracking method (Trackingdirect) and two indirect tracking methods leveraging population-based surrogacy relationships with the left atria (Trackingindirect_LA) or other target (Trackingindirect_target) were developed. Tracking performance was evaluated using transverse ECG-gated breathhold MRI images from 15 healthy and 10 AF participants. Geometric and volumetric tracking errors were calculated, defined as the difference between the ground-truth and tracked target centroids and volumes respectively. Transverse, breath-hold, noncardiac-gated cine images were acquired at 4 Hz in 5 healthy and 5 AF participants to qualitatively characterize tracking performance on images more comparable to MRILinac acquisitions. Results: The average 3D geometric tracking errors for Trackingdirect, Trackingindirect_LA and Trackingindirect_target respectively were 1.7 ± 1.2 mm, 1.6 ± 1.1 mm and 1.9 ± 1.3 mm in healthy participants and 1.7 ± 1.3 mm, 1.5 ± 1.0 mm and 1.7 ± 1.2 mm in AF participants. For Trackingdirect, 88% of analyzed images had 3D geometric tracking errors <3 mm and the average volume tracking error was 1.7 ± 1.3 cc. For Trackingdirect on non-cardiac-gated cine images, tracked targets overlapped organsat-risk or completely missed the target area on 2.2% and 0.08% of the images respectively. Conclusion: The feasibility of non-invasive MRI-guided tracking of cardiac-induced AF CR target motion was demonstrated for the first time, showing potential for improving AF CR treatment efficacy