ExperimentalReal-time magnetic resonance imaging–guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla
Introduction
Radiofrequency (RF) ablation1 has evolved from a primitive procedure to the mainstay of arrhythmia management it is today.2 As progress was made in the understanding of the mechanisms underlying arrhythmias, the limitations of fluoroscopy and conventional mapping techniques became apparent. Electroanatomic mapping allows for three-dimensional (3D) cardiac chamber reconstruction, spatial catheter localization, tissue characterization based on local electrograms (EGMs) and electrophysiologic tissue properties, and assessment of adequate RF energy delivery. Reduction in local tissue EGM voltage remains a widely used, albeit indirect, method for assessing adequate lesion formation. Electroanatomic mapping has become the cornerstone of modern, complex cardiac ablations.
A more effective endpoint for RF ablation would be direct, real-time (RT) visualization of myocardial destruction to assess lesion formation during ablation. Magnetic resonance imaging (MRI) allows for the assessment of location and extent of RF ablation lesion and of scar formation in the myocardium.3 However, assessment of lesion formation during RF energy delivery has remained elusive. Several groups have reported MRI tracking of catheters within the cardiac chambers with successful delivery of RF energy and postprocedure visualization of lesion formation in 1.5-Tesla scanners.4, 5, 6, 7 This is feasible using MRI; however, to the best of our knowledge, the ability to combine RT MRI tracking of electrophysiology (EP) catheters and recording of EGMs, with actual tissue and lesion formation visualization during RF ablation, has not been described yet. A system integrating these would prove valuable because it would allow visualization of a “hard” endpoint to ablation: direct, RT visualization of myocardial scar formation and its correlation with changes in tissue electrophysiologic properties.
In this study, we report a novel 3-Tesla RT MRI-guided catheter tracking system combined with a unique RT MRI sequence and a new T2-weighted (T2W) sequence, which have allowed us to successfully and safely position an EP catheter within the atria, record intracardiac EGMs, and ablate with simultaneously RT visualization of RF myocardial lesion as it is being formed.
Section snippets
Catheter description and energy delivery
A 110-cm 7F, 3-mm-tip, steerable, MRI-compatible ablation catheter (SurgiVision Inc., Irvine, CA, USA) was designed to deliver RF energy under 3-Tesla RT MRI. The catheter offered impedance monitoring and unipolar and bipolar EGM recording, and was able to deliver up to 40 W of energy in a power-controlled mode. It had four tracking microcoils. Figure 1A depicts a prototype EP-MRI catheter with a close-up view showing the catheter tip and the tracking coils in a deflected catheter.
Intracardiac
Catheter testing, navigation, and recording of intracardiac EGMs
The catheter underwent heat testing at 3 Tesla and found to be less than 2° above baseline during RT MRI. The tracking elements allowed for catheter visualization during navigation. Navigation was tried at different frame rates by different operators. It was found that 5.5 fps provided a reasonable balance between good visual resolution and imaging time for all operators. Hence, our RT MRI guidance was performed using an RT GRE pulse sequence at 5.5 fps.
Discussion
MRI-guided ablation within the atrium has recently been reported by other groups.6, 7 In one of these studies, MRI angiography of the atrium was acquired. The atrium surface was segmented, and RT catheter navigation was carried out using this 3D reconstruction. However, no images were acquired during ablation.6 Rather, immediately postablation, lesion formation was confirmed by LGE imaging. In the other study, the catheters were navigated using RT MRI sequences.7 However, there was no immediate
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S. Vijayakumar, Dr. Kholmovski, G. Payne, and Dr. Marrouche are partially supported by grant from SurgiVision Inc. Dr. Vij and M. Guttman are employees of SurgiVision Inc.