Cardiac Electrophysiology Under MRI Guidance: an Emerging Technology

Abstract

Interventional magnetic resonance imaging (MRI) is a growing field, and the strength of MRI guidance for procedures rests fundamentally in the high-contrast imaging of soft tissue structures. Combined with the avoidance of radiation exposure, the potential for functional assessment and the ability to exploit MR signals for calculation of the location of interventional instruments, it is clear that the implementation of interventional MRI will continue to grow. For general cardiac interventions, the visualisation of thin, mobile structures presents particular challenges for MRI guidance. Cardiac electrophysiological (EP) procedures add a further dimension, as the accurate detection of intracardiac electrograms must be performed in a highly active electromagnetic environment. This review focuses on the technical considerations for the performance of EP procedures under MRI guidance (MR-guided EP). Potential Benefits of MR-guided Electrophysiological ProceduresMRI techniques offer a high soft-tissue contrast-to-noise ratio (CNR) in comparison with that seen with X-ray, computed tomography (CT) and ultrasound. However, the environment can present challenges and is an expensive procedure; therefore, all the benefits of MR-guided EP must be fully considered to justify the additional difficulties and expense. Broadly speaking, these benefits can be divided into three main areas: improved precision of ablation targeting (substrate identification), improved intra-procedural guidance and improved assessment of ablation lesion formation. Emerging data also suggest that CMR imaging may be used to guide atrial ablation procedures. Although the atrial wall is thinner, native fibrosis and ablation scar can be identified using primarily threedimensional LGE techniques. 2,11 Findings from some studies have been interpreted to suggest that successful ablation of fibrotic regions, distant to the pulmonary veins (PVs), may help improve AF ablation success rates. 12 Similarly, atrial re-entrant circuits can be modelled in silico based on atrial scar location and can be used to inform ablation strategies. 13 Sites of PV reconnection have been identified using CMR, with successful ablation guided by the CMR-derived substrate, 14 but these findings have not been replicated in all studies. Substrate Identification 15To date, all studies that have used CMR-derived substrate identification to guide ablation have relied on fusion of the imaging to electroanatomic Abstract MR-guidance of electrophysiological (EP) procedures offers the potential for enhanced arrhythmia substrate assessment, improved procedural guidance and real-time assessment of ablation lesion formation. Accurate device tracking techniques, using both active and passive methods, have been developed to offer an interface similar to electroanatomic mapping platforms, and MR-compatible EP equipment continues to be developed. Progress to clinical implementation of these technically complex fields has been relatively slow over the last 10 years, but recent developments have...

Magnetic resonance imaging guidance for the optimization of ventricular tachycardia ablation

“…Translational changes due to patient movement, cardiac or respiratory motion can also lead to discrepancies between imaging and EAM. 57 Potential registration errors of even a few millimetres, as described in the image integration studies, could lead to misidentification of critical sites with potential consequences on the efficacy of ablation.…”

“…Respiratory and cardiac motion is also more accurately assessed by MRI than any other imaging modality—compensating for these factors during RF ablation could have potential implications for the safety and efficacy of ablation. 57 …”

“…Protons are surrounded by electrons which act to disturb the local magnetic field during MR scanning. 57 Following ablation there is an increase in temperature, breaking of inter-molecular hydrogen bonds and an increased density of free electrons. The change in PRFS using phase mapping has been used to quantify tissue temperature changes during ablation and demonstrated in mongrel dogs.…”

“…This tool however, represents an interesting step in exploiting acute physiological changes during ablation using MR to potentially titrate RF energy delivery. 57 Further validation work is warranted to see if the technique is feasible in patients during real-time MR-guided ablation.…”

“…Although there is much interest within the electrophysiology community in real-time MR-guided interventions, there are relatively few centres with the resources, expertise and technologies to perform such interventions. 57 …”