MRI Guided EP Testing and Ablation
Abstract & Commentary
By John P. DiMarco, MD, PhD
Source: Nazarian S, et al. Feasibility of real-time magnetic resonance imaging for catheter guidance in electrophysiology studies. Circulation. 2008;118:223-229.
In this report, Nazarian et al from the johns Hopkins Hospital describe a magnetic resonance imaging (MRI) compatible system for electrophysiologic study catheter placement. They desired to assess the feasibility of performing electrophysiologic studies using a real time MRI guidance system. The catheters used were referred to as either passive when they were visualized by metallic component MRI artifacts or active if they were visualized by an MRI signal received by the catheter. Preliminary animal models and two pilot human experiments are described. Both the active and passive catheters had a woven Dacron body, copper wires, and platinum electrodes. The passive catheters are recognized on MRI by a susceptibility artifact surrounding the electrode. Only the tip is visualized. The active tracking catheter uses a 64 MHz loop antenna extending along the entire catheter body from which the MRI signal is received. Nazarian et al designed custom shielding and filtering hardware for the catheter's data acquisition and stimulator. Filters were employed to protect electronics and to mitigate catheter heating. Additional filters were also used to reduce gradient signal-induced noise and limit the band width of the intracardiac electrograms. A 1.5-T MRI scanner manufactured by Siemens Medical Systems was used for these experiments. A fast gradient-recalled-echo (GRE) sequence was used to obtain standardized views. The experiments performed also included phantom heating studies in a stable acrylic box. Temperatures were recorded during scanning along the length of the catheters.
Animal experiments were conducted in ten adult mongrel dogs. Catheters were inserted from femoral vein sheaths and positioned in the right heart using MRI guidance. An active catheter was used at the right ventricular apex, and passive catheters were used for right atrial and His bundle recordings. After each experiment, the animals were euthanized and the hearts examined for accurate catheter positioning and potential damage during the procedure. Two patients were also studied. These patients had previously undergone cavotricuspid isthmus ablation for typical apical flutter using fluoroscopic guidance. These patients were then transferred to the MRI suite and catheter manipulation via MRI was performed.
Catheter heating was minimal. During standard GRE sequences, there was less than a 2° temperature rise on all measuring points along the catheter's length. In the canine experiments, all catheters could be successfully positioned at their intended target positions with an average time of 2-7 minutes. Post-mortem examination revealed small endocardial hemorrhages in two animals on the ventricular septum just beyond the tricuspid annulus. There was no evidence of thermal tissue injury on histologic examination, and Nazarian et al interpreted these as due to pressure. Active catheters could be more easily seen on MRI and required fewer real-time imaging planes for catheter tipped localization. Filtered electrograms were comparable to those obtained with standard catheters outside the MRI environment in both the canine and human studies. No failure to capture, or unexpected capture resulting from magnetic current induction, was noted.
Nazarian et al conclude that real time MRI imaging may be used to guide electrophysiologic studies in the future. The combination of anatomic data with electrophysiologic signals may aid in the future ablation of complex cardiac arrhythmias.
Commentary
Many standard electrophysiologic procedures require relatively little fluoroscopic time and can be completed with minimal radiation exposure to the operator or to the patient. However, complex ablation procedures such as atrial fibrillation ablation, ablation of complex atrial arrhythmias, and ventricular tachycardia ablation often require extensive catheter manipulation which, if done fluoroscopically, results in significant radiation exposure. Even today, many procedures use combinations of pre-procedure MRI or CT scans that are then merged with nonfluoroscopic catheter mapping systems during ablation procedures. Fluoroscopy, however, is still frequently required in these cases.
The studies presented here show that there is some potential for combining the superior anatomic detail provided by cardiac MRI with a real-time ability to position catheters for electrophysiologic studies without compromising the electrical signals obtained. If developed further, this would provide a system free of radiation exposure that has the potential to facilitate complex mapping and ablation procedures. These preliminary data are quite interesting, but further reductions in the time required for catheter manipulation will be required before this technique can be applied clinically. However, these data do illustrate that MRI guidance is feasible and we can expect rapid developments in this field.
In this report, Nazarian et al from the johns Hopkins Hospital describe a magnetic resonance imaging (MRI) compatible system for electrophysiologic study catheter placement.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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