Segmented delayed-enhancement MR imaging (a and c) reveals increased enhancement of left atrial tissue. Color 3D models (b and d) of MR images clearly show regions of abnormal enhancement (green) and healthy tissue (blue).
Images courtesy of the University of Utah School of Medicine.

Using MR imaging to measure scarring to the left atrium following radiofrequency ablation (RFA) for atrial fibrillation can predict treatment success. Researchers, including radiologists and cardiologists, published their findings in the Oct. 7, 2008, issue of the Journal of the American College of Cardiology.

Atrial fibrillation is a growing clinical problem. Of 5 million patients with atrial fibrillation, only around 38,000 are being treated with ablation, according to study co-author Nassir F. Marrouche, M.D., director of the cardiac electrophysiology laboratories and the atrial fibrillation program at the University of Utah Health Sciences Center in Salt Lake City.

To detect and quantify left atrial wall injury, investigators, led by Christopher J. McGann, M.D., and Eugene G. Kholmovski, Ph.D., performed a 3D delayed-enhanced cardiac MR imaging sequence before and three months after pulmonary vein antrum isolation in patients with atrial fibrillation.

"Noninvasive imaging of the left atrial wall with MR imaging is a recent advancement and a powerful tool to evaluate injury related to radiofrequency energy delivery during ablation," said Dr. Marrouche. "Results reported in our study suggest that the degree of left atrial wall injury predicts procedural outcome at three months."

Current Scar Assessment Techniques are Invasive

RFA appears effective in managing atrial fibrillation. The amount of the scar tissue created by RFA in the left atrium is an indicator of success; however, current scar assessment techniques are invasive. MR images have greater resolution than the 2D images now used to establish tissue damage, said Dr. Marrouche.

"Ablation is an intricate procedure with many complications," he said. "We built the MR imaging lab of the future with an MR imaging scanner and fluoroscopy. The goal is to complete the entire ablation procedure under MR, which we hope to do this year."

The University of Utah' s atrial fibrillation imaging group is the first in the world to integrate electrophysiology, radiology and image processing into one group, said Dr. Marrouche. "Today we can see the shadow of the catheter with fluoroscopy, but we can't see the wall of the heart or the changes we are creating on the wall or how deep these changes are," he said. "In the future, we may be able to plan ablation using MR imaging to identify the heart muscle cells that need to be destroyed."

Study Among First to Apply MR to Nonischemic Disease

MR has great utility and potential for evaluation of cardiac disease, said Gautham Reddy, M.D., a professor of radiology and vice-chair for education at the University of Washington School of Medicine in Seattle. Dr. Reddy led a session, Cardiac CT Mentored Case Review—Pulmonary Veins and Pericardium, at RSNA 2008.

3D MR imaging color models of varying degrees of enhanced/fibrotic (green and yellow) tissue preprocedure in four patients.

Determining the extent of tissue damage can help in determining if the patient is a good candidate for ablation.

3D MR color models of varying degrees of scar (orange) tissue three months postprocedure in four patients.

3D visualization and segmentation of the acquired MR image allows for the overall extent of scarring to be evaluated.

Delayed-enhancement MR techniques are currently used to assess ischemic cardiac disease, said Dr. Reddy, and the Utah researchers' study is one of several that have shown applications of delayed-enhancement MR imaging to nonischemic cardiac disease.

"This is an innovative approach, using MR to assess the amount of scar tissue after the RFA," said Dr. Reddy. "The scar tissue is not a complication, but the desired effect of ablation treatment. They have found that the more damage to the left atrial wall, the more successful the procedure in preventing recurrence of atrial fibrillation."

There is great potential to use the noninvasive procedure to evaluate the success of the ablation, said Dr. Reddy. "This is an innovative technique," he said. "In the past, success was measured by more invasive procedures, but MR takes us away from that. This is a breakthrough noninvasive way of evaluating the outcome of the procedure.

This RSNA 2008 session of Cardiac CT Mentored Case Review focused on pulmonary veins and pericardium. Session leader Gautham Reddy, M.D., said using MR to assess the amount of scar tissue after radiofrequency ablation for atrial fibrillation is an innovative approach. Radiologists will play an important role in implementing the practice clinically, Dr. Reddy said.

"I think radiologists will play an important role in MR evaluation following RFA for atrial fibrillation," Dr. Reddy continued. "The two lead authors on this paper are in the Department of Radiology and worked with their cardiology colleagues. I think radiologists will definitely have new roles that we didn' t have in the past. There is a lot of potential for radiologists in clinical practice in cardiac MR as well as in other research applications."

Dr. Marrouche said he expects that in three or four years, every electrophysiology lab may have at least one MR scanner. "The future is doing ablation and MR together, so we have exact confirmation of lesions," he said.

Added Dr. Reddy: "This can be put into practice relatively quickly because delayed-enhancement MR is already an established technique. Applying the technique to new indications is a simple step. I think this could see widespread use in a short amount of time."