Biomedical engineers at the Johns Hopkins University have extended magnetic resonance imaging (MRI) into cardiac surgery for the first time with a new procedure to help prevent rapid and irregular heart rates.
The novel technique uses MRI instead of X-rays to guide radio frequency cardiac ablation, a minimally invasive surgical procedure that blocks abnormal electrical pathways in heart muscle. The MRI technique makes catheter-based intervention more accurate and reliable, which should reduce overall costs, and eliminates the current hazard of radiation exposure.
MRI has been used primarily for diagnosis throughout the human body, and is increasingly being used to guide therapy. Until now, however, MRI-guided therapies have not targeted the heart. The new approach, says lead investigator Albert Lardo, Ph.D., represents "a significant shift away from X-ray-guided techniques and establishes the feasibility of performing cardiovascular intervention under MRI guidance." Lardo presented some of his findings at a scientific conference of the American Heart Association in New Orleans last month. The new technique was also described in a recent issue of the AHA journal Circulation.
In recent years, radio frequency (RF) cardiac ablation has become a standard therapy to treat basic types of arrhythmia, or abnormal heart rate. The procedure benefits about one out of every 13 arrhythmia patients each year in the United States. During RF ablation, doctors make a small incision in the arm or leg and insert a special electrode catheter, essentially a long, flexible wire, through an artery to the heart and close to the trouble spot. As radio frequency energy passes through the catheter, the tip of the wire heats up and burns the heart tissue, creating a small, elongated lesion. The lesions and subsequent scar tissue, which do not conduct electrical impulses, essentially form roadblocks for the abnormal impulses responsible for the arrhythmia.
One limitation of X-ray-guided RF ablation, however, is that it is difficult to confirm the optimal location to place the lesions. These optimal conditions are essential to the surgery's success; if the lesions have gaps, are too thin or are misaligned, the problematic electrical impulses could eventually reroute themselves, and the arrhythmia could return.
"X-ray is inherently a two-dimensional projection imaging technique," says Lardo, "which means that there is a lot of anatomic ambiguity. It's very difficult to know in 3-D where you are." Also, as the lesions are made, X-rays are deficient at distinguishing between the lesions and the heart tissue.
"Although we have bioelectric feedback, we're just kind of shooting in the dark," he says.
Not surprisingly, arrhythmia reoccurrence rates are high, says Lardo. Physicians may try to tilt the odds in their favor, sometimes creating as many as 50 to 60 lesions in only a 2- to 3-millimeter space, "but the goal is to make as few lesions as possible and still achieve conduction block," he says.
"With MRI, you can overcome all those limitations," says Lardo. "We know precisely where the catheter is so we gain a lot of confidence in being able to put the lesion in the right spot. Not only that, but we have a way to monitor the formation of the lesion s it happens, and watch it grow. If the image tells us that the lesion isn't big enough, we can go back to the same spot and deliver another burn."
Fewer and more precise and effective lesions should reduce the overall costs of the surgery, says Lardo. The time to perform the procedure should be reduced, and the need for a second procedure nearly eliminated.
MRI is generally considered a safe technique because no X-rays are used, thus eliminating any radiation exposure to the patient and physician. Also, the electromagnetic fields do not, by themselves, cause tissue damage. But despite MRI's advantages, Lardo had to overcome several hurdles to develop the new technique.
For one, the catheter could be made from only nonmagnetic materials, such as gold, platinum or copper. Nonmagnetic materials not only help reduce image distortion, but because conventional MRI uses a monstrous electromagnet, any magnetic components "would be pulled out of your hand," says Lardo.
Another problem was signal interference. When creating lesions, the ablation catheter emits radio frequency energy. At the same time, the MRI system is acquiring an image of the heart. "MRI is based on radio frequency energy," says Lardo, "so there is indeed a problem with interference."
One solution was to perform the ablation and imaging in a staged fashion, one after the other. But to take full advantage of the technology, Lardo and his team developed imaging filters that will allow both steps to be performed simultaneously.
The technique also relies on a catheter antennae. By placing the antennae next to the ablation coil in the catheter, it improves the resolution and helps eliminate the noise.
The next step is human clinical trials, which await approval from the Food and Drug Administration.
The research was supported in part by grants from The Whitaker Foundation, the American Heart Association, the National Institutes of Health, and Surgi-Vision, Inc.