MRA gains from advances in high-field hardware

High-field scanners have much to offer proponents of MR angiography. Moving from 1.5T to 3T should theoretically double the signal-to-noise ratio, which can boost the all-important spatial resolution. Acquisition of images with a 3T magnet also means longer T1 tissue relaxation times. This in turn should translate into stronger signals from contrast agents.

Yet not all radiology departments have access to 3T systems. Sales of 3T scanners may be increasing steadily in both Europe and the U.S., but until existing technology requires replacement, 1.5T systems are likely to remain the mainstay of most radiology departments.

Even institutions with ready access to 3T MR have held back from switching their entire clinical MRA workload to a higher field strength scanner. The benefits of 3T imaging are balanced by a set of limitations. Signal gains and longer T1 relaxation times have to be tempered against the practicalities of working with an extremely strong magnetic field.

One issue is the difficulty of maintaining field homogeneity within stronger magnet bores. Better engineering is helping iron out this problem, but many 3T acquisitions must still be performed within a smaller field-of-view. Pockets of signal loss can also be observed, owing to the so-called dielectric resonance effect. This effect, which is due to the shorter wavelength of radiofrequency signal at 3T, is greatest when intensive RF sequences are applied.

Another well-known drawback associated with high-field MRI is the dramatic increase in RF energy absorbed by patients during scanning. Doubling the field strength from 1.5T to 3T causes a fourfold increase in the specific absorption rate. This rise in SAR translates into a greater heating effect in patients' tissue and hence greater potential for discomfort or even burns. Such tissue temperature rises are usually negligible. At 3T, however, field strength inhomogeneities can lead to local SAR "hot spots." This risk can be minimized by keeping the frequency and power of RF energy as low as reasonably possible.

"SAR can be a big limitation for MRA because you tend to use very high performance techniques that result in a fair amount of RF deposition," said Dr. James Carr, director of cardiovascular imaging at Northwestern University Medical School in Chicago. "So with 3T, sometimes you have to back off a little bit on some of the more aggressive parameters."

Dr. Paul Finn, a professor of radiology and medicine at the University of California, Los Angeles Medical Center Cardiovascular Research Laboratory, was wary of RF heating effects and artifacts when moving his MRA protocols to 3T. So far, however, radiologists at UCLA have observed few difficulties with their amended sequences. Standard contrast-enhanced MRA examinations involve very fast 3D, short TR, and short TE sequences. Reducing the flip angle for 3T MRA helps prevent problems with increasing SAR, he said.

"The only place we have noticed some limitations has been the groin for lower extremity MRA. You occasionally get some dielectric resonance problems there, which we are addressing and should not be a long-term limitation," Finn said.

Performing MRA at 3T also offers the potential to lower contrast doses, according to Finn. UCLA radiologists have not studied this systematically. It is possible, however, that 3T MRA could be performed with 60% or 70% of the dose administered for the same examination at 1.5T. As hardware platforms improve, doses administered at 1.5T may decrease too.

MATURING TECHNOLOGY

If sequences can be tweaked to cater for 3T MR physics, why the delay in moving more MRA to 3T? The reason is that 1.5T scanners now do an excellent job at imaging the body's vasculature. Moving from 1.5T to 3T may offer a simple doubling in field strength, but today's 1.5T systems have been engineered to maximize their potential. The advent of parallel imaging is widely regarded to be a far more significant boon to contrast-enhanced MRA than 3T scanners. Advances in coil design have also helped make the most of an MR signal. Until this technology transfers from 1.5T, radiologists are in no rush to move their MRA caseloads.

The relative lack of 3T-compatible peripheral vascular coils has been a significant limitation to performing higher field strength MRA, according to Dr. Tim Leiner, a radiologist at Maastricht University Hospital in the Netherlands. He now uses a coil that covers patients from their diaphragm to their feet when performing peripheral MRA at 1.5T. This, combined with parallel imaging parameters, has resulted in excellent image quality.

"The most important thing when doing MRA is to start out with the right technical equipment. This means surface coils, at least for the lower legs; centric k-space filling, and parallel imaging," he said.

Carr has also held back from performing clinical MRA at 3T, as the 1.5T system at Northwestern provided more advanced functionality. That could change, however, with the imminent arrival of a 3T scanner with multichannel imaging capability and multiple coils. A proportion of clinical cases may be shifted to 3T along with the existing research MRA caseload.

"The multichannel and multiple coil setup is going to be much more conducive to doing MRA at 3T," he said. "It may allow us to go incredibly quickly and get very high-resolution images over the next couple of years."

Dr. Martin Prince, chief of MRI at the Presbyterian Hospital and Weill Cornell Medical Center in New York City, shares the view that top-notch scanner engineering and advanced coil technology are essential if the benefits of 3T are to be realized. Unpredictable artifacts have decreased considerably following an upgrade to 3T facilities at the New York City site. Development of phased-array coils and parallel imaging at 3T should improve image quality further, he said.

Dr. Jonathan Gillard, a neuroradiologist at Cambridge University Hospitals Foundation Trust, U.K., offers cautious optimism on the diagnostic benefits of 3T.

"It has come to the stage now where 3T systems are starting to mature enough such that you can get better quality studies and actually use 3T to its maximum advantage," he said. "But you have to be cautious with it. It is whether you can make that change deliver."