Diagnostic Imaging
July 2003

TECH WATCH

Ultrahigh-field MR seeks new details at 7T and 9.4T

Experimental systems at multiple sites may serve as pathfinders to new applications

The chemical signs of disease lie hidden in the signal of clinical MR scanners. Some cannot be seen directly at 1.5T; their presence might be indicated only by a drop in signal. Others are lost in the noise that surrounds them.

These clues to disease can, however, be seen at 7T or 9.4T, and once they are found and documented, they might be spotted in routine clinical studies. This is the raison d'etre behind a new breed of superscanners being developed by GE, Philips, and Siemens.

Philips Medical Systems plans to produce 10 units operating at 7T and has received orders for five. The first is slated for installation next year, but all will likely be installed by the end of 2006.

Siemens has one 7T system installed at the Center for Magnetic Resonance Research (CMMR) at the University of Minnesota and a second at Massachusetts General Hospital. Two more are on order.

A 7T GE system installed at the National Institutes of Health this spring is scheduled to begin operation in midsummer. The company is also implementing a 9.4T system at the University of Illinois, and the CMMR began installing a 9.4T system in early May.

It is no coincidence that ultrahigh-field imaging has become popular about the same time that 3T scanners are being commercialized, said J. Thomas Vaughan, Ph.D., director of the CMMR engineering core.

"Progress in this field is incremental," he said.

Vaughn and his staff take a "Lego" approach to MR, cobbling their systems together from different vendors and fitting the pieces into novel constructions. This kind of ad hoc construction has been Vaughan's trademark as an engineer since the early days of MR.

"We get where we are going a lot sooner because we don't have to wait for a major manufacturer to develop the system," he said.

Vaughan's latest project, a 9.4T system, is under construction. When it begins operating, the CMMR will be king of the MR hill. It might have to share that distinction fairly soon, however, when the 9.4T system at the University of Illinois begins operating later this year.

For the time being, Ohio State University has the ultimate MR scanner for human imaging. Dr. Gregory Christoforidis, an assistant professor of radiology, has used the 8T system at OSU to look at microvasculature deep in the brain, identifying vessels less than 100 microns in diameter in both healthy subjects and in patients with gliomas. This visualization is achieved using the magnetic susceptibility of deoxyhemoglobin, an impossibility at 1.5T. But lower fields can also benefit from research done on superscanners.

"When we go back to 1.5T images, we see that areas where the signal drops off seem to correspond to the microvascularity we see at 8T," Christoforidis said.

The results are speculative, he said. The correlations involve perfusion images at 1.5T and direct visualization at 8T and, therefore, need more detailed documentation.

"We haven't been able to prove it yet because we are way too early in our research," he said.

Correlating human studies with animal research could help pave the way. Dr. Michael Knopp, chair of Novartis and director of imaging research at OSU, believes that research at 7T and 8T is the key to capitalizing on advances made over the past several years in ultrahigh-field small-animal imaging.

"Animal studies have been indicative that ultrahigh-field imaging will improve our structural and functional imaging," Knopp said. "Now we hope to transfer this into clinical applications."

Part of that effort will be a collaboration with Philips Medical Systems, announced in May, that will lead to the installation of a 7T scanner. It will be used in the university's effort to develop a molecular imaging infrastructure.

John Patrick, Philips' director of MR business development, said the OSU research done at 8T will provide the foundation for developing clinical capability on the Philips 7T unit slated for delivery. That system may be especially well suited for the development of pharmaceuticals. Clinical opportunities will be clearer as Philips completes its plan to site 10 such systems in the next three years.

"As more machines get into the field, there will be a whole new basis of work upon which to build," Patrick said.

Kyle Salem, Ph.D., Siemens' MR research collaborations manager, expects that research performed on 7T scanners will illuminate findings obscured at 3T or 1.5T. Images at 7T that demonstrate pathologies might be matched with correlates in images taken at lower fields, he said. Sent to investigators or posted on the Internet, they could serve as references for making diagnoses.

"I believe 7T will be the magnifying glass that allows us to find what we are looking for at other fields," Salem said.

The first findings will probably address neuroimaging because much of the ultrahigh-field research focuses on the brain. Higher field strengths deposit more energy in the body. The smaller volume of tissue in the head requires fewer trade-offs than, for example, cardiac or abdominal studies, said Larry Wald, Ph.D., director of the NMR core at MGH. Investigators at MGH have used 7T to examine the mechanisms of drug addiction.

Problems such as energy deposition will eventually be solved by more efficient radio-frequency coils, Wald said.

"We bought a magnet big enough for the whole body," he said.