Siemens' 7T MRI system

Is 7-Tesla MRI where 3-Tesla was 10 years ago?

January 12, 2012
by Brendon Nafziger, DOTmed News Associate Editor
As new applications for cardiac imaging emerge, an industry scientist says clinical 7T MRI might one day come into its own.

German scientists have developed new techniques for capturing a heart beat with an ultra-high field MRI whose power is thousands of times stronger than the Earth's magnetic field.

Currently, all clinical work is done with 1.5T or 3T MRI units. But scientists are exploring how to scan the heart with a powerful and expensive, experimental 7-Tesla MRI machine.

To do so, the scientists had to invent new radiofrequency coils to capture images from the body and other technology to get around the difficulty of synchronizing the scan with the heart's motion.

If successful, 7T MRI heart imaging could mean sharper, clearer pictures of the heart to help doctors better diagnose disease.

Although the scientists involved in the project caution that the technology is still in its early stages, they hope the results also show the possible clinical potential of 7T, which right now is up for debate.

"7T is like 3T 10 years ago," James Meng, R&D director of MR at Siemens Healthcare, which is involved in the MRI heart studies, told DOTmed News. "If you talked about 3T 10 years ago, no one would believe it would become the clinical instrument it is today."

For now, these investigational devices -- none has been cleared by the Food and Drug Administration for clinical use -- are quite rare.

Only about 40 7T machines are installed worldwide, according to Meng. Siemens said it has installed almost 15 in the United States. Philips Healthcare, another manufacture of whole-body 7T MRI, said it has 9 installed, with a backlog for two more orders. GE Healthcare is the other main company in this sector. The company said it has installed 7T MRIs in a few countries.

Slower than expected

The units are scarce, in part, because they're expensive. The rule of thumb for MRIs is that adding 1 Tesla to the instrument adds another $1 million to the price tag. However, this has been disputed, and manufacturers generally do not comment on price.

The modality has also not advanced as fast as was previously forecast. In 2006, a Philips executive predicted that by 2009, the 7T market would see $700 million in accumulated sales, or 100 systems sold, according to a Diagnostic Imaging report. "I think that that has not happened up until now," Ken White, Philips' MR program manager, told DOTmed News. "But the potential is certainly there in the future."

New research

The potential comes from a host of fields where 7T MRI might help doctors learn more information than they can with scanners currently available in hospitals and imaging centers.

Although it has not been clinically proven to be superior, 7T MRI offers a theoretical benefit over weaker magnets: an increased signal-to-noise ratio. This could lead to higher resolution or faster scans.

"You could get a better delineation of, for example, brain structures," White said. "So that allows you to visualize disease that's not as easily visualized, possibly cannot be visualized at 3T, and potentially allows earlier diagnosis and treatment."

Neurology is, actually, one of the leading fields for the high-powered magnetic technology, especially functional magnetic resonance imaging. In 2010, researchers published an atlas of the brain using 7T MRI scans. And just last year, researchers used a 7T device to better detect the brain scarring that can trigger epilepsy.

Other promising applications include orthopedics and magnetic resonance angiography, especially non-contrast-based MRA.

New coils needed

But to take advantage of 7T MRIs, scientists need to first develop coils -- which help capture the patient images -- that can work with their much higher field strength.

This has been challenging, in part because when making coils for the new system, it's harder to achieve what's known as B1 homogeneity.

"B1 is the time varying magnetic field generated by the RF coil which is used to generate the image. Better B1 homogeneity contributes to better image quality," White explained.

At higher field strengths, such as in 7T MRIs, dielectric effects influence the homogeneity of the B1 field, White added. This can result in shading and other contrast effects. But scientists can mitigate this effect by using multiple transmit channels at the same time, with independent control of phase and amplitude to perform what's known as "B1 shimming."

Capturing the heart

Philips said it continues to develop new coils -- both in-house and with third parties. And progress is being made on the coil front, including for heart imaging.

For instance, Siemens and its academic collaborators said they've developed a multi-channel RF coil able to acquire heart images.

According to a study published last March in the Journal of Magnetic Resonance Imaging, researchers with the Berlin Ultrahigh Field Facility developed four-channel cardiac transceiver coils for the 7T MRI. The project was the result of a joint collaboration with scientists from Siemens and Charité-Universitätsmedizin Berlin, one of Germany's biggest university hospitals, and Max-Delbrück-Center for Molecular Medicine Berlin-Buch, a German research center.

The coil wasn't the only challenge, though. The scientists also had to create a triggering device designed to synchronize cardiac imaging with heart motion -- and one that was compatible with 7T MRI's strong magnetic fields. Previously, "the challenge of synchronization of data acquisition with the cardiac cycle" has been a major impediment for cardiovascular MRI, the scientists involved with the device wrote last fall in Journal of Cardiovascular Magnetic Resonance.

But, Prof. Thoralf Niendorf, the investigator of the study, said his team was able to correlate image exposure with the heartbeat.

It is, of course, an early step, and the scientists said there were many, many more to go before 7T MRI is ever used routinely on patients.