Speed is more important than ever in MRI. Opportunities in cardiac and brain imaging require that data be acquired rapidly. Because the most advanced applications can be accomplished only at high field, patient comfort depends on getting out of the
Speed is more important than ever in MRI. Opportunities in cardiac and brain imaging require that data be acquired rapidly. Because the most advanced applications can be accomplished only at high field, patient comfort depends on getting out of the cylindrical magnet as quickly as possible. Traditionally, faster and more powerful gradient coils with higher slew rates met the need for speed, but this approach is running up against human physiology, as the highest performance coils get closer to causing nerve stimulation effects.
Increasingly, MRI vendors are moving toward the use of sensitivity encoding (SENSE) as a way to reduce scan time without jeopardizing safety. Philips Medical Systems introduced the technology two years ago as the brainchild of its luminary group at the Institute of Biomedical Engineering, University and ETH in Zurich, Switzerland. Since then, the number of Philips sites using SENSE has expanded, as has the number of potential applications. About 50 Philips scanners are equipped with SENSE, and more are expected before the end of this year, as Philips’ sales effort gears up following the March launch of the product.
The technology has been successfully applied in contrast-enhanced (CE) MR angiography as well as cardiac and abdominal scanning. The Zurich developers of the technology are studying its use on a Philips 3-tesla Intera system installed earlier this year. The higher signal-to-noise ratio possible with 3 tesla and the faster scanning speeds achieved with SENSE are being combined for cardiac functional imaging and coronary MRA.
“Three tesla already provides time benefits because of its high signal-to-noise ratio,” said Andres van Est, Philips applications manager. “But 3 tesla is more susceptible to artifacts. SENSE shortens the echo train lengths, which makes 3 tesla less susceptible to those artifacts. So it’s cleaning up the images.”
Philips is not the only company with access to this technology, however, and competitors are gunning for the lead position. Toshiba’s work-in-progress SPEEDER is undergoing clinical tests in Japan. GE’s ASSET (array spatial sensitivity encoding technique) is running at more than a dozen clinical sites, according to the company. ASSET versions designed to operate at 1.5 and 3 tesla are scheduled for commercial release in the third quarter of this year.
“Philips got a jump start, because they developed it on one of their sites initially, but it was essentially a research tool,” said Dr. Amit Kakar, GE’s manager of MR advanced development. “It’s taken them time to ship SENSE to commercial sites.”
The names are different, but the approach is the same. SENSE, ASSET, and SPEEDER acquire MRI data in parallel. They use spatially varying signal acquired using different phased-array elements to decode lines of k-space.
“I wouldn’t be able to point out any difference between ASSET and SENSE,” Kakar said. “ASSET is basically a sensitivity encoding technique.”
SENSE-based imaging has obvious clinical advantages, which are achieved by accelerating the acquisition of data. But this technique is anything but fast when it comes to image generation. Sensitivity encoding is computing intensive, Kakar said.
“You’ve got to be sure that the system you are using has parallel processing, because while the data are being reconstructed, you should be able to do some other work in your routine imaging,” he said.
There are other limitations. Because temporal and spatial resolution are interchangeable, it has been difficult to measure exactly the benefit of using SENSE in terms of image quality, Kakar said. Another limitation is susceptibility to small field-of-view artifacts. These challenges give Philips an advantage over the competition, said Guido P. Stomp, global field marketing director for Philips MR.
“We designed our systems years back with SENSE in mind, so the installed base can easily upgrade to SENSE by adding software and coils,” he said. “It’s not easy (for our competitors) to catch up.”
They have plenty of reasons to try. The benefits of using SENSE are most obvious in cardiac and abdominal studies, but there are other applications. GE and other vendors are exploring the use of SENSE in diffusion-weighted neuroimaging, as well as spectroscopy. Toshiba is looking especially hard at SPEEDER as a means for reducing scan time in CEMRA exams, according to Joseph Fritz, Ph.D., senior manager of clinical development at Toshiba America Medical Systems.
Philips has the most experience working with the technology. Pediatric applications are proving especially attractive. Studies at Texas Children’s Hospital in Houston have demonstrated that SENSE cuts in half the time needed for CEMRA of thoracic and abdominal vasculature, eliminating the need for children to hold their breath during the exam. An added benefit is the simultaneous acquisition of data with contrast injection, which negates the need for bolus timing. This is particularly important for children, whose faster blood flow and smaller size make using a contrast test bolus much more difficult than with adults.
Other Philips sites report clinical advantages. Bruderkrankenhaus in Trier, Germany, reduced the number of breath-holds needed for CEMRA from three or four to one. Puget Sound VA Health Care System in Seattle cut CEMRA scan times by 33%. Reducing the number of breath-holds is important when evaluating the liver and pancreas. With SENSE, the staff at Erasme Hospital in Brussels, Belgium, could acquire 25 slices of the abdomen in 15 seconds-a time span longer than most patients can hold their breath.
SENSE may have its greatest impact in cardiac imaging. Standard MR coronary angiograms typically require 10 to 20 minutes. SENSE cut that time in half at St. Luke’s Episcopal Hospital in Houston. Boosting spatial resolution may be the most important consideration in cardiac perfusion scans, as shown in studies at the Gasthuisberg University Hospital in Leuven, Belgium. Alternatively, physicians there have used SENSE to increase temporal resolution in cardiac scanning, which may be necessary to distinguish the myocardium from blood when doing volumetric cine studies.
“SENSE can be applied with any pulse sequence,” Stomp said. “So it’s not constrained to a specific application.”
While Philips appears to have the lead in clinical experience, GE and Toshiba are making plans to bridge that gap. ASSET is generating good results in neuro and renal angiography, according to Kakar. Toshiba luminaries are using the technique to dramatically cut the time needed for MR angiography. SPEEDER and ASSET also provide the option of increasing spatial or temporal resolution.
“We’re seeing these arrays speeding up exams by three, four, and five times,” Toshiba’s Fritz said. “Or you can get twice the field-of-view in the same amount of time (with a conventional scan).”
Optimal speed is achieved by using SPEEDER in combination with other advanced techniques, particularly SuperFASE, a very short echo half-Fourier sequence that allows MR angiography of vessels in the abdomen and chest without the use of contrast media. When SuperFASE and SPEEDER are combined, high-resolution vascular exams can be acquired in less than two minutes, he said.
Toshiba was among the early developers of sensitivity encoding, Fritz said. A paper on the subject was published in the early 1990s. Other vendors also conducted early developmental work, according to Kakar.
“Most companies had SENSE for quite a while, but they weren’t ‘productizing,’” Kakar said.
Spurred by Philips’ decision to commercialize the technique and work by the company’s Zurich luminaries to document its clinical benefits, major vendors are hopping onboard. Stomp believes industry acceptance will help Philips.
“One of the big advantages is that now there will be worldwide recognition of what we’ve done in the investigation of SENSE,” he said.