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3–tesla MRI bests 1.5–tesla in body and brain

High field strength machines provide faster and better scanning

By Karen Sandrick

As good as 1.5–tesla MR images are, they just can’t characterize some subtle abnormalities as well as 3 tesla can. Take the 1.5–tesla scan of a 14–year–old seizure victim that was normal except for an area of ventricular dilation. It wasn’t until radiologists at Massachusetts General Hospital performed 3–tesla MRI that they could spot heterotopic gray matter associated with a developmental problem.

High field strength MRI delineates structures that have not been seen before, such as blood vessels as small as 200 to 300 microns, drilling down to the ultra–structural level. But 3–tesla MRI is more than a glorified, high–tech microscope. According to radiologists who have been testing high field strength MRI in clinical settings, 3–tesla machines can do anything a workhorse 1.5–tesla scanner can, and do it faster and better.

Three–tesla scanning in the brain is a no–brainer, said Dr. Robert Lenkinski, director of experimental radiology and the 3–tesla MRI program at Beth Israel Deaconess Medical Center. Conventional brain imaging at 3 tesla can be completed in the same exam time and can achieve a higher signal–to–noise ratio than at 1.5 tesla.

With a three– to four–fold higher SNR, 3–tesla MRI more precisely localizes areas of activation, enabling accurate mapping of brain function in patients more than 90% of the time. Three–tesla MRI also makes potentially insensitive techniques clinically robust, said Dr. Keith Thulborn, director of MR research at the University of Illinois at Chicago.

In the body, the SNR of a 3–tesla body coil is about the same as that of a 1.5–tesla phased–array coil. Adding a surface coil gives much more signal–to–noise headroom, which allows faster image acquisition and patient throughput or higher image resolution, revealing fine anatomic details and physiologic parameters, said Dr. Neal Rofsky, director of MRI at Beth Israel.

But while 3–tesla clinical systems have the same user interface as their 1.5–tesla predecessors, they take a little getting used to. Radiologists need to adjust to 3 tesla’s longer T1–weighted imaging, modify doses of contrast medium, and take care not to misinterpret data because of differences in contrast. So there is a learning curve, although not a steep one. Current opinion is that 3–tesla MRI will become the clinical standard, initially in neuroimaging, and eventually throughout the body.

3–Tesla in the Brain

Massachusetts General Hospital purchased two head–only, 3–tesla MRI scanners about six years ago, primarily for research purposes. Since late September, MGH has been devoting two mornings every week to 3–tesla clinical neuroradiology imaging, mainly involving tough cases in the head, such as the need to rule out myxoadenoma in a patient precociously pubescent.

“The demand for high–performance imaging is demonstrating clear medical benefit in cases where we can’t see lesions well enough with 1.5 tesla, such as scanning for developmental abnormalities or performing high–resolution MR angiography, diffusion or perfusion imaging,” said Dr. Gregory Sorensen, associate director of the MGH NMR Center.

The University of Illinois at Chicago and the University of Zurich in Switzerland have purchased head–only, 3–tesla scanners for research. They are performing high field strength clinical studies two days a week for conventional brain imaging, including visualization of tumors, identification of epileptic foci, and delineation of vascular disease.

The biggest payoff for 3 tesla, said Thulborn, is in presurgical planning to avoid invasive angiography or direct cortical mapping at the time of surgery. Coupled with anatomic detail, 3–tesla perfusion studies plot a patient’s physiology up to the edge of a lesion, and diffusion tensor images trace white–matter tracks that must be preserved, he said.

“The signal changes that we look at with blood oxygenation level–dependent contrast at 1.5 tesla is on the order of 1% to 2%,” Thulborn said. “That signal change goes to 3% to 5% at 3 tesla, meaning that you can do reliable individual patient mapping and interpretation.”

A second major clinical neuroimaging category for 3–tesla MRI at UIC involves monitoring the effects of rehabilitation in patients with cognitive impairment after successful acute treatment of acquired brain injury. Thulborn explained that some patients do not return to normal after acquired brain injury because of neurological damage to cognitive functions. A patient can’t read, for example, because a traumatic head injury has interfered with the visual space processing that allows coordination of eye movements.

UIC’s cognitive medicine program, headed by clinical psychologist Dr. Linda Laatsch, provides cognitive rehabilitation therapy to restore specific functions. Laatsch and her staff do not try to teach a patient how to read again but how to move the eyes across the page.

“It’s quite clear that patients with cognitive difficulties after acute brain injury have abnormal patterns of brain activation,” Thulborn said. “As they go through cognitive behavioral therapy, their patterns of activation return toward normal, and we can monitor how treatment engages cognitive processes to improve performance.”

Nevertheless, Thulborn has been unable to run a full 3–tesla neuroradiology schedule with a scanner limited to the head. Now that his scanner has been equipped with a neurovascular receive–only coil, he can extend high field strength imaging to the neck and examine the blood vessels feeding the brain as well as the circle of Willis. When a phased array spine coil is designed, he will have a full–service, 3–tesla neuroradiology operation that includes the thoracic and lumbar spine.

Whole–Body 3 Tesla

Since the first clinical whole–body, 3–tesla MRI was installed at Beth Israel, it has been used like 1.5–tesla machines, but even beyond. Because of the enhanced resolution of images and the ability to factor in physiology, 3–tesla imaging has not only captured finer details of anatomy, it has improved the separation of spectroscopic peaks of chemical species in the prostate. It has also identified small lesions in the breast, evaluated peripheral vascular disease, and tackled phosphorus imaging, Rofsky said.

Beth Israel is also focusing 3–tesla MRI on different targets. Rofsky et al are hoping extremely rapid imaging at 3 tesla will eliminate the need for breath–holding and open the door to routine screening in the abdomen where safety, speed, and throughput are paramount.

“It’s our belief that, in the abdomen with noncontrast 3–tesla MRI, we will exceed the capacity of noncontrast CT to detect early disease, and without exposing patients to ionizing radiation. The machine also is easy to use because you don’t have to position external coils. The patient just gets on the table in the scanner, you do your exam, and then take them out,” Rofsky said.

High field strength MRI can perform super–fast imaging without sacrificing image quality. As Rofsky explained, speed in the past was achieved by gradient structure, but now other technologies such as parallel imaging remove hardware constraints.

Parallel imaging, used in sensitivity encoding (SENSE) and first brought to market by Philips, can speed up any MRI sequence by a factor of two, three, four, or more, depending on the number of elements in the RF coil, said Dr. Jacques Coumans, Philips’ global marketing manager for MR.

The SENSE factor on standard Philips 1.5–tesla machines can double sequencing speed at most. Experience with SENSE and 3–tesla MRI at Zurich indicates that parallel imaging can raise speeds by a factor of six or eight, bringing metabolic mapping within reach.

“Spectroscopic imaging usually takes 10 to 12 minutes. With SENSE techniques and 3–tesla MRI, you increase resolution and still have a decent exam time. A minute or two, and your spectra are done,” Coumans said.

Mechanical Machinations

The first round of 3–tesla MR scanners, designed for research, didn’t have clinical front ends, and that was just fine for investigators who love to tinker with innovative imaging protocols. But for clinicians like Sorensen, 3–tesla prototypes reduced productivity because they lacked access to turbo spin–echo, fast spin–echo, and other sequences.

New 3–tesla machines from most manufacturers, however, have standardized software platforms across an entire MR product line. So most 3–tesla machines have the same user interfaces and parameters as 1.5–tesla scanners. T2–weighted imaging is exactly the same at 1.5 tesla and 3 tesla; MR angiography is pretty much the same at both field strengths; and other pulse sequences have been redesigned to operate efficiently at 3 tesla. Radiologists therefore can often take an imaging protocol, recompile the pulse sequences on the 3–tesla unit, and take advantage of high field strength.

The transition from 1.5– to 3–tesla body imaging has actually been easier from an engineering as well as an optimization perspective because manufacturers have overcome the problem of the specific absorption rate (SAR).

“The concern with 3–tesla MRI was that with increased radio–frequency exposure, you would ultimately run into physiologic human limitations that would restrict the number of slices or the speed at which you could scan,” said Dr. David Weber, manager of MR growth programs at GE Medical Systems.

But with software and body–coil engineering designs, clinical 3–tesla MRIs can match virtually all 1.5–tesla SAR performance standards, he said.

Manufacturers have optimized entire 3–tesla systems–not just field strength–to preserve the benefits of extra signal to noise. As Sorensen explained, his prototype 3–tesla scanner had such lackluster gradient performance that he could get better diffusion images at 1.5 tesla. But new 3–tesla machines have much higher slew rates of 400 millitesla per msec and higher gradient performance of at least 4 gauss per cm.

High field strength MRI nevertheless is not a simple plug–in. Radiologists can’t take every single 1.5–tesla protocol, install it into a 3–tesla machine, and get reliable, consistent protocols, Rofsky said.

“Protocols have to be modified because of the higher RF,” said Dr. Peter Boesiger, a physicist at the University of Zurich. “RF power more or less goes with the square of the field strength. That means we use four times more RF power, which makes a difference with many sequences because of specific absorption rate limitations.”

T1–weighted imaging requires special attention because it is slightly longer at 3 tesla and therefore produces greater inflow enhancement and background suppression. Radiologists consequently need to modify TR and TE and alter the dose of contrast medium. Thulborn has been reducing the amount of contrast medium by half.

“Full–contrast doses are disturbingly black and white, so you have to adjust the contrast if you are doing hard–copy films, or to window through the dynamic range to get the full benefit on PACS,” Thulborn said.

Because of the difference in contrast between 1.5 and 3 tesla, radiologists also need to refine their interpretation of data. Contrast enhancement of the dura, which indicates meningitis or carcinomatosis at 1.5 tesla, is common on 3–tesla images. If radiologists are not careful, they can mistake normal tissue for white–matter disease, Thulborn said.

The increased SNR of high field strength MRI offers radiologists the luxury of choosing between maximizing throughput or image resolution.

“Extra SNR is a little like currency. Radiologists can choose to spend it on extra–high–resolution scans; they can spend it on shorter exam times by not bumping up the matrix size; or they can spend it on different contrast–to–noise ratios that they couldn’t do before,” Sorensen said.

According to Thulborn, routine brain imaging at 3 tesla can be done more efficiently than at 1.5 tesla. A standard 3–tesla MRI brain scan, including fast spin–echo, FLAIR, gradient–echo, and high–resolution pre– and postcontrast whole brain imaging, can be completed in 30 minutes and still leave time to turn the table around.

What’s Next?

Over the next few years, 3–tesla MRI likely will follow the same trajectory as any new technology.

“A group of leaders will always be ready to take on new technology. In this case, it’s the major medical schools and university–based hospitals. In a few years, the rest will follow,” said Dr. Yuri Wedmid, manager of MR programs for Siemens.

Enough data today support 3–tesla MRI as the principal technology for brain imaging in academic medical centers that balance clinical applications and research. And high field strength MRI, at least in the short term, will have targeted applications in the body to provide highly detailed images and spectroscopy of small organs, such as the prostate, or minuscule lesions that lie buried behind complex anatomy.

As for private imaging centers, Lenkinski believes that many private practices will adopt the technology in the next two years to gain a competitive edge.

Ms. Sandrick is a freelance writer in Chicago.

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