7T MRI Sharpens Its Focus

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Source: 3T-7T-MRI_1327085268.jpg - 3T-7T MRI
MR image quality compared at 3T (top) and 7T (bottom) using nearly identical acquisitions and quadrature transmit volume radiofrequency coils. Signal/noise is improved ~2x at 7T compared with 3T.
A handful of research sites across the U.S. are deploying 7T MRI systems to investigate the mighty magnets for neurological, vascular and orthopedic imaging applications. 7T MRI provides a higher signal-to-noise ratio than 3T systems, delivering images with resolution of a few hundred microns. It could open doors to new, highly specialized clinical applications. However, 7T systems come with hefty actual and operational price tags as protocols and processes for ultra-high field MRI are far from plug-and-play.

In 2004, University of California, San Francisco (UCSF) was one of the first sites in the U.S. to install a 7T system. For the last six years, the departments of neuroradiology and neurology have collaborated on technical and clinical research applications. “The primary advantages of 7T MRI are its higher signal-to-noise ratio, different  image contrast and spectral resolution,” says Christopher P. Hess, MD, PhD, In 2004, the University of California, San Francisco (UCSF) installed a 7T system and since then, the departments of neuroradiology and neurology have collaborated on technical and clinical research applications. “The primary advantages of 7T MRI are its higher signal-to-noise ratio, different image contrast and spectral resolution,” says Christopher P. Hess, MD, PhD, neuroradiology chief at San Francisco VA Medical Center.  

William D. Rooney, PhD, director of Advanced Imaging Research Center at Oregon Health & Science University in Portland, adds, “Signal-to-noise ratio is our bread and butter in MRI. The more of it we have, the more we can do.”

Much of the work with 7T has focused on neuroimaging. The UCSF magnet is housed next to a 3T system, and a primary emphasis has been comparing 3T with 7T. For example, 7T delivers much better spectroscopy and shows the margins and vascularity of brain tumors with greater detail than 3T. It also offers a window into the brains of brain tumor survivors.

“Brain tumor patients are living longer, and there may be collateral damage such as neuropsychiatric deficits or cognitive issues following radiation therapy or chemotherapy,” explains Hess. While a 3T magnet may show a few small microhemorrhages related to radiation, a 7T study reveals hundreds of microhemorrhages. “As we understand the physiology of how radiation therapy affects the brain, we can better tailor treatment based on the more vulnerable areas of the brain,” says Hess.

Rooney and his colleagues have leveraged 7T to characterize blood vessel changes associated with multiple sclerosis. The system delivers improved sensitivity for detecting low levels of MRI contrast agent. “This allows us to see more subtle pathological changes in tissue at 7T, and in standard applications, it allows us to use lower contrast doses, which is important because the risk of contrast administration is not zero. It’s always better to use as little as you need to get the information you want,” says Rooney. The researchers have used 7T MRI data to measure the movement of water across different compartmental boundaries in tissues, which also may be significant as it may provide information on cellular metabolism.

Plugging the gaps

The sweet spot of 7T likely will fall in the few gaps left by its 1.5T and 3T siblings. For example, surgical planning for temporal lobe epilepsy seems to be an unmet need. “Current clinical imaging, 1.5T and 3T MRI, is pretty good at finding the most common pathology for temporal lobe epilepsy in a general way, but not at showing the exact extent of pathology,” says Thomas R. Henry, MD, a neurologist at University of Minnesota in Minneapolis.

1.5T and 3T systems do not produce reliable images of the major hippocampal structures because of submillimetric dimensions and limited MR contrast. Meanwhile, surgical treatment requires precise localization to determine whether or not critical functions, such as memory, language or motor skills would be affected. Henry and colleagues have investigated using 7T MR images to inform surgical planning for patients with temporal lobe epilepsy.

The researchers hypothesized that the higher-field system could show subregional distributions of hippocampal atrophy and allow detection of associated malformations. Indeed, harnessing the additional signal-to-noise ratio of 7T MRI provided better visualization of subtle alterations in patients with mild degrees of hippocampal sclerosis, according to a study published online July 11, 2011, in Radiology.

Beyond neuroimaging

While the lion’s share of 7T research has targeted the brain, research into additional applications is starting to take shape. At the Advanced Imaging Research Center, researchers are looking at the liver.
Liver imaging utilizes the Carbon-13 (C13) isotope to analyze glycogen utilization and assess how different therapies affect patients with endocrine disorders. Ultimately, 7T MRI may help physicians assess therapeutic efficacy and better tailor treatments to various patient populations.

Another promising area is vascular imaging, specifically arteriovenous malformations and aneurysms. As in other early applications, 7T provides much finer depiction of the anatomy. “We’re seeing arteries that we aren’t accustomed to see at 3T, such as the lenticulostriate arteries and small arteries off the middle cerebral, anterior cerebral and posterior cerebral arteries,” says Hess. The clinical payoff is that these arteries are believed to be responsible for lacunar infarcts and hypertensive hemorrhage. The next step is a study of patients with hypertension to see if the data can be used to predict hemorrhage or stroke in patients with underlying vascular disease.

Researchers at the University of Iowa Hospitals and Clinics in Iowa City will join the 7T club late in 2012 via an $8 million National Institutes of Health grant.

Approximately 75 percent of the initial work will be focused on neuroimaging applications, according to Vincent A. Magnotta, PhD, principal investigator on the grant. However, a few of the nearly 33 research projects extend beyond the brain.
“One major area of research centers on using T2 mapping to assess cartilage after traumatic injury and the patient’s susceptibility to osteoarthritis,” says Magnotta. As therapies to delay the onset of osteoarthritis are developed, the ability to accurately predict which patients are likely to develop the condition is critical.

Like other potential 7T applications, this fills a niche rather than a universal need. While the spatial resolution of 1.5T and 3T systems may suffice for imaging cartilage in the knee, it is very difficult to visualize cartilage in smaller structures, such as the ankle, says Magnotta.

The fine print

As with any new imaging system, 7T brings engineering, technical and economic challenges. The FDA has not approved any coils for 7T MRI. UCSF researchers construct custom coils for their 7T system, and the Advanced Imaging Research Center houses a lab to build coils.

The need for custom coils is a symptom of one of the primary challenges of 7T MRI: cost. With a price tag of $1 million per Tesla, sites need to weigh whether it is worth investing in one 7T system or two 3T magnets, says Hess. Another economic factor is administrative overhead. A 3T MRI can be managed by a technologist, but a 7T system requires a vendor representative to handle the additional complexity, a physicist to develop coils and pulse sequences and a handful of post-doctoral and graduate students to conduct research.

On the technical front, 7T MRI provides enhanced sensitivity to susceptibility, which is a plus when researchers are looking for susceptibility. But macroscopic susceptibility caused by bony structures and air bone interfaces in the brain can produce artifacts and make it difficult to evaluate the cortex and medial temporal lobes, says Hess.

As 7T systems come into focus, it seems clear that the improved signal-to-noise ratio and resolution show promise for a few specialized applications, allowing sites that can master the magnet to provide imaging data to fine-tune surgical and radiation therapy planning and possibly inform prognosis for various patient conditions. HI