7T MRI Sharpens Its Focus

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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.
Source: William D. Rooney, PhD, and John Grinstead, PhD.

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