The history of MRI has been marked by a push toward ever-higher fields in order to obtain greater sensitivity and contrast. As examples of the benefits of stronger magnets, the signal-to-noise ratio of brain images is at least 1.6 times higher at 7T than at 4T,1 and use of a 7T magnet improves the sensitivity of MRI to blood flow and oxygen utilization in the brain.2 Moreover, because signal strength increases with field strength, high-field MR spectroscopy is able to detect molecules that are present in lower concentrations and to pinpoint the sites occupied by the molecules more accurately.3
Some specialists are exploring the capabilities of 7T magnets. One site involved in this work is the Massachusetts General Hospital NMR Center, Boston, where a system has been installed with funding from the US Office of National Drug Control Policy. The research has two goals: to improve basic understanding of drug addiction and to identify pathways for effective clinical intervention. Lawrence L. Wald, PhD, chief physicist at the center, notes that, “At 7T, we can study brain function on a finer spatial scale. We see 7T scanners as the Hubble Telescope of brain-function studies.”
At the University of Minnesota’s Center for Magnetic Resonance Research, Minneapolis, the 7T scanner likewise is focused on functional brain imaging, but Michael Garwood, PhD, professor of radiology, has two MR spectroscopy projects related to breast cancer that he is working to move from 4T MRI to 7T MRI. The first project will follow 400 women with breast lesions to determine how well the presence of choline-containing compounds predicts whether a lesion is malignant. “If MR spectroscopy can, indeed, give us greater accuracy in differentiating benign from malignant lesions, we will be able to eliminate a large number of breast biopsies,” Garwood points out.
The second study uses MR spectroscopy to measure phosphocholine for early determination of responsiveness to neoadjuvant chemotherapy. MR spectroscopy may have advantages over the current method, positron-emission tomography (PET) with fluorodeoxyglucose, for this purpose. “MR spectroscopy is cheaper than PET and does not involve radioactivity,” Garwood says. “It also has higher spatial resolution and provides anatomic information with the same scanner.”
At present, there are some special siting and patient-safety issues associated with 7T magnets. “The fringe field of a 7T magnet is enormous,” Wald says. “Our room contains 0.36 kilotonnes of iron in slabs 56 cm thick, and it resembles a battleship. As these systems become commercial products, however, the manufacturers certainly will develop less difficult means of containing the fields.”
Garwood notes, “There are some transient effects of ultrahigh fields. If you move the patients too quickly, they may feel dizzy or have a metallic taste in their mouths, but you can avoid these problems by moving them slowly. There also may be issues of tissue heating, but these, too, are being dealt with now.”
A 7T magnet may sound like an impractical research tool, but, as Wald says, “Until a few years ago, 3T MRI was only a research tool; 7T MRI could well follow a similar path.”
– Judith Gunn Bronson, MS
- Vaughan JT, Garwood M, Collins CM, et al. 7T vs 4T: RF power, homogeneity, and signal-to-noise comparison in head images. Magn Reson Med. 2001;46:24-30.
- Yacoub E, Schmuel A, Pfeuffer J, et al. Investigation of initial dip in fMRI at 7 tesla. NMR Biomed. 2001;14:408-412.
- Gruetter R, Garwood M, Ugurbil K, Seaquist ER. Observation of resolved glucose signals in 1H NMR spectra of the human brain at 4 tesla. Magn Reson Med. 1996;36:1-6.