3T broadens scope of MR spectroscopy

April 15, 2005

MR spectroscopy has struggled for acceptance in mainstream radiology. Although MRS data are often used to answer neuroradiological questions, its poor resolution has failed to impress the wider radiological community.

MR spectroscopy has struggled for acceptance in mainstream radiology. Although MRS data are often used to answer neuroradiological questions, its poor resolution has failed to impress the wider radiological community.

With the growth of 3T, MRS practitioners are exploring how a stronger field could aid their technique. The higher signal should, in theory, make spectra easier to interpret because peaks are farther apart and higher relative to the baseline noise. Greater SNR should also improve spectral resolution.

The benefits of 3T for MRS are already being realized in body imaging as well as neuroradiology. The 3T scanner at Beth Israel Deaconess Medical Center in Boston has enabled radiologists there to acquire breath-hold spectroscopy data of renal carcinomas, with a view to aiding tumor characterization. Failure to freeze motion during MRS means that voxel sampling is erratic, said Dr. Neil Rofsky, chief of MRI at the center.

Other investigators are using the higher SNR to test MRS with different chemical elements. MRS typically images the chemical shift from hydrogen atoms (protons) in key biological markers such as N-acetylaspartate (NAA), lactate, and choline. Because signal from elements other than hydrogen is generally weaker, a higher SNR is needed.

Prof. Dr. Arend Heerschap, head of experimental biomedical MR at the University Medical Center St. Radboud in Nijmegen, the Netherlands, uses chemical shift patterns from fluorine to help assess uptake of novel therapies in the liver to treat colon cancer metastases. Previous attempts to measure uptake at 1.5T could be performed only on tumors very close to the surface coil, he said. Increasing the field strength allows measurement throughout the liver.

"F-MRS is nearly impossible at 1.5T," he said. "With nonproton MRS, there is an urgent need for higher sensitivity, so higher field strengths are very welcome."

Prof. Dr. Juergen Hennig, scientific director of the radiology department at University Hospital Freiburg in Germany, is studying the potential of carbon-13 and phosphorus in 3T MRS. Previous studies performed at 2T showed that P-MRS is a good surrogate parameter of organ viability in transplants, he said.

Moving from 1.5T to 3T involves its own share of new challenges. Areas with a brain-bone interface, such as the brain stem and thalamus, need particular care, said Dr. Hui Mao, an assistant professor of radiology at Emory University in Atlanta.

"Sometimes problems that you get with H-MRS at 1.5T are amplified when you move to 3T," he said.

Spectral patterns derived from 3T MRS may appear unfamiliar and difficult to interpret at first, said Dr. Bruce Daniel, an assistant professor of radiology at Stanford University.

"The citrate peak, for example, goes from being a single peak to a collection of peaks, some that are pointing upward and some that are downward. It can be quite difficult to resolve," he said.

Rofsky believes, however, that an experienced MR spectroscopist would have few problems interpreting 3T data. The real skill is to look beyond the data familiar at 1.5T and gain added information from peaks not visible at lower field strengths, he said. Close examination of 3T H-MRS data from the breath-hold studies at Beth Israel Deaconess has shown a decrease in free cholesterol and fatty acids in renal cell carcinoma. Neither of these metabolites is generally studied at 1.5T.

"If you only look at NAA and choline, you won't really expand the technology. You have to function in a different paradigm," he said.

The move toward metabolite mapping over wider tissue volumes, rather than production of individual spectra, should help push MRS further into mainstream, according to Rofsky. The maps are derived from data in adjacent voxels, which can then be color coded to highlight alternative metabolites. Beth Israel Deaconess researchers are acquiring P-MRS maps of choline from the feet of patients with diabetes. Superimposition of these maps on standard T1- and T2-weighted MR images reveals the relationship between severe nephropathy and blunted muscle metabolism.

Dr. Neeraj Chepuri, a consultant neuroradiologist at Abbott Northwestern Hospital in Minneapolis, concurs that the future of 3T MRS lies with metabolite maps. But postprocessing software to produce these maps must be more widely available and compatible with standard radiology IT systems so that MRS can be integrated into daily workflow, he said.