Ultrahigh field magnetic resonance imaging and spectroscopy.

Abstract

1. IntroductionIn our laboratory, the effort to pursue high magneticfields has been intricately tied to our interest in developingmethods for the acquisition of physiological and biochem-ical information noninvasively using the nuclear spins of thewater molecules and metabolites in the human body. In thiseffort, a relatively recent and unique accomplishment hasbeen the introduction of the ability to map human brainfunction noninvasively. The concurrent and independentwork performed at the University of Minnesota, Center forMagnetic Resonance Research [1] and at MGH [2], was, inour case, conducted at 4 Tesla. It was one of the firstexperiments performed at 4 Tesla in our laboratory. Today,functional images with subcentimeter resolution of the en-tire human brain can be generated in single subjects and indata acquisition times of several minutes using 1.5 TeslaMRI scanners that are often employed in hospitals for clin-ical diagnosis. However, there have been advantages inusing significantly higher magnetic fields such as 4 Tesla,and recently 7 Tesla in humans, and 9.4 Tesla in animalmodels. Similarly, over the last two-and-a-half decades,spectroscopy studies in intact cells have proven to be rich inbiochemical information. However, the most useful of thesestudies were performed in isolated cells or perfused tissues.Only recently, they were extended to small animal modelsusing high magnetic fields. In human applications, spectros-copy efforts pursued at the commonly available magneticfield of 1.5 Tesla were in general unable to produce datacomparable in information to the high field perfusedorgan or animal model data. This has changed with theavailability of ultrahigh magnetic fields for human appli-cations. While the use of very high magnetic fields suchas 7 Tesla in human studies is still in its infancy, the datagathered to date suggest that there are significant gainsfor spectroscopy studies in general and some of theseaccomplishments relevant to high magnetic fields arereviewed in article.2. Imaging using the hydrogen nucleus2.1. Signal-to-noise ratio (SNR)In all NMR experiments, especially in in vivo applica-tions, gains in SNR are the key to extending the applicationsof this phenomenon to new frontiers in research. SNR gainscan be achieved in going to higher fields. SNR, however,becomes rather complex when high magnetic fields (hencehigh frequencies) are considered with lossy biologic sam-ples such as the human body and the human head. Therelationship between SNR and resonance frequency, ,orequivalently field strength has been examined for biologicsamples in numerous studies [3–9], predicting increaseswith field strength. At high frequencies such as 170 MHz(