Review articleUltrahigh field magnetic resonance imaging and spectroscopy
Introduction
In our laboratory, the effort to pursue high magnetic fields has been intricately tied to our interest in developing methods for the acquisition of physiological and biochemical information noninvasively using the nuclear spins of the water molecules and metabolites in the human body. In this effort, a relatively recent and unique accomplishment has been the introduction of the ability to map human brain function noninvasively. The concurrent and independent work performed at the University of Minnesota, Center for Magnetic Resonance Research [1] and at MGH [2], was, in our case, conducted at 4 Tesla. It was one of the first experiments performed at 4 Tesla in our laboratory. Today, functional images with subcentimeter resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1.5 Tesla MRI scanners that are often employed in hospitals for clinical diagnosis. However, there have been advantages in using significantly higher magnetic fields such as 4 Tesla, and recently 7 Tesla in humans, and 9.4 Tesla in animal models. Similarly, over the last two-and-a-half decades, spectroscopy studies in intact cells have proven to be rich in biochemical information. However, the most useful of these studies were performed in isolated cells or perfused tissues. Only recently, they were extended to small animal models using high magnetic fields. In human applications, spectroscopy efforts pursued at the commonly available magnetic field of 1.5 Tesla were in general unable to produce data comparable in information to the high field perfused organ or animal model data. This has changed with the availability of ultrahigh magnetic fields for human applications. While the use of very high magnetic fields such as 7 Tesla in human studies is still in its infancy, the data gathered to date suggest that there are significant gains for spectroscopy studies in general and some of these accomplishments relevant to high magnetic fields are reviewed in article.
Section snippets
Signal-to-noise ratio (SNR)
In all NMR experiments, especially in in vivo applications, gains in SNR are the key to extending the applications of this phenomenon to new frontiers in research. SNR gains can be achieved in going to higher fields. SNR, however, becomes rather complex when high magnetic fields (hence high frequencies) are considered with lossy biologic samples such as the human body and the human head. The relationship between SNR and resonance frequency, ω, or equivalently field strength has been examined
Imaging using low gyromagnetic nuclei
Low gyromagnetic nuclei can in principle be used to obtain unique biologic information. Atoms with magnetic nuclei such as phosphorus, oxygen, carbon (with magnetic isotopes 31P, 17O, 13C) all appear in abundance within cells. Ions such as sodium and potassium (with magnetic isotopes 23Na, 33K) are critical in cellular function. However, these low gyromagnetic nuclei suffer from limitation in sensitivity. Not only is the inherent MR sensitivity for detection of these nuclei low because of their
Spectroscopy at high magnetic fields
SNR gains somewhere between linear as observed for the 1H spins and near quadratic as seen for 17O nucleus will also be valid for spectroscopy studies that often utilize 1H, 31P, 13C nuclei. Spectroscopy studies with these nuclei, however, will also benefit significantly from unique advantages besides improved SNR at higher field strengths. Higher Bo fields yield improved resolution due to chemical shift and spectral simplifications of coupled spin systems.
In 1H spectra obtained from the human
Conclusion
Very high magnetic fields (4 Tesla and above) have been used in human biomedical research only recently and in only few groups. The results so far suggest that while indeed technical challenges increase substantially, there are also significant gains that can be realized in many applications ranging from proton imaging to spectroscopy.[47], [48], [49], [50], [51]
Acknowledgements
The work reported here from the Center for Magnetic Resonance Research, University of Minnesota, was supported by the National Research Resources (NCRR) division of the National Institutes of Health Grant P41 RR08079.
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