The Promise of High-Field-Strength MR Imaging

The application of MR imaging in medicine and basic research has seen a steady growth in the field strength of the magnets. Subsequent to the installation of the first few high-field-strength (ie, ≥ 3T) systems, which were mainly developed for improved MR spectroscopy, functional MR imaging (fMRI) became the dominant driving force behind their proliferation. It was quickly recognized, however, that numerous other MR applications could benefit substantially from the increased field strength, and there are now several 7T systems either running or at some stage of being brought on-line. In addition, an 8T system has been operational for several years, and there are even plans underway for the installation of 9.4T and higher human systems. These high-field-strength systems are at the leading edge of technology development in MR applications, and they are proving to be well worth the effort. This proliferation of high-field-strength magnets has led to improved applications in just about every area of MR, from basic science research laboratories to the clinic. Indeed, it is expected that within the next few years 3T scanners could account for more than a quarter of the clinical MR market.

The reasons for this continued advance in field strength are many. One of the most obvious benefits is improved signal-to-noise ratio (SNR). The intrinsic SNR scales linearly with static magnetic field (1), but in reality the actual SNR achievable is somewhat lower than the intrinsic SNR gain, mainly because of hardware limitations. It is expected that these limitations will be overcome with further research and that the full improvement in SNR obtainable with higher field magnets will eventually be realized.

Many other areas of improved MR applications are evident in relation to increased magnetic field strengths. One of these can be found in the improved applications of contrast reagents, because, as the field strength increases, the detection threshold decreases. This is a strong effect with no real theoretical limit and has significant implications, particularly in the emerging field of molecular imaging.

Another more obvious advantage of high-field-strength MR is the enhanced measurement of susceptibility-induced relaxation, which has led to improvements in fMRI. Not only does the contrast-to-noise ratio improve, but the spatial definition of the signal intensity also improves.

We also expect major advantages in high-field-strength applications to spectroscopy (2). Spectroscopy at high field strengths is enhanced by the increase in SNR, and higher field strengths afford improved spatial resolution in spectroscopy. Finally, increased spectral dispersion will provide more reliable quantification and additional sensitivity gains.

The article by Dasher et al in this issue of the AJNR points to yet another area of improved MR applications afforded by high-field-strength magnets, namely, the significant improvement attainable in spatial resolution and contrast. In their report, the authors present MR images acquired at 8T of the microvasculature of the live human brain as well as the embalmed and unembalmed postmortem human brain. The ability to identify the microvasculature in human brain at a resolution that allows close comparison to histology has significant implications in many fields of CNS disorders and specifically in the treatment of reperfusion injury and in the physiology of solid tumors and angiogenesis. There is every reason to believe that our continued efforts to push the envelope of high-field-strength applications, like the examples presented in Dasher et al’s article, will open new vistas in what appears to be a never-ending array of basic science research and clinical applications.

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