Potential and Challenges with Ultrahigh Field MRI

Ultrahigh field MRI at 7-9.4 T in human-sized whole-body systems has become a new forefront of MRI. Following initial demonstrations of the feasibility at academic institutions (Robitaille et al., 2000; Ugurbil et al., 2003), the major MRI vendors have now embraced the potential of these systems, and embarked on the development of commercial ultrahigh field systems.

The principal advantage of increased magnetic field strength is that the signal-to-noise ratio (SNR) increases with increasing field strength, and that this SNR gain may, in turn, be used to either increase spatial or temporal resolution in proton MRI, or to significantly increase the sensitivity of MRI with other nuclei such as 13C, 19F, 23Na, and 31P (Lei et al., 2003). Furthermore, chemical shift spectral dispersion is increased, thus improving the detection of metabolites by allowing for better differentiation of spectral lines in MR spectroscopy. Finally, magnetic susceptibility effects from paramagnetic material increase with increasing field strength, and this provides advanced contrast mechanisms for naturally occurring paramagnetic material such as deoxyhemoglobin and tissue iron. Increased sensitivity to deoxyhemoglobin content presents a mechanism for improved depiction of venous vasculature (Reichenbach et al., 1998; Reichenbach and Haacke, 2001), and is the basis for blood oxygen level dependent (BOLD) neurofunctional MRI (Yacoub et al., 2003). Measurement of tissue iron may allow for noninvasive assessment of various conditions altering iron content in the brain, including normal aging and neurodegenerative diseases.

These potential advantages are offset not only by the increased cost for large high-field magnets, but by several fundamental physical conditions that presently pose serious challenges for ultrahigh field MRI, including artifacts from inhomo-geneous B0 and B1 fields, different tissue relaxation times, and safety concerns.

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