DMI shows metabolism of acetate and glucose in the brain and liver and reveals the Warburg effect in patients with brain tumors.
Comprehensive and quantitative measurements of T 1 and T 2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T 1 ) and decrease (T 2 ) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T 2 relaxation and a strong increase of T 1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T 1 /T 2 -based anatomical MRI quickly levels off beyond ϳ7 T and may actually decrease at higher magnetic fields. Magnetic fields strengths for in vivo MRI and MRS have seen a steady increase, and are currently up to 9.4 T for humans and Ͼ14 T for animals. This drive has largely been fueled by the greatly improved contrast-to-noise (CNR) ratio of functional MRI (fMRI) techniques, as well as the linear increase in signal-to-noise (SNR) ratio and spectral resolution with increasing field strength. However, although T 1 and T 2 relaxation parameters play an important role in the actual SNR and resolution, they are often ignored in discussions of high-field NMR. While the fielddependent trends for T 1 and T 2 relaxation of water have been quantitatively established for lower magnetic fields (1), no comprehensive and quantitative relaxation parameters are available for the range of high magnetic field strengths (Ͼ3 T) currently used in MR research laboratories. Some studies have reported T 1 and T 2 relaxation at one magnetic field (e.g., Refs. 2 and 3), but it is difficult to establish any quantitative trends because of large variations among laboratories. Knowledge about metabolite and macromolecule proton T 1 and T 2 relaxation times is even more limited, with few single field measurements (4 -6) and even fewer quantitative magnetic field comparisons available (7). Here we present comprehensive and quantitative measurements of T 1 and T 2 relaxation of water, metabolites, and macromolecules in rat brain under nearidentical experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T). Besides establishing the contribution of relaxation on sensitivity and resolution, these data will also be valuable for determining optimal image T 1 /T 2 contrast, establishing optimal acquisition parameters for quantitative MRS, and determining optimal inversion delays for macromolecule detection or suppression. MATERIALS AND METHODS GeneralAll experiments were performed on 1) a 4.0 T Bruker magnet equipped with 15-cm-diameter gradients (128 mT/m in 150 s; Magnex Scientific, Oxford, UK), 2) a 9.4 T Magnex magnet equipped with 9-cm-diameter gradients (490 mT/m in 175 s; Resonance Research Inc., Billerica, MA, USA), or 3) a 11.74 T actively-pumped Magnex magnet equipped with 9-cm-di...
High quality magnetic field homogenization of the human brain (i.e. shimming) for MR imaging and spectroscopy is a demanding task. The susceptibility differences between air and tissue are a longstanding problem as they induce complex field distortions in the prefrontal cortex and the temporal lobes. To date, the theoretical gains of high field MR have only been realized partially in the human brain due to limited magnetic field homogeneity. A novel shimming technique for the human brain is presented that is based on the combination of non-orthogonal basis fields from 48 individual, circular coils. Custom-built amplifier electronics enabled the dynamic application of the multi-coil shim fields in a slice-specific fashion. Dynamic multi-coil (DMC) shimming is shown to eliminate most of the magnetic field inhomogeneity apparent in the human brain at 7 Tesla and provided improved performance compared to state-of-the-art dynamic shim updating with zero through third order spherical harmonic functions. The novel technique paves the way for high field MR applications of the human brain for which excellent magnetic field homogeneity is a prerequisite.
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