3.0 T Proton MR Spectroscopy

Abstract

Proton MR Spectroscopy at 3.0 T Increased S/N and chemical shiftAt high magnetic field strength, spectroscopy improves with respect to lower field MR devices thanks to an increase in the signal/noise ratio (S/N) and resolution of the chemical shift (figure 3) 1-7 .In theory, the S/N should double on moving from 1.5 T to 3.0 T systems, but in practice the improvement is only around 20-46%. The linear rise in the S/N ratio expected with 3.0 T MR units would only be possible if other factors like T1 and T2 values (T1 is increased, T2 is decreased) or the RF effect failed to change significantly. To this should be added the lower coil sensitivity and poorer homogeneity of its volume, less shimming and misregistration errors caused by the wider chemical shift which can degrade the quality of the spectra 4-6 .In terms of increased resolution of the chemical shift, although the position of spectrum peaks is constant on the ppm scale (e.g. NAA at 2.02 ppm), the resonance frequency is different (for NAA 127 Hz at 1.5 T, 254 Hz at 3.0 T). The greater amplitude of the chemical shift frequency range of brain metabolites yields better spectral resolution and hence a more clear-cut separation between adjacent peaks which may overlap at lower magnetic field strength. In practice, this major advantage cannot be fully exploited in in vivo spectroscopy studies as the linewidth (in Hz) of the metabolites increases by at least a factor of two so that the separation between individual adjacent peaks is not radically improved using 3.0 T MR systems. The increase in linewidth is proportional to the reduction in T2 values and the increase in local magnetic field inhomogeneity at high field strength. Inhomogeneity can be reducing by more appropriate higher grade shimming. In addition, if the spatial dimensions of field inhomogeneities are within the range of voxel size, narrower linewidths can be obtained by using small voxels (less than 0.4 ml, i.e., 0.7×0.7×0.7 mmc) 8 . INTRODUCTION -Proton MR spectroscopy (MRS)consists in the in vivo study of some of the steps involved in brain metabolism. Proton MRS displays low molecular weight brain metabolites in the form of spectra whose shape basically depends on metabolite concentration (more than 0.1-1 mM) which varies in different pathological conditions. Proton MRS is an adjunct to routine MR investigation offering information useful for tissue characterization. The main metabolites detected vary according to the acquisition parameters (TR, TE) and type of impulse sequence adopted (STEAM, PRESS), and include: N-acetyl-aspartate (NAA, neuronal marker) at 2.01 ppm, choline (Cho, membrane marker) at 3.22 ppm, creatine/phosphocreatine (Cr/PCr, cell density index) at 3.5 ppm, myoinositol (mI, glial marker) at 3.56 ppm and when present lactic acid (Lac, anaerobic glycolysis index) and lipids (Lip, necrosis index). Two techniques, single voxel and multivoxel, are used depending on the spatial localisation procedure. Single voxel MRS acquires the spectrum of a preselected brain volume of interest, w...