Proton magnetic resonance spectroscopy (MRS) has been widely used for the assessment of brain metabolites in gliomas. Compared with previous studies that have used 1.5 T and a quadrature head coil, the use of the field strength of 3 T with an 8-channel coil was able to provide an increase in signal-to-noise ratio (SNR) and better spectral resolution. The improved SNR could be used to either decrease acquisition time or improve spatial resolution.
A general marker of brain tumors is the elevation of choline containing compound (Cho) and the reduction of N-acetyl aspartate (NAA). Our study has shown that the variations in metabolite intensity between tumor and normal brain in patients with gliomas are caused by both changes in absolute metabolite concentrations and increases in T2 relaxation times for Cho and creatine (Cr). Longer T2 relaxation value of Cho in gliomas means that the ratio of Cho/NAA that is observed in long echo spectra is larger than that in short echo spectra, which suggests that the contrast in metabolite ratios between tumor and normal tissue would be greater at longer echo times.
Although more metabolites are observed at short echo times (<40ms), peak overlap and complex coupling patterns make it difficult to isolate individual components. A J-resolved MRSI sequence with localization in three spatial dimensions was implemented within a clinically feasible scan time at 3 T (23 min). It allows simultaneous detection of Cho, Cr, NAA, glutamate, myo-Inositol and lactate at 3 T, as well as evaluation of T2 values of singlets.
With the availability of the improved hardware and techniques, in vivo 1H-MRS can offer new possibilities at the even higher field strength, 7 T. Our study has demonstrated that brain metabolite SNR was improved at 7 T relative to 3 T, but the increase in SNR observed in our initial studies was less than linear with respect to B0, primarily due to differences in linewidth. Furthermore, the differences in relaxation times mean that longer TR and shorter TE are needed at 7 T in order to take advantage of the gain in SNR due to increased B0.