Figure 3. FLAIR MRI and proton MRSI of a low grade brain tumor (oligodendroglioma) involving the mesial left frontal lobe. The lesion exhibits a decreased signal from N-acetyl aspartate, but only minor changes in choline and creatine.

Although this is only a small pilot study, it does suggest that there are spectral differences between low- and high-grade tumors, and that brain tumors virtually always show reduced NAA levels. This is consistent with the concept and experimental data that suggest that NAA is primarily located in neuronal tissue. These results are consistent with some, but not all, previous reports that high-grade brain tumors show higher levels of Cho than low-grade tumors (41,42). However, some of the prior studies differed in terms of patient population (e.g. including patients with prior treatments, or tumors of different type) and/or technique (e.g. single voxel spectroscopy). One feature of the current study was that MRSI scans were inspected for the greatest metabolic abnormality (highest Cho signal), which in some cases was located on the rim of the lesion. This may partially account for differences with prior SV-MRS studies that placed the MRS voxel over the center of the lesion, which in high grade primary or metastatic tumors may be necrotic (and therefore have low levels of all metabolites (except lipids)) (40). Further studies in larger numbers of patients will be required to substantiate these preliminary results.

3.2. Quantitation Using Inhomogeneous Receiver Coil Arrays

In recent years, the use of phased-array receiver coils for brain MRI has become quite common (43). Phased-array coils are attractive in that they provide substantial SNR gains compared to conventional volume resonators (typically about 100% improvement in superficial brain locations closest to the coil elements), and can also be used to reduce scan time through parallel imaging methodologies such as "SENSE" (44). These same imaging advantages also apply to MRS and MRSI applications.

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