When performing MRS, the most prominent peak is that of water. In order to evaluate the aforementioned metabolites, water suppression must be performed, usually by using "Chemical Shift Selective Imaging", or CHESS [5].

Magnetic resonance spectroscopy is usually evaluated by determining absolute metabolic ratios [6]. Normal metabolite ratios are (Fig. 1):

• Cho/Cr up to 1.2, while values above 1.5 are considered abnormal,
• NAA/Cr up to 2.0, while values below 1.6 are considered abnormal,
• NAA/Cho up to 1.6. 

Increased cell membrane synthesis and accelerated cell proliferation in brain tumors are associated with the choline peak elevation, while the creatine peak reduction reflects changes in the metabolism. 

Elevated Cho/Cr ratio is typical for both low and high-grade gliomas (Fig. 2), while it is associated with a more prominent decrease in NAA  concentration in high-grade gliomas (Fig. 3), compatible with neuronal dysfunction and/or destruction. Increased concentration of mI is detected in low-grade gliomas, since this metabolite reflects myelin breakdown. The presence of the lactate peak (Fig. 4, indicating anaerobic glycolysis) is associated with WHO grade IV gliomas (GBM) [7].

Exceptions that may cause misinterpretation are anaplastic oligodendrogliomas and multicentric, non-enhancing GBMs.  Multivoxel MRS (CSI) is able to highlight intratumor metabolic differences and foci with higher dedifferentiation (Fig. 4 and 5). Markedly elevated lipid peak together with high Cho peaks are evident in metastatic lesions (Fig. 6).

MRS patterns of several conditions that mimic tumors on conventional MRI are also presented:

• pyogenic and fungal abscesses (Fig. 7, 8),
• demyelination (Fig. 9),
• progressive multifocal leukoencephalopathy (Fig. 10),
• venous infarcts (Fig. 11).

Extraaxial (non-glial) and axial (glial) neoplasms can be differentiated based on their differences in respective spectra. Non-glial neoplasms present with "choline only" spectra, while Cr peak is either not detected, or appears at the bottom of Cho peak (while glial neoplasms show these two peaks as separate) [1, 9]. NAA peaks are not detected in non-glial neoplasms. (Fig. 12, 13)

Differentiation between recurrent high-grade gliomas and radiation necrosis on conventional MRI is often exceedingly difficult. Disruption of the blood-brain barrier makes contrast enhancement evident in both cases. Recurrent neoplasm (Fig. 14) shows significantly increased Cho/Cr ratios (>1.7) compared to radiation necrosis (Fig. 15)[8].