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Background
Magnetic resonance spectroscopy (MRS) is a technique that characterizes tissue based on metabolic content. In conjunction with magnetic resonance imaging (MRI), MRS can identify metabolic spectra for common cerebral pathologies. When lesion identification on conventional MR sequences is difficult, MRS can often refine the differential diagnosis helping to distinguish between gliomas, nonglial tumors, post-radiation changes, demyelinating conditions, and infection. Accurate interpretation of MRS requires an understanding of the basic science principles in spectrum acquisition, normal parenchymal spectrum, metabolic signature of various pathologies, and intrinsic limitations of MRS. Frequently it is paired with MR perfusion to avoid common diagnostic pitfalls that result from different pathologies that may have similar appearing spectra.

Educational Goals / Teaching Points
This educational exhibit aims to describe the basic principles of MRS; identify common peaks and what metabolites they represent; display the MRS appearance of glial and nonglial tumors, post-radiation changes, white matter disease, infection, and ischemia; share common diagnostic pitfalls; and understand limitations of MRS.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
MRS determines the concentration of various metabolites within a defined volume of tissue and displays this data as a spectrum on a two-dimensional grid, where the y-axis is the amplitude, and the x-axis is a measure of the resonance frequencies in parts per million. The radiologist chooses a representative voxel within the area of interest and an additional control voxel. Prior to interpretation of a spectrum, a quality control check should be performed to ascertain proper placement of the voxel. MRS gathers data in the 1-5 ppm range. N-acetyl aspartate (NAA) is a metabolite that is unique to neurons in adults and is found in oligodendrocytes. In a normal sample, this is the largest peak at 2 ppm. Creatine demonstrates peaks at 3.03 and 3.94 ppm and is slightly higher in gray matter. Choline, which is involved in synthesis of phospholipids and neurotransmitters (acetylcholine), has a peak of 3.2 ppm. Increased choline often reflects increased cell membrane turnover. The amplitude of these metabolites in conjunction with lipids, lactate, myoinositol, 2-hydroxyglutarate, and others aid in developing accurate differential diagnoses for a lesion in question.

Conclusion
It is helpful for the training radiologist to become familiar with MRS as a technique that when performed within an MR examination, can increase the diagnostic yield of the study, and potentially expedite management. Accurate interpretation should include recognition of common diagnostic pitfalls and the potential needed for additional sequences such as MR perfusion to separate disease entities with overlapping spectra.