Published online by Cambridge University Press: 04 August 2010
Imaging of brain tumors has evolved into a multimodal tool providing improved diagnostic and prognostic accuracy, fundamental in disease monitoring and assessing response to therapy.
Proton MR spectroscopic imaging (1H-MRSI) combines the spatial localization capabilities of MR imaging with the biochemical information of 1H-MR spectroscopy, and provides a valuable clinical tool for brain tumors by depicting metabolic changes reflective of cellular density, anaplasia, and mitotic index.
Choline is elevated in all tumor types due to altered membrane metabolism, and shows correlation with cellular density and indices of cell proliferation. N-acetyl-aspartate (NAA) decreases with tumor infiltration and substitution of normal neural and glial cells. The Cho/NAA ratio is, therefore, a useful parameter particularly in most adult and pediatric primary brain tumors, with a higher ratio correlating with higher cell density and generally associated with a poor prognosis.
While 1H-MRS can show different metabolic patterns in different tumor types, it is not used as a primary diagnostic tool.
Increasing Cho/NAA and Cho/Cr ratios in serial exams of a primary astrocytoma are suggestive of transformation to a higher grade. By following metabolic changes, 1H-MRS can be useful in monitoring disease progression or response to therapy.
1H-MRSI allows the evaluation of spatial heterogeneity and the macroscopic boundary of a mass, and may provide guidance for targeted biopsy, surgery, or therapy.
1H-MRS studies can also be particularly useful in distinguishing neoplastic from non-neoplastic lesions, and differentiating recurrent tumor from predominantly delayed radiation necrosis.