MR Metabolic Imaging of Brain Lesions: Lessons and Caveats

Abstract

Magnetic resonance (MR) spectroscopic imaging is rapidly becoming a widespread clinical modality for assessing brain lesions. The clinical benefit of this metabolic imaging technique has been demonstrated by a large number of studies in a wide range of neuropathologic conditions. At the University of California San Francisco and many other institutions, MR spectroscopic imaging is routinely added to brain tumor MR imaging examinations for a combined metabolic-anatomic assessment of tumor extent prior to and following therapy (1–3). It is important to note that the diagnosis in these patients is already established and the clinical question being answered is what is the extent of abnormal metabolism, not what is the pathologic cause. MR spectroscopic imaging can be useful, as well, in differential diagnosis, but especially for this purpose it is important to understand the limitations and contraindications of this technology. This issue of Academic Radiology contains an article by Pomper et al (4) entitled “Quantitative MR Spectroscopic Imaging of Brain Lesions in Patients with AIDS: Correlation with [11C-Methyl]thymidine PET and Thallium-201 SPECT” that raises several important issues regarding the use of MR spectroscopic imaging in the diagnosis of focal brain lesions. Since the current clinical standard of contrast material–enhanced MR imaging is inadequate to reliably differentiate central nervous system lesions in patients with acquired immunodeficiency syndrome (AIDS), metabolic imaging techniques have been applied to improve radiologic diagnosis. Prior studies have demonstrated some success but also substantial false-positive and false-negative rates for single photon emission computed tomography (SPECT) (5) and falsepositive results for fluorodeoxyglucose (FDG) positron emission tomography (PET) (6). MR spectroscopy has also been investigated for differentiating brain lesions in AIDS patients with disparate results. In one study by Chang et al (7), a significant difference in metabolite levels was observed between each group for 11 toxoplasmosis abscesses, 12 progressive multifocal leukoencephalopathic lesions, eight lymphomas, and four cryptococcomas. Choline–to–creatine and choline–to–N-acetyl aspartate ratios were significantly elevated in patients with lymphoma, and in toxoplasmosis all metabolites were reduced with the exception of substantial elevation in lactate and lipid resonances. Other spectral differences differentiated the other lesion types, as well. This led to the conclusion that MR spectroscopy was a “sensitive and potentially specific noninvasive adjunctive method for differential diagnosis of focal brain lesions in AIDS.” Other studies, however, including the study of Pomper et al (4) published in this issue, have found MR spectroscopy to be much less specific and not reliable on its own to characterize brain lesions in these patients. Chinn et al (8) conducted an MR spectroscopic study of 27 lesions (18 toxoplasmosis regions and nine lymphomas) and found no statistically significant difference in the metabolite levels between these lesion types. The spectra from all lesions were significantly different from those of normal brain, but there was substantial overlap in metabolite levels between the two lesion groups. Although the postulated increase in choline–to–creatine ratio was observed in all lymphomas, elevation of this ratio was also noted in 11 of 15 toxoplasmosis abscesses. Lipid peaks were increased in 10 of 15 toxoplasmosis regions but also in four Acad Radiol 2002; 9:377–378