MR Spectroscopy and Brain Tumours

Intracranial space occupying lesions can be brain neoplasms, abscesses or infarcts. Primary brain neoplasms mainly arise from glial tissue and are thus called gliomas. The 3 principal varieties of gliomas are astrocytomas, oligodendrogliomas and ependymomas. Gliomas can sometimes be difficult to differentiate from infective lesions such as abscesses. MR spectroscopy can help differentiate these lesions by measuring the relative concentration of metabolites in and around lesions.

The two most useful metabolites are choline and N-acetyl aspartate (NAA). Choline is a component of cell membranes and is thus increased in areas of rapidly dividing cells such as tumours, and is decreased where there is cell destruction such as in abscesses. Choline levels are unchanged in infarcts.

NAA is a metabolite of glutamate, and is produced by neurons. It will be decreased whenever there is paucity of neurons due to destruction. Thus NAA levels are reduced in areas of gliomas, abscesses or infacts.

Some investigators use the ratio of choline:NAA to predict lesions that may be gliomas. A choline:NAA ratio>2 is thought to be specific for gliomas. While the ratio may be high in brain metastases, in the case of primary brain tumours, the ratio remains high in watershed areas around the glioma, as these tumours infiltrate beyond their radiologically apparent borders, while metastases are truly cicumscribed.

Other metabolites such as lactate or creatine can also be measured by MR spectrscopy. Lactate is increased when their is anaerobic activity, and is thus expressed in tumours, infarcts as well as abscesses.

These principles are illustrated in the figure.