[Functional imaging in brain tumors]

Over the last two decades, the routine imaging methods available for the diagnosis and monitoring of intracranial tumors are computer tomography and magnetic resonance imaging. Both procedures provide precise anatomical delineation of the lesions, determine changes in tumor volume in response to therapy. However, they are not effective in differentiating low-grade from high-grade lesions and residual or recurrent tumor from radiation/chemo necrosis. Radionuclide procedures (positron emission tomography and single photon emission computer tomography), especially positron emission tomography is an example of a technique with high sensitivity and accuracy that has the potential to yield the information necessary not only to provide a means for diagnosis of tumors based on altered tissue metabolism but also to serve as a tool for monitoring the effects of the therapy. Measurements of the rate of glucose metabolism by 18F-FDG uptake in brain lesions are the most frequent applications in order to discriminate low-grade from high-grade lesions and tumor recurrence from radiation necrosis. 11C-methionine positron emission tomography is used to delineate the boundaries of tumors, providing information of value in directing stereotactic biopsy, planning the approach and extent of brain surgery, and permitting differentiation of the metabolizing neoplasm from simple disruption of the blood-brain barrier. Another substrate, 11C-thymidine has been used to measure nucleotide metabolism and utilization in DNA synthesis. Activation studies are useful in patients suffering from lesions near to or in the eloquent areas. It is likely that multiple positron emission tomography radioligands and multimodality imaging are the standards for diagnosis and monitoring of the effects of therapeutic intervention.