Principles and current trends in the correlative evaluation of glioma with advanced MRI techniques and PET

Cerebral gliomas comprise a heterogeneous group of primary neoplasms of the central nervous system, representing a signifcant cause of cancer morbidity and mortality. Contrast-enhanced magnetic resonance imaging (MRI) is paramount for identifying structural brain abnormalities related to the development of gliomas. Although morphological MRI remains the current standard of care for initial diagnostic workup, surgical planning, monitoring therapy response and surveillance during follow-up, it is rather diffcult to define tumor grade and boundaries and to assess response to radiochemotherapy solely by contrast-enhancement, due to a variety of factors influencing blood-brain barrier (BBB) permeability and contrast agent distribution. The nature of a lesion lies beyond often misleading gross structural patterns, down to the cellular and molecular level, hence the imaging techniques of advanced multimodal MRI and positron emission tomography (PET) have emerged to provide critical non-invasive insight into the underlying biology of primary brain cancer. Out of the various PET radiotracers, labeled amino acids are of particular significance due to their non-dependency on BBB disruption to reach glioma cells and their excellent tumor-to-background contrast. After discussing the basic imaging principles of MR perfusion, diffusion, spectroscopy and PET in glioma, this review focuses on the correlative imaging with amino acid PET and advanced MRI techniques in tumor grading and staging, in guiding stereotactic biopsy and surgical excision and in assessing therapy response, post-therapy surveillance and prognosis. Lastly, a reference is made on the expanding availability of integrated PET/MRI systems and the resulting benefits of simultaneous image acquisition.