In this paper, we review the applications of functional magnetic resonance imaging (MRI) for target delineation and critical organ avoidance for brain radiotherapy. In this article we distinguish functional MRI from brain functional MRI (fMRI). Functional MRI includes magnetic resonance spectroscopic imaging (MRSI), perfusion MRI, diffusion tensor imaging (DTI) and brain fMRI. These functional MRI modalities can provide unique metabolic, pathological and physiological information that are not available in anatomic MRI and can potentially improve the treatment outcomes of brain tumors. For example, both choline (Cho) to N-acetylaspartate (NAA) and Cho to creatine (Cr) ratios from MRSI increase with increasing tumor malignancy and can be used to grade gliomas. Relative cerebral blood volume (rCBV) measurements from dynamic susceptibility contrast perfusion magnetic resonance imaging (DSC MRI) are superior to conventional contrast-enhanced MRI in predicting tumor biology and may be even superior to pathologic assessment in predicting patient clinical outcomes. Brain fMRI can help identify and avoid functionally critical areas when constructing treatment plans for brain radiotherapy. In the past, functional MRI measurements have not been routinely used in a clinical arena due to the experimental nature of these imaging modalities. As these methods become more commonly used and effective image co-registration algorithms become available, integration of functional MRI into the treatment process of brain radiotherapy now appears to be clinically feasible, at least in major medical centers.

Key words: Functional imaging, Magnetic resonance spectroscopy, Perfusion imaging, Gliomas, Radiotherapy, Diffusion MRI, Brain fMRI.