Diffusion imaging with MR tractography for brain tumor surgery

Abstract

In the last 15 years, advances in neurosurgery, neuroradi-ology and neuro-oncology have dramatically changed management of brain tumors, especially of gliomas seated in eloquent areas carrying a higher risk for permanent postoperative neurological deficits. This chapter provides clinically relevant and practical information and a review of the current literature from glioma biology through MR diffusion basic principles and current theories about connectivity of eloquent brain systems to clinical application of MR tractography, so that the reader can get a thorough interdisciplinary impression of the state of the art. In contrast to brain metastases and meningiomas, gliomas extensively infiltrate the extracellular space of the gray and white matter, changing the anatomic and functional properties of the brain. MR diffusion imaging has great potentials to contribute to disclose the mechanisms of interaction between gliomas and the host tissue. Diffusion tensor imaging (DTI) is the most established and validated clinical application of MR tractography, and it is increasingly performed in large medical centers. More advanced diffusion MR acquisition schemes such as high angular resolution diffusion imaging (HARDI) and more sophisticated tractography algorithms such as spherical deconvolution (SD) and Q-ball imaging (QBI) have been developed to overcome DTI limitations. The community is beginning to apply the advanced methods in presurgical mapping. A detailed understanding of the relationship between eloquent white matter fascicles and infiltrating gliomas is mandatory to correctly plan a resection and interpret the functional neurophysiological responses recorded during intraoperative monitoring (IOM) with electromyography (EMG), motor evoked potential (MEP), and direct intra-operative electrical stimulation (IES). It should be emphasized that MR diffusion tractography provides anatomical, not functional, information. The neurosurgical community is increasingly recognizing the value of MR diffusion imaging with tractography in evaluating patients with gliomas. MR tractography is a great educational tool for neurosurgeons and neurora-diologists. Presurgical visualization of eloquent fascicles in the proximity of a mass has been associated with a higher probability of total resection in low- and high-grade gliomas. Postoperative MR tractography is increasingly used to correlate postoperative deficits with white matter anatomy and guide rehabilitation strategies. This chapter presents optimized clinical presurgical HARDI protocols and tractography methods for visualization of the major white matter tracts that are part of the motor, language, and visuospatial attention systems. Practical examples of how to interpret MR tractography findings are given, and illustrative cases with typical and atypical presurgical findings are presented. Complementary applications with functional MR imaging (fMRI) are highlighted. Finally, the clinical value and limitations of pre-surgical MR diffusion imaging are discussed.