Presurgical Functional MRI and Diffusion Tensor Imaging

Abstract

Functional magnetic resonance imaging (fMRI) provides noninvasive localization and lateralization of specific brain functions by measuring local hemodynamic changes coupled to neuronal activation (neurovascular coupling) during active stimulation of the brain or in its resting state. Different magnetic properties of oxygenated (diamagnetic) and deoxygenated (paramagnetic) hemoglobin are exploited to generate the blood oxygen level-dependent (BOLD) contrast. Diffusion tensor magnetic resonance imaging (DTI) measures anisotropic (directional) diffusion of protons along myelinated fibers and thereby provides detailed information on the white matter architecture. Specific white matter tracts can be reconstructed using diffusion tensor tractography (DTT). During the last three decades both novel MR modalities have revolutionized the imaging research on human brain function and structure and its connectivity under physiological and pathological conditions.

Task-based (TB) presurgical fMRI in patients with brain tumors or epilepsies—with first clinical applications dating back to the mid-1990s—represents the best established and validated clinical application and has been integrated into the presurgical imaging workflow of leading neurocenters. Specific tasks need to be performed by the patient during the actual fMRI measurements activating the brain areas in question for surgery. Usually, essential cortical motor and language areas are localized and the language-dominant hemisphere is determined prior to treatment. Memory, auditory, somatosensory, visual, or other functions have also been investigated, but played a quantitatively minor role in presurgical fMRI.

Resting-state (RS) fMRI measures synchronized fluctuations in BOLD signals related to spontaneous brain activation. Besides localizing so-called eloquent functional brain areas various “resting-state networks” can be identified and analyzed for functional connectivity including changes related to the brain’s disease. Since its introduction to presurgical functional neuroimaging arena—roughly a decade ago—RS-fMRI has seen a tremendous evolution, demonstrated promising results, and has relevant advantages for clinical application: Cooperation is not required and patients with severe functional deficits can be investigated. Time-consuming and demanding individual patient training prior to the fMRI exams is not necessary. Application is possible under sedation or anesthesia directly before or even during neurosurgery with ultrafast image processing at hand. However, the data available on validity, reliability, and potential patient benefit of presurgical RS-fMRI are still limited and require further investigation.

DTI and DTT provide complementary data on functionally important white matter connections to be spared during treatment, e.g., the pyramidal tract (motor function), the arcuate fascicle (language), the optic radiation (vision), and others. This imaging information is indispensable to establish the best possible treatment for each individual patient, and to achieve the ultimate goal of brain surgery: function-preserving complete or most radical removal of the pathology. To this end fMRI and DTI are integrated meaningfully into functional neuronavigation.

Meanwhile, relevant medical societies (e.g., the American Society for Functional Neuroradiology, ASFNR) have released guidelines and recommendations on imaging procedures and data evaluation for presurgical fMRI and DTI. However, all clinical functional neuroimaging modalities are far from being simple “push-button” techniques. Responsible presurgical application requires profound knowledge of the imaging techniques employed, underlying functional neuroanatomy, physiology and pathology, neuroplastic alterations, influencing factors, artifacts, pitfalls, validity, and limitations. Finally, information on other—alternative or complementary—functional neuroimaging modalities and their multimodal integration is essential.