Pre-Operative Functional Mapping in Patients with Brain Tumors by fMRI and MEG: Advantages and Disadvantages in the Use of One Technique over the Other

SIMPLE SUMMARY: Providing an accurate localization of the brain functions before proceeding with neurosurgery is of vital importance to spare the removal of crucial brain areas. In this review paper, we investigated how two techniques, namely functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG), can serve this purpose, with specific reference to pre-operative assessment of sensorimotor and language functions in patients with brain tumors. First, we explored the methodological strengths and issues of each technique from data acquisition, to data processing and result display, also providing a temporal overview of these technical aspects and potential strategies developed to overcome the main issues. Then, we identified 16 studies that made pre-operative assessments by both techniques and accurately scrutinized them. Overall, despite the potential limitations associated with one technique or the other, the majority of these studies underlined the reliable and complementary use of fMRI and MEG. For these reasons, with the aim of the most reliable pre-operative assessment as possible, we recommend the combined use of both techniques, favored by the recent technological advances making MEG use more feasible. ABSTRACT: Pre-operative mapping of brain functions is crucial to plan neurosurgery and investigate potential plasticity processes. Due to its availability, functional magnetic resonance imaging (fMRI) is widely used for this purpose; on the other hand, the demanding cost and maintenance limit the use of magnetoencephalography (MEG), despite several studies reporting its accuracy in localizing brain functions of interest in patient populations. In this review paper, we discuss the strengths and weaknesses of both techniques, from a methodological perspective first; then, we scrutinized and commented on the findings from 16 studies, identified by a database search, that made pre-operative assessments using both techniques in patients with brain tumors. We commented on the results by accounting for study limitations associated with small sample sizes and variability in the used tasks. Overall, we found that, although some studies reported the superiority for MEG, the majority of them underlined the complementary use of these techniques and suggested assessment using both. Indeed, both fMRI and MEG present some disadvantages, although the development of novel devices and processing procedures has enabled ever more accurate assessments. In particular, the development of new, more feasible MEG devices will allow widespread availability of this technique and its routinely combined use with fMRI.