INTRODUCTION
paradigm. Sensory mapping using fMRI is more difficult than the reliable motor activation as the BOLD signal is low and frequently shows overlap with M1 (52). Since fMRI is based on the hemodynamic response, it is thought to be less powerful in lesions that lead to changes in the vascular auto-regulation, such as gliomas or cerebral ischemia (18,37). This could result in false positive and false negative detection of cortical sensorimotor areas in some patients (20,21,32,34) as compared with intraoperative mapping. In addition, its temporal resolution makes discrimination between premotor, motor, and somatosensory components associated with hand movements difficult (32). Consequently, interpretation of fMRI maps may be more challenging in patients with brain lesions than in healthy subjects (21). Furthermore, it is still unclear to what extent the resection border could approach the functional MR-representation, due to the lack of a standardized approach or multicenter randomized trials addressing this issue. In cases where fMRI is not possible or feasible, especially in pediatric patients, patients with claustrophobia, carrying ferromagnetic material, or harboring vascular abnormalities other brain mapping modalities can be considered as an option.
MULTIMODAL IMAGING MEG detects electrical activity in the brain with millisecond temporal resolution and fMRI measures hemodynamic phenomena. As a result of measuring different physiological functions, discrepancies may be expected. Therefore, it is important to be aware of the strengths and limitations of each modality when comparing different imaging modalities. The first comparison of MEG and fMRI was performed with a visual motion paradigm in 1999 (1). Later, other combined both modalities for sensorimotor research in healthy subjects (4,33,53) but also in patients (31,32). In neurosurgical patients, studies showed that combining (sensorimotor) fMRI and MEG may increase the localization reliability and that MEG may be superior to fMRI in some patients with unclear fMRI localization (20,23,32). Usually prominent activation of nonprimary areas in fMRI could lead to false localization of the CS in some cases. However, despite these differences, the increased reliability of combined sensorimotor mapping may be useful in neurosurgical practice to avoid postoperative deficits.
FUNCTIONAL SENSORIMOTOR REORGANIZATION MEG can be used to localize the primary sensorimotor cortex in relation to intracranial lesions and address whether so-called functional reorganization or cerebral plasticity exists. In healthy subjects, simple motor tasks elicit contralateral activation of M1 and somatosensory stimulation causes early S1 activation. Patients with intracranial lesions can show altered patterns of activation, varying from subtle shifts from the functional region involved to complete lateralization (10,54,58).
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