Presurgical Tool for Mapping Brain Function

D. Cevolani, R. Agati, M. Leonardi

Until not many years ago, the only reliable ways of mapping of brain eloquent areas were invasive methods, such as intraoperative cortical stimulation and somato-sensory evoked potentials. Invasive methods are accurate, but time-consuming during the surgical procedure, often reducing the mapping analysis to „just sufficient" knowledge to enable to surgeon to proceed [36].

Obviously, the goal of neurosurgery is to maximize resection while preserving important brain functions. With this aim, it is important to provide the surgeon with all the available information to identify the eloquent cortex preoperatively; it is a well-known phenomenon that many tumours and their surrounding oedema cause a significant mass effect, which may markedly distort the cortical anatomy and make classical anatomical landmarks useless.

The presurgical use of functional magnetic resonance imaging (fMRI) paradigms enables the neurosurgeon to be given a complete mapping of brain eloquent areas before surgery, thus making the surgeon aware of the actual situation. Consequently, the surgeon may plan the surgery and decide the strategy of approach preoperatively, including the question of whether to operate or not [13].

The fMRI technique has a high spatial and temporal resolution, a non-invasive character and is safe (the source of the signal is endogenous and MRI has no known risks), thus also allowing the patient follow-up. It enables a correct definition of the relationships between, for instance, a tumour and the adjacent eloquent cortex. It is noteworthy that, sometimes, the intraoperative mapping by direct cortical stimulation is unsuccessful, especially when testing higher cognitive functions such as language, which requires the patient to be awake and not sedated as during the surgery [15]. In these cases, fMRI information becomes not only an invaluable help, but also avoids the chance of a „surprise" during the surgical procedure [3].

Finally, fMRI can detect functional cortical reorganization and plasticity, namely the displacement of brain function from one location to another [14, 27]. This phenomenon has clear-cut implications for the surgical management of the patient.

In this paper, we focus our attention on: the phenomena at the root of eloquent brain maps, the description of some activation paradigms, and their main presurgical application.

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