Functional MRI in pre-surgical planning: Case study and cautionary notes

Abstract

Functional magnetic resonance imaging (fMRI) is an MRI technique capable of estimating regional brain activity during a predefined cognitive task. The number of publications using fMRI has increased exponentially 1 since the technique was first introduced over 20 years ago. 2 A PubMed search using the keywords 'fMRI OR ('functional MRI')' at the time of writing, identified 31 176 results, with 28 882 of these having been published in 2011. Although fMRI has contributed significantly to our understanding of neurocognitive function, the number of resulting clinical applications has been limited. Currently, the most promising direct clinical application is in pre-surgical planning, where fMRI is used to identify the spatial relationship between important functional areas in the brain and the area to be resected. 3 The fMRI results can thus influence the entry point and trajectory of the surgical intervention to avoid damaging eloquent cortical areas. These are cortical areas in which injury causes symptomatic cognitive or motor deficits and therefore, if avoided, could greatly influence surgical outcome. The basis of fMRI is that deoxygenated blood is magnetic whereas oxygenated blood is not. The presence of deoxyhaemoglobin in the tissue being imaged therefore perturbs the magnetic field, resulting in a signal loss as measured by T2*-weighted MR imaging. Performing a task in the MRI scanner causes increased neuronal activity, resulting in increased energy utilisation at the synapse, and consequently local increases in blood flow to the brain regions that are recruited during the task. This characteristic is known as the blood oxygen level dependent (BOLD) effect. The signal change measured by fMRI is consequently not a direct measure of neuronal activity, but rather a combination of flow, metabolism, blood volume and blood oxygenation. This signal change is very small, in the order of 1 -2% of the total signal. fMRI scans typically involve alternating periods of rest and activity over a total scan period of 5 -10 minutes, to improve the sensitivity of the technique. The colourful activation patterns associated with fMRI indicate the statistical likelihood that the regional blood flow is synchronous with the predefined periods of rest and activity. The most commonly used fMRI tasks for neurosurgical planning are tactile, motor, language and visual. 4