Functional magnetic resonance imaging (fMRI) has become a common tool for presurgical sensorimotor mapping, and is a significant preoperative asset for tumors located adjacent to the central sulcus. fMRI has changed surgical options for many patients. This noninvasive tool allows for easy display and integration with other neuroimaging techniques. Although fMRI is a useful preoperative tool, it is not perfect. Tumors that affect the normal vascular coupling of neuronal activity will affect fMRI measurements. This article discusses the usefulness of blood oxygen level dependent (BOLD) fMRI with regard to preoperative motor mapping.

Keywords

fMRI sensorimotor mapping

Intraoperative motor mapping

BOLD fMRI

Key points

Introduction

In the early 1990s, functional magnetic resonance imaging (fMRI) entered neuroimaging as a unique resource in the arsenal of preoperative planning tools for patients with brain tumors. fMRI is a technique that takes advantage of the differences in magnetic susceptibility between oxyhemoglobin and deoxyhemoglobin. It is a less invasive neuroimaging method than its positron emission tomography (PET) predecessor, given that the contrast agent is endogenous.1 fMRI is possible because oxyhemoglobin has a magnetic resonance (MR) signal different from that of than deoxyhemoglobin. When a task is performed, oxygenated blood in excess of the amount needed (termed luxury perfusion) is delivered to the active area. The difference in magnetic susceptibility between deoxyhemoglobin concentrations and oxyhemoglobin concentrations creates the signal in functional imaging. This effect is termed the blood oxygen level dependent (BOLD) signal. fMRI provides good spatial localization (as low as 1 mm) and temporal acquisition resolution (as low as 1 second) but is limited by the resolution of the hemodynamic response (8–30 seconds). The superior spatial resolution is particularly advantageous for mapping peritumoral eloquent areas for treatment planning.2

fMRI can effectively map the sensory and motor areas. The motor gyrus is somatotopically organized, with all body parts represented in a way that is preserved across different people. fMRI can provide a multidimensional map in a single mapping session. Such maps of sensorimotor function help the surgeon to assess the risks of surgery and guide intraoperative mapping techniques.2,3 The primary motor and sensory areas in fMRI are of particular interest for surgical planning, because iatrogenic damage to these areas can cause permanent neurologic deficits. As a result the precise localization of various motor and sensory areas is useful, particularly in the setting of a space-occupying lesion.

Primary motor and sensory cortices are distinct in the functions they subserve. However, as a result of significant neuronal reciprocity in the region, injury to either can result in a mixed motor/sensory deficit. For example, injury to the primary motor gyrus usually leads to a permanent, largely irreversible paresis.4 Injury to the sensory cortex, while producing the expected sensory perceptual deficits, can also lead to a similar type of paresis seen with injury to the motor strip as a result of the lack of proprioceptive information. A variety of other deficits is seen with injury to the postcentral gyrus, depending on whether the left or right hemisphere is damaged. Some of these include 2-point discrimination, astereognosis (inability to discern objects by feeling them), and agraphism (inability to write). This article discusses the usefulness of BOLD fMRI with regard to preoperative motor mapping.

Anatomic organization of the sensorimotor system

The 4 main regions that subserve motor control that are of interest to neurosurgeons are the primary motor cortex, the primary sensory cortex, the premotor cortex, and the supplementary motor area (SMA). The motor and sensory gyri taken together are often referred to as one larger area termed the primary sensorimotor cortex.5