Presurgical functional localization of primary somatosensory cortex by dipole tracing method of scalp-skull-brain head model applied to somatosensory evoked potential

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Abstract

The aim of the present study was to explore the utility of dipole tracing (DT) of a scalp-skull-brain (SSB) head model in preoperative functional localization of the human brain. Nine patients who underwent surgery of mass lesions around the central sulcus (CS) were employed. By using SSB/DT, dipole source location of early cortical components of the somatosensory evoked potential (SEP) was estimated before surgery. Motor cortex, CS and primary somatosensory cortex were determined by cortical SEP during surgery. After surgery precise functional mapping was reproduced in MRI, and the accuracy of DT was evaluated by measuring the distance between estimated dipole source and the posterior bank of the CS. We defined this distance as localization error of DT. In 4 cases without structural change around the sensorimotor cortex, localization error ranged from 1 to 4 mm with an average of 2 mm. In 5 cases with structural alteration of sensorimotor cortex, localization error ranged from 6 to 10 mm with an average of 8 mm. The difference in localization error between the two groups was statistically significant, and may have been caused by changes of conductance near sensorimotor cortex in the latter group. Functional localization by DT was accurate and useful. But localization error could not be ignored in cases with structural alteration in the sensorimotor cortex.

Introduction

It is important to ascertain the location of the central sulcus (CS) before and during surgery in order to plan and perform safe surgical resection of a space-occupying lesion around the sensorimotor cortex. Though in some cases the CS can be estimated in magnetic resonance images (MRI) (Ebeling et al., 1989, Berger et al., 1990), it is sometimes very difficult to identify the CS when it is displaced by the lesion. In such cases functional localization by electrophysiological methods are required.

Electrical stimulation of cerebral cortex or somatosensory evoked potential (SEP) recorded from the cortical surface (cortical SEP) (Wood et al., 1988, Allison et al., 1989) are reliable tools for identifying the CS during surgery. But they are invasive and preoperative planning cannot be informed by these methods.

Dipole tracing (DT) is a non-invasive method by which electrical activity recorded by conventional electroencephalography can be approximated as 1 or 2 equivalent dipoles (Scherg and Von Cramon, 1986, Homma et al., 1987). Dipole location and vector moment are estimated by inverse solution method. We applied the DT method with a scalp-skull-brain (SSB) head model (Homma et al., 1994, Homma et al., 1995) to SEP (SEP-DT) in order to determine functional localization of the brain. Dipole locations and vector moments of early cortical components of the SEP were estimated under the assumption that the location of dipole source is generated in area 3b of the primary somatosensory cortex. In the present study we demonstrated the value of SEP-DT in presurgical planning and assessed the estimation accuracy using intraoperative cortical SEP. Electrophysiological factors which modified the accuracy of SEP-DT are also discussed.

Section snippets

Patient population

Nine patients (4 males and 5 females) were studied. All of them had space-occupying lesions around the sensorimotor cortex detected in standard MRI studies and were candidates for surgical resection. Clinical characteristics are described in Table 1 and Table 2. There were four cases of meningioma and three cases of astrocytoma, one case of metastatic brain tumor and one case of arachnoid cyst. All the patients were informed about the purpose and the design of this study.

Preoperative SEP-DT study

As the principles and

Case 4

This 74-year-old woman was healthy until she had a seizure involving her right hand. MRI revealed left sided parasagittal extra-axial mass lesion. Results of the preoperative SEP-DT study are shown in Fig. 1. The dipole source of N20-P20 was located on the cortical surface 5 cm lateral to the midline on the left side (Fig. 1B). The vector moment is directed frontomedially. Before surgery we expected that the tumor would not touch the motor cortex and would be removed without neurological

Discussion

During resection of a mass lesion in the central area of the brain, it is helpful to identify the somatosensory and motor cortex pre- and intra-operatively. As a tool of functional mapping, electrical stimulation of the cerebral cortex and SEPs recorded from the cortical surface electrodes are reliable (Wood et al., 1988, Allison et al., 1989), and allow good surgical results (Gregorie et al., 1984, Ebeling et al., 1992). But these techniques are invasive and cannot be utilized for preoperative

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