Localization of the motor hand area using transcranial magnetic stimulation and functional magnetic resonance imaging
Introduction
Transcranial magnetic stimulation (TMS) allows the selective stimulation of a particular cortical area using a magnetic field pulse (Barker et al., 1985). Focal TMS is a convenient tool for non-invasive and painless mapping of the somatotopical organization of the motor cortex (Rossini and Caramia, 1988, Cohen et al., 1989, Brasil-Neto et al., 1992, Fuhr et al., 1992, Wilson et al., 1993, Mortifee et al., 1994). The spatial validity of TMS-based mapping of the motor cortex has recently been demonstrated by comparing the TMS-derived maps with the results of direct electrical stimulation of the exposed cortical surface in patients undergoing neurosurgical procedures (Krings et al., 1997a, Krings et al., 1997b). TMS has been successfully used in investigating the short- and long-term reorganization of the cortical motor output maps after hemispherectomy (Benecke et al., 1991), spinal cord lesions (Topka et al., 1991), limb amputation (Cohen et al., 1991a), transient anesthesia of limbs and fingers (Brasil-Neto et al., 1994, Rossini et al., 1996) and prolonged immobilization (Liepert et al., 1995).
With the use of T2 weighted magnetic resonance imaging (MRI) sequences in subjects performing specific tasks, local changes in blood oxygenation and cerebral perfusion can be visualized. Such functional MRI (fMRI) brain maps have been obtained for the human motor cortex (Kwong et al., 1992, Connelly et al., 1993, Constable et al., 1993, Kim et al., 1993, Lai et al., 1993, Schad et al., 1993). Using fMRI, a precise anatomical location of the human hand area of the motor cortex is possible. When a subject uses his hand, an activation in a characteristically shaped part of the contralateral precentral gyrus can be seen. This structure, which has been named the ‘hand knob’, resembles a Greek omega or epsilon in the axial and a hook in the sagittal plane (Yousry et al., 1997). The hand knob corresponds exactly to that part of the precentral gyrus in front of the middle genu of the central sulcus, which was first described at the beginning of the century by Testut (1911).
The spatial coordinates of a point on a subject's scalp can be coupled to the coordinates of the radiological images using a frameless neuronavigational system. Such a system allows an excellent 3 dimensional orientation through real-time graphical-anatomical interaction and visualizes any position on the skull in relation to the anatomical brain structure below it (Spetzger et al., 1995, Spetzger et al., 1997).
The aim of our study was to compare the motor cortex representation of intrinsic hand muscles using TMS and fMRI and to correlate the maps so obtained with the anatomical structure of the motor cortex as revealed by T1 weighted MRI.
Section snippets
Subjects and methods
Four normal right-handed subjects (age 26–36 years, all males) participated in the study. The protocol was approved by the local ethics committee and all subjects gave their informed consent for the study.
Magnetic resonance imaging
A localized significant (P<0.001) increase in signal intensity was found in all subjects, which projected into the brain parenchyma of both the pre- and the post-central gyrus, contralateral to the activated (right) hand. A significant activation of the supplementary motor area (SMA) was revealed in 3 subjects. All activated sites in the precentral gyrus were located on the knob-like structure of the posterior face (Fig. 2). The hand knob was epsilon-shaped in two subjects and had the form of
Discussion
In this study, the exactness of TMS motor output cortical maps of the intrinsic hand muscles was evaluated using a non-invasive imaging technique. In order to achieve this goal, the correlation between neuroimaging (MRI and fMRI) and TMS cortical mapping was established with a frameless neuronavigational system. The accuracy of the primary data depends on the quality of the neuroradiological investigation (Guthrie, 1994). The practical accuracy of the whole system is defined by several features
Acknowledgements
This study was supported by the ‘Interdisziplinäres Zentrum Prävention und Kompensation von Störungen des ZNS’ and the ‘START’ program of the University Hospital Aachen. The authors are grateful to Dr. Stuart Fellows (University Hospital, Aachen) for editorial assistance.
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