Rapid identification of the primary motor area by using magnetic resonance axonography

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Abstract
To identify the primary motor area (PMA) quickly and correctly, the authors used magnetic resonance (MR) axonography, including anisotropic diffusion-weighted (DW) MR imaging and three-dimensional anisotropic contrast (3DAC) imaging, which was performed to visualize the corticospinal tract mainly originating from the PMA. All studies were obtained in 10 normal volunteers and in 17 patients with brain tumors affecting the central motor system. Data sets of anisotropic DW imaging and anatomical and functional (f)MR imaging were acquired while the participants executed simple hand movements. Offline processing of 3DAC MR axonography images was subsequently done to extract only the anisotropic components of the tract fibers. Somatosensory evoked fields (SSEFs) and intraoperative cortical somatosensory evoked potentials (SSEPs) were recorded after electrical stimulation of the median nerve. In normal volunteers, anisotropic DW imaging, 3DAC imaging, fMR imaging, and magnetoencephalography consistently localized the PMA in both hemispheres. In contrast, fMR imaging and SSEFs failed to identify the PMA in seven and one of the 17 patients, respectively, because of cortical dysfunctions due to brain tumor. The anisotropic DW imaging data acquired within 30 seconds with no patient tasks successfully identified the PMA in 12 patients, and failed in five patients because of the lesions involving the frontal lobe. The anisotropic axonal components were distinctly visualized on 3DAC images and indicated the PMA location, which was confirmed on intraoperative SSEPs in all 17 affected hemispheres. Swift and noninvasive PMA identification by rapid scanning with MR axonography is a promising method for routine clinical use and is especially beneficial for patients who have severe cortical dysfunction in the PMA.

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