Toshiki Yoshimine, Amami Kato, Masaaki Taniguchi, Hirotomo Ninomiya, Masayuki Hirata, Yasuo Nii, Motohiko Maruno, Norio Hirabuki*, Hironobu Nakamura*, Jung Kyo Lee^, Tae Sung Ko^^ and Toru Hayakawa
We have developed a translucence stereoscopy method for displaying the distribution of multiple interictal epileptiform discharges within the intracranial space. The epileptiform discharges, measured using a wholehead magnetoencephalography system, were modeled by a least-squares method to obtain the equivalent current dipoles. The dipoles were located in the stereo pair of intracranial images composed of translucent brain slices at several selected levels. The technique demonstrated clearly the distribution of interictal dipoles within the brain in three patients. Three dimensional understanding of the intracranial distribution of multiple dipoles in one image is valuable in analyzing the intracerebral neurophysiological events in epileptic patients. [Neurol Res 1998; 20: 572-576]
Keywords: Stereoscopy; magnetoencephalography; epilepsy; interictal spike; epileptiform discharge; Landau-Kleffner syndrome
Multichannel magnetoencephalography (MEG) is useful in localizing the epileptiform discharges in the interictal phase of epilepsy1-4 . Using a least-squares method, an equivalent current dipole can be modeled for the epileptiform discharge, which can be displayed in patient magnetic resonance images (MRI). The analysis of interictal epileptiform discharge is important in understanding the neurophysiological mechanism of the interictal phenomena and also potentially useful in analyzing the lesion for ablative surgery2-4. Since the interictal dipoles localize at many different points, almost the same number of MRI sections are necessary to display these locations2-5. The spatial distribution of dipoles displayed in numerous different MRI sections are not easy to understand. In the present study, we loaded the dipoles on a stereo pair of translucent MRI images to improve the three-dimensional understanding of the distribution of the interictal dipoles.
Patients and methods
Three patients with medically intractable epilepsy were studied with MEG for interictal spike activities. A 41-year-old man (Patient 1) had a one-and-a-half-year history of simple partial seizures (SPS) with sensory symptoms on the left extremities as well as occasional secondary generalized seizures. He had a large contusion in the right parietal lobe caused by a traffic accident two-and-a-half years earlier (Figure 3, bottom). A 39year-old woman (Patient 2) experienced SPS as well as complex partial seizures (CPS) for 32 years. She also complained of a paroxysmal flickering visual sensation in recent months. MRI and cerebral angiography demonstrated a small arteriovenous malformation (AVM) and a small hematoma cavity at the base of the right occipital lobe (Figure 4, bottom). A six-and-a-halfyear-old boy (Patient 3) had a 3-year history of CPS, and experienced verbal auditory dysfunction for 2 years. MRI was negative. The clinical data of these patients are summarized in Table 1.
MEG was recorded with a 64-channel, whole head system (CTF Systems Inc., Port Coquitlam, BC, Canada)6 after reduction of antiepileptics (Patients 1 and 2) or under moderate sedation (Patient 3). MEG data were recorded at a sample rate of 625 Hz and passed through a 2-30 Hz off-line filter. Epileptiform discharges were analyzed with a single dipole source model using a simplex-based least squares minimization algorithm (Figure 1)7. Dipoles with least-squares errors less than 15%-25% were selected for localization on the MR images. Tl-weighted MR images consisting of 124 sequential sagittal slices at 1.5 mm thickness were obtained with a 1.5 T MR unit (Signa Horizon, GE Medical Systems, Milwaukee, WI, USA). The MRI data were integrated with the MEG data using three fiducial skin markers (Figure 2)8. The combined MEG/MRI data were processed with an image processing system comprising a workstation and personal computers (J 200, Hewlett Packard; Power Macintosh 8600/250 and 7600/200) equipped with graphic software packages (AVS, KGT, Tokyo, Japan; Adobe Photoshop 4.0J, Adobe Systems Inc., Mountain View, CA, USA). Cortical slices at several levels were arbitrarily selected and segmented semiautomatically9. They were differentiated by color, made translucent, and combined in a single coordinate system together with the dipoles. A stereo pair was obtained by rotating the image by 20-30 degrees.
A stereo pair of translucent stereoscopic images was obtained for all three patients. In Patient 1, a total of 10 dipoles were obtained, which were distributed in the brain surrounding the traumatic cavity in the left posterior temporal to lower parietal lobes (Figure 3). All except one were located posteriorly, inferiorly or medially to the cavity. One was located anteriorly. The orientation of dipole vectors was variable. In Patient 2, of the 32 dipoles obtained, 25 were located in the midtemporal to lower frontoparietal lobes and 7 in the medial occipital lobe of the right hemisphere (Figure 4). The dipoles in the upper temporal region were oriented mostly downward. In Patient 3, the 19 dipoles obtained were distributed bilaterally, but 10 of them were localized in the deep Sylvian region of the left hemisphere and 5 within the right thalamus or close to it (Figure 5). Most dipoles in the deep Sylvian region were oriented downward.
DISCUSSION AND CONCLUSION
The translucence stereoscopy technique facilitated understanding of the spatial distribution of the interictal epileptiform discharges. The orientation of the dipole vectors was also clearly demonstrated. In Patient 1, the dipoles were distributed in a rather wide area around the traumatic cavity, but most of them were located posterior to, inferior to, or medial to, but not anterior to the cavity. This distribution pattern accords well with the patient's sensory seizures. The seizure pattern in Patient 2 was rather complex but may be explained by the two major areas of dipole distribution: the dipoles in the midtemporal region are probably related to the long history of CPS, and the dipoles in the medial occipital lobe to the recent visual symptoms. The AVM and the old hematoma were removed, resulting in the disappearance of paroxysmal visual symptoms, but the frequency of CPS was not reduced. Temporal lobectomy is currently scheduled. In Patient 3, many dipoles were distributed in the deep Sylvian region, a finding that coincides roughly with previous reports of LandauKleffner syndrome10-11. The precise position of each dipole is, however, difficult to assess in the stereoscopic image. Most dipoles seem to be located in the insular cortex, but this is not absolutely clear.
In the human brain, visual perception of threedimensional space depends on a variety of information sources, such as stereopsis, convergence, motion parallax, shading, etc.12-13. In the present display system, the spatial perception is mainly dependent on stereopsis. Deferentially colored, translucent brain slices also help recognition of the spatial layout of dipoles and brain structures. However, natural and precise perception of depth is not obtained with a fixed accommodation stimulus by a simple stereo pair of two-dimensional images14. The present technique is good for general understanding of the distribution of dipoles, but the precise location of each dipole will be best assessed on original MRI. Thus, these two kinds of images seem complementary.
Theoretically, MEG localization of interictal epileptiform discharges has an advantage over EEG localization because the magnetic field is not distorted by the cranial structures 2,4,15. The analysis of interictal epileptiform discharges, especially the temporal changes in activity, will be valuable in elucidating the mechanism of intracerebral propagation of epileptic activitiesa1,16. Future application of translucence stereoscopy for the temporospatial analysis of interictal epileptiform discharges will further help the understanding of neurophysiological mechanisms involved in epileptic activity.
This study was supported in part by the Osaka Medical Research Foundation for Incurable Diseases.
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Correspondence and reprint requests to: Toshiki Yoshimine, MD, PhD, Department of Neurosurgery, Osaka University Medical School, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan. Accepted for publication May 1998.
Department of Neurosurgery, *Department of Radiology, Osaka University Medical School, Osaka, Japan Department of Neurological Surgery, : Department of Pediatrics, University of Ulsan, College of Medicine, Asan Medical Center, Seoul, Korea
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