Abstracts

Rationale: Clinical 1.5 and 3 Tesla magnetic resonance imaging (MRI) often detects hippocampal sclerosis (HS) in mesial temporal lobe epilepsy (TLE). Focal hippocampal dysplasia associated with HS has been detected histopathologically in TLE surgical specimens. We hypothesized that 7 T MRI might detect hippocampal malformations with HS in TLE, with increased contrast and submillimetric spatial resolution. Methods: We acquired T1- and T2-weighted 7 T brain MRI in 11 healthy subjects and 8 unilateral TLE patients, who consented with IRB approval. Patients had scalp EEG ictal onsets over one temporal lobe, which was ipsilateral to hippocampal atrophy or T2 increases on clinical MRI. T1-weighted, 3-dimensional, magnetization-prepared, rapid acquisition, gradient-recalled-echo (0.8x0.8x0.8mm3 resolution) sequences imaged the whole brain. T2-weighted, turbo spin echo (0.25x0.25x1.2 mm3) sequences imaged the entire hippocampus, in contiguous oblique coronal slices. Data were analyzed qualitatively to define morphology and count hippocampal head digitations. Results: Each 7 T image set in healthy and TLE subjects showed submillimetric hippocampal shape and structures including the hippocampal striation (intrahippocampal white matter which separates Ammon s horn and dentate gyrus), and the alveus, on coronal images. Among 22 hippocampi in healthy subjects, one hippocampus had a single digitation and the others had 2-3 digitations of the hippocampal head. All TLE subjects had 0 or 1 hippocampal digitation on the epileptogenic side, and 3 also had 0 or 1 digitation contralaterally. Malrotations of the hippocampal body were observed in 3 TLE and 4 healthy subjects. (Clinical MRI did not consistently detect the hippocampal striation, alveus, and other structures that were visible at 7 T. Clinical MRI did not consistently detect paucity of digitations, and malrotations, which were detected at 7 T.) The Figure shows T2-weighted coronal 7 T images through the hippocampal heads (upper two panels) and bodies (lower panel) in a healthy (c9) and TLE (p1) subject; black arrows indicate the hippocampal striation with normal digitations (c9) and bilaterally absent digitations (p1), and the white arrow indicates malrotation of the hippocampal body (contralateral to HS, in p1). Conclusions: Ultrahigh field MR images defined internal and external hippocampal morphology more clearly than did clinical MRI. Hypoplasia of the hippocampal head may be highly associated with HS, but can occur contralateral to HS in TLE patients. Absence or paucity of digitations of the hippocampal head may represent a specific deformity of hippocampal morphology in mesial TLE. Histopathological correlation will be required to determine whether this deformity is an MRI sign of hippocampal dysplasia. Malrotation of the hippocampal body may be a normal variant of hippocampal morphology. Improved contrast and submillimetric spatial resolution at 7 T should considerably enhance future pathophysiological research and perhaps surgical planning in TLE. Acknowledgments: Supported by NIH P41 RR008079 & P30 NS057091, and the Keck Foundation.