Lactate and NAA Mapping Assessed by Proton NMR Chemical‐Shift Imaging in Seizures

Abstract

Purpose: To observe metabolic maps of lactate and N‐acetyl asparate (NAA) by proton nuclear magnetic resonance (NMR) in vivo in the rat brain during the acute stage of hemispheric seizures. Methods: Adult male Sprague‐Dawley rats were anesthetized with isoflurane and were examined by using a 2.0‐T CSI Omega system. Seizures were elicited through transient opening of the blood‐brain barrier (BBB) in the left hemisphere by injection of a hyperosmolar solution of mannitol into the left internal carotid artery, following an intravenous injection of bicuculline methiodide. Both hemispheres were then observed from an axial view before and after the induction of seizures by using the three‐dimensional chemical shift imaging (CSI) technique. Proton CSI data were obtained from a 40 × 40 mm2 FOV, divided into an 8 × 8 matrix, by using water suppression, fat suppression, and outer volume suppression, thus creating a metabolic map. The opening of the BBB was confirmed as a transient increase in signal intensity in the left hemisphere on T,‐weighted MR images. Results: Neither the intravenous injection of bicuculline methiodide nor the transient opening of the BBB alone caused seizure activity. The rats developed tonic‐clonic seizures after the transient opening of the BBB after injection of bicuculline methiodide. The left hemisphere showed periodic spikes on the scalp EEG ‐15 min after the opening. Metabolic images showed an accumulation of lactate in the left cerebral hemisphere at 1 and 3 h after seizure induction. Metabolic images of lactate, however, detected no abnormalities in the hemisphere 24 h after the seizure. No clear concurrent changes in the NAA map were observed in this study. The cerebellar hemispheres showed no changes of either the lactate or NAA metabolic images. Conclusions: We found a transient accumulation of lactate in the cerebral hemisphere due to abnormal neuronal excitation after its evocation with bicuculline methiodide. The metabolic maps were obtained in vivo with a noninvasive procedure. We suggest that lactate can be a reversible indicator for NMR metabolic mapping, showing the focus of abnormal neuronal excitation in the acute stages of clinical seizures.