Rapid alterations in diffusion-weighted images with anatomic correlates in a rodent model of status epilepticus

Abstract

Background and purpose: Diffusion-weighted MR imaging has emerged as a noninvasive tool for the detection of regional neuronal damage. We hypothesize that changes in diffusion-weighted images will correlate with pathophysiologic alterations caused by pilocarpine-induced status epilepticus.

Methods: MR images of brain tissues were examined in vivo by use of T2- and diffusion-weighted imaging at 3, 6, 12, and 24 hours after pilocarpine-induced seizures. Histologic verification of neuronal damage was also performed after imaging to assess the extent and the time course of neuronal cell death.

Results: The piriform cortex, amygdala, and retrosplenial (and somatosensory) cortex displayed significant apparent diffusion coefficient (ADC) decreases 12 hours after seizure initiation. In contrast, an ADC rise of 19% was observed in the hippocampus 24 hours after seizure induction. Histologic data from the piriform cortex and amygdala confirmed severe neuronal loss, whereas hippocampal damage was much less pronounced at 12 hours. Interestingly, very little histologic damage was seen in the retrosplenial cortex.

Conclusion: This study capitalized on diffusion-weighted imaging as a sensitive technique for the early identification of seizure-induced neuronal damage and differentiation of regional severity of these alterations. Hippocampal neuropathology is slower and longer in duration (approximately 7 days), while the piriform cortex and amygdala exhibit very rapid neurodegenerative alterations (approximately 24 hours) after pilocarpine-induced status epilepticus. These histologic changes are reflected in opposing ADC values within these regions.

fig 1.

ROIs used for quantitative analysis. A, Representative control image on which ROIs were outlined using Chesire image processing software. B, Schematic drawing of a rat brain at similar level with the identical ROIs superimposed, illustrating the accuracy of our ROIs. ROIs were defined as: retrosplenial cortex (RC), hippocampus (Hippo), thalamus (Thal), amygdala (Amy), piriform cortex (PC)

fig 2.

Diffusion maps. Diffusion maps were generated from unweighted (b = 0) and weighted (b = 1228 s/cm²) images. The b factor refers to the effective diffusion weighting; it is a function of the amplitude of the gradients and their duration (see Methods for details). It is readily apparent that at 12 and 24 hours there are widespread changes in signal intensity in the hippocampus and piriform cortex. These alterations are quantified by an increase in ADC in the hippocampus (arrowheads), whereas there is a decrease in the piriform cortex (arrows) and retrosplenial cortex (*). Note that the retrosplenial cortex returns to control ADC levels by 24 hours.

fig 3.

Normalized ADC values after pilocarpine-induced seizures. Contrasting ADC (with standard error of the mean) changes were found between different limbic regions. The piriform cortex (and amygdala) show significant ADC decreases as early as 12 hours that are still present at 24 hours. Conversely, the hippocampus displays a slow increase in ADC that is significantly elevated at 24 hours. The retrosplenial cortex shows a decrease in ADC that peaks at 12 hours but returns to control levels by 24 hours.

fig 4.

Cresyl violet and silver impregnation stain of the hilar region of the dentate gyrus. A, Cresyl violet–stained sections in a control rat after imaging showing no loss of neurons. (M-molecular region, G-granule cell layer, H-hilus, CA3-pyramidal cells of the hippocampal CA3 layer). B, Twenty-four hours after the induction of seizures, there is a pronounced loss of neurons in the hilus seen in cresyl violet–stained sections. C, The silver degeneration stain in an adjacent section, from the same control animal in A, confirms the lack of neuronal loss within the hilus of the dentate gyrus. Neuronal degeneration can be visualized as darkly stained neurons; this control section lacks any darkly stained neurons. D, The widespread loss of hilar neurons is confirmed after staining with silver degeneration methods. Arrowheads denote several neurons that are clearly in the process of degenerating. Note that the other regions, including the CA3 region, did not have significant neuronal degeneration. (Scale bar: 100 microns)

fig 5.

Histologic assessment of neuronal damage in the piriform cortex and amygdala 12 hours after pilocarpine-induced seizures. A, Cresyl violet–stained section from a control animal. Note dark neuronal staining in layers II-III and in the nuclei of the amygdala. B, There is marked neuronal loss and vacuolization of layer II-III in the piriform cortex (arrows). C, Silver degeneration staining confirms the lack of neuronal loss in this section from a control animal. D, Twelve hours after the induction of SE, there are numerous darkly stained neurons in layer II-III of the piriform cortex (arrowheads) and some nuclei of the amygdala. Arrows indicate widespread loss of pyramidal cells in the piriform cortex. (Scale bar: 100 microns)

fig 6.

Silver and cresyl violet staining of retrosplenial cortex. Histologic assessment of the retrosplenial cortex following pilocarpine-induced seizures. A, Control (A), 12-hour (B), and 24-hour (C) cresyl violet–stained sections after pilocarpine-induced seizures. Note the apparent swelling in cortical layers III-VI (arrow) at 12 hours, which appears to remit to control levels by 24 hours. Control sections (D) stained for silver degeneration show no neuronal degeneration, whereas 12-hour tissue sections (E) depict moderate neuronal death (arrowheads). By 24 hours (F), there is no overt neuronal degeneration. There are occasional neurons that appear to be undergoing neuronal death (arrowheads) at this time point. (Scale bar: 100 microns)