Changes in the apparent diffusion coefficient of water and T2 relaxation time in gerbil hippocampus after mild ischemia

Selective neuronal vulnerability of human hippocampal CA1 neurons: lesion evolution, temporal course, and pattern of hippocampal damage in diffusion-weighted MR imaging

The CA1 (cornu ammonis) region of hippocampus is selectively vulnerable to a variety of metabolic and cytotoxic insults, which is mirrored in a delayed neuronal death of CA1 neurons. The basis and mechanisms of this regional susceptibility of CA1 neurons are poorly understood, and the correlates in human diseases affecting the hippocampus are not clear. Adopting a translational approach, the lesion evolution, temporal course, pattern of diffusion changes, and damage in hippocampal CA1 in acute neurologic disorders were studied using high-resolution magnetic resonance imaging. In patients with hippocampal ischemia (n=50), limbic encephalitis (n=30), after status epilepticus (n=17), and transient global amnesia (n=53), the CA1 region was selectively affected compared with other CA regions of the hippocampus. CA1 neurons exhibited a maximum decrease of apparent diffusion coefficient (ADC) 48 to 72 hours after the insult, irrespective of the nature of the insult. Hypoxic-ischemic insults led to a significant lower ADC suggesting that the ischemic insult results in a stronger impairment of cellular metabolism. The evolution of diffusion changes show that CA1 diffusion lesions mirror the delayed time course of the pathophysiologic cascade typically observed in animal models. Studying the imaging correlates of hippocampal damage in humans provides valuable insight into the pathophysiology and neurobiology of the hippocampus.

Transient ADC change precedes persistent neuronal death in hypoxic–ischemic model in immature rats

A brief ischemia causes delayed neuronal death (DND) in some areas vulnerable to ischemia. Additionally, it causes a transient reduction in the apparent diffusion coefficients (ADCs) obtained from diffusion-weighted magnetic resonance imaging (DWI), which is a powerful tool to detect ischemic changes in the brain at a very early stage. The present study examined long-term histopathological changes in the hippocampal neurons up to 30 days after a very mild hypoxic-ischemic (HI) insult in immature rats. Three-week-old male rats were subjected to 15- and 30-min HI insults (15-min HI and 30-min HI) and serial DWI was performed. Only animals whose ADC reduction pattern was transient were examined histopathologically. ADCs decreased significantly during the insult, and the ADC values of 30-min HI group were significantly lower than those of 15-min HI group. Ischemic neuronal changes were observed up to 30 days after the insult in 30-min HI group, although ADCs in the chronic stage were within the normal range. In addition, neuron density in 30-min HI group was significantly lower in the chronic stage (on days 14 and 30) than in 15-min HI group. A very mild hypoxia-ischemia followed by a transient ADC reduction causes persistent neuronal death, which can be predicted by measuring ADCs during the acute insult.