To produce a minor stroke consisting of a small infarct with mild ischemic damage in peri-infarct regions and then determine whether standard magnetic resonance (MR) imaging will detect additional injurious effects produced by a second diffuse mild ischemic insult.A transient ischemic attack or minor stroke often precedes a second stroke. The ischemic damage can involve selective cell death that is not usually detectable using anatomical MR imaging ^1,2. Strokes also often produce a core of ischemic damage with peri-infarct regions of scattered necrosis. Unknown is whether such peri-infarct regions compared to normal tissue are differentially susceptible to damage from a second mild ischemic insult and whether cellular changes are produced that are more readily detected using standard MRI – e.g. T2 weighted imaging. The aim of the present study was to better understand the MR diagnosis of ischemic injury produced by multiple ischemic insults, with a focus on what occurs with a recurrent insult in a peri-infarct region containing scattered cell death. A mild photothrombosis (PT) procedure was used to elicit an initial ischemic insult consisting of a small superficial cortical infarct overlying a peri-infarct region of scattered necrosis. This was followed 1,2 or 7 days later by a relatively short transient middle cerebral artery occlusion (MCAO) of 30 min using a microclip to produce diffuse mild cortical ischemia within the middle cerebral artery territory (n=10) ². Control rats (n=6) were subjected to a sham PT procedure (surgery but no illumination) followed by transient MCAO. Ischemic damage was assessed using MR imaging and microscopic assessment of paraffin sections stained with hematoxylin and eosin. MR scans were acquired 24 hr after each ischemic insult using a 9.4T Bruker MR system. A T2 spin echo scan (10ms echo spacing, 32 echoes) was acquired and used to calculate a T2 map. Changes in T2 were compared in slices containing the PT lesion and in a slice adjacent to the photothrombotic infarct. Regions of interest included the PT lesion, a peri-lesion region, normal non-ischemic cortex and cortex within the middle cerebral artery territory (non PT).

In all control rats (n=6/6) a sham PT procedure followed by transient MCAO resulted in mild ischemic changes consisting of scattered cell death within the middle cerebral artery territory (not associated with T2 changes). A minor photothrombotic stroke was produced in all 10 rats and was visible in T2 images as a hyperintense core in superficial cortex (mean area of 0.7+0.2 mm²) overlying an area of less intense T2 changes (e.g. Fig 1-3 A). The hyperintense and milder T2 changes were respectively associated with infarction and scattered ischemic cell death.

Following the second ischemic episode there was often increased severity of the MCAO ischemic injury compared to the (Sham plus MCAO) control group. Within the middle cerebral artery territory one animal had a large hemorrhage and infarction, whereas 4 had predominantly incomplete infarction and 5 predominantly scattered cell death. Regions with partial and complete infarction had increased T2 on MR images (e.g. Fig 1 C,D,yellow arrows) whereas T2 was normal in regions with scattered necrosis (T2=103.2+1.2 % control).

In regions near photothrombotic lesions compared to normal regions, there was evidence for an interaction between the initial mild ischemic injury and the second mild MCAO (e.g. Fig 1 and 2, C,D red arrows). Inspection of the MR images revealed that regions near the PT lesion (selected from the MR acquired 24h post PT) had increased T2 relative to the T2 measured in the MCAO ischemic (non PT) cortex. Such an enhanced peri-infarct region post MCAO was visible in 1 and 2 d groups (mean T2 of 21.8+10.2% greater than that in ipsilateral/non PT cortex). In contrast there was no visible enhancement in peri-infarct regions in animals with MCAO 7 d following PT (p<0.02 vs 1 and 2 d). These regions of increased T2 corresponded to areas of increased histological injury that comprised increased vacuolation.

The present results indicate that peri-infarct regions with mild ischemic changes are susceptible to a second acute (1 or 2 day later) ischemic event. The combined ischemic damage produces cellular changes that are detected using T2 imaging and correspond to increased histological damage that include increased vacuolation. With a longer one week recovery between insults, there was no T2 increase and less ischemic damage in the peri-infarct region. Better understanding of multiple transient injurious ischemic events and their diagnosis is relevant for improved management and treatment of patients with recurrent transient ischemic insults.