Rodent stroke induced by photochemical occlusion of proximal middle cerebral artery: Evolution monitored with MR imaging and histopathology

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Abstract

Purpose

To longitudinally investigate stroke in rats after photothrombotic occlusion of proximal middle cerebral artery (MCA) with magnetic resonance imaging (MRI) in correlation with histopathology.

Materials and methods

Forty-two rats were subjected to photochemical MCA occlusion and MRI at 1.5 T, and sacrificed in seven groups (n = 6 each) at the following time points: 1, 3, 6 and 12 h, and at day 1, 3 and 9. T2-weighted (T2WI) and diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) map was performed in all rats. Contrast-enhanced T1-weighted imaging (CE-T1WI) was compared to intravital staining with Evans blue in one group for assessing blood–brain barrier (BBB) integrity. The brain was stained histochemically with triphenyl tetrazolium chloride (TTC) and processed for pathological assessment. The evolutional changes of relative lesion volume, signal intensity (SI), and the BBB integrity on MRI with corresponding histopathology were evaluated.

Results

The ischemic lesion volume reached a maximum around 12 h to day 1 as visualized successively by DWI, ADC map and T2WI, implicating the evolving pathology from cytotoxic edema through vasogenic edema to tissue death. The ADC of brain infarction underwent a significant reversion after 12 h, reflecting the colliquative necrosis. On CE-T1WI, BBB leakage peaked at 6 h and at day 3 with a transitional partial recovery around 24 h. The infarct volume on T2WI, DWI and ADC map matched well with that on TTC staining at 12 h and at day 1 (p > 0.05).

Conclusion

The evolution of the present photothrombotic stroke model in rats could be characterized by MRI. The obtained information may help longitudinal studies of cerebral ischemia and anti-stroke agents using the same model.

Introduction

Being pioneered by Watson et al. [1], photochemically induced thrombotic (PIT) stroke represents a general method that elicits various rodent models of focal cerebral ischemia. According to different approaches employed, the PIT stroke models can actually be classified into four types: type A, end-artery occlusion in the cortex, which is the classical type of the PIT model [2], [3], [4], [5], [6], [7]; type B, the “ring” model, which features certain “tissue at risk” or ischemic penumbra within the “ring” shape of photochemically induced cortical lesion [8], [9], [10], with both types A and B based on the occlusion of microvessels in the cortex; type C, middle cerebral artery (MCA) occlusion, which can be further divided into the proximal MCA occlusion (type C1) [11] and distal MCA occlusion (type C2) [12], [13], [14]; type D, common carotid artery occlusion, which causes multiple cerebral infarcts [15]. Since MCA territory is the most frequently insulted area for focal cerebral infarcts in clinic, type C PIT models most closely replicate the situations of stroke in humans [16]. Unlike other PIT models that are extensively reported in literature [1], [2], [3], [4], [5], [6], [8], [9], [10], [12], [13], [14], the proximal MCA occlusion or the type C1 PIT model has not been sufficiently characterized since its first introduction in 1993 [11].

Magnetic resonance imaging (MRI) proves to be pivotal in experimental stroke studies due to its in vivo, non-invasive and non-destructive features with excellent spatial and temporal resolution [6], [16], [17], [18]. Previous studies have shown that the PIT model of proximal MCA occlusion is advantageous over other models for its reproducible larger ischemic lesion and the possibility of reperfusion. Therefore, it appears ideal for studying “tissue at risk” or ischemic penumbra and for evaluating novel anti-stroke drugs particularly with MRI [19], [20], [21], [22]. Recently, a delayed perfusion phenomenon at MRI [23] and its relation to the collateral circulation, spontaneous reperfusion, and ischemic penumbra has been observed using this PIT model [24]. However, whether the pathological evolution of this stroke model differs from other models has not been systemically studied by in vivo MRI.

Therefore, the purpose of the present study was to further investigate the morphological changes of this particular PIT stroke model in rats using a clinical MRI scanner correlated with histopathology in a longitudinal manner.

Section snippets

Animal preparation

In compliance with the current institutional regulations for use and care of laboratory animals, 42 male Sprague–Dawley (SD) rats weighting 230–350 g were included in this study. Anesthesia was made with initial inhalation of 4% isoflurane for 3 min and maintained with 2% isoflurane in a mixture of 20% oxygen and 80% room air. Body temperature was kept at 37.5 ± 0.5 °C with a heating pad during the surgical operation. The left proximal MCA was occluded by a photothrombotic approach as detailed

Temporal evolution of cerebral ischemic volume on MR imaging

The ischemic lesion normally appeared first at the ventral part of the basal ganglia and then spread to the entire basal ganglia and ipsilateral cortex. At 1 h after MCA occlusion, the relative ischemic lesion volume depicted on T2WI was significantly smaller than that on DWI and ADC map. The lesion size gradually expanded and reached a maximum at day 1 on T2WI, DWI and ADC map similarly. The lesion remained almost unchanged up to day 3 followed by a significant decrease at day 9 on all MR

Comparisons with other photothrombosis stroke models

Despite similar basic principle and pathology, our PIT model with proximal MCA occlusion demonstrated a larger ischemic lesion covering both the cortex and striatum compared to other photothrombotic stroke models that affect only peripheral areas [4], [6], [7]. The main reason is that the proximal left MCA at the olfactory tract, rather than the end-artery [2], [3], [4], [5], [6], [7], [8], [9], [10] or distal MCA [12], [13], [14] chosen elsewhere, was thrombotically occluded with light

Summary

The evolution of the present photothrombotic stroke model in rats has been studied by MRI and histopathology, and compared with other photochemical stroke models. Better understanding of the serial changes on MRI and pathology in this stroke model may benefit research in cerebral ischemia and anti-stroke agents using MRI. Furthermore, the use of a commercial 1.5 T magnet could improve the interpretation of the results in this study and give them more relevance towards the clinical scenarios.

Acknowledgement

We thank Dr. Hilde Vandenhout for her help in manuscript editing.

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