Direct Thrombus Imaging as a Means to Control the Variability of Mouse Embolic Infarct Models The Role of Optical Molecular Imaging

Background and Purpose-High experimental variability in mouse embolic stroke models could mask the effects of experimental treatments. We hypothesized that imaging thrombus directly would allow this variability to be controlled. Methods-We optically labeled thrombi with a near-infrared fluorescent (NIRF) probe C15 that is covalently linked to fibrin by factor-XIIIa. Labeled thrombus was injected into the left distal internal carotid artery (ICA) of C57/BL6 mice (n=47), near its bifurcation, and laser-Doppler cerebral-blood-flow (CBF) was assessed for 30 minutes. NIRF thrombus imaging was done ex vivo at 24 hours. Results-CBF variably decreased to 43.9 +/- 17.3% at 5 minutes (rCBF; 11.2 similar to 80.4%). NIRF thrombus imaging at 24 hours showed variability in distribution (ICA bifurcation, adjacent and/or remote areas) and burden (2279 +/- 1270 pixels; 0 similar to 5940 pixels). Final infarct size was also variable (21.0 +/- 10.3%; 4.7 similar to 60.3% of the bihemispheric volume). Despite this heterogeneity, a strong thrombus-infarct correlation was maintained. The left hemispheric target infarct size (% of the hemisphere) correlated with thrombus burden, as a stronger predictor of infarct volume (P<0.001, r=0.50) than rCBF (P=0.02, r=-0.34). The infarct size was best predicted by a combination of thrombus imaging and CBF: left-hemispheric big-thrombi (>1865 pixels)/low-rCBF (<= 42%) had an infarct volume of 56.9 +/- 10.4% (n=12), big-thrombi/high-rCBF had 45.9 +/- 23.5% (n=11), small-thrombi/low-rCBF 35.7 +/- 17.3% (n=11) and small-thrombi/ high-rCBF 27.3 +/- 16.4% (n=12). Conclusions-This is the first study to demonstrate that the highly heterogeneous nature of the mouse embolic stroke model can be characterized and managed by using near-infrared fluorescent thrombus imaging combined with CBF monitoring to stratify animals into useful subgroups. (Stroke. 2011;42:3566-3573.)