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No studies have determined the effect of differences in pial collateral extent (number and diameter), independent of differences in environmental factors and unknown genetic factors, on severity of stroke. We examined ischemic tissue evolution during acute stroke, as measured by magnetic resonance imaging and histology, by comparing 2 congenic mouse strains with otherwise identical genetic backgrounds but with different alleles of the Determinant of collateral extent-1 (Dce1) genetic locus. We also optimized magnetic resonance perfusion and diffusion–deficit thresholds by using histological measures of ischemic tissue.Perfusion, diffusion, and T2-weighted magnetic resonance imaging were performed on collateral-poor (congenic-Bc) and collateral-rich (congenic-B6) mice at 1, 5, and 24 hours after permanent middle cerebral artery occlusion. Magnetic resonance imaging–derived penumbra and ischemic core volumes were confirmed by histology in a subset of mice at 5 and 24 hours after permanent middle cerebral artery occlusion.Although perfusion-deficit volumes were similar between strains 1 hour after permanent middle cerebral artery occlusion, diffusion-deficit volumes were 32% smaller in collateral-rich mice. At 5 hours, collateral-rich mice had markedly restored perfusion patterns showing reduced perfusion-deficit volumes, smaller infarct volumes, and smaller perfusion–diffusion mismatch volumes compared with the collateral-poor mice (P<0.05). At 24 hours, collateral-rich mice had 45% smaller T2-weighted lesion volumes (P<0.005) than collateral-poor mice, with no difference in perfusion–diffusion mismatch volumes because of penumbral death occurring 5 to 24 hours after permanent middle cerebral artery occlusion in collateral-poor mice.Variation in collateral extent significantly alters infarct volume expansion, transiently affects perfusion and diffusion magnetic resonance imaging signatures, and impacts salvage of ischemic penumbra after stroke onset.