Comparison of Magnetic Resonance Imaging and Histology in Collagenase‐induced Hemorrhage in the Rat

Complex changes in the appearance of blood on magnetic resonance images (MRIs) complicate interpretation of intracerebral hemorrhage. In this study serial MRIs of intracerebral hemorrhage were obtained at 1.5 T, using a recently developed model for brain hemorrhage, and the MRI findings were compared with histological findings. Hemorrhages were induced in 24 adult rats by the stereotactic injection of 0.5 unit of bacterial collagenase into the caudate/putamen. Initially, there was hypointensity seen on both T1- and T2-weighted images. Conversion to hyperintensity began on both T1- and T2-weighted images around 10 hours, when red blood cell lysis was observed histologically. By 24 hours, complete conversion to marked hyperintensity had occurred. Calculated T2 values increased twofold to fivefold after lysis, indicating that T2 effects dominate the conversion to hyperintensity seen after red blood cell lysis. At 24 hours there was a rim of intact red blood cells around the lesion, which was hypointense on MRI at a time when iron stains were negative. Fresh blood appears hypointense on T1- and T2-weighted images both at the early stages of a bleed and at later stages when rebleeding occurs. Collagenase-induced hemorrhage is a useful animal model to follow the evolution of paramagnetic effects of blood on MRI.