An ischemia penumbra model for micro-PET studies

Objectives Rat middle cerebral artery occlusion models are frequently used to study the pathophysiology of focal ischemia but infract size and location are difficult to control. Well defined and reproducible ischemic lesions can be achieved in rodents with direct cortical injection of endothelin-1 (ET1). Little however is known about the pathophysiology of this injection model. We conducted a multitracer microPET experiment using a CBF, a hypoxia and a viability marker to 1) characterize the time course of ischemia in the ET1 model and to 2) test the hypothesis that the model provides a penumbra (tissue at risk of infarction).

Methods 8 Sprague Dawley rats underwent serial dynamic PET scans with 11-C-Flumazenil (FMZ) and 18-F-MISO (MISO) of 60 and 120 minutes duration. In 5 animals, focal ischemia was induced by ET1 injection into the left motor cortex, 3 animals were controls. PET scanning was performed on a CTI Concorde microPET R4 scanner 3h (n=2) and 4h (n=3) post ET1 injection. Standardized volumes of interest (44 mm3) were placed in the center of the injection site and in the same position of the unaffected hemisphere. SUVR were obtained as surrogate marker for rCBF (FMZ uptake 0-20 minutes, FMZflow), tissue viability (FMZ binding 40-60 minutes, FMZbdg) and hypoxia (FMISO retention, 100-120min, MISOret).

Results In ET1 injected animals FMZflow was significantly lower with and MISOret higher than in controls (table). FMZbdg in ischemic rats however was not significantly different from control animals. Severity of hypoxia was linearly related to time of ischemia onset (r2=0.89, p<0.01) with maximum SUVR at 6h post ischemia.

Conclusions The ET1-injection model produces reproducible focal hypoperfusion and hypoxia. Hypoxia develops slowly up to 6h post ischemia and does not seem to compromise tissue viability. It may thus offer a prolonged experimental window to study the ischemic penumbra with microPET.