Spatial and temporal modulation of perfusion in the rat ovary measured by arterial spin labeling MRI

The hemodynamic changes triggered by luteinizing hormone/human chorionic gonadotropin (LH/hCG) in ovaries of immature pregnant mare serum gonadotropins (PMSG)-primed female Wistar rats were followed by pulsed arterial spin labeling magnetic resonance imaging. Decreased perfusion was monitored in the first 2 hours after administration of hCG followed by a transient significant rise in perfusion. Subsequently, constant ovarian perfusion of 10.9 +/- 4.3 mL min(-1) g(-1) was maintained during the exponential increase in ovarian volume. However, ovarian perfusion was not uniform, and prior to ovulation poorly perfused regions were detected that were assigned to the follicular fluid in preovulatory follicles. This result implied that in the time scale of seconds, corresponding to the T1 relaxation time of water in the follicular fluid, exchange of arterial water with water in the follicular fluid was negligible. Along with the drop in the levels of high-energy phosphate metabolites detected by 31P nuclear magnetic resonance spectroscopy and the shift to glycolytic metabolism, these results support the hypothesis that physiological hypoxia could play a role in large preovulatory follicles as part of the normal ovarian cycle.