Evaluation of Intrarenal Distribution of Ultrasmall Superparamagnetic Iron Oxide Particles by Magnetic Resonance Imaging and Modification by Furosemide and Water Restriction

Abstract

The steady-state intrarenal distribution of ultrasmall superparamagnetic iron oxide (USPIO) particles in the cortex, the outer medulla (OM), and the inner medulla (IM) was investigated using magnetic resonance imaging in the normal rabbit kidney and in situations that modify the corticopapillary osmotic gradient. Experiments were performed on rabbit kidneys with T2-weighted spin-echo sequence and T2-weighted gradient-recalled-echo sequence. The intravenous dose was 27 μmole/kg of iron in all rabbits. Three groups were studied: normally hydrated rabbits (n=5), after water restriction (n=5) to increase the osmotic gradient, and after furosemide injection (n=5) to decrease the osmotic gradient. The signal intensity (SI) was quantified by region of interest placed on the cortex, the OM and the IM. In normally hydrated rabbits, a maximal decrease of the SI after USPIO was noted in the medulla, without significant difference between the OM and the IM on spin-echo sequences. The decrease of the SI was maximal in the IM on gradient-recalled echo sequences. In dehydrated animals, the maximum SI loss was in the OM. The furosemide-induced transient enhancement of the IM lasted a few minutes. The observed SI changes due to the distribution of the USPIO between OM and IM were not based on modifications of the osmotic gradient. The authors suggest that these SI changes were mostly dependent on the vascular density.