Sensitivity of USPIO-Enhanced R2 Imaging to Dynamic Blood Volume Changes in the Rat Kidney

Purpose: To determine whether MRI in combination with an intravascular contrast agent is sensitive to pharmacologically induced vasodilation and vasoconstriction in the rat kidney. Materials and. Methods: R-2 imaging was performed in 25 Sprague Dawley rats at 3 Tesla in the presence of ferumoxytol, an ultrasmall superparamagnetic iron oxide (USPIO) agent. with a long plasma half-life. R-2 changes were measured following manipulation of blood volume by intravenous administration of adenosine, a short-acting vasodilator, or N-G-nitro-L-arginine methyl ester (L-NAME), a long-acting nitric oxide synthase inhibitor with known vasoconstrictive effects. As a control, R-2 responses to adenosine and L-NAME were also examined in the absence of ferumoxytol. Results: In the presence of ferumoxytol, adenosine induced a significant increase in R-2, while L-NAME produced a reduction, although the latter was not statistically significant. Control experiments revealed small R-2 changes in till: absence of ferumoxytol. An incidental finding was that the cross-sectional area of the kidney also varied dynamically with adenosine and L-NAME. Conclusion: Our results suggest that ferumoxytol-enhanced R-2 imaging is sensitive to adenosine-induced vasodilation. The responses to L-NAME, however, were not statistically significant. The variations in kidney size and the R-2 changes in the absence of ferumoxytol may reflect alterations in the volume of the renal tubules.