Sensitivity of USPIO‐enhanced R 2 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 the 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. J. Magn. Reson. Imaging 2011;33:1091–1099. © 2011 Wiley‐Liss, Inc.