Sensitivity of USPIO‐enhanced R₂ imaging to dynamic blood volume changes in the rat kidney

Abstract

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 ferumoxytolenhanced 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.

Evaluation of Intrarenal Oxygenation in Iodinated Contrast-Induced Acute Kidney Injury–Susceptible Rats by Blood Oxygen Level–Dependent Magnetic Resonance Imaging

“…Changes in R 2 are potential confounders as demonstrated by recent report 34 because R 2 could change with compartmental volumes (tubular, vascular, and interstitial compartments), vasoactive drugs such as the ones used in this study could influence R 2 and, hence, R 2*. Interestingly, a previous report 35 indicated that L-NAME, which would be considered a vasoconstrictor, resulted in an increase in tubular volume and, therefore, in a decrease in R 2. On the basis of all the previously mentioned scenarios, the expected changes due to blood volume, hemoglobin, and R 2 may not only be non–dominant contributors, but their effects may actually be counteracting the large increases observed in R 2*.…”

Sensitivity of arterial spin labeling perfusion MRI to pharmacologically induced perfusion changes in rat kidneys

“…The dosage of the vasoactive drugs in this study was chosen based on a previously published animal study looking for blood volume changes by contrast enhanced MRI . During our initial testing, we observed little (less than 3%) cortical perfusion change (data not shown) with an adenosine dose rate of 0.5 mg/kg/min . Subsequently, we doubled the dose rate of adenosine to 1.0 mg/kg/min.…”

Current MRI techniques for the assessment of renal disease

“…With proper kinetic analysis renal blood flow, glomerular filtration rate, total or capillary renal blood volume (or the volume of distribution) and plasma and tubular mean transit times may be assessed with these MR imaging [12–15]. Intravascular iron-oxide agents can also be used to dynamically assess vascular changes following pharmacologic interventions [16] or assess the patency and architecture of the vasculature following renal transplants [17]. In the context of cancer imaging, iron-oxide or Gadolinium agents are also used to quantify (map) the mean vessel size and vascular architecture [18, 19].…”