The MRI‐measured arterial input function resulting from a bolus injection of Gd‐DTPA in a rat model of stroke slightly underestimates that of Gd‐[14C]DTPA and marginally overestimates the blood‐to‐brain influx rate constant determined by Patlak plots

The hypothesis that the arterial input function (AIF) of gadolinium‐diethylenetriaminepentaacetic acid injected by intravenous bolus and measured by the change in the T₁‐relaxation rate (ΔR₁; R₁ = 1/T₁) of superior sagittal sinus blood (AIF‐I) approximates the AIF of ¹⁴C‐labeled gadolinium‐diethylenetriaminepentaacetic acid measured in arterial blood (reference AIF) was tested in a rat stroke model (n = 13). Contrary to the hypothesis, the initial part of the ΔR₁‐time curve was underestimated, and the area under the normalized curve for AIF‐I was about 15% lower than that for the reference AIF. Hypothetical AIFs for gadolinium‐diethylenetriaminepentaacetic acid were derived from the reference AIF values and averaged to obtain a cohort‐averaged AIF. Influx rate constants (K_i) and proton distribution volumes at zero time (V_p + V_o) were estimated with Patlak plots of AIF‐I, hypothetical AIFs, and cohort‐averaged AIFs and tissue ΔR₁ data. For the regions of interest, the K_is estimated with AIF‐I were slightly but not significantly higher than those obtained with hypothetical AIFs and cohort‐averaged AIF. In contrast, V_p + V_o was significantly higher when calculated with AIF‐I. Similar estimates of K_i and V_p + V_o were obtained with hypothetical AIFs and cohort‐averaged AIF. In summary, AIF‐I underestimated the reference AIF; this shortcoming had little effect on the K_i calculated by Patlak plot but produced a significant overestimation of V_p + V_o. Magn Reson Med 63:1502–1509, 2010. © 2010 Wiley‐Liss, Inc.