Accurate and precise measurement of renal filtration and vascular parameters using DCE-MRI and a 3-compartment model.

aorta , Cp glom , Ct are the time-dependent concentrations in vd, aortic plasma, glomerular plasma, and kidney tissue respectively. vp, vb and vd are the fractional volumes of glomerular plasma, glomerular blood and the distribution space for tracer extracted from the blood (principally the tubules). The delay and dispersion for plasma-borne tracer travelling from the aorta to the glomeruli are described by the Glomerular Impulse Response Function (GIRF) g(t). F1 is the tracer extraction rate per unit volume (mmole min -1 mL -1 ) from the glomerular plasma by the kidney; F1=K trans Cp glom ; K trans is the transfer constant 3 from glomerular plasma to kidney (GFR per unit volume of tissue). MRI modelling: There were 4 free parameters: vb, K trans , and 2 delay and dispersion parameters which defined the GIRF. Three GIRF shapes were investigated: instant exponential decay; delayed exponential decay; and delayed gaussian. Blood flow F was estimated from the peak, and also using F=vb/MTT (MTT=mean transit time). Filtration Fraction FF=K trans /F. Perfusion mode (using tissue data up to the post-bolus dip) enabled F and vb to be estimated with minimal influence of filtration. MRI analysis: AIF's were found from the descending aorta. Blood T10 =1.4 s; parenchyma T10 =1.2 s. Relaxivity r1=4.5 s -1 mM -1 . Hct = 41%. Kidney volume was measured from ≈8 slices in each kidney. Normal group (n=15) mean and sd were found. Instrumental sd was found from the repeats using Bland Altmann analysis. Body Surface Area was estimated from BSA (m²) = 0.0235 Height(cm) 0.422 Weight(kg) 0.514 . 4 Standardised kidney volumes (for BSA=1.73m 2 ) were calculated. Results: Delayed exponential and gaussian GIRF's fitted better (rms residual ≈3-4%) than instant exponential; gaussian values are shown in the table. Cortical ROI's gave higher values of filtration, and lower MTT's, than parenchyma, as expected. However parenchymal vb (44%) and F were unexpectedly higher than cortical vb (35%) and F. Perfusion mode gave better fits that uptake mode (residuals ≈1% lower); however reproducibility and perfusion parameters were the same.