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Figure 8 SI-time curves obtained in a patient with left urinary obstruction. (A) Anatomic image showing dilatation of the left pyelocaliceal system. (B) SI-time curves obtained from regions-of-interest drawn on the entire renal parenchyma (excluding pyelocaliceal system) on each side, showing three phases: a first abrupt ascending segment followed by a first peak, corresponding to the "vascular-to-glomerular first-pass" or cortical vascular phase; a second slowly ascending segment, ended by a second peak, corresponding to the glomerulotubular phase; and a slowly descending segment, corresponding to the predominant excretory function and the so-called ''excretory phase.'' Areas under the curve have been drawn to calculate split renal function. Abbreviation: SI, signal intensity.

clearance is not reliable (low muscle mass, obese, etc.) (46), and clinical protocols requiring a reliable and reproducible renal function estimation.

GFR measurement can be obtained by two types of techniques.

1. Creatinine clearance estimated from formulae that avoid urinary collection, such as the Cockcroft and Gault and the Modification of Diet in Renal Disease study. However, these formulae have several limitations related to variations of plasma creatinine level, which are around 10%; tubular secretion of creatinine in advanced renal failure may lead to overesti-mation of GFR and reduction of creatinine excretion that occurs with age due to a decrease in skeletal muscle mass. Furthermore, in acute or rapidly progressing renal failure, these formulae provide inaccurate information when GFR is rapidly changing (47).

2. Tracer clearance, using either nonradioactive (such as inulin or iothala-mate) or radioactive tracers (51Cr-ethylenediamine tetraacetic acid or 99mTc-DTPA). These procedures are not widely used because they are time consuming and require several blood samplings, with or without urine collection.

SKGFR Based on Intrarenal Kinetics. The requirements to quantify GFR are identical to those previously listed for perfusion measurements using T1 agents. By introducing advanced models that take into account both the first passage of blood and the filtration part, it is possible to simultaneously calculate both RBF and singlekidney GFR (SKGFR).

To calculate GFR, several compartmental models have been proposed: the Rutland-Patlak model (16,48), the cortical compartment model (49), and an extensive multicompartmental model (45) including several cortical and medullary compartment models. According to Annet et al. (50), the cortical compartment model seems to provide more accurate results than the Patlak plot applied to the cortex alone or to the entire parenchyma. However, results from these studies are precluded by some limitations, which include nonuniform SI either from section to section (48) or between time-points (45), inaccurate or absence of movement correction, and inadequate conversion of SI into concentration (49). Developments are progressing for using sequences providing uniform SI in space and time, for applying automatic movement correction to improve the quality of SI time curves, and to display 3-D maps of GFR on a voxel-by-voxel basis (Fig. 9). However, these methods still require validation with reference methods in large human populations with normal and decreased renal function.

Single-Kidney Extraction Fraction. A quantitative method of in vivo measurement of the single-kidney extraction fraction (EF) and SKGFR was proposed by Dumoulin et al. (51) and applied to experimental studies by Niendorf et al. (52-54). This method is based on the measurement of T1 within flowing arterial and venous blood (Look-Locker method) during continuous Gd infusion where the steady-state equation can be used:

where RBF is the renal blood flow and Hct is the hematocrit value of the blood. The EF is calculated from the equation:

Figure 9 GFR map of a kidney transplant obtained after dynamic T1w Gd-enhanced sequence and application of the Rutland-Patlak plot on a voxel-by voxel basis. Abbreviations: T1w, T1-weighted; GFR, glomerular filtration; Gd, gadolinium.

where T1a is T1 in the renal artery, T1v is T1 in the vein, and T10 is the T1 in the blood without Gd. Once EF is calculated for each kidney, values of SKGFR can be calculated if RBF is known. Preliminary animal studies have shown concordant results compared to those calculated from inulin clearance.

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