Fig. 1.

Magnetic resonance urogram in a 3-month-old boy with antenatal hydronephrosis. a Axial T2-weighted MR image shows mild dilatation of the left renal pelvis without caliectasis. The renal parenchyma appears normal, with preservation of the corticomedullary differentiation and normal increased signal intensity in the renal medulla. The assessment of the renal parenchyma is the first step in interpreting quantitative renal MR data. b Coronal post-contrast T1-weighted maximum-intensity projection (MIP) MR image derived from the early cortical phase shows contrast agent within the aorta as well as brisk and symmetrical enhancement of the renal parenchyma. The initial contrast enhancement of the kidneys occurs predominantly from renal perfusion, with the contrast agent localized to the vascular compartments of the kidneys. This is followed by gradual increase in parenchymal enhancement secondary to glomerular filtration of the contrast agent and tubular reabsorption of water. The renal outlines are smooth and the kidneys’ size is similar. c Coronal post-contrast T1-weighted MIP image derived from the late nephrographic phase, obtained 45 s later than the arterial phase seen in (b), demonstrates enhancement of both the cortex and medulla. The gadolinium concentration in the vascular compartment is decreasing while the contrast agent in the nephron is increasing and becoming more concentrated. The signal intensity of the medulla exceeds that of the cortex as a result of tubular concentration. d Coronal post-contrast T1-weighted MIP image acquired 60 s after the vascular phase shows initial excretion into the calyces. The appearance of the contrast agent in the calyces is essentially simultaneous, demonstrating no evidence of increased pressure in the renal pelvis. The mean transit time (MTT) on the right was 54 s and on the left 56 s, both within the normal range. The calyceal transit time (CTT) is a visual marker used in initial classification of a hydronephrotic kidney that can be quantified using the MTT. e Maximum-intensity projection image acquired 10 min after the initial vascular phase shows contrast agent filling the renal pelvis and ureters bilaterally. The right side is normal, with a renal transit time of 2 min 45 s. There is mild dilatation of the left renal pelvis with transition in caliber at the level of the ureteropelvic junction (UPJ) and a normal-size left ureter. The renal transit time on the left was slightly delayed when compared to the right, with contrast agent appearing in the left ureter at 3 min 30 s. This was in the normal range and no evidence suggested physiologically significant obstruction. f Normal signal intensity versus time curves for both kidneys demonstrate similar morphology reflecting the perfusion, concentration and excretion of the contrast agent. It is important to remember that the signal intensity curves represent only contrast agent within the renal parenchyma because any signal in the collecting system is excluded from the analysis. The red curve represents the signal in the aorta and is reflective of the contrast bolus with abrupt initial upstroke followed by an exponential decay curve. The initial upstroke of the curves for both kidneys (A) is related to renal perfusion. The flatter but still increasing second portion of the curve (B) represents glomerular filtration and tubular concentration of the contrast agent. Once the contrast agent begins to be excreted, there is a gentle decrease in the parenchymal contrast agent, paralleling the signal in the aorta (C). The quantitative measures of renal function were normal: vDRF L:R = 49:51; pDRF L:R = 48:52; unit Patlak L:R = 0.27:0.28 mL/min/cm³; asymmetry index = 0.0; MTT, L=56 s, R=52 s. L left, pDRF Patlak differential renal function, R right, vDRF volumetric differential renal function