Renal transit time

The plasma disappearance of Tc-DTPA can be described by a double exponential function (McAfee et al. 1979). One component (0.99) is excreted with a biological halftime of 100 min, the rest (0.01) with a half-time of 7 days. The fraction excreted by the kidneys is 1.0, and the renal transit time is 5 min. For abnormal cases, the retention half-time of the major component is increased (1,000 min) and so is the transit time (20 min). 

The kidneys, bladder wall, and adrenals are most exposed organs. MAG3 is rapidly distributed in the extracellular fluid and excreted entirely by the renal system. The renal transit time is approximately 4 min, as is for OlH (Bubeck et al. 1987). 

When renal function is bilaterally impaired, it is assumed that the clearance rate of MAG3 is one tenth of that for the normal case, the renal transit time is increased to 20 min, and a fraction of 0.04 is taken up in the liver. 

Fig.1.2.1a-f. Normal MR urogram in 3-month-old boy with antenatal hydronephrosis. Images a-c show same slice from each of three separate volume acquisitions, whereas d-f show MIP projections derived from the same three separate time points, a and d show the cortical phase, b and e were acquired 60 s later and demonstrate enhancement of both the cortex and medulla with the signal intensity of the medulla exceeding the cortex, c and f were acquired 115 s after the vascular phase and show excretion into the calyces, renal pelvis and ureters. The renal transit time was 2 min and 20 s bilaterally and the volumetric DRF was 51 49... 

Fig. 1.2.5a-c. Renal transit time calculation. The renal transit time is calculated from the MIP images of each volume acquisition. Each acquisition has a unique time stamp depending on the MR vendor. Initially, the time of cortical perfusion is determined as 160215 (a) and is subtracted from the time points when contrast is identified in the proximal ureter. This is done for both kidneys (R=160407 and L=160615) so that the RTT is calculated on the right as 1 min and 58 s (b) and 4 min on the left (c)... 

Fig. 1.2.11a,b. A 3-month-old boy with non-obstructive megaureter and simple ureteroceles, a Volume-rendered image showing bilateral hydroureteronephrosis. The renal transit time was normal bilaterally, b Detailed imaging of the ureterovesical junction showing simple ureteroceles... 

Hydronephrotic kidneys that cannot accommodate a fluid challenge develop signs of decompensation on MR urography. These include a delayed, dense nephrogram, delayed calyceal and renal transit times and a difference between the pDRF and vDRF >4. 

Fig. 1.2.13a,b. Ectopic, hydronephrotic and hypoplastic kidney, a Coronal post-contrast MIP showing a pelvic kidney on the right. The vDRF was calculated at 32% on the right, b Delayed images demonstrate hydronephrosis, but no evidence of obstruction was seen with a renal transit time less than 4 min... 

Jones RA, Perez-Brayfield MR, Kirsch AJ, Grattan-Smith JD (2004) Renal transit time With MR urography in children. Radiology 233 41-50... 

Renal blood (or plasma) flow (p-aminohippuric acid clearance or ultrasonic transit time flowmetry)... 

Test results are affected by defective intestinal absorption caused by intestinal disease or variations in transit time, by impaired hepatic conjugation caused by liver disease, and by impaired renal excretion. To compensate for these possible errors, a control substance (e.g., PABA) may be given on a second day, or alternatively, C-PABA or p-aminosalicylic acid (PAS) can be given orally with the NBT-PABA. Low recovery of the control substance in the urine indicates probable decreased intestinal absorption or decreased renal excretion. About 60% (range 48% to 72%) of the orally administered dose is recovered in the urine normally. In pancreatic insufficiency, PABA excretion is significantly decreased. The result is then calculated as a pancreatic excretion index (PEI) as follows ... 

These equations contain the usual input-output terms of compartmental mass balances and also a simple first-order renal clearance, which is close to inulin clearance for MTX. The Ri are tissue-to-plasma distribution ratios to account for protein binding. The volumes V, and flows, Q, are known from recorded anatomy and physiology. Other parameters are defined as follows kK, renal clearance kL, saturable rate of drug transport into bile KLy saturation constant for bile transport ko, saturable rate of intestinal absorption Kq, saturation constant for intestinal absorption , nonsaturable rate of intestinal absorption kF, reciprocal of nominal transit time in small intestine. 

Other parameters are of great interest for a global approach of renal safety fractional excretion of Na+ and K+, renal blood flow (p-aminohip-puric acid clearance or ultrasonic transit-time flowmetry), enzymuria (which could allow differential location of toxic injuries), proteinuria. 

The sites of microinjection were determined by measurement of lis-samine green tubular transit times (8). Three puncture sites were taken into consideration. "Early" and "late" proximal respectively correspond to the first or second and the last proximal convolution to appear on the renal surface and "distal" was a superficial convolution of a distal tubule. A microinjection was considered to be technically satisfactory if the rate of injection was maintained constant, no reflux and no visible tubular dilatation occurred and if recovery of injected inulin was 100 jh 5 %. The rate of microinjection was adjusted to approximate single nephron filtration rates. 

For each patient the key features derived from MR urography include calculation of differential renal function (both volume and Patlak) (vDRF and pDRF), signal versus time curves for each kidney and the aorta, individual kidney GFR index of each kidney, concentration and excretion from each compartment, renal and calyceal transit times and overall anatomic diagnosis. It is important to understand that we have two methods to determine the differential renal function one based on volume and one based on the individual kidney GFR as determined by the Patlak plot. In most cases these are symmetric however, when there is a difference in these two measures of DRF it implies a change in renal hemodynamics that may ultimately provide information about which kidneys will benefit from surgery. 

The parameters routinely assessed by MR urography include the overall anatomic diagnosis, vDRF and pDRF, renal and calyceal transit times, signal intensity versus time curves and individual GFR index for each kidney. 

The second parameter that maybe obtained from a renographic curve is the evaluation of tracer molecule transport through the entire nephron, known as the transit time. A normal transit time (approximately 3 min) excludes renal obstruction. Abnormal transit time indicates urine statis. It is not possible on the basis of a prolonged transit time to differentiate obstruction from simple dilatation of the collecting system. 

Pseudothrombosis of the iliac vein (Fig. 7.28) has been described following simultaneous pancreas-kidney transplantation with bilateral revascularization to the respective iliac vessels (Gupta et al. 2002). Pseudothrombosis results from delayed venous opacification of the iliac vein ipsilateral to the pancreatic graft as compared to the contralateral side of the renal graft. This phenomenon results from longer transit time and reduced blood flow to the pancreas as compared to the kidney. Pseudothrombosis can also involve the ipsilateral iliac vein below the vascular anastomoses of the pancre-... 

Moderate atypia and atypical hyperplasia of the urothelium were first described in 4 pieces of nephroureterectomies performed in 3 CHN patients prior or at time of transplantation [28]. Then, three cases of cancers of the urinary tract were reported the first case, a 28 year old woman with CHN, developed two papillary transitional cell carcinomas in the posterior bladder wall 12 months after a renal transplantation [31] the second case, a 42 year old woman with CHN, presented with hematuria secondary to a papillary transitional cell carcinoma of the right pelvis [32]. The third case was a 49 year old woman previously published as... 

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