PD48-06: Mean transit time: a novel marker to identify obstructive uropathy in the setting hydronephrosis using functional magnetic resonance urography.

Karmon Janssen, Richard Jones, Damien Grattan-Smith, Andrew Kirsch, Jennifer Lovin*, Atlanta, GA

Introduction: The physiology of hydronephrosis is complex, and a well-established definition of obstruction is lacking. Functional magnetic resonance urography (fMRU) has the capacity to use multi-compartment models of tracer kinetics to assess renal pathophysiology. Mean transit time (MTT) is a mathematical calculation of the contrast agent in the tubular compartment and characterizes the passage of the agent through the tubules. The purpose of this study was to define MTT in normal kidneys and to assess its use in children with ureteropelvic junction obstruction (UPJO) before and after pyeloplasty.

Methods: We retrospectively evaluated the MTT in 38 children with normal appearing renal units (n=76). There were 14 boys and 24 girls with ages ranging from 0.3 to 17.4 years. The MTT was calculated using the two-compartment Annet model of renal tracer kinetics. We then compared the normal renal units (n=76) to children with unilateral hydronephrosis and delayed dense nephrograms (n= 37 renal units) on fMRU who underwent successful pyeloplasty.

Results: The mean value of MTT for normal renal units (n=76) was 53.9+9.4 (s). There was no correlation between MTT and renal volume. In the 37 renal units with delayed dense nephrograms and a contralateral normal kidney, the mean MTT of the preoperative hydronephrotic kidney was 149±88 (s), while the contralateral normal kidney the mean MTT was 48±14 (s). After successful pyeloplasty, the mean MTT was 47.9±14 (s) and 59±21 (s) (p < 0.001) for the hydronephrotic and contralateral kidneys, respectively (Graph 1). Preoperative and postoperative MTT in the normal kidneys were statistically different (p=0.004).

Conclusions: The MTT is a novel functional parameter that is thought to be a marker of tubular dysfunction and is shown to measure reversible hemodynamic effects associated with renal obstruction. The ability to accurately measure normal MTT is robust. While MTT is significantly delayed in UPJO, MTT returns to normal following successful pyeloplasty. The prolonged MTT in the normal kidney following pyeloplasty is thought to be related to a shift in renal perfusion to the previously obstructed kidney. MTT has the potential to quantitate severity of obstruction.

Source of Funding: none