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A Novel Method of Identifying Ureteropelvic Junction Obstruction: A Comparative Study of Imaging Modalities in Assessing Renal Function and Degree of Obstruction in a Swine Model.
Courtney K. Rowe, M.D., Brian J. Minnillo, M.D., Young K. Hong, M.D., Alan B. Retik, M.D., Hiep T. Nguyen, M.D.
Children's Hospital Boston, Boston, MA

Purpose: There remains no gold standard for diagnosing clinically significant ureteropelvic junction obstruction (UPJO). Established radiological modalities differentiate varying degrees of obstruction; however they have limitations in functionality, applicability, and/or comprehensiveness. Furthermore, there is often radiation exposure, which has long-term consequences especially in children. Current research to improve imaging modalities for UPJO, such as Doppler-US, gadolinium-DTPA MRI, and contrast-enhanced CT renography, has been unfruitful or insufficient. We propose the novel use of dynamic fluorescent imaging of the urinary tract in assessing UPJO, enabling more accurate diagnostic and prognostic determination than currently available imaging modalities.

Methods: Twenty swine underwent partial or complete unilateral ureteral obstruction: 5 complete obstruction, 5 partial, 5 complete obstruction for 2 days followed by release of obstruction, and 5 sham. Groups were survived for 3-7 days(short term), 10-14 days(long term), and 28-30 days (chronic dilation without obstruction). Imaging with US, MAG-3 renography, MRI urography, and fluorescence was performed after ligation. Renal fluorescence parameters (perfusion time, peak time, relative renal fluorescence (RRf), ureteral transit time, fluorescent intensity) were calculated and compared with each radiologic parameter (i.e. t½, %residual).
Results: We demonstrated that partial and complete obstruction could be validated and distinguished by US, MAG-3 and MRI findings and confirmed by histology. Using fluorescence imaging, distinct vascular and urinary parameters were identified in partial and complete obstruction compared to controls. Subjective assessment of images obtained from fluorescent imaging provided anatomic details similar to MRI and US. Unlike IVP, CT and fluoroscopy, detailed visualization of urinary flow through the area of obstruction could be performed continuously without concern of radiation exposure (Figure1). Measurement of fluorescent parameters provided a more objective assessment of renal function and degree of obstruction similar to renography and MRI. Perfusion time and peak time increased significantly with the degree of obstruction, demonstrating the decreased function of the affected kidney. Moreover, the degree of obstruction was correlated with decreasing absolute renal fluorescence emitted from the obstructed kidney and decreasing RRf of the obstructed kidney, similar to the findings on renography (Figure2). These parameters are dependent on renal blood flow and function.

Figure 1: Genhance™ probe demonstrating urine flow progression from proximal to distal ureter in a swine with partial obstruction (top left to bottom right).
Figure 2: Comparison of Mean RRF(MAG-3) vs. RRf (Genhance)

Conclusion: Using a large animal model that is rapidly translatable to clinical practice, we demonstrate that dynamic fluorescent imaging can determine the degree of ureteral obstruction based upon subjective (real-time visualization of urinary flow) and objective measurements. Fluorescent imaging combines the advantages of current imaging modalities to provide a potentially more accurate method of assessing structural and functional implications of clinically significant UPJO.

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