The Importance of Obtaining Conjugate Views on Renographic Evaluation of Large Hydronephrotic Kidneys: An In Vitro and Ex Vivo Analysis

PURPOSE: It is postulated that significant obstruction leads to decreased ipsilateral renal function. However, maintained or even increased differential renal function is often seen in patients with large hydronephrotic kidneys. The reason for such a phenomenon is unclear. We designed an in vitro and ex vivo experimental model that permits the controlled orientation of thinned renal parenchyma and background activity during gamma camera detection. MATERIALS AND METHODS: A 5-step experimental study was designed with balloons or reservoirs containing known amounts of radioisotope. A balloon was incrementally compressed between 2 Plexiglas sheets (step 1). Simultaneous measurement of radio emission was done of 5 glass beakers of varying diameters filled with the same amount of water and (99m)Tc (step 2). A single beaker containing diluted (99m)Tc was interfaced with air (step 3) or water (step 4) at different distances from the gamma camera. Pig kidneys previously injected with dimercapto-succinic acid were removed and scanned (step 5). One of the kidneys was then progressively sliced, thinning its parenchyma, and sliced and nonsliced kidneys were simultaneously scanned again. RESULTS: Progressively increased counts were detected as the projected surface area of the radioactive balloons (step 1) and beakers (step 2) increased. Counts detected were inversely proportional to the amount of water interfaced between the container and the gamma camera. Significantly more radiation was detectable in sliced kidneys containing dimercapto-succinic acid than in intact control kidneys. CONCLUSIONS: These findings suggest that the artifactual supranormal differential renal function that is observed during renographic evaluation of large hydronephrotic kidneys actually exists. Parenchymal proximity and distribution in relation to the pelvis are critical determinants. These findings suggest that the conjugate view technique may be more suitable for renographic evaluation of large hydronephrotic kidneys.