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Normalized Residual Activity: Usual Values and Robustness of the Method

Department of Radioisotopes, Centre Hospitalo-Universitaire St. Pierre, Brussels, Belgium

The objectives of this study were 2-fold: first, to investigate the robustness of the normalized residual activity (NORA), a parameter that has recently been proposed for the estimation of renal emptying during renography; and second, to define the usual values of NORA in 2 categories of kidneys—those with a normal renogram and those that are dilated but definitely unobstructed. Methods: NORA was defined as the renal activity at a given moment (end of renogram, end of furosemide acquisition, image after micturition) divided by the renal activity between 1 and 2 min. Two variables that might influence the results of NORA were evaluated: the choice of background correction, and an error in the estimation of the 1- to 2-min renal activity. To estimate the values of NORA in usual clinical conditions, 2 sets of data were analyzed: normal kidneys with a normal renogram pattern, and dilated but definitely unobstructed kidneys. Results: Using a perirenal or a subrenal background correction, NORA was, on average, 67% or 83%, respectively, of the value obtained without background correction. The use of a renal activity of 1 min 20 s to 2 min 20 s instead of a 1- to 2-min activity resulted in a systematic 10%–15% underestimation of NORA. The 90th percentile values of NORA were, in the normal group, 0.70 at 20 min, 0.23 at the end of the furosemide test, and 0.10 after micturition. In the kidneys that had undergone surgery, the 90th percentile values were 3.92 at 20 min, 2.91 at the end of the furosemide test, and 1.99 after micturition. A good correlation was observed between NORA and output efficiency. Conclusion: If adequately standardized, NORA is a robust and simple parameter that allows evaluation of renal emptying at any time of the acquisition. One should be aware of the fact that high NORA values, corresponding to poor renal emptying, may be observed in the operated unobstructed kidneys, even after micturition.

Key Words: renogram • normalized residual activity • renal drainage • renal output • hydronephrosis

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