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Aberrant 131I orthoiodohippurate renal plasma flow values were obtained when our image-based method was moved from an old to a new gamma camera. We suspected inadequate collimation as the reason for this problem because we observed increased septal penetration and scatter. Camera effects were largely eliminated because both our old and new cameras had W thick crystals. We report here the effects of two high and three medium energy collimators upon the fraction of syringe image count beyond the syringe image, and upon the correlation of these non-image count fractions with geometric collimator parameters. We also report the effects of these same collimators upon kidney-to-background count ratios, using patient relevant abdominal phantoms. The percentage counts beyond the syringe image varied from 33 to 57. The phantom kidney-to-background ratios varied from 3.7 to 2.0. We conclude that image-based quantitative measurements are strongly dependent on collimator selection. The data also provide a user basis for collimator evaluation, selection and design, and have implications for 131I and 111In antibody imaging.