To demonstrate in a porcine model that reflux during embolotherapy can be relatively quantified (ie, as embolization efficiency) and that nontarget embolization can be eliminated by using an antireflux microcatheter. Renal artery embolization was performed with radiopaque tantalum microspheres (concentration of 1 g/20 mL) in three swine. Second-order right renal arteries (n = 3) underwent embolization with a 3-F antireflux catheter, and second-order left renal arteries (n = 3) underwent embolization with a 4-F end-hole catheter as a control. After embolization, kidneys were explanted and underwent micro-computed tomographic (microCT) imaging. Three-dimensional volumetric and multiplanar imaging of the kidneys was performed to assess vascular distribution. Digital Imaging and Communications in Medicine data were analyzed, with a threshold algorithm used to create binary images. The number of positive values in a region of interest in the target embolized tissue (upper pole or lower pole) and the nontarget adjacent tissue was determined, and embolization efficiency was calculated. Wilcoxon rank-sum statistical analysis was performed to compare nontarget embolization between infusion catheters. All renal arteries underwent successful embolization with tantalum microspheres, with 20 mL (1 g) administered in all dose deliveries. MicroCT provided high-resolution visualization of the renal parenchyma at 70-μm resolution. In control renal arteries, a standard 4-F end-hole catheter had an embolization efficiency of 72%± 13. In experimental renal arteries, the antireflux microcatheter had an embolization efficiency of 99.9%± 1.0 (P< . 05). A significant decrease in nontarget embolization (ie, reduction in reflux) was possible with an antireflux microcatheter compared with a conventional end-hole catheter.