Purpose: To develop and characterize radiopaque embolization microspheres capable of in vivo detection with intraprocedural fluoroscopy and computed tomography (CT) imaging and to evaluate their spatial distribution inside target tissues during and after transcatheter embolization. Materials and Methods: Polyvinyl alcohol hydrogel microspheres were loaded with Lipiodol and examined for iodine content, stability of loading, and conspicuity with fluoroscopy and CT in vitro. Transcatheter embolization of swine liver and kidney was performed with the radiopaque microspheres and spatial distribution was evaluated with intraprocedural fluoroscopy and CT. Ex vivo evaluation was performed with light microscopy and micro-CT. Results: In vitro analyses demonstrated that radiopaque microspheres could be loaded with sufficient iodine content to be detected with routine fluoroscopy and CT imaging and that such loading was relatively stable. Radiopaque microspheres were visible in vivo with fluoroscopy and CT during transcatheter embolization. CT imaging during embolization procedures demonstrated a dose-dependent relationship in the number and size of visualized embolized arteries. Imaging features of radiopaque microsphere distribution inside target tissues correlated well with ex vivo light microscopic and micro-CT evaluation of microsphere distribution. Conclusions: Radiopaque embolization microspheres are visualized during transcatheter embolization with routine intraprocedural fluoroscopy and CT. These radiopaque microspheres provided the three-dimensional spatial distribution of embolic material inside target organs during the procedure, and therefore can provide real-time intraprocedural feedback for the interventional radiologist. These microspheres may be useful for demonstrating the influence of material and technical variability in transcatheter embolization in addition to providing intraprocedural identification of tissue at risk of undertreatment. © 2010 SIR.