Residual Oil‐Based Contrast Medium Mimicking a Metallic Foreign Body After Cesarean Section: Diagnostic Value of AI ‐Assisted 3D CT Analysis and Prenatal MRI

ABSTRACT A 36‐year‐old woman underwent an elective cesarean section at 37 weeks of gestation. A postoperative abdominal X‐ray unexpectedly revealed a 20 mm hyperattenuating pelvic lesion. Computed tomography (CT) demonstrated a dense structure exceeding 3000 Hounsfield units with metallic‐like artifacts, suggesting a retained metallic object. Intraoperative video review and instrument count confirmed no missing items. Three‐dimensional CT reconstruction and open‐source artificial intelligence (AI)–based shape analysis found no similarity to metallic instruments. Review of the patient's history revealed prior hysterosalpingography with oil‐based iodinated contrast medium. Retrospective assessment of prenatal magnetic resonance imaging (MRI) showed a cystic lesion with high signal intensity on T 1 ‐ and T 2 ‐weighted images, with signal suppression on fat‐suppressed T 1 ‐weighted imaging, consistent with fatty components. The lesion was diagnosed as a pseudocyst due to residual contrast medium. This case demonstrates that oil‐based contrast can mimic metallic density, and that AI‐assisted three‐dimensional CT and MRI review can prevent unnecessary reoperation.