Background <p>Intraoperative cholangiography (IOC) is widely used to delineate biliary anatomy and confirm ductal integrity during liver resection. However, conventional radiographic IOC requires a C-arm set-up, may interrupt the operative workflow, and exposes patients and operating-room personnel to ionizing radiation. Contrast-enhanced intraoperative ultrasonic cholangiography (CE-IOUSC) combines intrabiliary injection of highly diluted perfluorobutane microbubbles (Sonazoid^®) with contrast harmonic ultrasonography, enabling real-time visualization that can be repeated on demand without radiation. We hypothesize that CE-IOUSC provides superior procedural convenience while maintaining biliary visualization and safety.</p> Methods <p>This is a single-center, open-label, randomized, parallel-group, phase III comparative trial. Adults (≥ 20 years) scheduled for liver resection, including living donors, are eligible when cholecystectomy and transcystic catheterization via the cystic duct are feasible. After informed consent, participants are randomized 1:1 (stratified by liver condition: normal vs. impaired) to CE-IOUSC or conventional radiographic IOC using a computer-generated sequence. In both arms, cholangiography is performed twice—before and after bile duct division—with additional imaging permitted as clinically required. Conventional IOC uses diluted iothalamate contrast (Urografin^®) under C-arm fluoroscopy. CE-IOUSC uses 1,000-fold diluted Sonazoid^® administered through the cystic duct and evaluated using intraoperative ultrasonography in contrast mode. Cross-over “rescue” cholangiography with the alternative modality is allowed when the assigned method is unsuccessful or inadequate. The primary endpoint is the cumulative time required to complete intraoperative cholangiography, including any additional imaging and repeated attempts. Secondary endpoints include surgeon-rated usability (5-point Likert questionnaire completed by the primary operator immediately after surgery), detectability of biliary structures (segmental visualization rates assessed by predefined criteria), postoperative biliary complications (bile leak or stricture graded by Clavien–Dindo), cross-over rescue efficacy, and safety outcomes (procedure-related adverse events and perioperative laboratory changes). Outcomes will be analyzed primarily on an intention-to-treat basis.</p> Discussion <p>This trial will quantify whether CE-IOUSC improves the efficiency and usability of intraoperative biliary imaging without compromising visualization or patient safety. The results will inform evidence-based adoption of ultrasound-based cholangiography as a practical alternative to conventional radiographic IOC in liver resection.</p> Trial registration <p>Japan Registry of Clinical Trials (jRCT), jRCTs051230202.</p>