An Automated Mechatronic System for Registration of Three-Dimensional Ultrasound Images in Cervical Brachytherapy Procedures.

Abstract

High dose rate brachytherapy is a standard treatment for cervical cancer; however, intraoperative visualization remains limited by modality-specific constraints and operator variability. We describe a novel integrated mechatronic system for automated registration and fusion of three-dimensional transrectal (3D TRUS) and transabdominal (3D TAUS) ultrasound images, designed to improve image-guided cervical brachytherapy. The system combines a tracked, four-degree-of-freedom mechatronic 3D TAUS arm with an existing 3D TRUS device and introduces a mechanical referencing tool to enable spatial alignment between the two imaging systems. Forward kinematic tracking of the 3D TAUS arm was implemented using Denavit-Hartenberg parameters, with tracking errors corrected using a linear regression-based transformation. After correction, sub-millimeter tracking error was achieved across clinically relevant configurations. System performance, including geometric fidelity and volumetric accuracy, was evaluated using a custom multi-purpose imaging phantom. All observed geometric errors in the registered 3D US images were sub-millimeter, and volumetric measurements of agar-based inclusions deviated from nominal values by no more than 1.3 cm$^{3}$. The fused 3D TRUS and 3D TAUS images enhanced phantom visualization by mitigating shadow artifacts and expanding the field-of-view, enabling accurate segmentation. The proposed system enables reproducible, minimally operator-dependent ultrasound image fusion with clinically acceptable geometric and volumetric accuracy. This approach supports ultrasound-based image guidance and potential treatment planning in cervical brachytherapy, offering a cost-effective alternative to CT or MRI, particularly in settings with limited access to advanced imaging technologies.