Ultrasound-Guided Real-Time Spinal Motion Visualization for Spinal Instability Assessment

TL;DR

This work addresses the need for real-time, three-dimensional visualization of spinal motion with reduced radiation exposure. It couples a single preoperative CBCT spine model with intraoperative robotic ultrasound, using a coarse-to-fine registration and a hierarchical, attenuated vertebral motion framework to simulate bending via a parameter s ∈ [−S_{max}, S_{max}]. Registration refinement with axis-biased ICP and motion tracking through RAFT optical flow enable continuous 3D visualization by interpolating poses between neutral and maximally bent configurations. Validation on a bendable spine phantom shows clinically acceptable registration accuracy (mean ~2.67 mm, median ~1.94 mm) and motion-estimation errors (mean ~3.17 mm, median ~2.24 mm), with improved intermediate-state accuracy (mean ~2.64 mm, median ~2.01 mm). The proposed radiation-reduced system offers a promising 3D alternative to dynamic X-ray for assessing spinal instability, though more extensive in-vivo validation is needed.

Abstract

Purpose: Spinal instability is a widespread condition that causes pain, fatigue, and restricted mobility, profoundly affecting patients' quality of life. In clinical practice, the gold standard for diagnosis is dynamic X-ray imaging. However, X-ray provides only 2D motion information, while 3D modalities such as computed tomography (CT) or cone beam computed tomography (CBCT) cannot efficiently capture motion. Therefore, there is a need for a system capable of visualizing real-time 3D spinal motion while minimizing radiation exposure. Methods: We propose ultrasound as an auxiliary modality for 3D spine visualization. Due to acoustic limitations, ultrasound captures only the superficial spinal surface. Therefore, the partially compounded ultrasound volume is registered to preoperative 3D imaging. In this study, CBCT provides the neutral spine configuration, while robotic ultrasound acquisition is performed at maximal spinal bending. A kinematic model is applied to the CBCT-derived spine model for coarse registration, followed by ICP for fine registration, with kinematic parameters optimized based on the registration results. Real-time ultrasound motion tracking is then used to estimate continuous 3D spinal motion by interpolating between the neutral and maximally bent states. Results: The pipeline was evaluated on a bendable 3D-printed lumbar spine phantom. The registration error was $1.941 \pm 0.199$ mm and the interpolated spinal motion error was $2.01 \pm 0.309$ mm (median). Conclusion: The proposed robotic ultrasound framework enables radiation-reduced, real-time 3D visualization of spinal motion, offering a promising 3D alternative to conventional dynamic X-ray imaging for assessing spinal instability.

Figures (3)

• Figure 1: Overview of our proposed system.(a) Experimental setup and movable spine phantom. (b) Pre-calibration procedure. (c) Ultrasound acquisition and segmentation at maximal spinal bending with 3D compounding. (d) A kinematic model was initialized and applied to the CBCT volume to simulate spinal motion. (e) Workflow of coarse and fine registration, kinematic parameter optimization, and motion tracking using RAFT optical flow. (f) Comparison of compounded ultrasound, bent CBCT model, and ground truth visualization.
• Figure 2: Transformation chain of system calibration. This chain illustrates the relationships among the coordinate systems. Specifically, {R} denotes the robot frame, {U} the ultrasound frame, {C} the CBCT volume frame, and {O} the LoopX optical tracking camera frame.
• Figure 3: Visualization of registration. (a), (b), (c) show the right lateral, left lateral, and flexion bending, respectively. The green line indicates the bending direction in each state.