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.

翻译：目的：脊柱不稳是一种普遍存在的病症，会导致疼痛、疲劳和活动受限，严重影响患者的生活质量。在临床实践中，诊断的金标准是动态X射线成像。然而，X射线仅能提供二维运动信息，而计算机断层扫描（CT）或锥形束计算机断层扫描（CBCT）等三维模态无法有效捕捉运动。因此，需要一种能够在最小化辐射暴露的同时实现实时三维脊柱运动可视化的系统。方法：我们提出将超声作为三维脊柱可视化的辅助模态。由于声学限制，超声仅能捕捉脊柱浅表表面。因此，将部分复合的超声体积与术前三维成像进行配准。在本研究中，CBCT提供中立位脊柱构型，而机器人超声采集则在脊柱最大弯曲时进行。首先将运动学模型应用于CBCT导出的脊柱模型进行粗配准，随后使用迭代最近点算法进行精配准，并根据配准结果优化运动学参数。然后，利用实时超声运动跟踪，通过在中立位和最大弯曲状态之间进行插值，来估计连续的三维脊柱运动。结果：该流程在一个可弯曲的3D打印腰椎体模上进行了评估。配准误差为$1.941 \pm 0.199$ mm，插值脊柱运动误差为$2.01 \pm 0.309$ mm（中位数）。结论：所提出的机器人超声框架能够实现辐射剂量降低的实时三维脊柱运动可视化，为评估脊柱不稳提供了一种有前景的、可替代传统动态X射线成像的三维方案。