Assessment of spinal curvatures in static postures using localized 3D ultrasound: A proof-of-concept study

Abstract

<div>Measurement of vertebral spinal motion is a difficult endeavor given the lack of reachable bony landmarks for placement of reflective markers for common optoelectronic systems and large skin movement artifacts along the spine affecting the accuracy of surface-mounted techniques. The objective of this proof-of-concept study was to develop an experimental framework to assess spinal curvatures with high accuracy by using ultrasound (US) imaging. A phantom spine model (T1-T12) immersed in water was scanned using both Magnetic resonance imaging (MRI) and three dimensional (3D) US. The US scans were obtained for the phantom spine in different postures while the US probe was localized by three electromagnetic tracking sensors (NDI system) attached to the probe head and later identified on the acquired US volumes via visual inspection. Spinal landmarks were selected on the US images and validated using US-MRI registration. Their position was then related to the position of the tracking sensors, enabling the precise localization of landmarks and reconstruction of spinal curvatures in a global reference frame. The obtained anatomical reconstructions were further refined using an automatic registration algorithm and validated by an experienced sonographer (ground-truth solution). Results of this study show that localized 3D US enables tracking of spinal landmarks and allows for an accurate reconstruction of static postures. A high reliability in sensor identification of 0.49 mm was found on average while the inter-observer variability was strongly correlated to the intra-observer error with maximum deviance of 0.8 mm. The NDI-based spine reconstruction had a mean error of 5.6 mm compared to the ground-truth, which is significantly lower than errors obtained by current gold standards (>10 mm). Combining US probe tracking and semi-automatic registration methods anatomical reconstructions comparable to clinical standards were obtained. This study implies that US holds the potential for advancing vertebral motion assessment and the reconstruction of spinal curvatures. In future, this could overcome limitations of marker-based approaches.</div>