Three-dimensional assessment of scoliosis based on ultrasound data

Abstract

ABSTRACT In this study, an approach was proposed to assess the 3D scoliotic deformity based on ultrasound data. The 3D spine model was reconstructed by using a freehand 3D ultrasound imaging system. The geometric torsion was then calculated from the reconstructed spine model. A thoracic spine phantom set at a given pose was used in the experiment. The geometric torsion of the spine phantom calculated from the freehand ultrasound imaging system was 0.041 mm -1 which was close to that calculated from th e biplanar radiographs (0.025 mm -1 ). Therefore, ultrasound is a promising technique for the 3D assessment of scoliosis. Keywords: Geometric torsion, freehand 3D ultrasound, scoliosis, spine. 1. INTRODUCTION Idiopathic scoliosis is a spinal disorder that generates a complex three-dimensional (3D) deformation of the spine for which there is no known cause [1]. About 2-4% of the adolescent population has some degree of scoliosis [2]. Currently, the Cobb angle [3], measured on two-dimensional (2D) radiogra phs, is the gold standard to evaluate scoliosis severity. However, because of the 3D nature of scoliosis, the 3D index of scoliotic deformity is clinically important. The geometric torsion is a 3D measurement. Po ncet et al. [4] evaluated the relevance of geometric torsion as a 3D index of scoliosis. Their study demonstrates that geometric torsion is an intrinsic property of a curved line and can be used to characterize the 3D shape of the scoliotic spine. Our previous study [5] also demonstrates that 3D geometric torsion reveals structural differences that are not apparent in the Cobb measurement. To calculate the geometric torsion 3D reconstruction of the spine is needed. Accurate 3D reconstruction can be performed using computed tomography (CT). However, a large number of scans are required to generate a 3D image of the whole spine with adequate resolution, which subjects the patient to a considerable radiation dose. Many studies developed approaches to reconstruct the 3D spine from radiographs. These X-ray based techniques still inevitably expose patients to harmful ionizing radiation. Spine reconstruction can also be performed with magnetic resonance imaging (MRI). Once surgery has been performed, though, the presen ce of nonferromagnetic metallic implants produces artifacts and creates suboptimal images. Also, the use of MRI is costly. Therefore, ultrasound is an attractive method because it is cost effective, portable, easy to operate, and has no radiation risk. Cheung et al. [6] developed a 3D ultrasound system for scoliosis assessment. They reconstructed the 3D spinal model from the landmarks assigned on the spinous processes and laminae on ultrasound images. Their study compared the spine deformation angle measured by X-ray Cobb’s method w ith that by the projected ma rkers from the 3D spine, and showed that the mean different between the results obtained by the two methods was 2.2°. Nguyen et al. [7] developed an image processing method to reconstruct the posterior surf ace of vertebrae from 3D ultr asound data. They reported that the Cobb angle measured from the in-vivo ultrasound image was 4° different from the radiograph. In this study, we reconstructed the vertebra surface from ultrasound data and calculated the 3D geometric torsion.