Development of 3D ultrasound system for assessing adolescent idiopathic scoliosis

Abstract

Adolescent idiopathic scoliosis (AIS) is a common spinal disease. The prevalence of AIS is 2 to 4 % of the youngster population between 10 and 16 years old in the US. In Hong Kong, the prevalence of AIS is about 3%. With spinal deformity in coronal plane more than 10° measured by Cobb’s method, the medical condition is defined as scoliosis. Cobb’s method has been regarded as a gold standard in scoliosis diagnosis over decades. The major reason causing etiology of AIS is still remained unclearly. For the patients with more skeletally and sexually immature, the spinal curvature progression occurs in greater probability. Curve progression is one of the most common long-term sequelae of untreated AIS. Bracing and physiotherapy are common non-surgical treatment for preventing curve progression. The patients warrant a series of X-ray examination in which the patient undergoes regularly radiograph of whole spine every 4 to 6 months until skeletal maturity is reached. Surprisingly, less than 1% of the screened population and less than 10% of those patients with Cobb angle greater than 10° are warranted treatment. This previous finding suggests that 90% or more AIS patients are subject to unnecessary radiation. As radiation hazard of radiograph, frequency of assessment is limited, making disease progression monitoring and treatment outcome measure more difficult. Clearly, screen of teenager, monitoring of AIS, lifelong follow-up, outcome measure of therapy and treatment of scoliosis using non-radiation technologies are the glaring need to be achieved. Non-radiation approaches for curvature assessment have been developed such as Quantec system using surface topographical technology, Orthoscan Ortelius 800 using electromagnetic spatial sensing technology, and Upright multi-position MRI. However, none of them have been widely used due to the low accuracy or high cost. Ultrasound imaging is a low cost, radiation-free, and widely available modality, and thus it is a potential method for assessing scoliosis. The objective of this study is to develop a freehand 3D ultrasound system for the assessment of scoliosis and to establish the measurement protocol of using the system. The 3D ultrasound system was comprised of an ultrasound scanner using a probe with a width 92 mm and frequency range of 5-10 MHz, a custom-made supporting frame, an electromagnetic spatial sensing device, a desktop PC installed with a video capture card and a dedicated program for image and data collection, processing, visualization, volume reconstruction, analysis, and assessment. Using the obtained ultrasound images and their corresponding spatial data, different methods have been developed to measure the spine deformity, including 3D image stack approach and volume project approach. The 3D image stack approach used the manually identified transverse processes (TP) for measurement, while the volume project approach used reconstructed image to form an X-ray like projection to identify TP and spine column profile. Tests on spine phantoms and human subjects were conducted for evaluating the system performance. Four flexible spinal column phantoms featured with soft intervertebral discs allowing deformation were used in the phantom tests. The spine phantom was deformed to simulate scoliosis, and each was deformed four times to stimulate different spinal curvatures and imaged using X-ray. Therefore, a total of 16 phantom stimulated spinal curvatures were tested, using both X-ray Cobb's method and the 3D ultrasound method. 36 subjects (age: 31.1±14.7) were recruited for scoliosis measurement tests, and another 11 subjects participated in the intra- and inter- observer tests for both scanning and image analysis. The results obtained using the 3D ultrasound method was compared with the Cobb's angle obtained using X-ray measurement. The results of the phantom tests showed that there was a strong correlation (R²=0.7586, p<0.001) between the Cobb’s angle and the angle obtained using the 3D image stack approach where TP was used as a reference. The results also demonstrated excellent intra-observer (ICC=0.99, p<0.001) and inter-observer (ICC=0.89) repeatability. For the subjects, the results also demonstrate good correlation (R²=0.6806, p<0.001) between the results obtained by the two methods. Intra-observer (ICC=0.565) and inter-observer (ICC=0.75) repeatability were relatively poor in comparison with those of phantom tests. For the subject tests using the volume projection approach with the spine column profile as a reference for measurement, the results showed that there was a strong correlation (R²=0.7903, p<0.001) between the Cobb's angle and the result obtained by the new method. The intra-observer (ICC=0.99, p<0.001) and inter-observer (ICC=0.919, p<0.001) repeatability were very high for this method. For the volume projection approach using TP as a reference, the results showed a strong correlation (R²=0.7779, p<0.001) between the Cobb's angle and the new measurement. The intra-observer (ICC=0.98, p<0.001) and inter-observer (ICC=0.961, p<0.001) repeatability were also very high. Further analysis also demonstrated that intra- and inter-observer repeatability was very high for both scanning and data analysis when they were analyzed separately. Using 3D ultrasound techniques, a non-radiation AIS assessment system has been successfully developed together with a number of novel image analysis approaches for the measurement of spine curvature. It has been demonstrated that the measurement using the new system was very reliable when the volume project approach was used and the correlation between the Cobb's angle and the result obtained using the new method was very high. The results suggested that the 3D ultrasound system could be a potential complementary tool for Cobb's method. Future improvement is necessary to facilitate the new system to be a clinical tool for the screening and assessment of scoliosis, particularly AIS. The future development works should include a portable version of the system, automatic angle calculation algorithm, real-time volume reconstruction and projection, etc. It is believed that the developed system integrated with the new features will provide clinicians with an unprecedented powerful imaging and analyzing tool for the assessment of scoliosis.