Ultrasound Imaging of Hip Displacement  in Children with Cerebral Palsy
Creators: Pham, Tu T
Publisher: name: University of Alberta Library
Year: 2024

<p>Hip displacement is a prevalent musculoskeletal disorder in children with cerebral palsy (CP). Reimers’ migration percentage (MP) derived on anteroposterior pelvis X-ray is the current means to diagnose and monitor hip displacement. Migration percentage signifies the ratio of the distance between the lateral femoral head (FH) and the acetabular margin (A) to the width of FH (B). Because of the concern of cumulative ionizing radiation in young children during hip surveillance, ultrasound (US) has been proposed to assess hip displacement. This thesis presents the development and validation of using 2D US to image hip displacement and reports the investigation process of applying 3D US for assessing hip displacement.   Prior to the development of the hip US approach, various radiological indexes used to evaluate hip disorders in children with CP using different imaging modalities were reviewed. Plain X-ray is the most used modality to monitor hip disorders. Computed tomography offers 3D views of the hip but is limitedly used in children due to high doses of ionizing radiation. Low-dose radiation EOS X-ray system, which can acquire frontal and lateral images simultaneously in standing and sitting acquisition positions, is not applicable for most children with CP who have limited physical ability. Magnetic resonance imaging (MRI) and US are less used because MRI is expensive, and US is operator-dependent. Despite all the limitations, most diagnostic indexes measured on these imaging modalities have demonstrated good to excellent reliability.    To eliminate the time-consuming and subjective process of manual MP measurements, a machine learning (ML) approach to automatically measure MP on X-ray was developed. Pre-trained convolutional neural networks were fine-tuned to predict anatomical hip landmarks on radiographs to calculate the MP values. The intra-class correlation coefficient with 95% confidence interval (ICC[2,1], 95% CI) for the comparison between ML-based and manual MPs was 0.91 (0.86–0.94) whereas the accuracy of the ML method was 4.5 ± 4.3%.    To develop the US method, 3D-printed hip phantoms were used in an in vitro study to determine the accuracy of MP measurements. With the 2D US approach, the hip phantoms were imaged in the coronal view to measure distance A and imaged in the transverse view to estimate width B by a circle fitting method. The MPUS had mean absolute difference and standard deviation (MAD ± SD) of 1.1 ± 0.9% when referenced to the MPs measured on 3D designs and 0.7 ± 0.7% when compared to the MPX-ray. The US approach was further validated in an in vivo study. The results showed that the US method had good to excellent test-retest reliability of the US scans (ICC[2,1] = 0.90), intra-rater (ICC[2,1] = 0.94), and inter-rater (ICC[2,1] = 0.82) reliability. The MAD ± SD was 6.9 ± 5.6% when comparing the MPUS to the manual MPX-ray and was 8.3 ± 6.1% when pairing the MPUS with the automatic MPX-ray predicted by ML.  To investigate the 3D US method, an electromagnetic (EM) tracking system was used to track the position and orientation of the US scanner. The optimum tracking configuration and accuracy of the EM system were determined in laboratory experiments. Under the configuration yielding the maximum accuracy, the mean errors of the EM system were 0.46 ± 0.43 mm in position and 0.5 ± 0.19º in rotation.  A phantom study was conducted using the EM-tracked-US scanner for 3D hip US. The 3D US images of the hip phantoms were well visualized, and the MPs measured on the reconstructed US images were within a 3% error compared to the 3D models. However, when the approach was tested on children with CP, the hip anatomy was not well visualized in the reconstructed images. Hence, more work is needed to bring the 3D US to clinical use.    In summary, this thesis reported the development and validation of a 2D US method to assess hip displacement using both in vitro and in vivo data. The results demonstrated good to excellent reliability (95% CI of ICC: 0.71–0.97) and good accuracy of 7.6 ± 5.9%. Although the tested 3D US approach had not been successful in evaluating hip displacement in children, the foundation work has been developed. </p>