3D ultrasound to quantify lateral hip displacement in children with cerebral palsy: a validation study

A fully automated measurement of migration percentage on ultrasound images in children with cerebral palsy

Migration percentage (MP) is the gold standard to assess the severity of hip displacement in children with cerebral palsy, which is measured on anteroposterior hip radiographs. Recently, the ultrasound (US) method has been developed as a safe alternative imaging modality to image and monitor children's hips. However, measuring MP on US images (MPUS) is time-consuming, challenging, and user-dependent. This study aimed to develop machine learning algorithms to automatically measure MPUS and validate the algorithms with MPXray. A combination of signal filtering, convolution neural networks (CNNs), and UNets was applied to segment the regions of interest (ROI), detect edges or feature points, and select the desired US frames. A total of 62 hips including both coronal and transverse scans per hip were acquired, out of which 36 with applying augmentation method were utilized for training, 8 for validation, and 18 for testing. The intraclass correlation coefficient (ICC2,1) and the mean absolute difference (MAD) between the automated MPUS versus manual MPXray were 0.86 and 6.5% ± 5.5%, respectively. To report the MPUS, it took an average of 104 s/hip. This preliminary result demonstrated that MPUS was able to extract automatically within 2 min with a clinical acceptance accuracy (10%).

Reliability and Accuracy of Ultrasound Measurement of Hip Displacement in Children with Cerebral Palsy

OBJECTIVE

Hip migration percentage (MP) measured on anteroposterior pelvis radiographs is the gold standard to assess the severity of hip displacement in children with cerebral palsy (CP). Repeated exposure of these children to ionizing radiation under a hip surveillance program is undesirable. Recently, a semi-automatic approach to measure MPUS on ultrasound (US) images was validated in a phantom study. This pilot in vivo study applied the previous phantom method and aimed to determine the reliability and accuracy of the MPUS.

METHODS

Thirty-four children (23 boys and 11 girls) aged 8.9 ± 3.1 y old and diagnosed with CP were recruited. A total of 59 hips were scanned once, while 43 of these were scanned twice to evaluate the test-retest reliability. Two raters (R1 and R2) manually measured MPUS; procedures included selecting images of interest, cropping a region of interest and removing soft tissues on hip US images. Custom software was developed to measure MP automatically after the manual pre-image processing.

RESULTS

The intra-class correlation coefficients (ICC2,1) for the test-retest (R1), intra-rater (R1) and inter-rater (R1 vs R2) reliabilities were 0.90, 0.94 and 0.82, respectively. The standard error of measurement of MPUS for all three evaluations was ≤3.0%. The mean absolute difference between MPUS and MPX-ray and the percentage of MPUS within clinical acceptance error of 10% for R1 and R2 were (R1: 6.2% ± 4.9%, 84.7%) and (R2: 7.6% ± 6.1%, 73.7%), respectively.

CONCLUSION

This study demonstrated that US scans were repeatable and MPUS could be measured reliably and accurately.

Ultrasound Imaging of Hip Displacement in Children With Cerebral Palsy

OBJECTIVE

An approach to estimation of hip displacement on ultrasound (US) images is described. Its accuracy is validated through numerical simulation, an in vitro study with 3-D-printed hip phantoms and pilot in vivo data.

METHODS

A diagnostic index, migration percentage (MP), is defined by the ratio of acetabulum-femoral head distance to femoral head width. The acetabulum-femoral head distance could be measured directly on hip US images, while the femoral head width was estimated from the diameter of a best-fit circle. Simulation was performed to evaluate the accuracy of circle fitting with noiseless and noisy data. Surface roughness was also considered. Nine hip phantoms (three different sizes of femur head × three MP values) and 10 US hip images were used in this study.

RESULTS

The maximum diameter error was 16.1 ± 8.5% when the roughness and noise were 20% of the original radius and 20% of the wavelet peak, respectively. In the phantom study, the percentage errors of MPs between the 3-D-design US and X-ray US were 0.3%-6.6% and 0.0%-5.7%, respectively. From the pilot clinical trial, the mean absolute difference between the X-ray-US MPs was 3.5 ± 2.8% (1%-9%).

CONCLUSION

This study indicates that the US method can be used to evaluate hip displacement in children.

Imaging Parameters of Hip Dysplasia in Cerebral Palsy: A Systematic Review

Background

Cerebral Palsy is the leading cause of childhood physical disability globally. The motor disorders of CP are often associated with musculoskeletal anomalies, of which hip displacement is the second most common abnormality after abnormalities of foot and ankle. Various radiological parameters have been described in the literature which detects and quantifies hip dysplasia, with MP being the current gold standard. This study aims to review these radiological indicators of hip dysplasia in children with cerebral palsy from the published literature.

Methods

A literature search using PubMed, Embase, and Google Scholar was done on 15th June 2021 focusing on surveillance of hip dysplasia in cerebral palsy. The studies to be included were to have used anyone or more radiological parameter for detection of hip dysplasia with the use of any of the radiological methods.

Results

The initial search yielded 1184 results. After the screening of the abstracts and full texts, a final of 30 studies was included for this systematic review. The majority of the studies were graded as Level 3 evidence (16/30), followed by Level 2 studies (14/30). X-ray was the most common modality of detection of dysplasia followed by CT scan, ultrasonography, and arthrogram. The reproducibility of the various parameters shows good to excellent intraclass coefficients.

Conclusions

Parameters other than MP can be used to screen hips in CP. This would be useful in patients in whom either the lateral acetabular edge is not discernible on a plain anteroposterior radiograph or there are issues in the positioning of the patient. Additional views and structures can be visualized which can lead to improved screening and planning. Further investigations are required to appreciate the full potential of these parameters and how they can be better utilized.

Graphical abstract

2D Ultrasound Validation to Assess the Accuracy of Hip Displacement Measurement: A Phantom Study

Hip displacement is a common orthopedic abnormality in children with cerebral palsy and is assessed on anteroposterior pelvic radiographs during surveillance. Repeated exposure to ionizing radiation is a major concern of cancer risks for children. Ultrasound (US) has been proposed to image the hips. The severity of hip displacement is measured by the Reimers' migration percentage (MP), which is calculated by the ratio of the femoral head distance from the acetabulum to the width of the femoral head. Methods have been published to estimate MP from the US hip images in literature; however, validation for accuracy has not been reported. This study aimed to determine the accuracy of the 2D ultrasound techniques using two 3D printed hip phantoms with known MP values. The MPs estimated from the US images were compared with those measured from the X-ray images. Based on the experimental results, the US measurements had a maximum absolute discrepancy of 2.2% as compared to 9.8% from the X-ray measurements for the MP. The study on phantoms has showed the proposed US approach is promising with better accuracy and without ionizing radiation.Clinical Relevance - If the accuracy is proved to be at least as good as the current X-ray gold standard, the proposed US method will provide a modality of choice to pediatric patients for hip displacement diagnostics and hip surveillance, especially those with cerebral palsy. The method will be free of ionizing radiation and therefore significantly improve the pediatric patient care.