Author: Walaa Abdelfadeel, Reagan Thomas Dugan, Tiffany Kinsey, Cameron Kofler, Zheng Feng Lu, Ingrid S. Reiser, Gregory Stacy, Sara Wallace 👨🔬
Affiliation: University of Chicago Medicine, University of Chicago 🌍
Purpose: To develop an accurate, vendor-neutral method for estimating geometric magnification at the plane of the hip joint on standing anterior-posterior (AP) radiographs for digital templating of total hip arthroplasty procedures.
Methods: Markers of known size were placed on an acrylic block anterior to an anthropomorphic phantom and posterior to the phantom on the wall bucky surface. Geometric magnification at the plane of the hip joint is estimated using the measured magnification of the markers and a ratio for the coronal plane of the hip joint relative to the anteroposterior thickness of the patient, estimated to be 0.45 based on analysis of CT scans published by King et al in 2009. To assess the method, AP radiographs of the phantom were acquired in the wall bucky configuration. The phantom and markers were arranged to simulate different coronal plane ratios and anteroposterior thicknesses of 30- and 45-cm. Magnification was estimated from measurements of the two markers in PACS. Hip joint diameter was measured in digital templating software using the two-marker method and the default 115% magnification, and were compared to ground truth measured on CT. Each acquisition was acquired three times for averaging.
Results: Analysis indicated that error in measured magnification and hip joint diameter increased as the true patient coronal plane ratio diverged from the assumed value. The magnitude of the errors was comparable for both simulated anteroposterior patient thicknesses using the two-marker method, with a maximum error in measured diameter of 1.74-mm for both. Using the fixed magnification method results in greater errors for both thicknesses with maximum errors of 2.76-mm for the 30-cm phantom and 7.76-mm for the 45-cm phantom.
Conclusion: This work indicates that a two-marker method for estimating magnification of the hip joint in digital templating can provide more accurate procedure planning.