Established acetabular radiological reference values can be reliably transferred to reconstructed parallel-beam 2D images from ultra-low-dose pelvic CT

Background: The anteroposterior radiograph of the pelvis is essential for diagnosing hip pathologies. Radiograph-like projections reconstructed as cone-beam images from CT data correlate strongly with conventional radiographs. However, CT inherently uses parallel x-rays rather than a cone-beam geometry. This study aims to determine whether parallel-beam radiograph-like projections from CT provide comparable reference values to cone-beam reconstructions. Methods: 63 patients (126 hips) undergoing CT for symptomatic hip pathologies without prior hip surgery were included. From the same CT data, cone-beam and parallel-beam radiograph-like images were reconstructed using a standardized algorithm. Reference values, including lateral center-edge angle (LCEA), medial center-edge angle (MCEA), acetabular index (AI), acetabular arc (AA), extrusion index (EI), crossover sign, and posterior wall sign, were measured on both projection types, eliminating technical bias. Two observers performed all measurements twice to assess inter- and intra-observer reliability, and intraclass correlation coefficients (ICCs) were calculated. Conclusion: Parallel-beam radiograph-like projections demonstrate good to excellent reliability (ICCs: 0.89-0.99) for key reference values of hip pathologies compared to cone-beam radiograph-like projections from the same CT data. These findings suggest parallel beam projections can be reliably used with established reference values for conventional radiographs.