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. 2025 Dec 28:74:73-78.
doi: 10.1016/j.jor.2025.12.063. eCollection 2026 Apr.

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

Arnaud Klopfenstein  1 , Thomas Marth  2   3 , Stefan Sommer  3   4 , Reto Sutter  2 , Patrick O Zingg  1 , Dominik Kaiser  1
Affiliations

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

Arnaud Klopfenstein et al. J Orthop. .

Abstract

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.

Results: 126 hips were analyzed: 52 with LCEA <22° ("acetabular undercoverage"), 49 with LCEA 22°-33° ("normal coverage"), and 25 with LCEA >33° ("acetabular overcoverage"). ICCs between observers and between projection types demonstrated good to excellent reliability for all reference values (0.89-0.99).

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.

Keywords: Acetabulum; Computed tomography; Hip joint; Radiographic image interpretation.

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Conflict of interest statement

Stefan Sommer is employed by Siemens Healthineers, all the authors declares that they have no conflicts of interest. Balgrist University Hospital and Balgrist Campus each have an academic research collaboration with Siemens Healthineers. Balgrist University Hospital also has an academic research collaboration with Bayer.

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