Single-Phantom Ct-Referenced Evaluation of Residual Measurement Bias in Full-Length Lower-Limb Radiography After Sid/Sod Calibration

Purpose: This single-phantom CT-referenced technical-validation study evaluated residual length and angular measurement bias after source-to-image distance/source-to-object distance (SID/SOD) calibration in full-length lower-limb radiography using system/protocol configurations representing rotational stitching and slot-scanning. Methods: A whole-body anthropomorphic phantom was imaged once with each of six system/protocol configurations: four rotational-stitching configurations and two slot-scanning protocols. CT-derived coronal-plane measurements from three-dimensional landmark localization served as the reference standard. Three attending radiologists performed measurements across three sessions. SID/SOD-calibrated femoral, tibial, and full-length mechanical-axis lengths; mechanical lateral distal femoral angle (mLDFA); medial proximal tibial angle (MPTA); hip-knee-ankle angle (HKA); joint line convergence angle (JLCA); and SID/SOD-calibrated 10.00 mm steel bead diameter were analyzed. Results: Slot-scanning protocols showed small negative full-length biases (Eagle Eye HQ, −5.34[Formula: see text]mm; Eagle Eye HS, −5.14[Formula: see text]mm), whereas rotational-stitching configurations showed positive full-length biases ranging from 28.66[Formula: see text]mm to 57.19[Formula: see text]mm. Angular bias behaved differently: mean mLDFA bias ranged from −0.33[Formula: see text] to 0.85[Formula: see text], MPTA from 1.57[Formula: see text] to 2.21[Formula: see text], HKA from −0.31[Formula: see text] to 0.42[Formula: see text], and JLCA from −2.08[Formula: see text] to −1.37[Formula: see text]. Mean SID/SOD-calibrated steel bead diameter bias ranged from 0.14[Formula: see text]mm to 0.88 mm; bead enlargement was smaller in the tested slot-scanning protocols, although their full-length skeletal bias was slightly negative. Conclusions: In this single-phantom technical validation, SID/SOD calibration left system/protocol-specific residual length bias. Length measurements require local verification before cross-system comparison. Angular measurements showed parameter-specific behavior and require separate validation from linear length measurements. Steel bead diameter measurements may serve as a practical quality-assurance indicator of local linear scale behavior, but they do not replace anatomical phantom validation.