Intrinsic Tolerance in C-Arm Imaging: How Extrinsic Re-optimization Preserves 3D Reconstruction Accuracy

2026-03-14

AI Summary

This study addresses the challenge of inaccurate intrinsic parameter estimation in clinical calibration of C-arm X-ray systems, which typically limits 3D reconstruction accuracy. The authors discover that C-arm imaging systems exhibit inherent tolerance to intrinsic parameter errors and propose that re-optimizing only the extrinsic parameters can effectively compensate for substantial intrinsic inaccuracies. This approach significantly relaxes calibration requirements while preserving sub-millimeter reconstruction precision. Validated through both simulation and real-world experiments on multiple commercial C-arm systems, the method demonstrates robust performance: even with a focal length error as large as 500 pixels (approximately 100 mm), the average 3D reconstruction error remains below 0.2 mm; similarly, a principal point offset of 200 pixels increases reprojection error by less than 0.5 pixels.

Technology Category

• Machine Learning: Calibration & Uncertainty Quantification
• Computer Vision: 3D Computer Vision
• Intelligent Robots: State Estimation

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• Systems and Infrastructure for Web, Mobile and WoT: Experiences and lessons learnt from Web-based algorithms and system deployments
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Abstract

\textbf{Purpose:} C-arm fluoroscopy's 3D reconstruction relies on accurate intrinsic calibration, which is often challenging in clinical practice. This study ensures high-precision reconstruction accuracy by re-optimizing the extrinsic parameters to compensate for intrinsic calibration errors. \noindent\textbf{Methods:} We conducted both simulation and real-world experiments using five commercial C-arm systems. Intrinsic parameters were perturbed in controlled increments. Focal length was increased by 100 to 700 pixels ($\approx$20 mm to 140 mm) and principal point by 20 to 200 pixels. For each perturbation, we (1) reconstructed 3D points from known phantom geometries, (2) re-estimated extrinsic poses using standard optimization, and (3) measured reconstruction and reprojection errors relative to ground truth. \noindent\textbf{Results:} Even with focal length errors up to 500 pixels ($\approx$100 mm, assuming a nominal focal length of $\sim$1000 mm), mean 3D reconstruction error remained under 0.2 mm. Larger focal length deviations (700 pixels) elevated error to only $\approx$0.3 mm. Principal point shifts up to 200 pixels introduced negligible reconstruction error once extrinsic parameters were re-optimized, with reprojection error increases below 0.5 pixels. \noindent\textbf{Conclusion:} Moderate errors in intrinsic calibration can be effectively mitigated by extrinsic re-optimization, preserving submillimeter 3D reconstruction accuracy. This intrinsic tolerance suggests a practical pathway to relax calibration precision requirements, thereby simplifying C-arm system setup and reducing clinical workflow burden without compromising performance.

Problem

• C-arm imaging
• intrinsic calibration
• 3D reconstruction accuracy
• extrinsic re-optimization
• calibration error

Innovation

• intrinsic tolerance
• extrinsic re-optimization
• C-arm imaging
• 3D reconstruction accuracy
• calibration error compensation