Differentiable Forward and Back-Projector for Rigid Motion Estimation in X-ray Imaging

TL;DR

This paper introduces a general, analytical differentiable forward/back-projector framework for X-ray imaging, enabling scalable, accurate, and memory-efficient rigid motion estimation with significant speed and quality improvements.

Contribution

It presents a novel analytical gradient formulation for forward/back-projection, applicable across projector types, improving efficiency and accuracy in X-ray imaging tasks.

Findings

Achieves ~8x speedup in 2D/3D registration

Enhances image sharpness and structural fidelity in CT reconstruction

Demonstrates superior efficiency over existing methods

Abstract

Objective: In this work, we propose a framework for differentiable forward and back-projector that enables scalable, accurate, and memory-efficient gradient computation for rigid motion estimation tasks. Methods: Unlike existing approaches that rely on auto-differentiation or that are restricted to specific projector types, our method is based on a general analytical gradient formulation for forward/backprojection in the continuous domain. A key insight is that the gradients of both forward and back-projection can be expressed directly in terms of the forward and back-projection operations themselves, providing a unified gradient computation scheme across different projector types. Leveraging this analytical formulation, we develop a discretized implementation with an acceleration strategy that balances computational speed and memory usage. Results: Simulation studies illustrate the numerical accuracy and computational efficiency of the proposed algorithm. Experiments demonstrates the effectiveness of this approach for multiple X-ray imaging tasks we conducted. In 2D/3D registration, the proposed method achieves ~8x speedup over an existing differentiable forward projector while maintaining comparable accuracy. In motion-compensated analytical reconstruction and cone-beam CT geometry calibration, the proposed method enhances image sharpness and structural fidelity on real phantom data while showing significant efficiency advantages over existing gradient-free and gradient-based solutions. Conclusion: The proposed differentiable projectors enable effective and efficient gradient-based solutions for X-ray imaging tasks requiring rigid motion estimation.