Fast intraoperative 2D/3D head registration for image-guided interventions using biplane x-ray angiography

Two-dimensional to three-dimensional (2D/3D) registration is critical in image-guided interventions, particularly in vascular procedures such as endovascular therapy (EVT), where accurate alignment between preoperative 3D images and intraoperative 2D x-ray angiography can improve procedural safety and precision. However, achieving both real-time and precise 2D/3D registration remains challenging due to high computational demands.

This study aims to develop a fast 2D/3D registration method that directly optimizes the in-plane alignment between digitally reconstructed radiographs (DRRs) and x-ray angiography images, thereby eliminating the need for forward projection at each objective function evaluation and improving computational efficiency without compromising registration accuracy.

We propose a registration method that solves an alternative 2D/2D registration problem to estimate in-plane transformation parameters and iteratively updates the underlying 2D/3D registration parameters. The method incorporates a skull region-of-interest (ROI) map to reduce anatomical mismatches between DRRs and angiography images, and maximizes a similarity metric, normalized cross-correlation (NCC), in an iterative manner. The algorithm was evaluated using both numerical simulations (XCAT phantom) and clinical angiography data, comparing performance to an intensity-based 2D/3D registration and a version of our method without skull ROI guidance. The success rate was evaluated differently depending on the data source: based on the NCC in simulations, and on parameter accuracy compared to manual estimations by two experts in clinical data. Also, registration parameter accuracy, runtime, number of forward projections, and convergence behavior were assessed.

In the numerical simulation, the proposed method reduced runtime by more than 10 $\times$ compared to conventional 2D/3D registration, while maintaining an equivalent success rate. In the clinical study, it achieved runtime reduction of approximately 6 $\times$ and a 100% success rate in non-truncated cases, where the head was mostly captured within the x-ray angiography field of view, and demonstrated strong visual agreement with the manual estimation results. In truncated cases, although the registration performance decreased across all methods, the proposed method outperformed the 2D/3D registration.

We proposed a fast 2D/3D registration method that achieved significantly reduced runtime and improved accuracy. Despite being limited to rigid registration, it shows promise for real-time clinical use with further optimization.