PerCal: A Novel Permanent Calibration Method for Spinal Surgery Navigation

The X-ray machine serves as a crucial auxiliary tool in surgical settings, collaborating with surgical navigation systems to facilitate intraoperative 2-D fluoroscopic image navigation and provide a reliable foundation for 2-D–3-D registration navigation technology. The attainment of precise navigation for both 2-D fluoroscopic images and 3-D preoperative CT/MRI images hinges upon the efficient and accurate calibration of X-ray machine. Conventional calibration techniques for X-ray machines, whether conducted online or offline, are characterized by excessively intricate calibration processes, imposing significant time demands during surgery and resulting in diminished surgical efficiency. We propose a novel permanent calibration (PerCal) strategy employing a self-developed X-ray source tracker (TRKR-S), image plate tracker (TRKR-IP), and portable calibrator (CAL-P), aiming to streamline surgical procedures and enhance overall surgical efficiency. This strategy reframes the X-ray machine calibration challenge into the determination of the spatial position of the X-ray source in relation to TRKR-S and the 2-D fluoroscopic image relative to TRKR-IP. In the intraoperative navigation scenario, solely collecting position information of TRKR-S and TRKR-IP under the optical camera during perspective scanning enables real-time and precise intraoperative 2-D fluoroscopic image navigation. Meanwhile, the initialization mechanism for the projection (IM-Proj) coordinate frame has been proposed to ensure the effective solution of the PerCal problem. Furthermore, the membership criterion for 2-D/3-D (MC-2D/3D) marker points has been formulated with the aim of expeditiously and precisely determining the mapping correlation between spatial marker points within the 3-D calibration scene and their corresponding marker points on the 2-D fluoroscopic image. The efficacy of the proposed PerCal strategy combining IM-Proj and MC-2D/3D is validated through ablation experiments using a G-Type Arm, confirming its calibration accuracy and timeliness. The AMSE errors for frontal and lateral perspectives were denoted as
$0.332~ \pm ~0.026$
and
$0.53~ \pm ~0.023$
mm, respectively. Notably, it necessitates only one calibration session to attain permanent reuse of calibration data in spinal surgery navigation. The navigation registration time is confirmed not to exceed 10 s, affirming the accuracy and efficiency of PerCal.