基于投影约束的C臂外参数优化方法

C臂外参数计算是手术导航系统实现图像空间与手术空间映射的关键环节.标记点提取误差将使外参数偏离真值.采用投影方法系统分析选点误差对外参数计算的影响,发现相机光轴方向平移参数收敛性弱、易受误差干扰.利用不同平面上投影距离之比作为约束对线性方法所得外参数进行修正.借鉴二/三维图像配准方法,采用修正后的估计外参数与已标定内参数进行重投影,根据几何方法直接求出外参数修正量,避免反复迭代优化.仿真结果表明,该标定方法对误差具有一定免疫性,能够提高外参数精度. 

Reproject and correct optimization in determining C-arm extrinsic parameters based on geometry constraints

The precision of C-arm extrinsic parameters is critical to computer assisted surgery systems to implement the registration of X-ray image and surgery physical space intraoperatively.Feature point extraction errors may cause deviation of extrinsic parameters using current calibration methods. A projection method was adopted to literally analyze the influence of point error to the precision of extrinsic parameters and among the parameters the translation along the optical axis was found to be less convergent and prone to fall in false optimum state.A statistical projection ratio applying geometry constraints was presented as a restriction to rectify the initial parameter estimated using linear method along the optical axis,so that a coarse transformation between the world coordinate and the camera coordinate can be determined.A 'reproject and correct' approach was proposed to correct the estimated extrinsic parameters based on the reprojection of the physical space to images with intrinsic parameters preoperatively determined and calculate the extrinsic parameters directly without iteration.Simulation results show that this method is more robust and precise compared to the current linear calibration method.