Optimising Camera–ChArUco Geometry for Motion Compensation in Standing Equine CBCT

Standing equine cone-beam CT (CBCT) enables diagnostic imaging in weight-bearing conditions, reducing the need for general anaesthesia, but residual postural sway during acquisition can introduce motion artefacts that degrade image quality. External optical tracking based on a ChArUco fiducial and an auxiliary RGB camera is a practical strategy for projection-wise motion compensation; however, the impact of camera–marker geometry on pose-estimation performance is not well characterised. This study evaluates how viewing angle and working distance affect ChArUco-based pose estimation in a controlled CBCT-motivated setting. Pose estimates were obtained with an Intel RealSense D435 RGB sensor and OpenCV, using dedicated mechanical fixtures to vary viewing angle in 1° increments and to adjust the camera-to-board distance in 5cm steps. Accuracy and precision were quantified using mean absolute error with respect to ground truth and the standard deviation across repeated measurements. Continuous and cyclic acquisition protocols yielded comparable errors, indicating that repeated repositioning did not introduce substantial additional variability. Viewing-angle experiments revealed a consistent accuracy–precision trade-off for rotation estimation: frontal views minimise mean absolute error but exhibit the highest variability, whereas increasingly oblique views reduce variability at the expense of larger mean error. Increasing working distance was associated with larger standard deviations, particularly affecting depth repeatability. These results provide practical guidance for selecting camera placement and nominal viewpoints when deploying ChArUco-based tracking for motion-aware standing equine CBCT/CT workflows.