Oliver Hornung

Experimental evaluation of the accuracy at the C-arm pose estimation with x-ray images

C-arm X-ray systems need a high spatial accuracy for applications like cone beam computed tomography and 2D/3D overlay. One way to achieve the needed precision is a model-based calibration of the C-arm system. For such a calibration a kinematic and dynamic model of the system is constructed whose parameters are computed by pose measurements of the C-arm. Instead of common measurement systems used for a model-based calibration for robots like laser trackers, we use X-ray images of a calibration phantom to measure the C-arm pose. By the direct use of the imaging system, we overcome registration errors between the measurement device and the C-arm system. The C-arm pose measurement by X-ray imaging, the new measurement technique, has to be evaluated to check if the measurement accuracy is sufficient for the model-based calibration regarding the two mentioned applications. The scope of this work is a real world evaluation of the C-arm pose measurement accuracy with X-ray images of a calibration phantom using relative phantom movements and a laser tracker as ground truth.C-arm X-ray systems need a high spatial accuracy for applications like cone beam computed tomography and 2D/3D overlay. One way to achieve the needed precision is a model-based calibration of the C-arm system. For such a calibration a kinematic and dynamic model of the system is constructed whose parameters are computed by pose measurements of the C-arm. Instead of common measurement systems used for a model-based calibration for robots like laser trackers, we use X-ray images of a calibration phantom to measure the C-arm pose. By the direct use of the imaging system, we overcome registration errors between the measurement device and the C-arm system. The C-arm pose measurement by X-ray imaging, the new measurement technique, has to be evaluated to check if the measurement accuracy is sufficient for the model-based calibration regarding the two mentioned applications. The scope of this work is a real world evaluation of the C-arm pose measurement accuracy with X-ray images of a calibration phantom using relative phantom movements and a laser tracker as ground truth.

A realistic X-ray simulation for C-arm geometry calibration

Applications like cone beam computed tomographies (CBCTs) or 2D-3D overlays need a high geometrical accuracy of the C-arm system. In order to achieve this, geometry calibrations are performed to increase the accuracy given by the kinematics of the system. Commonly X-ray images of a phantom with known geometry are taken for the calibration. The images, together with a 3D model of the phantom, serve as input for an optimizer, which estimates the pose of the C-arm relative to the phantom. Afterwards, the estimates are used to increase the geometrical accuracy of the system. Inaccuracies due to real world effects appear, e.g. manufacturing or assembly inaccuracies of the phantom, the position of the X-ray tube, or the pose of the detector. To evaluate these factors separately a simulation is helpful, which needs to be as realistic as possible. To achieve this we defined three requirements, which have to be fulfilled: realistic noise, realistic absolute errors and similar error distributions within the working volume. By means of these criteria we investigate if our simulation mirrors a real world C-arm pose measurement for C-arm geometry calibration sufficiently.