Automatic standard plane adjustment on mobile C-Arm CT images of the calcaneus using atlas-based feature registration

Intraarticular fractures of the calcaneus are routinely treated by open reduction and internal fixation followed by intraoperative imaging to validate the repositioning of bone fragments. C-Arm CT offers surgeons the possibility to directly verify the alignment of the fracture parts in 3D. Although the device provides more mobility, there is no sufficient information about the device-to-patient orientation for standard plane reconstruction. Hence, physicians have to manually align the image planes in a position that intersects with the articular surfaces. This can be a time-consuming step and imprecise adjustments lead to diagnostic errors. We address this issue by introducing novel semi-/automatic methods for adjustment of the standard planes on mobile C-Arm CT images. With the semi-automatic method, physicians can quickly adjust the planes by setting six points based on anatomical landmarks. The automatic method reconstructs the standard planes in two steps, first SURF keypoints (2D and newly introduced pseudo-3D) are generated for each image slice; secondly, these features are registered to an atlas point set and the parameters of the image planes are transformed accordingly. The accuracy of our method was evaluated on 51 mobile C-Arm CT images from clinical routine with manually adjusted standard planes by three physicians of different expertise. The average time of the experts (46s) deviated from the intermediate user (55s) by 9 seconds. By applying 2D SURF keypoints 88% of the articular surfaces were intersected correctly by the transformed standard planes with a calculation time of 10 seconds. The pseudo-3D features performed even better with 91% and 8 seconds.