Ultrasound sonography becomes more relevant in the context of future minimal invasive computer assisted orthopaedic surgery. For transcutaneous non invasive registration an A-Mode ultrasound features several advantages in comparison to the B-mode technology. Apart from low costs the A-Mode technique offers simple realization of real time processing. Further A-Mode ultrasound allows reliable detection of bone-tissue surfaces and eliminates the need for manual segmentation.
The first step is to perform a CT scan of the patients region of interest (ROI). After reconstructing a surface voxel model using the computer tomographic slices several matching techniques are applied to solve the registration problem. A paired point matching with at least three bone landmarks must be performed to calculate an initial transformation of the bone co-ordinate system relative to the co-ordinate system of the tracking instrument. To increase the accuracy of the registration a second matching technique must be applied. The ultrasound probe is used to transcutaneous palpate the surface of the bone. Based on the initially calculated spatial position of the bone an autopilot function is used to maintain a perpendicular alignment of the ultrasound beam relative to the palpated surface of the bone. These digitized bone points are used to undertake a surface based registration using the iterative closest point algorithm. To decrease the time of performance a KD-tree data structure with small bucket size is applied.
To simulate the ultrasound parameters of soft tissue a gelatine model is used. After performing the final surface registration the averaged translation error was 0.55 mm. The averaged rotation error along the axial, sagittal and transversal axes was found to be lower then 0.6 Â°.