PURPOSE: Spatially tracked ultrasound-guided needle insertions may require electromagnetic sensors to be clipped on the needle and ultrasound probe if not already embedded in the tools. It is assumed that switching the electromagnetic sensor clip does not impact the accuracy of the computed calibration. We propose an experimental process to determine whether or not devices should be calibrated on a more frequent basis. METHODS: We performed 250 calibrations. Of these, 125 were performed on the needle and 125 on the ultrasound. Every five calibrations, the tracking clip was removed and reattached. Every 25 calibrations, the tracking clip was exchanged for an identical 3D-printed model. From the resulting transform matrices, coordinate transformations were computed. Data reproducibility was analyzed through looking at the difference between mean and grand mean, standard deviation and the Shapiro-Wilks normality constant. Data was graphically displayed to visualize differences in calibrations in different directions. RESULTS: For the needle calibrations, transformations parallel to the tracking clip and perpendicular to the needle demonstrated the greatest deviation. For the ultrasound calibrations, transformations perpendicular to the sound propagation demonstrated the greatest deviation. CONCLUSION: Needle and ultrasound calibrations are reproducible when changing the tracking clip. These devices do not need to be calibrated on a more frequent basis. Caution should be taken to minimize confounding variables such as bending the needle or ultrasound beam width at the time of calibration. KEY WORDS: Calibration, tracking, reproducibility, electromagnetic, spatial, ultrasound-guided needle navigation, transformation, standard deviation.
PURPOSE: Neurosurgical registration using optical tracking in prone position is problematic due to a lack of anatomical landmarks on the posterior skull. The current method of registration involves insertion of screws into the skull. Surface registration using ultrasound has been proposed as a less invasive method of registration. Obtaining full access to the posterior skull would require patient hair removal, which is not favored by patients as it can cause an increased risk of surgical site infection and a less aesthetic outcome. We performed ultrasound scans on participants with no hair removal to evaluate the visibility of the mastoid processes and occipital base of the posterior skull in ultrasound imaging. METHODS: Participants were scanned using a linear and a curvilinear ultrasound probe. Scans were taken at the maximum and minimum frequency of each probe. Ultrasound scans captured the region around each mastoid process, the external occipital protuberance, and the occipital base of the skull. Scans were recorded using the Sequences extension in 3D Slicer and replayed for visual analysis. RESULTS: At its minimum frequency, the linear probe was found to have identifiable bone surfaces with some level of uncertainty. At its maximum frequency, clear identification of the mastoid processes and occipital base was possible. The curvilinear probe did not allow identification of bone surfaces in the ultrasound image. CONCLUSION: A linear probe at a high frequency provides clearly identifiable bone surfaces, allowing for the selection of points used in an iterative closest point algorithm for surface registration.
PURPOSE: Tracked navigation has become prevalent in neurosurgery. Problems with registration of a patient and a preoperative image arise when the patient is in a prone position. Surfaces accessible to optical tracking on the back of the head are unreliable for registration. We investigated the accuracy of surface-based registration using points accessible through tracked ultrasound. Using ultrasound allows access to bone surfaces that are not available through optical tracking. Tracked ultrasound could eliminate the need to work (i) under the table for registration and (ii) adjust the tracker between surgery and registration. In addition, tracked ultrasound could provide a non-invasive method in comparison to an alternative method of registration involving screw implantation. METHODS: A phantom study was performed to test the feasibility of tracked ultrasound for registration. An initial registration was performed to partially align the pre-operative computer tomography data and skull phantom. The initial registration was performed by an anatomical landmark registration. Surface points accessible by tracked ultrasound were collected and used to perform an Iterative Closest Point Algorithm. RESULTS: When the surface registration was compared to a ground truth landmark registration, the average TRE was found to be 1.6±0.1mm and the average distance of points off the skull surface was 0.6±0.1mm. CONCLUSION: The use of tracked ultrasound is feasible for registration of patients in prone position and eliminates the need to perform registration under the table. The translational component of error found was minimal. Therefore, the amount of TRE in registration is due to a rotational component of error.