Reproducibility of freehand calibrations for ultrasound-guided needle navigation.

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.