Ultrasound-based navigation of robotic drilling at the lateral skull base

Introduction: Robotic surgery at the lateral skull base must be navigated due to the noble adjacent structures. The accuracy of CT based intraoperative navigation is limited by the resolution of the CT scan itself, but could be substantially improved by using ultrasound scanning. Material and Methods: 24 human skull bone specimen fixed in formaldehyde were acoustically characterized by an ultrasound transmission system in water at the frequencies 1, 2.25, 5 and 7.5 MHz. Results: Sound velocity c in skull bone was 2593 m/s (SD 276). The acoustic dampening coefficient ß was 2.8875 dB/mm*MHz (SD 0.424, ±7%). The best of the tested frequencies proved to be 2.25 MHz, as it allows scanning down to a depth of 8 mm while giving axial resolution of 0.6 mm in standard data processing. Thereby, it was possible to automatically detect the inner bony contour and to create a 3D map of the scanned bone (shown below).