Ultrasound Guided Surgery: Image Processing and Navigation

The process of 3D freehand ultrasound acquisition consists of two steps: scanning and reconstruction. Prior to scanning, a position sensor is attached to the probe for tagging each image with its position and orientation in space. A crucial step prior to reconstruction and visualization is calibration of the ultrasound probe with the attached position sensor, i.e. determining the position and orientation of the scan plane relative to the position sensor. We propose a method for probe calibration that involves the use of an easy to build and easy to use calibration phantom, comprising a water tank and a thin planar nylon mesh membrane. The submerged membrane is scanned by freehand, covering all degrees of freedom for movement in space, and the following calculations are performed automatically, including the edge detections. The technique is denoted the Freehand Membrane Scan Method. The accuracy of the method is assessed by 3D scanning of a phantom with 27 precision made thread crosses in known locations, then compared to another method developed earlier by our research group, the Membrane Alignment Method.