Image-based method for in-vivo freehand ultrasound calibration

For freehand ultrasound systems, a calibration method is necessary to locate the position and orientation of a 2D B-mode ultrasound image plane with respect to a position sensor attached to the transducer. In addition, the acquisition time discrepancy between the position measurements and the image frames has the be computed. We developed a new method that adresses both of these problems, based on the fact that a freehand ultrasound system establishes consistent 3D data of an arbitrary object. Two angulated sweeps of any object containing well visible structures are recorded, each at a different orientation. A non-linear optimization strategy maximizes the similarity of 2D ultrasound images from one sweep to reconstructions computed from the other sweep. No designated phantom is required for this calibration. The process can be performed in vivo on the patient. We evaluated our method using freehand acquisitions on both a phantom and the human liver. The accuracy of the approach was validated using a 3D ultrasound probe as a known reference geometry.