An efficient calibration method for freehand 3-D ultrasound imaging systems.

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.