Calibration of 2D Ultrasound in 3D space for Robotic biopsies

Freehand Ultrasound technique is widely used in intraoperative biopsy procedures for detecting the volumes of interest. Freehand ultrasound probe is faster and flexible with 6 degrees of freedom. Thats why the imaging system must be calibrated in 3D space before integrating it with Robotics Biopsy System. In this paper we present a 3D space calibration method using a multipoint cross-wire phantom. The Ultrasound probe is attached to a robotic manipulator arm which moves it over the phantom in precise steps of distances and angles. The position and orientation of the probe is tracked by an optical tracking system. Optical markers are placed on the probe, phantom tank and the validation needle. The optical tracking system returns the position and orientation of the reference frames attached to these optical markers. The location of threads with reference to the frame of Ultrasound probe is found using this information. These values and the values returned by a mathematical model of the calibration box are used to construct the calibration matrix. The whole system is automated so it can process high number of frames which makes it rapid and more accurate. This process is used to calibrate the space for an automated needle insertion biopsy robot. The accuracy of the system was checked by a validation needle in 3D space. RMS error of the experiment groups on average was 1.67mm.