Calibration of Free-hand 3D Ultrasound Image Based on Magnetic Tracker

3D ultrasound image can provide more information than that of 2D ultrasound,which will help doctor locate the focus position accurately.In the process of free-hand 3D ultrasound image based on magnetic tracker,a receiver is attached to the probe for directly tracking its position and orientation to the transmitter coordinate.Further calibration is still required to achieve the position and orientation of each image. The goal of calibration is to determine the spatial transformation of the image coordinate with respect to the receiver coordinate.Firstly,the phantom was scanned through.Secondly,the feature point was picked up through manual recognition and measure.Lastly,the expected spatial transformation between the probe and the image was smoothed out from a large number of data.A "N" style wire phantom was design and produced.The phantom leaned in the gutter,and was scanned from different position and orientation.On each image the middle point on the diagonal was extracted as feature point.After tracking and feature extraction,the spatial transformation matrix was decided through least square method.It is proved that the calibration method was convenient to use,and the calibration precision could meet the demand of ultrasound guide,laying a good foundation for further 3D reconstruction.