3-D calibration of ultrasound image for rural MRCAS

To reduce the cost of Medical Robotics and Computer Assisted Surgery (MRCAS) in rural medical of china, the researchers have focused on MRCAS investigation. They are inclined to select ultrasonography as medical image guidance because of its low cost, noninvasiveness and accuracy. 3-D calibration of ultrasound image is a very important technique of MRCAS. To improve the calibration precision, a method based on grid-string phantom and rapid algorithm for coordinate transformation is investigated. It accurately defines the spatial relationship between the US image plane and an external tracking device attached to the US probe. The phantom consists of two layers of grid-string planar arrays. The cross of the strings can rapidly guide the user to align the probe accurately for imaging; the cross coordinates are directly used in the algorithm for homologous points matching without any transformations. The calibration parameters are calculated by matching of homologous points in the image and the tracking device, then, the 2-D US image can be converted into 3-D US coordinates. The results show that this method can accelerate the speed of the calibration, simplify the calibration algorithm, reduce the influences of the US beam thickness on the precision and obtain the normal calibration precision and reconstruction accuracy of the 3-D US system.