Time of flight and FMCW catheter localization

This work uses ultrasound signals to track the 3D location of a catheter. Our system provides 3D coordinate data and not images. The coordinates can be registered with preoperative, 3D CT image data to provide a ?GPS? like navigation system for catheter based minimally invasive surgery. We used three ultrasound piston transducers as transmitters and held these transmitters at fixed locations. We then moved a custom catheter based transducer within the transmitted ultrasound field. Time of flight (TOF) or frequency modulated continuous wave (FMCW) range-finding measured the distance between each transmitting transducer and the catheter. We used trilateration to calculate 3D coordinates for the catheter given the spatial coordinates of the transmitters. We performed the localization procedure in a DI water tank and a porcine tissue phantom. In the tank experiment, we shifted the catheter in 1 mm increments for two 10 cm runs, once towards the plane of the transmitters (axially) and once parallel to the plane of the transmitters (laterally). We evaluated performance with linear regression. For TOF, the slope and linear regression fits were m =0.976 with Spearman correlation coefficient R_s = 0.993 and m=0.709 with R_s = 1 for the axial and lateral translation coordinates, respectively. For FMCW, the values were m = 1.004 with R_s = 1 and m= 0.928 with R_s = 0.996, all statistically significant with p <0.001. We prepared the porcine tissue phantom by implanting a model silicone aorta beneath the skin surface and imaged it with a CT scan. We advanced the catheter laterally into the phantom and used TOF range-finding to track the position. We then plotted the coordinates on the CT images (Fig. 1). Despite crude registration methods, results demonstrated agreement between the anatomical environment and the tracked path.