# Ultrasound Navigation for Transcatheter Aortic Stent Deployment Using Global and Local Information
## 2 Materials and Methods
### A Global 3D Navigation Map Based on 2D US-3D MR Image Registration
#### 2.2.1 Multi-Feature Particle Filter Tracking Method for the 3D Trajectory of the Catheter
Dark yellow volume is lesion; (**e**) virtual visual image from endovascular view; (**f**) collected IVUS image. Comparatively, in the case of a global map plus local navigation information, the user can be guided to insert catheter along the preoperative planning path. Meanwhile, the local navigation information (Figure 10d-f), further provides the user with critical details of the target point. Firstly, to evaluate the vessel reconstruction error of local navigation information, we calculated the average outer radius of the automatically reconstructed aortic model from segmented IVUS images, and compared this average outer radius with the actual radius of aortic phantom. The resolution of IVUS images is 0.0175 mm and the acquired reconstruction error of local information is 0.2217 mm. Secondly, to evaluate the targeting error of catheter intervention, we captured coronal and sagittal X-ray projections when the catheter was navigated to the target by using global plus local information. These projections images are measured (See Figure 11b) and the acquired targeting error is 1.7214 mm. This experiment shows that the new US navigation system which integrates global and local information can improve upon the navigation system that relies on a global navigation map alone. ## 4 Discussion

The aim of this paper is to present a radiation-free US navigation system for transcatheter aortic stent deployment by combining MR images with two types of US images including IVUS images and common US images. The proposed US navigation system can provide surgeons with not only a global navigation map of heart and surrounding tissue, but also morphological characteristics of aorta to assist stent deployment. There are two key parts in our system: (1) a global navigation map from 2D US-3D MR image registration; (2) local morphological navigation information based on US images tracking and IVUS images segmentation. Recently, 2D US images have been widely used in minimally invasive cardiac procedures due to its real-time imaging capabilities. However, it is difficult to relate the 2D US images to anatomical context due to the limit of image quality. 3D US image may overcome this problem to some degree, but it performs 3D imaging at the cost of decreased temporal resolution and only provides a narrow

Figure 11: (**a**) Two perpendicular X-ray projections were taken to verify the targeting distance between the catheter arrival position and target point during catheter intervention under the global guidance alone; (**b**) Two perpendicular X-ray projections under the global map plus local navigation. field of view. Therefore, we propose to use high-quality 3D context from MR images to enhance 2D US images through image registration, providing a global navigation map for surgeons. To solve the difficulty of 2D US-3D MR image registration, we develop a novel registration method based on calibrations of 2D and 3D US probes. On the one hand, we acquire the small probes' calibration reproducibility errors (calibration error of 2D US probe: 0.61 mm, calibration error of 3D US probe: 1.42 mm). On the other hand, in Figure 6b, the contours of ventricles in 2D US and 3D MR images achieve a good agreement after registration, which qualitatively demonstrates that the proposed 2D US-3D MR image registration method is effective. A TRE of 2.70 mm is obtained for US and MR image registration. Through registration, interpretability of 2D US images is improved within the 3D anatomical context provided by MR images. Therefore, a global navigation map by integrating 2D US and 3D MR images is achieved (Figure 10a). Local high-resolution navigation can reveal morphological characteristics of the aorta, especially the information on the target lesion. The vessel reconstruction error of local information is reduced by 80% compared to global information (error: 0.2217 mm versus 1.1241 mm). In addition, in the catheter intervention experiment, the targeting error of global navigation is 2.8701 mm.