# Ultrasound Navigation for Transcatheter Aortic Stent Deployment Using Global and Local Information
## 1 Introduction  
Placement of endovascular stent has become a preferred option for treatment of acute aorta dissection and aortosclerosis, because it can reduce the risk of infection and patient trauma [1]. Stent placement can replace conventional medical treatment for the majority of patients. During the procedures, surgeons insert a catheter into aortic lesions to finish the placement of stent. However, incorrect placement of the stent can cause delayed complications. There are two critical factors which directly influence the accuracy of stent deployment: (1) information on morphological characteristics of the aorta, especially calcification or lesion areas, which can greatly assist surgeons to determine the size of stent [2]; (2) intuitive three-dimensional (3D) navigation images which can reflect intraoperative information of cardiac structure [3]. Conventional stent placement, which is typically performed under X-ray, remains challenging, due to several shortcomings of fluoroscopic guidance. Firstly, repeated radiations influence the health of patients and clinical staff. Contrast agents probably increase the risk of iatrogenic renal injury for patients. Secondly, although fluoroscopy can visualize the catheter excellently, fluoroscopy lacks depth perception and cannot directly visualize anatomic structures [4]. Thirdly, 2D (two-dimensional) fluoroscopy images cannot provide a quantitative analysis of the vessel'smorphological characteristics [5]. To solve these problems, realizing 3D intuitive image navigation is useful for catheter intervention and stent placement. Several solutions have been proposed for performing 3D navigation imaging. Some authors have suggested using other 3D imaging modalities as alternatives to fluoroscopy. For example, intraoperative magnetic resonance (MR) and computed tomography (CT) imaging technologies [6; 7] have been applied, but their usage is very limited because of relatively low temporal resolution and inflexibility in the workflow. Another approach is the transfer of preoperative CT volume into the intraoperative environment by fusing CT with fluoroscopy. [PERSON] et al. have used feature-based rigid registration to combine 3D preoperative CT with intraoperative 2D fluoroscopy for endovascular navigation [8]. Although this approach may potentially provide 3D information, it still exposes the interventionist and patient to harmful radiation. To solve the above problem, some researches proposed using electromagnetic (EM) tracking rather than relying on X-ray for navigation, because EM tracking can collect 3D position information by assembling an EM sensor on the catheter. [PERSON] et al. [9] use EM approaches to track catheter position and overlay the position onto previously acquired CT or MR data during the procedures. [PERSON] et al. propose combining EM technologies with virtual visual feedback for surgical navigation [10]. [PERSON] et al. also present a navigation method by combining multi-slice CT with EM navigation for endovascular intervention [11]. These studies achieve the overlay between catheter tip and preoperative images, but cannot detect movements of soft tissue during the surgery. In addition, some non-ionizing radiation imaging methods have been used for radiation-free navigation. Real-time ultrasound (US) images may assist endovascular intervention and navigation. For example, an US-based catheter localization method [12] and a guidance technology based on US image registration [13] have been proposed for the minimally invasive endovascular surgical navigation. Furthermore, [PERSON] et al. propose a visualization navigation system that integrates US imaging with preoperative anatomical models for catheter intervention of oral cancers [14]. [PERSON] et al. make use of biplane US to guide trans-catheter aortic valve implantation deployment in a phantom study [15]. However, it is difficult that a single cardiac US image provides sufficient information for doctors.