# 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
When the inserted catheter approaches the lesion of aorta, users need to focus on the local navigation information. With local navigation, users obtain more accurate distance information between lesion and inserted catheter. When the distance is close to zero, the catheter is opened to release the stent. 4. In this process, local information including virtual visual images of endovascular view and collected IVUS image is also displayed. This navigation information can also help the surgeon judge the severity of lesion. ## 3 Experiments and Results

Our experiment platform (as shown in Figure 5) includes the following components. (1) an optical tracking system (Polaris, Northern Digital Inc., Waterloo, ON, Canada) to collect the pose information of US probes; (2) an MR compatible multimodality heart phantom (SHELLEY Medical, London, ON, Canada), which contains a left ventricle (LV) and a right ventricle (RV). An aortic phantom with fixed radius (outer radius: 4.5000 mm; inner radius: 3.5000 mm) is right above the heart phantom; (3) an US system (iU22 xMATRIX, Philips, Amsterdam, The Netherlands) with a 2D linear array probe (S5-1, Philips, Amsterdam, The Netherlands). The size of 2D US images is 600 \(\times\) 800 pixels with resolution 0.2382 mm \(\times\) 0.2382 mm; (4) an IVUS system (Boston Scientific Galaxy 2 system, Marlborough, MA, USA) with a 40 MHz Atlantis SR IVUS probe. The collected IVUS images are 8-bits, 512 \(\times\) 512 with in-frame resolution 0.0175 mm\(\times\) 0.0175 mm; (5) a 7.5-F diameter catheter ([PERSON], Boston Scientific, Marlborough, MA, USA). The IVUS probe and stent are inserted into aortic phantom along with the catheter; (6) preoperative 3D MR images of phantom which are collected with a MR scanner (Philips Achieva 3.0T TX, Amsterdam, The Netherlands). The size of the MR images is 480 \(\times\) 480 \(\times\) 300 voxels with resolution 0.4871 mm \(\times\) 0.4871 mm and slice thickness 1.6000 mm; (7) the image processing technologies in our navigation system are developed using mixed program based on C++ and Matlab (Matlab2014a, MathWorks, Natick, MA, USA). The navigation system runs under Windows 7, on an Intel Core i7 computer with 16 GB RAM (Random Access Memory). Figure 5: The in vitro experiment platform of aortic and cardiac phantom. The navigation system contains two key parts including the acquirement of global navigation map based on 2D US-3D MR image registration, and local high-resolution navigation information based on US images tracking and IVUS image segmentation. A set of experiments was conducted to evaluate these two parts (Sections 3.1 and 3.2). In particular, many image-processing methods applied in these two parts were evaluated individually. After the analyses of these two key parts, an in vitro experiment of catheter intervention (Section 3.3) was conducted to confirm that the integration of global and local information is significant. ### Evaluation of the Global 3D Navigation Map

The global 3D navigation map is based on 2D US-3D MR image registration. And the proposed 2D US-3D MR images registration method relies on effective calibration results of 2D and 3D US probes. Therefore, the evaluation of global navigation map contains the verification of probe calibrations and assessment of 2D US-3D MR image registration. #### 3.1.1 Calibrations of 2D and 3D US Probes

Calibration reproducibility (CR) error measures repeatability of a probe calibration method when it is performed on a new set of images [28]. CR error is the Euclidian distance between two calibration transformations (\(\mathrm{T}^{\mathrm{PR}^{i}}_{\mathrm{US}}\) and \(\mathrm{T}^{\mathrm{PR}^{i}}_{\mathrm{US}}\)) of the same US image point \(P^{\mathrm{US}}\).