Catheter-based cardiac ablation has gained increasing popularity in the treat- ment of heart arrhythmias during the past two decades. It has become the most widely accepted interventional treatment for cardiac arrhythmias with a high success rate [2], [3]. However, it is considered a challenging interventional procedure which may take up to 4 hours [4]. The major complexity arises from the fact that guidance and control of the distal tip of a catheter inside a beating heart is difficult with- out proper 3D visualization, dexterous control of the flexible catheter and a sense of touch at the catheter tip. There are commercial software packages which provide 3D visualization of the heart and catheter using electro-anatomical mappings and/or pre-operative CT/MRI images of a patient’s heart. Azizian and Patel [5] proposed the use of online 3D ultrasound-based visualization of the heart and ablation catheter using transthoracic echocardiography (TTE) images.
Robotic catheter control systems have also been developed to provide dexter- ous manipulation of flexible ablation catheters. The SenseiR robotic catheter system (Hansen Medical, Mountain View, CA) provides a master/slave control system for remote navigation and control of an ablation catheter using a steerable sheath [6]; It also displays a rough estimation of the tip contact forces on the screen [2]. The NIOBER magnetic navigation system (Stereotaxis, St. Louis, MO) provides mag- netic navigation of a soft catheter [7]. Jayender et al. [8] proposed a master-slave
.A version of this chapter has been presented in IEEE International Conference on Robotics and Automation (ICRA), 2012 and has been published in [1]. [ c2012 IEEE]
robotic system which guides an active catheter instrumented with Shape Memory Alloy (SMA) actuators through the vasculature. However, none of these systems provide haptic feedback or any sort of force/impedance control of the catheter tip.
Studies show that the forces applied by the catheter tip to the heart tissue have a great impact on the outcome of the ablation procedure [2]. Insufficient force could lead to incomplete ablation, while excessive forces could result in complications such as cardiac perforation. Therefore, it would be very useful to provide the cardiologist with a measure of the contact force at the distal tip of the catheter to help him/her perform a faster and more reliable interventional procedure with less complications due to improper catheter/endocardium contact during ablation. It would also be useful to perform a hybrid position/force or impedance control on the catheter tip in order to regulate the applied force at a desired level while keeping contact at a target location on the beating heart wall. Recent work by Kesner and Howe [9] has proposed a robotic catheter system to assist with surgery performed inside the beating heart such as mitral valve annuloplasty. The proposed system benefits from a force sensor mounted right at the tip of the catheter and a force controller which aids in maintaining a constant contact force at the catheter tip during the procedure in spite of the beating heart motion.
Control of the contact force or impedance at the tip of a steerable catheter requires a model of its distal section. The problem of modeling a catheter has been studied by a number of research groups and several different methods have been pro- posed. Numerical approaches have been used for modeling and simulating a catheter inside the vasculature. Ikuta et al. [10] and Dawson et al. [11] proposed a multibody system for modeling long flexible instruments, such as catheters or endoscopes, in which the flexible device is modeled as a set of rigid links connected by flexible joints. Another technique was suggested by Kukuket al. [12], where all possible shapes for a discretized flexible instrument are found and then the shape which complies best with the physical and mechanical constraints is selected. Another approach was proposed by Koningset al. [13]. It considers a guidewire as a series of small rigid segments con- nected with joints and finds the shape of the guidewire within the vasculature based on the principle of minimization of energy, where energy is defined as a function of the
positions of all joints. Another category of numerical methods assumes that a wire- like object is composed of multiple beams and uses a finite element method (FEM) to model it [14–16]. Although FEM can be used to model the behavior of long flexible instruments, it is computationally expensive and may lead to inaccurate results if all the constraints are not known.
Continuum robot theory is another approach to modeling a catheter [17]. The behavior of continuum robots has been well studied and different models have been proposed [18–20]. Camarillo et al. [21] developed a model for tendon-driven contin- uum robots which can also be used for pull-wire catheters. In this model, a single section of the manipulator is considered as a cantilever beam which undergoes large deflection due to actuation by a single tendon. The displacements of tendons are em- ployed to determine the shape of the manipulator using a mapping between tendon displacements and the shape of the beam. However, this approach requires detailed information of the actuators of a continuum robot which is not available for a catheter. We model the distal section of a steerable ablation catheter as a beam that undergoes large deflections and investigate the performance of such a model. The developed model describes the in-plane bending of a catheter when the applied forces are also in the catheter plane. The pull-wire bending of the catheter is modeled with two different approaches and the results are compared. Notably, the proposed model does not require extensive knowledge of the internal structure of the catheter and it can be applied to an arbitrary steerable ablation catheter with a fairly simple calibra- tion step. An experimental setup is used to validate the proposed model empirically. This setup is described in Section 2.2. The mathematical model of a beam with large deflections along with the approaches taken to describe the pull-wire bending of the catheter tip is presented in Section 2.3. Section 2.4 reports experimental results which are then discussed in Section 2.5. Section 2.6 concludes the chapter with suggestions for future research.
(a) Experimental setup (b) Right-angled chessboard used for registering the coordinates in camera
images to global coordinates
Figure 2.1: Experimental setup.