Haptic instrumentation for an interventional radiology simulator

Abstract

Low trauma, reduced costs and fast recovery are only a few factors why minimally invasive surgery (MIS) is taking over classical surgical methods. Interventional Radiology (IR) is a minimally invasive procedure where thin tubular instruments are steered through the patient's vascular system under X-ray imaging. In order to perform these procedures, a radiologist has to be trained to master hand-eye coordination, instrument manipulation and procedure protocols. The importance of training IR procedures rises with the increasing variety and sophistication of the procedures performed using this approach. Apart from the traditional see one, do one, teach one model, radiologists have only had few mock devices with limitations in practical use. Training on animals is an alternative, but its use is hindered by high costs and ethical issues. The existing simulation systems all have major drawbacks: the requirement to use modified instruments, high inertia of the haptic feedback that creates a noticeably degraded dynamic behavior, or excessive friction that is not properly compensated. In this thesis we propose a quality training environment dedicated to IR. The system is composed of a virtual reality (VR) simulation of the patients anatomy linked to a robotic interface providing haptic force feedback. This thesis focuses on the requirements, design and prototyping of a specific haptic interface for the manipulation of catheters and guide wires. Translational tracking and force feedback on the catheter are provided by two cylinders forming a friction drive arrangement. The complete friction drive can be set in rotation with an additional motor providing torque feedback. A force and a torque sensor are integrated in the cylinders for direct measurement on the catheter enabling disturbance cancellation with a closed-loop force control strategy. One of the specific requirements during a simulated intervention is to measure the position of the tip of a guide wire inside a catheter. This is a so called in tube problem. The guide wire is detected at the entrance of the simulator and then grasped by a micro-gripper. Forces and movements of the guidewire moving inside the catheter are then transmitted through a gripper and along a rod to an external unit. This thesis also reports a study of several micro-gripper designs developed and tested on a haptic force feedback interface developed at the LSRO-EPFL. A virtual reality environment has been developed to provide real-time visualisation and force feedback. Several different software routines are integrated in the system. During an IR procedure the tip of an employed instrument interacts with a wall of blood vessels and a repulsive force is transmitted to the user. Force feedback helps radiologist to orient and navigate the inserted instrument in the vessel. This thesis presents a novel method for measurement of internal constraints during an IR procedure. The measurements help to determine the maximum interaction forces between the catheter and the wall of the blood vessel and to observe the shape of dynamic interaction during manipulation. In order to recreate a realistic simulation environment it is necessary to derive the elastic properties and behavior of arteries when contact with an instrument occurs. A novel method for measurement of mechanical properties of blood vessels is proposed in this work. Information derived with this method is also very useful to determine coherent boundary conditions for hemodynamic simulations. Measurements of the modulus of elasticity of an artery are carried out by measuring the deformations due to the inflation of an angioplasty balloon catheter used for IR procedures. A test bench, consisting of an angioplasty balloon, a PVA model and an actuator used to inflate the balloon, is developed for realization of experiments. The pressure-volume curve during the inflation of the balloon is observed. The Young's modulus is derived with an analytical model of the measurement system. The results are then analyzed and compared to existing data from literature.