Despite the requirement of a minithoracotomy, transapical aortic valve implantation is a relatively easy, safe, and straightforward technique. The short and direct access route allows excellent alignment between the prosthesis and the aortic root. With the assistance of the visualization of the active and passive markers on the devices in the MRI, the orientation and positioning of the implanted valve are more precise and predictable.
Real-time MRI with proper parameter values provides excellent visualization for intraoperative guidance of aortic valve replacement on the beating heart. It provides better image quality and a complete view of the entire volume of interest more than other competing imaging methods, such as fluoroscopy/angiography, in which some anatomic structures are not visible, and echocardiography, in which the field of view is small and can be obscured by calcification which is frequently the source of the valvular problem. MRI-guided surgery also allows direct functional assessments to be made before, during, and immediately after valve implantation that are not obtainable by conventional imaging alone. However, the presence of a strong magnetic field of MRI scanner demands all the devices used must be MRI safe and compatible.
Both self-expanding and balloon-expandable prostheses are used in TAVR. In our experience, self-expanding stents were easier to position and deploy thus leading to fewer complications during transapical aortic valve replacement. The intrinsic radial force of the self-expanding stent allows for even expansion of the prosthesis. As a result, the orientation of the implanted valve is more predictable. The self-expanding stent can be retrieved and repositioned before it is fully expanded; this aids precise placement and diminishes the risk and embolization. The self-expanding stent, with its specific geometric design, handles torsion better, while the balloon-expandable stent has no elasticity and the material is relatively soft leading to more frequent strut fractures.
Robot assistance can reduce the cognitive load on the physician with improved accuracy and repeatability in transapical valve replacement under MRI guidance. The high magnetic field and the confined space of an MR scanner present many technical challenges. The mechatronic components including actuators, sensors, and controllers must be able to work in an accurate, stable, and robust way in an MR environment. Materials used for a robotic system should have low magnetic susceptibilities (comparable with air, water, or human tissue), low electrical conductibility, adequate mechanical strength, and good manufacturing properties.
The robotic system has been tested on a stable phantom. This phantom is not an ideal replica of the beating heart; but with proper anatomical dimension between the aortic annulus and the apex, it provides a reasonable situation to validate the coordinated working of the different components of the integrated system before preclinical experimentation.
The control strategy and the human machine interface for MRI compatible robot systems for medical interventions need to be studied. In the engineering of robots for medical applications, detailed analyses of the functions of the entire system, that is, robot, interfaces and application, taken as single entity, are arguably more important than the individual performance of the subsystems (robot, surgeon, interfaces, and application, separately). Thus, having a combination of more than one interface such as; an image-guided interface, console guided interface, or hands-on interface based on the specific application might yield a higher performance from the entire system.