3.1 System Architecture
An image-guided robotic system has been designed to assist the procedure of transapical aortic valve replacement. illustrates the system architecture. It comprises two major sub-systems: the imaging system and the robotic system. The imaging system consists of a standard commercial MRI scanner connected with a customized computer for real-time 3D image reconstruction. The robotic system includes a planning interface and a compound manipulator with its control interface. Preoperatively, the subject undergoes a MRI scan to determine the aortic annular diameter, coronary ostial anatomy, and apical location. The slices are sent to the planning interface of the robotic system for the surgeon to review and determine the positioning and the trajectory of the delivery device.
After calibration and registration, information from pre-operative planning is transformed into the robot coordinate frame. The positioning module is moved to the position such that the delivery device is aligned with the planned trajectory. The surgeon then controls the valve delivery module that rigidly affixed to the positioning module to manipulate the delivery device to place the valve into aortic annulus inside MRI scanner by remote control. Interactive rtMRI provides the surgeon real-time feedback of the progress.
3.2 Imaging System
A real-time MRI interactive system [11
] is used for intervention. This system consists of an interactive user interface, and operating room large screen display, specialized pulse sequences, and customized image reconstruction software. With this system, multiple (up to 3) oblique planes can be imaged and displayed simultaneously at their respective 3D locations. Three oblique image planes that used in our intervention are 1) an image plane that passes through the apex, the aortic valve and the left main coronary artery (LM), 2) an image plane that passes through the apex, the aortic valve and the right coronary artery (RCA), 3) an image plane that is on the aortic annulus. The rendering can be rotated on the in-room display to match the orientation of patient. shows the 3D rendering with three image planes viewing. Image slices can be repositioned and added or omitted as needed. The MRI tissue contrast can be interactively channeled by toggling saturation pulses off/on to highlight selected objects.
Real-time multiple slice imaging with 3-D rendering on the custom reconstruction computer
3.3 Robotic System
shows the configuration of our robotic system. It comprises two components: a 5-DOF positioning module and a 3-DOF valve delivery module. An MRI compatible Innomotion robotic arm (Innomedic, Herxheim, Germany) is employed to move and maintain the valve delivery device along with its planed trajectory. This robotic arm was originally developed for precise needle insertion for MR-guided therapy of spinal diseases [5
]. It has 5-DOFs with a remote center of motion (RCM) structure and its configuration fits into a standard closed MRI scanner. This fully MRI compatible robot is built of nonmagnetic materials. It is actuated by pneumatic motors and uses MR-compatible optical encoders for positioning control. In the current stage, the robotic arm is just used as a positioning tool. It can be upgraded to track the patient motion during the procedure. A newly developed 3-DOF valve delivery module is attached to the robotic arm. This compact robotic module is designed for manipulating and placing the valve into a beating heart inside MRI scanner by remote control.
Picture of the robotic system for MRI guided transapical AVR. It shows an Innomotion arm for the positioning module and the prototype of the new developed valve delivery module affixed on the robotic arm.
3.4 Valve Delivery Module
We have developed a 3-DOF valve delivery module to provide precise and reproducible positioning of bioprosthetic aortic valve via transapical approach [12
]. This independent module can be attached to a robotic arm or to a passive arm. The valve delivery module consists of a sterilizable valve delivery unit and an active mechanism that provides the essential manipulation of the delivery device for a valve placement. The valve delivery unit includes a delivery device, a trocar, and a trocar adaptor. The delivery device consists of a straight plastic rod, outside of which is a sheath protecting the prosthetic valve before it is deployed. The translation and rotation of the delivery device directly relate to the translation and rotation of the bioprosthesis. The active mechanism is affixed on the Innomotion robotic arm. The delivery device and the trocar are mounted on the active mechanism of the valve delivery module and the end-effector of the Innomotion arm respectively.
The active mechanism includes three axes: the translation axis A, the rotation axis B and the insertion axis C. The operations of these axes are independent. The order of operation is interchangeable. The translation axis provides linear displacement of the delivery device along its axis. The translation axis can travel at speeds up to 10mm/sec and provide 90mm maximum travel length. The rotation axis allows the delivery device rotating around its axis to change the orientation of the prosthesis relative to coronary ostia before it is deployed. This axis travels at speeds up to 5deg/sec and the travel range is ±60 degree. The third one is the insertion axis. It actuates the relative movement of the inner rod and sheath of the delivery device to advance the prosthesis outside of the sheath.
To maintain image quality and prevent local heating in the proximity of the patient, all parts of the prototype module were made from non-conductive plastic materials such as Delrin, Ultem, and Polyetheretherketones (PEEK). Pneumatic actuators were used to actuate all three axes. The MR compatible pneumatic cylinders were made with glass bores, graphite pistons, brass shafts and plastic housings (Airpel, Norwalk, CT). Magneto-translucent fiber-optical linear and rotary incremental encoders made of glass and ceramics (Innomedic, Herxheim, Germany) were used to gather positioning information.
A 2-GHz Pentium IV IBM PC-compatible machine running Windows XP with an XMP-SynqNet-PCI hardware interface (Motion Engineering Inc, USA) installed was used for control. A PIV (proportional position loop integral and proportional velocity) controller run on the board was used for servoing the valve delivery module movement. Remote Motion Block (RMB) was used to connect conventional analog drives of the pneumatic servo valves into a SynqNet motion network system. The control PC was placed outside the MR room, and the RMB with pneumatic servo valves and other pneumatic supplies was placed outside the 5 Gauss line inside the MR room.
3.5 System Integration and User Interfaces
The Innomotion robotic arm is mounted on the side-rails of the MRI table. The robotic arm is calibrated based on four fiducial markers rigidly attached to its end-effector. These fiducial markers are composed of diluted gadolinium-DTPA.
We developed a graphic user interface for the pre-operative surgical plan. The surgeon first inserts a trocar with an adaptor into the apex of the heart. With a MRI scan, MR images are sent via DICOM protocol to the control PC and planning of the device trajectory is performed by clicking on the images in the user interface. The trajectory is transformed to the registered robot coordinate frame. The surgeon loads the delivery device with the prosthetic valve and inserts the delivery device into the trocar. The robotic arm with the active mechanism of the valve delivery module affixed is moved to the position. The trocar and the delivery device are then mounted on the end-effector and the active mechanism of the delivery module respectively. Thus, direct access to the aortic valve is created. We also developed a simple interface for module control. It provides coarse and fine two-level control on the translation and rotation axes. The surgeon determines and inputs the translation and rotation values and clicks the coarse control buttons to move the prosthetic valve to the correct place, he/she can tune the position and orientation by clicking the fine control buttons. Once the bio-prosthetic valve has reached the desired position, the surgeon clicks the insertion button to advance the valve out from the delivery device and inflates the balloon to deploy the valve.