Real-time Magnetic Resonance Gradient-based Propulsion of a Wireless Microdevice Using Pre-Acquired Roadmap and Dedicated Software Architecture

Real-time software platform design for in-vivo navigation of a small ferromagnetic device in a swine carotid artery using a magnetic resonance imaging system

Using an 1.5T Siemens clinical Magnetic Resonance Imaging system (MRI), a 1.5 mm diameter ferromagnetic bead is moved across a pre-planned path in the carotid artery of a 25 kg living swine. The software architecture for the navigation and path planning is herein described. Using the real-time feedback capabilities of recent MRIs, the device is moved, controlled and tracked using the magnetic gradients coils already present for imaging purposes. Navigation of the ferromagnetic device has been achieved with a peak velocity of about 13 cm/s through a set of pre established 11 waypoints. The dedicated software architecture presented in this paper lies in a modified real-time MRI imaging sequence. The dedicated architecture permits the navigation of the ferromagnetic bead with an operating frequency of 24 Hz Real-time control of the magnetic core is achieved through the implementation of a simple 2D PID controller incorporated in the presented software platform.

In vivo MR-tracking based on magnetic signature selective excitation

A novel magnetic resonance (MR)-tracking method specifically developed to locate the ferromagnetic core of an untethered microdevice, microrobot, or nanorobot for navigation or closed-loop control purpose is described. The tracking method relies on the application of radio-frequency (RF) excitation signals tuned to the equipotential magnetic curves generated by the magnetic signature of the object being tracked. Positive contrast projections are obtained with reference to the position of the magnetic source. A correlation function performed on only one k-space line for each of the three axes and corresponding to three projections, is necessary to obtain a 3-D location of the device. In this study, the effects of the sphere size and the RF frequency offset were investigated in order to find the best contrast noise ratio (CNR) for tracking. Resolution and precision were also investigated by proper measurement of the position of a ferromagnetic sphere by magnetic resonance imaging (MRI) acquisition and by comparing them with the real position. This method is also tested for a moving marker where the positions found by MRI projections were compared with the ones taken with a camera. In vitro and in vivo experiments show the operation of the technique in tortuous phantom and in animal models. Although the method was developed in the prospect of new interventional MR-guided endovascular operations based on miniature untethered devices, it could also be used as a passive tracking method using tools such as catheters or guide wires.

Impact of the MRI-based Navigation System Constraints on the Step Response Using a PID Controller

This paper presents the identification and the study of the impact of a real-time MRI-based propulsion and tracking system constraints on the step response using a discrete PID controller. A simplified model for such a system accounts for these constraints such as the time delay, the sampling period, and the blood velocity. This application is intended for minimally invasive operations within the human cardiovascular system.