The feasibility for in vivo navigation of untethered devices or robots is demonstrated with the control and tracking of a 1.5 mm diameter ferromagnetic bead in the carotid artery of a living swine using a clinical magnetic resonance imaging ͑MRI͒ platform. Navigation is achieved by inducing displacement forces from the three orthogonal slice selection and signal encoding gradient coils of a standard MRI system. The proposed method performs automatic tracking, propulsion, and computer control sequences at a sufficient rate to allow navigation along preplanned paths in the blood circulatory system. This technique expands the range of applications in MRI-based interventions.

Medical nanorobotics exploits nanometer-scale components and phenomena with robotics to provide new medical diagnostic and interventional tools. Here, the architecture and main specifications of a novel medical interventional platform based on nanorobotics and nanomedicine, and suited to target regions inaccessible to catheterization are described. The robotic platform uses magnetic resonance imaging (MRI) for feeding back information to a controller responsible for the real-time control and navigation along pre-planned paths in the blood vessels of untethered magnetic carriers, nanorobots, and/or magnetotactic bacteria (MTB) loaded with sensory or therapeutic agents acting like a wireless robotic arm, manipulator, or other extensions necessary to perform specific remote tasks. Unlike known magnetic targeting methods, the present platform allows us to reach locations deep in the human body while enhancing targeting efficacy using real-time navigational or trajectory control. The paper describes several versions of the platform upgraded through additional software and hardware modules allowing enhanced targeting efficacy and operations in very difficult locations such as tumoral lesions only accessible through complex microvasculature networks.

Abstract-This paper reports the use of a magnetic resonance imaging (MRI) system to propel a ferromagnetic core. The concept was studied for future development of microdevices designed to perform minimally invasive interventions in remote sites accessible through the human cardiovascular system. A mathematical model is described taking into account various parameters such as the size of blood vessels, the velocities and viscous properties of blood, the magnetic properties of the materials, the characteristics of MRI gradient coils, as well as the ratio between the diameter of a spherical core and the diameter of the blood vessels. The concept of magnetic propulsion by MRI is validated experimentally by measuring the flow velocities that magnetized spheres (carbon steel 1010/1020) can withstand inside cylindrical tubes under the different magnetic forces created with a Siemens Magnetom Vision 1.5 T MRI system. The differences between the velocities predicted by the theoretical model and the experiments are approximately 10%. The results indicate that with the technology available today for gradient coils used in clinical MRI systems, it is possible to generate sufficient gradients to propel a ferromagnetic sphere in the larger sections of the arterial system. In other words, the results show that in the larger blood vessels where the diameter of the microdevices could be as large as a couple a millimeters, the few tens of mT/m of gradients required for displacement against the relatively high blood flow rate is well within the limits of clinical MRI systems. On the other hand, although propulsion of a ferromagnetic core with diameter of 600 m may be possible with existing clinical MRI systems, gradient amplitudes of several T/m would be required to propel a much smaller ferromagnetic core in small vessels such as capillaries and additional gradient coils would be required to upgrade existing MRI systems for operations at such a scale.

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.