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Upgraded gradient coils can effectively enhance the MRI steering of magnetic microparticles in a branching channel. Applications of this method include MRI targeting of magnetic embolization agents for oncologic therapy. A magnetic suspension of Fe(3)O(4) magnetic particles was injected inside a y-shaped microfluidic channel. Magnetic gradients of 0, 50, 100, 200, and 400 mT/m were applied to the magnetic particles perpendicularly to the flow by a custom-built gradient coil inside a 1.5-T MRI scanner. Measurement of the steering ratio was performed both by video analyses and quantification of the mass of the particles collected at each outlet of the microfluidic channel, using atomic absorption spectroscopy. Magnetic particles steering ratios of 0.99 and 0.75 were reached with 400 mT/m gradient amplitude and measured by video analyses and atomic absorption spectroscopy, respectively. Experimental data shows that the steering ratio increases with higher magnetic gradients. Moreover, theory suggests that larger particles (or aggregates), higher magnetizations, and lower flows can also be used to improve the steering ratio. The technological limitation of the approach is that an MRI gradient amplitude increase to a few hundred milliteslas per meter is needed. A simple analytical method based on magnetophoretic velocity predictions and geometric considerations is proposed for steering ratio calculation., ((c) 2010 Wiley-Liss, Inc.)

The possibility of automatically navigating untethered microdevices or future nanorobots to conduct target endovascular interventions has been demonstrated by our group with the computer-controlled displacement of a magnetic sphere along a pre-planned path inside the carotid artery of a living swine. However, although the feasibility of propelling, tracking and performing real-time closed-loop control of an untethered ferromagnetic object inside a living animal model with a relatively close similarity to human anatomical conditions has been validated using a standard clinical Magnetic Resonance Imaging (MRI) system, little information has been published so far concerning the medical and technical protocol used. In fact, such a protocol developed within technological and physiological constraints was a key element in the success of the experiment. More precisely, special software modules were developed within the MRI software environment to offer an effective tool for experimenters interested in conducting such novel interventions. These additional software modules were also designed to assist an interventional radiologist in all critical real-time aspects that are executed at a speed beyond human capability, and include tracking, propulsion, event timing and closed-loop position control. These real-time tasks were necessary to avoid a loss of navigation control that could result in serious injury to the patient. Here, additional simulation and experimental results for microdevices designed to be targeted more towards the microvasculature have also been considered in the identification, validation and description of a specific sequence of events defining a new computer-assisted interventional protocol that provides the framework for future target interventions conducted in humans.

This paper shows that even a simple proportional-integral-derivative (PID) controller can be used in a clinical MRI system for real-time navigation of a ferromagnetic bead along a predefined trajectory. Although the PID controller has been validated in vivo in the artery of a living animal using a conventional clinical MRI platform, here the rectilinear navigation of a ferromagnetic bead is assessed experimentally along a two-dimensional (2D) path as well as the control of the bead in a pulsatile flow. The experimental results suggest the likelihood of controlling untethered microdevices or robots equipped with a ferromagnetic core inside complex pathways in the human body.

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 approximately 600 microm 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.

Steering micro-carriers being tracked by an MRI system may be very attractive in oncology. Here, iron oxide microparticles have been steered in a Y-shaped microchannel placed between a Maxwell pair (dB/dz=443 mT/m) located in the center of an MRI bore. A suspension of 10.82 microm iron oxide particles was injected into the channel and a magnetic gradient generated by the Maxwell pair was used to deflect their trajectory. The experimental results based on the percentage of particles retrieved at the targeted outlet during the experiments show that magnetic gradient steering in the human cardiovascular system within an MRI bore can be envisioned.

The Magnetic Resonance Submarine (MR-Sub) project is a first attempt to validate a new propulsion method for future small magnetically controlled microdevices suited for minimally invasive applications in blood vessels. A Magnetic Resonance Imaging (MRI) system provides the driving force in three dimensions to a ferromagnetic core that could be embedded onto a specialised microdevice. The paper describes preliminary tests made to match the magnetic force induced by an MRI system on a ferromagnetic sphere with the drag force it encompasses in a cylindrical tube. These tests provide a proof of concept demonstrating that this new method of propulsion is very promising within the constraints of such types of operations. This conclusion is based on specific measurements showing that 1010/1020 carbon steel spheres (3.175 mm and 2.381 mm in diameter) can withstand a maximum flow of 0.370 +/- 0.0064 l/min (19.5 cm/s) and 0.311 +/- 0.01209 l/min (16.4 cm/s) respectively when placed inside a 6.35 mm diameter PMMA tube and subjected to a 18 mT/m magnetic field gradient.