Adapting the clinical MRI software environment for real-time navigation of an endovascular untethered ferromagnetic bead for future endovascular interventions

Recent Progress in Magnetically Actuated Microrobots for Targeted Delivery of Therapeutic Agents

Therapeutic agents, such as drugs and cells, play an essential role in virtually every treatment of injury, illness, or disease. However, the conventional practices of drug delivery often result in undesirable side effects caused by drug overdose and off-target delivery. In the case of cell delivery, the survival rate of the transplanted cells is extremely low and difficulties with the administration route of cells remain a problem. Recently, magnetically actuated microrobots have started offering unique opportunities in targeted therapeutic delivery due to their tiny size and ability to access hard-to-reach lesions in a minimally invasive manner; considerable advances in this regard have been made over the past decade. Here, recent progress in magnetically actuated microrobots, developed for targeted drug/cell delivery, is presented, with a focus on their design features and mechanisms for controlled therapeutic release. Additionally, the practical challenges faced by the microrobots, and future research directions toward the swift bench-to-bedside translation of the microrobots are addressed.

A review of magnet systems for targeted drug delivery

Magnetic drug targeting is a method by which magnetic drug carriers in the body are manipulated by external magnetic fields to reach the target area. This method is potentially promising in applications for treatment of diseases like cancers, nervous system diseases, sudden sensorineural hearing loss, and so on, due to the advantages in that it can improve efficacy, reduce drug dosage and side effects. Therefore, it has received extensive attention in recent years. Successful magnetic drug targeting requires a good magnet system to guide the drug carriers to the target site. Up to date there have been many efforts to design the magnet systems for targeted drug delivery. However, there are few comprehensive reviews on these systems. Here we review the progresses made in this field. We summarized the systems already developed or proposed, and categorized them into two groups: static field magnet systems and varying field magnet systems. Based on the requirements for more powerful targeting performance, the prospects and the future research directions in this field are anticipated.

Superparamagnetic Iron Oxide Nanoparticles in Musculoskeletal Biology

The use of platelet-rich plasma and mesenchymal stem cells has garnered much attention in orthopedic medicine, focusing on the biological aspects of cell function. However, shortly after systemic delivery, or even a local injection, few of the transplanted stem cells or platelets remain at the target site. Improvement in delivery, and the ability to track and monitor injected cells, would greatly improve clinical translation. Nanoparticles can effectively and quickly label most cells in vitro, and evidence to date suggests such labeling does not compromise the proliferation or differentiation of cells. A specific type of nanoparticle, the superparamagnetic iron oxide nanoparticle (SPION), is already employed as a magnetic resonance imaging (MRI) contrast agent. SPIONs can be coupled with cells or bioactive molecules (antibodies, proteins, drugs, etc.) to form an injectable complex for in vivo use. The biocompatibility, magnetic properties, small size, and custom-made surface coatings also enable SPIONs to be used for delivering and monitoring of small molecules, drugs, and cells, specifically to muscle, bone, or cartilage. Because SPIONs consist of cores made of iron oxides, targeting of SPIONs to a specific muscle, bone, or joint in the body can be enhanced with the help of applied gradient magnetic fields. Moreover, MRI has a high sensitivity to SPIONs and can be used for noninvasive determination of successful delivery and monitoring distribution in vivo. Gaps remain in understanding how the physical and chemical properties of nanomaterials affect biological systems. Nonetheless, SPIONs hold great promise for regenerative medicine, and progress is being made rapidly toward clinical applications in orthopedic medicine.

MR imaging of therapeutic magnetic microcarriers guided by magnetic resonance navigation for targeted liver chemoembolization

PURPOSE

Magnetic resonance navigation (MRN), achieved with an upgraded MRI scanner, aims to guide new therapeutic magnetic microcarriers (TMMC) from their release in the hepatic vascular network to liver tumor. In this technical note, in vitro and in vivo MRI properties of TMMC, loaded with iron-cobalt nanoparticles and doxorubicin, are reported by following three objectives: (1) to evaluate the lengthening of echo-time (TE) on nano/microparticle imaging; (2) to characterize by MRI TMMC distribution in the liver; and (3) to confirm the feasibility of monitoring particle distribution in real time.

METHODS

Phantom studies were conducted to analyze nano/microparticle signals on T 2*-weighted gradient-echo (GRE) MR images according to sample weight and TE. Twelve animal experiments were used to determine in vivo MRI parameters. TMMC tracking was evaluated by magnetic resonance imaging (MRI) in four rabbits, which underwent MRN in the hepatic artery, three without steering, two in real-time, and three as blank controls. TMMC distribution in the right and left liver lobes, determined by ex vivo MR image analysis, was compared to the one obtained by cobalt level analysis.

RESULTS

TMMC induced a hypointense signal that overran the physical size of the sample on MR images. This signal, due to the nanoparticles embedded into the microparticles, increased significantly with echo-time and sample amount (p < 0.05). In vivo, without steering, contrast-to-noise ratio (CNR) values for the right and left lobes were similar. With MRN, the CNR in the targeted lobe was different from that in the untargeted lobe (p = 0.003). Ex vivo, TMMC distribution, based on MRI signal loss volume measurement, was correlated with that quantified by Co level analysis (r = 0.92). TMMC accumulation was tracked in real time with an 8-s GRE sequence.

CONCLUSIONS

MRI signal loss induced by TMMC can serve to track particle accumulation and to assess MRN efficiency.

An MRI-powered and controlled actuator technology for tetherless robotic interventions

This paper presents a novel actuation technology for robotically assisted MRI-guided interventional procedures. In the proposed approach, the MRI scanner is used to deliver power, estimate actuator state and perform closed-loop control. The actuators themselves are compact, inexpensive and wireless. Using needle driving as an example application, actuation principles and force production capabilities are examined. Actuator stability and performance are analyzed for the two cases of state estimation at the input versus the output of the actuator transmission. Closed-loop needle position control is achieved by interleaving imaging pulse sequences to estimate needle position (transmission output estimation) and propulsion pulse sequences to drive the actuator. A prototype needle driving robot is used to validate the proposed approach in a clinical MRI scanner.

Tracking and position control of an MRI-powered needle-insertion robot

The excellent imaging capabilities of MRI technology are standardizing this modality for a variety of interventional procedures. To assist radiologists, MRI compatible robots relying on traditional actuation technologies are being developed. Recently, a robot that is not only MRI compatible but also MRI powered was introduced. This surgical robot is imaged and actuated through interleaved MRI pulses, and can be controlled to perform automated needle insertion. Using the electromagnetic field generated by the MRI scanner, the robot can exercise adequate forces to puncture tissue. Towards the goal of automation, this paper reports results on tracking and control of an MRI-powered robot tagged with a fiducial marker. Tracking is achieved using non-selective RF pulses and balanced gradient readouts. To suppress the signal received from the tissue, spoiler gradients and background suppression are introduced. Their effects on tracking are quantified and are used to optimize the algorithm. Subsequently, a Kalman filter is employed for robustness. The developed algorithm is used to demonstrate position controlled needle insertion ex vivo.

Automatic slice positioning (ASP) for passive real-time tracking of interventional devices using projection-reconstruction imaging with echo-dephasing (PRIDE)

A novel and fast approach for passive real-time tracking of interventional devices using paramagnetic markers, termed "projection-reconstruction imaging with echo-dephasing" (PRIDE) is presented. PRIDE is based on the acquisition of echo-dephased projections along all three physical axes. Dephasing is preferably set to 4pi within each projection ensuring that background tissues do not contribute to signal formation and thus appear heavily suppressed. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Successful localization of the paramagnetic marker with PRIDE is demonstrated in vitro and in vivo in the presence of different types of off-resonance (air/tissue interfaces, main magnetic field inhomogeneities, etc). In order to utilize the PRIDE sequence for vascular interventional applications, it was interleaved with balanced steady-state free precession (bSSFP) to provide positional updates to the imaged slice using a dedicated real-time feedback link. Active slice positioning (ASP) with PRIDE is demonstrated in vitro, requiring approximately 20 ms for the positional update to the imaging sequence, comparable to existing active tracking methods.

MRI-based Medical Nanorobotic Platform for the Control of Magnetic Nanoparticles and Flagellated Bacteria for Target Interventions in Human Capillaries

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.

Magnetic navigation of an untethered micro device using four stationary coils

We introduce a magnetic navigation of a small magnet using four stationary coils. We used a Maxwell gradient coil to get magnetic propulsion force and three Helmholtz coils to control the moving direction of the magnet in the magnetic navigation. Using a three-channel coil driver with output capacity of 320A, we performed magnetic navigation of a small NdFeB magnet with the size of 10 mm x 10 mm x 12 mm on a horizontal plane. When navigated with a slow speed of about 1 mm/s, the magnet kept track of any arbitrary navigational path. We expect the proposed magnetic navigation method can be easily incorporated into the system for human applications since it does not use any moving coils.

Interventional cardiovascular magnetic resonance: still tantalizing

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."