Technical Note: Compact three-tesla magnetic resonance imager with high-performance gradients passes ACR image quality and acoustic noise tests.

PURPOSE: A compact, three-tesla magnetic resonance imaging (MRI) system has been developed. It features a 37 cm patient aperture, allowing the use of commercial receiver coils. Its design allows simultaneously for gradient amplitudes of 85 millitesla per meter (mT/m) sustained and 700 tesla per meter per second (T/m/s) slew rates. The size of the gradient system allows for these simultaneous performance targets to be achieved with little or no peripheral nerve stimulation, but also raises a concern about the geometric distortion as much of the imaging will be done near the system's maximum 26 cm field-of-view. Additionally, the fast switching capability raises acoustic noise concerns. This work evaluates the system for both the American College of Radiology's (ACR) MRI image quality protocol and the Food and Drug Administration's (FDA) nonsignificant risk (NSR) acoustic noise limits for MR. Passing these two tests is critical for clinical acceptance. METHODS: In this work, the gradient system was operated at the maximum amplitude and slew rate of 80 mT/m and 500 T/m/s, respectively. The geometric distortion correction was accomplished by iteratively determining up to the tenth order spherical harmonic coefficients using a fiducial phantom and position-tracking software, with seventh order correction utilized in the ACR test. Acoustic noise was measured with several standard clinical pulse sequences. RESULTS: The system passes all the ACR image quality tests. The acoustic noise as measured when the gradient coil was inserted into a whole-body MRI system conforms to the FDA NSR limits. CONCLUSIONS: The compact system simultaneously allows for high gradient amplitude and high slew rate. Geometric distortion concerns have been mitigated by extending the spherical harmonic correction to higher orders. Acoustic noise is within the FDA limits.

Peripheral nerve stimulation characteristics of an asymmetric head-only gradient coil compatible with a high-channel-count receiver array.

PURPOSE: To characterize peripheral nerve stimulation (PNS) of an asymmetric head-only gradient coil that is compatible with a commercial high-channel-count receive-only array. METHODS: Two prototypes of an asymmetric head-only gradient coil set with a 42-cm inner diameter were constructed for brain imaging at 3T with maximum performance specifications of up to 85 mT/m and 708 T/m/s. Tests were performed in 24 volunteers to measure PNS thresholds with the transverse (x = left-right; y = anterior-posterior [A/P]) gradient coils of both prototypes. Fourteen of these 24 volunteers were also tested for the z-gradient PNS in the second prototype and were scanned with high-slew-rate echo planar imaging (EPI) immediately after the PNS tests. RESULTS: For both prototypes, the y-gradient PNS threshold was markedly higher than the x-gradient threshold. The z-gradient threshold was intermediate between those for the x- and y-coils. Of the 24 volunteers, only two experienced y-gradient PNS at 80 mT/m and 500 T/m/s. All volunteers underwent the EPI scan without PNS when the readout direction was set to A/P. CONCLUSION: Measured PNS characteristics of asymmetric head-only gradient coil prototypes indicate that such coils, especially in the A/P direction, can be used for fast EPI readout in high-performance neuroimaging scans with substantially reduced PNS concerns compared with conventional whole body gradient coils. Magn Reson Med 76:1939-1950, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Prevalence and determinants of hypertension and associated cardiovascular risk factors: data from a population-based, cross-sectional survey in Saint Louis, Senegal.

BACKGROUND: The incidence of cardiovascular disease is growing worldwide and this is of major public health concern. In sub-Saharan Africa, there is a lack of epidemiological data on the prevalence and distribution of risk factors of cardiovascular disease. This study aimed at assessing the prevalence of hypertension and other cardiovascular risk factors among an urban Senegalese population. METHODS: Using an adaptation of the WHO STEPwise approach to chronic disease risk-factor surveillance, we conducted a population-based, cross-sectional survey from 3 to 30 May 2010 on 1 424 participants aged over 15 years. Socio-demographic and behavioural risk factors were collected in step 1. Physical anthropometryc measurements and blood pressure were documented in step 2. Blood tests (cholesterol, fasting blood glucose, and creatinine levels) were carried out in step 3. RESULTS: The prevalence of hypertension was 46% (95% CI: 43.4-48%), with a higher prevalence in females (47.9%) than males (41.7%) (p = 0.015), and 50% of these hypertensive were previously undiagnosed. Mean age was 53.6 years (SD: 15.8). In known cases of hypertension, the average length of its evolution was 6 years 9 months (range 1 month to 60 years). Hypertension was significantly associated with age (p = 0.001), socio-professional category (p = 0.003), dyslipidaemia (p < 0.001), obesity (p < 0.001), physical inactivity (p < 0.001), diabetes (p < 0.001) and stroke (p < 0.001). CONCLUSION: We found a high prevalence of hypertension and other cardiovascular risk factors in this population. There is need of a specific programme for the management and prevention of cardiovascular disease in this population.

Steering of aggregating magnetic microparticles using propulsion gradients coils in an MRI Scanner.

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.

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.

A computer-assisted protocol for endovascular target interventions using a clinical MRI system for controlling untethered microdevices and future nanorobots.

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.

Real-time MRI-based control of a ferromagnetic core for endovascular navigation.

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.

Interventional procedure based on nanorobots propelled and steered by flagellated magnetotactic bacteria for direct targeting of tumors in the human body.

Flagellated bacteria used as bio-actuators may prove to be efficient propulsion mechanisms for future hybrid medical nanorobots when operating in the microvasculature. Here, we briefly describe a medical interventional procedure where flagellated bacteria and more specifically MC-1 Magnetotactic Bacteria (MTB) can be used to propel and steer micro-devices and nanorobots under computer control to reach remote locations in the human body. In particular, we show through experimental results the potential of using MTB-tagged robots to deliver therapeutic agents to tumors even the ones located in deep regions of the human body. We also show that such bacterial nanorobots can be tracked inside the human body for enhanced targeting under computer guidance using MRI as imaging modality. MTB can not only be guided and controlled directly towards a specific target, but we also show experimentally that these flagellated bacterial nanorobots can be propelled and steered in vivo deeply through the interstitial region of a tumor. The targeting efficacy is increased when combined with larger ferromagnetic micro-carriers being propelled by magnetic gradients generated by a MRI platform to carry and release nanorobots propelled by a single flagellated bacterium near the arteriocapillar entry. Based on the experimental data obtained and the experience gathered during several experiments conducted in vivo with this new approach, a general medical interventional procedure is briefly described here in a biomedical engineering context.

Medical and technical protocol for automatic navigation of a wireless device in the carotid artery of a living swine using a standard clinical MRI system.

A 1.5 mm magnetic sphere was navigated automatically inside the carotid artery of a living swine. The propulsion force, tracking and real-time capabilities of a Magnetic Resonance Imaging (MRI) system were integrated into a closed loop control platform. The sphere was released using an endovascular catheter approach. Specially developed software is responsible for the tracking, propulsion, event timing and closed loop position control in order to follow a 10 roundtrips preplanned trajectory on a distance of 5 cm inside the right carotid artery of the animal. Experimental protocol linking the technical aspects of this in vivo assay is presented. In the context of this demonstration, many challenges which provide insights about concrete issues of future nanomedical interventions and interventional platforms have been identified and addressed.

Magnetic microparticle steering within the constraints of an MRI system: proof of concept of a novel targeting approach.

This paper presents a magnetic microparticle steering approach that relies on improved gradient coils for Magnetic Resonance Imaging (MRI) systems. A literature review exposes the motivation and advantages of this approach and leads to a description of the requirements for a set of dedicated steering gradient coils in comparison to standard imaging coils. An experimental set-up was developed to validate the mathematical models and the hypotheses arising from this targeting modality. Magnetite Fe(3)O(4) microparticles (dia. 10.9 microm) were steered in a Y-shaped 100 microm diameter microchannel between a Maxwell pair (dB/dz = 443 mT/m) located in the center of an MRI bore with 0.525 m/s mean fluid velocity (ten times faster than in arterioles with same diameter). Experimental results based on the percentage of particles retrieved at the targeted outlet show that the mathematical models developed provide an order of magnitude estimate of the magnetic gradient strengths required. Furthermore, these results establish a proof of concept of microparticle steering using magnetic gradients within an MRI bore for applications in the human cardiovascular system.

Method of propulsion of a ferromagnetic core in the cardiovascular system through magnetic gradients generated by an MRI system.

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.

Magnetic steering of iron oxide microparticles using propulsion gradient coils in MRI.

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.

Preliminary investigation of the feasibility of magnetic propulsion for future microdevices in blood vessels.

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.