Cylindrical meanderline radiofrequency coil for intravascular magnetic resonance studies of atherosclerotic plaque

Wireless, Battery-Free, and Fully Implantable Micro-Coil System for 7 T Brain MRI

An elegant solution for the concurrent transmission of data and power is essential for implantable wireless magnetic resonance imaging (MRI). This paper presents a self-tuned open interior microcoil (MC) antenna with three useful operating bands of 300 (7 T), 400, and 920 MHz, for blood vessel imaging, data telemetry, and efficient wireless transmission of power, respectively. The proposed open interior MC antenna contains two mirrorlike arms with diameters and lengths of 2.4 mm and 9.8 mm, respectively, to avoid blood flow blockage. To wirelessly show LED glow on a saline based phantom, the MC was fabricated on a flexible polyimide material and combined with a miniaturized rectifier and a micro-LED. Using a path gain, the power transfer efficiency (PTE) of the MC rotation was also analyzed. Additionally, the PTE was calculated for a range of distances between 25 and 60 mm, and a -27.1 dB PTE attained at a distance of of 30 mm. Based on the recommendations of the International Commission on Non-Ionizing Radiation Protection for human brain safety when exposed to radio-frequencies from external transmitter, a specific absorption rate analysis was analyzed. Measurements of the s-parameters were noted using a saline solution and blood vessel model to imitate a realistic human head. They were found to correlate reasonably with the simulated results.

The misunderstood meander: Redesigning MRI meander-line surface coils to reduce noise, increase uniformity, and eliminate image artifacts

Meander-line, or zig-zag, MRI surface coils theoretically promise spatially uniform fields with optimal field localization close to the coil. In reality, they suffer poorer than expected field localizations and acquired images are often highly inhomogeneous, plagued by repeating stripe-like signal-loss artifacts. We show that both these detrimental effects arise from coil design based on the same invalid approximation in the underlying theory. Here, the conventional approximation is corrected, yielding a modified coil design that validates the new theory by rectifying the above problems. Specifically, an easily implementable coil correction, which amounts to the addition of a single extra turn of wire, is introduced and shown to increase signal uniformity by an order of magnitude, eliminate image artifacts, and reduce unwanted signal interference from deeper within the sample by tightening the coil field localization to close to the coil, as intended for zig-zag designs. With independent optimization of coil size and imaging depth possible, such corrected meander-lines surface coils may be well suited for large area, near-surface imaging and spectroscopy applications.

Modeling Endovascular MRI Coil Coupling With Transmit RF Excitation

OBJECTIVE

To model inductive coupling of endovascular coils with transmit RF excitation for selecting coils for MRI-guided interventions.

METHODS

Independent and computationally efficient FEM models are developed for the endovascular coil, cable, transmit excitation, and imaging domain. Electromagnetic and circuit solvers are coupled to simulate net B₁ + fields and induced currents and voltages. Our models are validated using the Bloch-Siegert B₁ + mapping sequence for a series-tuned multimode coil, capable of tracking, wireless visualization, and high-resolution endovascular imaging.

RESULTS

Validation shows good agreement at 24-, 28-, and 34-μT background RF excitation within experimental limitations. Quantitative coil performance metrics agree with simulation. A parametric study demonstrates tradeoff in coil performance metrics when varying number of coil turns. Tracking, imaging, and wireless marker multimode coil features and their integration is demonstrated in a pig study.

CONCLUSION

Developed models for the multimode coil were successfully validated. Modeling for geometric optimization and coil selection serves as a precursor to time consuming and expensive experiments. Specific applications demonstrated include parametric optimization, coil selection for a cardiac intervention, and an animal imaging experiment.

SIGNIFICANCE

Our modular, adaptable, and computationally efficient modeling approach enables rapid comparison, selection, and optimization of inductively coupled coils for MRI-guided interventions.

Magneto-inductive catheter receiver for magnetic resonance imaging

A catheter-based RF receiver for internal magnetic resonance imaging is demonstrated. The device consists of a double-sided thin-film circuit, wrapped around a hollow catheter and sealed in place with heat-shrink tubing. Signals are detected using a resonant LC circuit at the catheter tip and transmitted along the catheter using an array of coupled LC circuits arranged as a magneto-inductive waveguide, a form of low frequency metamaterial. Coupling to a conventional RF system is accomplished using a demountable inductive transducer. Protection against external B 1 and E fields is obtained by using figure-of-eight elements with an electrical length shorter than that of an immersed dipole. The system is primarily designed for biliary imaging, can pass the biopsy channel of a side-opening duodenoscope, and is guidewire-compatible, potentially allowing clinicians to implement MR image guided procedures without changing their standard practice. Decoupling against B 1 and E fields is verified, and in vitro (1)H magnetic resonance imaging with submillimeter resolution is demonstrated at 1.5 T using phantoms.

Implanted, inductively-coupled, radiofrequency coils fabricated on flexible polymeric material: application to in vivo rat brain MRI at 7 T

Combined with high-field MRI scanners, small implanted coils allow for high resolution imaging with locally improved SNR, as compared to external coils. Small flexible implantable coils dedicated to in vivo MRI of the rat brain at 7 T were developed. Based on the Multi-turn Transmission Line Resonator design, they were fabricated with a Teflon substrate using copper micromolding process and a specific metal-polymer adhesion treatment. The implanted coils were made biocompatible by PolyDimethylSiloxane (PDMS) encapsulation. The use of low loss tangent material achieves low dielectric losses within the substrate and the use of the PDMS layer reduces the parasitic coupling with the surrounding media. An implanted coil was implemented in a 7 T MRI system using inductive coupling and a dedicated external pick-up coil for signal transmission. In vivo images of the rat brain acquired with in plane resolution of (150 μm)(2) thanks to the implanted coil revealed high SNR near the coil, allowing for the visualization of fine cerebral structures.

Surgically implantable magnetic resonance angiography coils improve resolution to allow visualization of blood flow dynamics

BACKGROUND

Magnetic resonance angiography (MRA) is clinically useful but of limited applicability to small animal models due to poor signal resolution, with typical voxel sizes of 1 mm(3) that are insufficient to analyze vessels of diameter <1 mm. We determined whether surgically implantable, extravascular MRA coils increase signal resolution adequately to examine blood flow dynamics

METHODS

A custom MRA coil was surgically implanted near the carotid artery of a New Zealand White rabbit. A stenosis was created in the carotid artery to induce complicated, non-laminar flow. Phase contrast images were obtained on multiple axial planes with 3T MRA and through-plane velocity profiles were calculated under laminar and complicated flow conditions. These velocity profiles were fit to a laminar flow model using ordinary least squares in order to quantify the degree of flow complication (Matlab). Flow was also measured with a Doppler flow probe; vessel diameters and flow velocities were compared with duplex ultrasound

RESULTS

Carotid artery blood flow was 24.7 +/- 2.6 ml/min prior to stenosis creation and reduced to 12.0 +/- 1.7 ml/min following injury (n=3). An MRA voxel size of 0.1 x 0.1 x 5 mm was achieved. The control carotid artery diameter was 1.9 +/- 0.1 mm, and cross-sectional images containing 318 +/- 22 voxels were acquired (n=26). Velocity profiles resembled laminar flow proximal to the stenosis, and then became more complicated just proximal and distal to the stenosis. Laminar flow conditions returned downstream of the stenosis

CONCLUSION

Implantable, extra-vascular coils enable small MRA voxel sizes to reproducibly calculate complex velocity profiles under both laminar and complicated flow in a small animal model. This technique may be applied to study blood flow dynamics of vessel remodeling and atherogenesis.

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.

Development of an active intravascular MR device with an optical transmission system

Magnetic resonance imaging (MRI) is a safe and reliable medical imaging method providing good soft tissue contrast while avoiding harmful ionizing radiation. It is highly desirable to use the MRI technology for interventional procedures. However, due to resonance effects that can result in tissue heating, long conducting cables must be avoided. Motivated by the need for more radio-frequency (RF) safety, we developed an optical transmission system for active intravascular MRI devices. An optical transmitter sends the MR signal via an optical fiber. A miniature optical modulator was designed to be integrated into a catheter tip. Furthermore, power is supplied optically to the transmitter. This system can target new medical applications, due to safe catheter tracking and safe intravascular imaging.

Imaging techniques for the vulnerable coronary plaque

The goal of this article is to illustrate the main invasive and noninvasive diagnostic modalities to image the vulnerable coronary plaque, which is responsible for acute coronary syndrome. The main epidemiologic and histological issues are briefly discussed in order to provide an adequate background. Comprehensive coronary atherosclerosis imaging should involve visualization of the entire coronary artery tree and plaque characterization, including three-dimensional morphology, relationship with the lumen, composition, vascular remodelling and presence of inflammation. No single technique provides such a comprehensive description, and no available modality extensively identifies the vulnerable plaque. In particular, we describe multislice computed tomography, which at present seems to be the most promising noninvasive tool for an exhaustive image-based quantification of coronary atherosclerosis.

Radio frequency coil technology for small-animal MRI

A review of the theory, technology, and use of radio frequency (RF) coils for small-animal MRI is presented. It includes a brief overview of MR signal-to-noise (S/N) analysis and discussions of the various coils commonly used in small-animal MR: surface coils, linear volume coils, birdcages, and their derivatives. The scope is limited to mid-range coils, i.e. coils where the product (fd) of the frequency f and the coil diameter d is in the range 2-30 MHz-m. Common applications include mouse brain and body coils from 125 to 750 MHz, rat body coils up to 500 MHz, and small surface coils at all fields. In this regime, all the sources of loss (coil, capacitor, sample, shield, and transmission lines) are important. All such losses may be accurately captured in some modern full-wave 3D electromagnetics software, and new simulation results are presented for a selection of surface coils using Microwave Studio 2006 by Computer Simulation Technology, showing the dramatic importance of the "lift-off effect". Standard linear circuit simulators have been shown to be useful in optimization of complex coil tuning and matching circuits. There appears to be considerable potential for trading S/N for speed using phased arrays, especially for a larger field of view. Circuit simulators are shown to be useful for optimal mismatching of ultra-low-noise preamps based on the enhancement-mode pseudomorphic high-electron-mobility transistor for optimal coil decoupling in phased arrays. Cryogenically cooled RF coils are shown to offer considerable opportunity for future gains in S/N in smaller samples.

Emerging vascular applications of magnetic resonance imaging: a picture is worth more than a thousand words

Vascular applications of magnetic resonance (MR) imaging are reviewed, with emphasis on algorithms that use nonpictorial information contained in the MR data set. Current clinical vascular practice generally limits use of MR angiography and three-dimensional vessel images to qualitative pictorial rendering without routinely using the available quantitative information contained within the MR data. This review is dedicated to recent advances that include characterization of vessel histology, assessment of carotid plaque vulnerability, characterization of blood flow dynamics, quantitative analysis of disease severity, and prediction of vascular intervention outcome. Examples from histologic preparation, in vitro and in vivo experiments, are discussed, with an emphasis on potential clinical applications and advances in acquisition technology.

Acquired diseases of the thoracic aorta: role of MRI and MRA

Diseases of the thoracic aorta can present with a broad clinical spectrum of symptoms and signs. Their prevalence appears to be increasing in western populations, most likely corresponding to aging and heightened clinical awareness but also influenced by the progress of high-resolution, noninvasive imaging modalities. Among them, MRI provides an excellent visualization of vascular structures and is well suited for evaluation of thoracic aorta disease. Currently, in many centers, noninvasive imaging modalities are the first choice in the cardiovascular system evaluation and diagnosis, reserving conventional angiography for use only before therapeutic intervention. Understanding the principle MRA techniques is essential for acquiring consistent diagnostic images. Basic technical considerations, which include fast spin-echo, fast gradient-echo, and MRA techniques with phase contrast and contrast-enhanced methods, are discussed and applied in the evaluation of acquired thoracic aorta diseases.