A new method to improve RF safety of implantable medical devices using inductive coupling at 3.0 T MRI.

OBJECTIVE: To enhance RF safety when implantable medical devices are located within the body coil but outside the imaging region by using a secondary resonator (SR) to reduce electric fields, the corresponding specific absorption rate (SAR), and temperature change during MRI. MATERIALS AND METHODS: This study was conducted using numerical simulations with an American Society for Testing and Materials (ASTM) phantom and adult human models of Ella and Duke from Virtual Family Models, along with corresponding experimental results of temperature change obtained using the ASTM phantom. The circular SR was designed with an inner diameter of 150 mm and a width of 6 mm. Experimental measurements were carried out using a 3 T Medical Implant Test System (MITS) body coil, electromagnetic (EM) field mapping probes, and an ASTM phantom. RESULTS: The magnitudes of B1+ (|B1+|) and SAR1g were reduced by 15.2% and 5.85% within the volume of interest (VoI) of an ASTM phantom, when a SR that generates opposing electromagnetic fields was utilized. Likewise, the Δ|B1+| and ΔSAR1g were reduced by up to 56.7% and 57.5% within the VoI of an Ella model containing a copper rod when an opposing SR was used. CONCLUSION: A novel method employing the designed SR, which generates opposing magnetic fields to partially shield a sample, has been proposed to mitigate the risk of induced-RF heating at the VoI through numerical simulations and corresponding experiments under various conditions at 3.0 T.

Comparison of SAR distribution of hip and knee implantable devices in 1.5T conventional cylindrical-bore and 1.2T open-bore vertical MRI systems.

PURPOSE: There is increasing use of open-bore vertical MR systems that consist of two planar RF coils. A recent study showed that the RF-induced heating of a neuromodulation device was much lower in the open-bore system at the brain and the chest imaging landmarks. This study focused on the hip and knee implants and compared the specific absorption rate (SAR) distribution in human models in a 1.2T open-bore coil with that of a 1.5T conventional birdcage coil. METHODS: Computational modeling results were compared against the measurement values using a saline phantom. The differences in RF exposure were examined between a 1.2T open-bore coil and a 1.5T conventional birdcage coil using SAR in an anatomical human model. RESULTS: Modeling setups were validated. The body placed closed to the coil elements led to high SAR values in the birdcage system compared with the open-bore system. CONCLUSION: Our computational modeling showed that the 1.2T planar system demonstrated a lower intensity of SAR distribution adjacent to hip and knee implants compared with the 1.5T conventional birdcage system.

Sensitivity and uniformity improvement of phased array MR images using inductive coupling and RF detuning circuits.

OBJECTIVE: To improve sensitivity and uniformity of MR images obtained using a phased array RF coil, an inductively coupled secondary resonator with RF detuning circuits at 300 MHz was designed. MATERIALS AND METHODS: A secondary resonator having detuning circuits to turn off the resonator during the transmit mode was constructed. The secondary resonator was located at the opposite side of the four-channel phased array to improve sensitivity and uniformity of the acquired MR images. Numerical simulations along with phantom and in vivo experiments were conducted to evaluate the designed secondary resonator. RESULTS: The numerical simulation results of |B1+| in a transmit mode showed that magnetic field uniformity would be decreased with a secondary resonator having no detuning circuits because of unwanted interferences between the transmit birdcage coil and the secondary resonator. The standard deviation (SD) of |B1+| was decreased 57% with a secondary resonator containing detuning circuits. The sensitivity and uniformity of |B1-| in the receive mode using a four-channel phased array were improved with the secondary resonator. Phantom experiments using a uniform saline phantom had 20% improvement of the mean signal intensity and 50% decrease in the SD with the secondary resonator. Mice with excess adipose tissue were imaged to demonstrate the utility of the secondary resonator. CONCLUSION: The designed secondary resonator having detuning circuits improved sensitivity and uniformity of mouse MR images acquired using the four-channel phased array.

Hyperpolarized 13C MRI: Path to Clinical Translation in Oncology.

This white paper discusses prospects for advancing hyperpolarization technology to better understand cancer metabolism, identify current obstacles to HP (hyperpolarized) 13C magnetic resonance imaging's (MRI's) widespread clinical use, and provide recommendations for overcoming them. Since the publication of the first NIH white paper on hyperpolarized 13C MRI in 2011, preclinical studies involving [1-13C]pyruvate as well a number of other 13C labeled metabolic substrates have demonstrated this technology's capacity to provide unique metabolic information. A dose-ranging study of HP [1-13C]pyruvate in patients with prostate cancer established safety and feasibility of this technique. Additional studies are ongoing in prostate, brain, breast, liver, cervical, and ovarian cancer. Technology for generating and delivering hyperpolarized agents has evolved, and new MR data acquisition sequences and improved MRI hardware have been developed. It will be important to continue investigation and development of existing and new probes in animal models. Improved polarization technology, efficient radiofrequency coils, and reliable pulse sequences are all important objectives to enable exploration of the technology in healthy control subjects and patient populations. It will be critical to determine how HP 13C MRI might fill existing needs in current clinical research and practice, and complement existing metabolic imaging modalities. Financial sponsorship and integration of academia, industry, and government efforts will be important factors in translating the technology for clinical research in oncology. This white paper is intended to provide recommendations with this goal in mind.

Improvement of 19F MR image uniformity in a mouse model of cellular therapy using inductive coupling.

OBJECTIVE: Improve 19F magnetic resonance imaging uniformity of perfluorocarbon (PFC)-labeled cells by using a secondary inductive resonator tuned to 287 MHz to enhance the induced radio frequency (RF) magnetic field (B1) at 7.05 T. MATERIALS AND METHODS: Following Faraday's induction law, the sign of induced B1 made by the secondary resonator can be changed depending on the tuning of the resonator. A secondary resonator located on the opposite side of the phantom of the 19F surface coil can be shown to enhance or subtract the induced B1 field, depending upon its tuning. RESULTS: The numerical simulation results of rotating transmit B1 magnitude (|B 1 + |) and corresponding experimental 19F images were compared without and with the secondary resonator. With the secondary resonator tuned to 287 MHz, improvements of |B 1 + | and 19F image uniformity were demonstrated. The use of the secondary resonator improved our ability to visualize transplanted cell location non-invasively over a period of 6 weeks. CONCLUSION: The secondary resonator tuned to enhance the induced B1 results in improved image uniformity in a pre-clinical application, enabling cell tracking of PFC-labeled cells with the secondary resonator.

Radio-Frequency Safety Assessment of Stents in Blood Vessels During Magnetic Resonance Imaging.

Purpose: The purpose of this study was to investigate the need for high-resolution detailed anatomical modeling to correctly estimate radio-frequency (RF) safety during magnetic resonance imaging (MRI). RF-induced heating near metallic implanted devices depends on the electric field tangential to the device (Etan ). Etan and specific absorption rate (SAR) were analyzed in blood vessels of an anatomical model to understand if a standard gel phantom accurately represents the potential heating in tissues due to passive vascular implants such as stents. Methods: A numerical model of an RF birdcage body coil and an anatomically realistic virtual patient with a native spatial resolution of 1 mm3 were used to simulate the in vivo electric field at 64 MHz (1.5 T MRI system). Maximum values of SAR inside the blood vessels were calculated and compared with peaks in a numerical model of the ASTM gel phantom to see if the results from the simplified and homogeneous gel phantom were comparable to the results from the anatomical model. Etan values were also calculated in selected stent trajectories inside blood vessels and compared with the ASTM result. Results: Peak SAR values in blood vessels were up to ten times higher than those found in the ASTM standard gel phantom. Peaks were found in clinically significant anatomical locations, where stents are implanted as per intended use. Furthermore, Etan results showed that volume-averaged SAR values might not be sufficient to assess RF safety. Conclusion: Computational modeling with a high-resolution anatomical model indicated higher values of the incident electric field compared to the standard testing approach. Further investigation will help develop a robust safety testing method which reflects clinically realistic conditions.

Improvement of Electromagnetic Field Distributions Using High Dielectric Constant (HDC) Materials for CTL-Spine MRI: Numerical Simulations and Experiments.

This study investigates the use of pads with high dielectric constant (HDC) materials to alter electromagnetic field distributions in patients during magnetic resonance imaging (MRI). The study was performed with numerical simulations and phantom measurements. An initial proof-of-concept and validation was performed using a phantom at 64 MHz, showing increases of up to 10% in electromagnetic field when using distilled water as the high dielectric material. Additionally, numerical simulations with computational models of human anatomy were performed at 128 MHz. Results of these simulations using barium titanate (BaTiO3) beads showed a 61% increase of [Formula: see text] with a quadrature driven RF coil and a 64% increase with a dual-transmit array. The presence of the HDC material also allowed for a decrease of SAR up to twofold (e.g., peak 10 g-averaged SAR from 54 to 22 W/kg with a quadrature driven RF coil and from 27 to 22 W/kg with a dual-transmit array using CaTiO3 powder at 128 MHz). The results of this study show that the use of HDC pads at 128 MHz for MRI spine applications could result in improved magnetic fields within the region of interest, while decreasing SAR outside the region.

RF Safety Evaluation of a Breast Tissue Expander Device for MRI: Numerical Simulation and Experiment.

This study describes the MRI-related radio frequency (RF) safety evaluation of breast tissue expander devices to establish safety criteria. Numerical simulations and experimental measurements were performed at 64 MHz with a gel phantom containing a breast expander. Additionally, computational modeling was performed (64 and 128 MHz) with an adult female model, containing a virtually implanted breast tissue expander device for four imaging landmark positions. The presence of the breast tissue expander device led to significant alterations in specific absorption rate (SAR) and|B1+|distributions. The main source of SAR alterations with the use of the breast expander device was the saline-filled pouch of the expander. Conversely, the variation of RF magnetic field (B1+) was mainly caused by the metallic port. The measured values of electric field magnitude did not increase significantly due to the introduction of the expander device. The maximum 1g- or 10g-averaged SAR values in tissues near the implant were lower than those expected in other regions of the patient body with normalization of both|B1+|equal to 2 µT at the coil isocenter and whole body averaged SAR equal to 4W/kg.

Effects of iron oxide nanoparticles on biological responses and MR imaging properties in human mammary healthy and breast cancer epithelial cells.

Superparamagnetic iron oxide nanoparticles (SPIONs, diameters >50 nm) have received great attention due to their promising use as magnetic resonance imaging (MRI) contrast agents. In this study, we evaluated the cellular uptake and biological responses in vitro of ultrasmall SPIONs (USPIONs, diameters < 50 nm). We compared the cellular responses between breast epithelia isolated from healthy and breast cancer donors after exposure to carboxy-terminated USPIONs (10 and 30 nm PEG-coated, 10 and 30 nm non-PEG-coated). The particles were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS) and gel electrophoresis. Cellular interactions with USPIONs were assessed by confocal microscopy and TEM. Cellular uptake of USPIONs was quantified using ICP-MS. Cell viability was measured by MTT and neutral red uptake assays. T2* weighted MRI scans were performed using a 7T scanner. Results demonstrated that cell association/internalization of USPIONs was size- and surface coating-dependent (PEG vs. non-PEG), and higher cellular uptake of 10 and 30 nm non-coated particles was observed in both cell types compared with PEG-coated particles. Cell uptake for 10 and 30 nm non-coated particles was higher in cancer cells from two of three tested donors compared to healthy cells from three donors. There was no significant cytotoxicity observed for all tested particles. Significantly enhanced MRI contrast was observed following exposure to 10 and 30 nm non-coated particles compared to PEG-coated particles in both cell types. In comparison, cancer cells showed more enhanced MRI signals when compared to normal cells. The data indicate that cell responses following exposure to USPIONs are dependent on particle properties. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1032-1042, 2016.

A Rotational Cylindrical fMRI Phantom for Image Quality Control.

PURPOSE: A novel phantom for image quality testing for functional magnetic resonance imaging (fMRI) scans is described. METHODS: The cylindrical, rotatable, ~4.5L phantom, with eight wedge-shaped compartments, is used to simulate rest and activated states. The compartments contain NiCl2 doped agar gel with alternating concentrations of agar (1.4%, 1.6%) to produce T1 and T2 values approximating brain grey matter. The Jacard index was used to compare the image distortions for echo planar imaging (EPI) and gradient recalled echo (GRE) scans. Contrast to noise ratio (CNR) was compared across the imaging volume for GRE and EPI. RESULTS: The mean T2 for the two agar concentrations were found to be 106.5±4.8, 94.5±4.7 ms, and T1 of 1500±40 and 1485±30 ms, respectively. The Jacard index for GRE was generally found to be higher than for EPI (0.95 versus 0.8). The CNR varied from 20 to 50 across the slices and echo times used for EPI scans, and from 20 to 40 across the slices for the GRE scans. The phantom provided a reproducible CNR over 25 days. CONCLUSIONS: The phantom provides a quantifiable signal change over a head-size imaging volume with EPI and GRE sequences, which was used for image quality assessment.

A novel method to decrease electric field and SAR using an external high dielectric sleeve at 3 T head MRI: numerical and experimental results.

Materials with high dielectric constant (HDC) have been used in high field MRI to decrease specific absorption rate (SAR), increase magnetic field intensity, and increase signal-to-noise ratio. In previous studies, the HDC materials were placed inside the RF coil decreasing the space available. This study describes an alternative approach that considers an HDC-based sleeve placed outside the RF coil. The effects of an HDC on the electromagnetic (EM) field were studied using numerical simulations with a coil unloaded and loaded with a human head model. In addition, experimental EM measurements at 128 MHz were performed inside a custom-made head coil, fitted with a distilled water sleeve. The numerical simulations showed up to 40% decrease in maximum 10 g-avg. SAR on the surface of the head model with an HDC material of barium titanate. Experimental measurements also showed up to 20% decrease of maximum electric field using an HDC material of distilled water. The proposed method can be incorporated in the design of high field transmit RF coils.

Analysis of cancer metabolism by imaging hyperpolarized nuclei: prospects for translation to clinical research.

A major challenge in cancer biology is to monitor and understand cancer metabolism in vivo with the goal of improved diagnosis and perhaps therapy. Because of the complexity of biochemical pathways, tracer methods are required for detecting specific enzyme-catalyzed reactions. Stable isotopes such as (13)C or (15)N with detection by nuclear magnetic resonance provide the necessary information about tissue biochemistry, but the crucial metabolites are present in low concentration and therefore are beyond the detection threshold of traditional magnetic resonance methods. A solution is to improve sensitivity by a factor of 10,000 or more by temporarily redistributing the populations of nuclear spins in a magnetic field, a process termed hyperpolarization. Although this effect is short-lived, hyperpolarized molecules can be generated in an aqueous solution and infused in vivo where metabolism generates products that can be imaged. This discovery lifts the primary constraint on magnetic resonance imaging for monitoring metabolism-poor sensitivity-while preserving the advantage of biochemical information. The purpose of this report was to briefly summarize the known abnormalities in cancer metabolism, the value and limitations of current imaging methods for metabolism, and the principles of hyperpolarization. Recent preclinical applications are described. Hyperpolarization technology is still in its infancy, and current polarizer equipment and methods are suboptimal. Nevertheless, there are no fundamental barriers to rapid translation of this exciting technology to clinical research and perhaps clinical care.