Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding.

PURPOSE: To optimize the design of radiofrequency (RF) shielding of transmit coils at 7T and reduce eddy currents generated on the RF shielding when imaging with rapid gradient waveforms. METHODS: One set of a four-element, 2 × 2 Tic-Tac-Toe head coil structure was selected and constructed to study eddy currents on the RF coil shielding. The generated eddy currents were quantitatively studied in the time and frequency domains. The RF characteristics were studied using the finite difference time domain method. Five different kinds of RF shielding were tested on a 7T MRI scanner with phantoms and in vivo human subjects. RESULTS: The eddy current simulation method was verified by the measurement results. Eddy currents induced by solid/intact and simple-structured slotted RF shielding significantly distorted the gradient fields. Echo-planar images, B1+ maps, and S matrix measurements verified that the proposed slot pattern suppressed the eddy currents while maintaining the RF characteristics of the transmit coil. CONCLUSION: The presented dual-optimization method could be used to design RF shielding and reduce the gradient field-induced eddy currents while maintaining the RF characteristics of the transmit coil

Biological effects of exposure to magnetic resonance imaging: an overview

The literature on biological effects of magnetic and electromagnetic fields commonly utilized in magnetic resonance imaging systems is surveyed here. After an introduction on the basic principles of magnetic resonance imaging and the electric and magnetic properties of biological tissues, the basic phenomena to understand the bio-effects are described in classical terms. Values of field strengths and frequencies commonly utilized in these diagnostic systems are reported in order to allow the integration of the specific literature on the bio-effects produced by magnetic resonance systems with the vast literature concerning the bio-effects produced by electromagnetic fields. This work gives an overview of the findings about the safety concerns of exposure to static magnetic fields, radio-frequency fields, and time varying magnetic field gradients, focusing primarily on the physics of the interactions between these electromagnetic fields and biological matter. The scientific literature is summarized, integrated, and critically analyzed with the help of authoritative reviews by recognized experts, international safety guidelines are also cited

RF Injection Network Development for Testing of Active Implantable Medical Devices Exposed to RF Fields in 1.5 T MRI Systems

© 2016 IEEE. This paper presents the design, construction, and testing of an RF injection network for MR-conditional medical testing of devices for use within 1.5 T MRI scanners (i.e., frequency of 63.4 MHz). The system was developed to meet the requirements of ISO/TS 10974:2018(E). A directional lumped element coupler, power splitter, an attenuator/isolator, low-pass filter, and high-pass filter were designed and implemented as part of the network. The RF injection network was developed in both a compact version implemented in a single PCB and discrete PCB version for use in different situations. The performance of each designed component was simulated and compared to measurement results. As an application example, a neuromodulation system was tested using the developed RF injection network for conductive emission testing

2 mm Radius Loop Antenna and Linear Active Balun for near Field Measurement of Magnetic Field in MRI-Conditional Testing of Medical Devices

© 2018 IEEE. This paper presents the design and construction of a magnetic field probe with an active balun for conditional testing of medical devices within a magnetic resonance imaging (MRI). The magnetic field probe was designed using a small loop antenna with 2 mm radius to have high spatial resolution and accuracy. It was tuned and matched at a center frequency of 128 MHz, which corresponds to 3 T MRI systems or equivalent radio frequency (RF) exposure systems for MRI-conditional testing of medical devices. An active balun with a differential transistor topology and a passive low-profile transformer were employed to boost the detected magnetic field signal level lead to higher sensitivity. It also has high input impedance that improves the decoupling of the probe to nearby medical devices. The designed magnetic field probe and active balun have been fabricated on a double-sided printed circuit board, FR4 thickness of 1.57 mm and a copper thickness of 35 μm, with overall footprint of 22 mm × 11 mm. A verification test setup was developed to generate a known field and calibrate the probe based upon an analytic calculation of field, FDTD simulation and a 10-mm radius passive tuned/matched loop antenna

Application and experimental validation of an integral method for simulation of gradient-induced eddy currents on conducting surfaces during magnetic resonance imaging.

The time-varying magnetic fields created by the gradient coils in magnetic resonance imaging can produce negative effects on image quality and the system itself. Additionally, they can be a limiting factor to the introduction of non-MR devices such as cardiac pacemakers, orthopedic implants, and surgical robotics. The ability to model the induced currents produced by the switching gradient fields is key to developing methods for reducing these unwanted interactions. In this work, a framework for the calculation of induced currents on conducting surface geometries is summarized. This procedure is then compared to two separate experiments: (1) the analysis of the decay of currents induced upon a conducting cylinder by an insert gradient set within a head only 7 T MR scanner; and (2) analysis of the heat deposited into a small conductor by a uniform switching magnetic field at multiple frequencies and two distinct conductor thicknesses. The method was shown to allow the accurate modeling of the induced time-varying field decay in the first case, and was able to provide accurate estimation of the rise in temperature in the second experiment to within 30% when the skin depth was greater than or equal to the thickness of the conductor

H-field probe with active balun for MRI-conditional testing of medical devices

This work presents the design and construction of a H-field probe with active balun for MRI (magnetic resonance imaging) conditional testing of medical devices. The H-field probe was designed using a small loop antenna with 2 mm radius to have high spatial resolution and sensitivity. It was tuned and matched at a center frequency of 128 MHz, which corresponds to 3 T MRI systems or equivalent RF exposure systems. An active balun with a gain 18.28 dB and noise figure of 0.5 dB was employed to boost the detected H-field signal level. The designed H-field probe and active balun have been fabricated on a double-sided printed circuit board (PCB), FR4 thickness of 1.57 mm and a copper thickness of 35 μm, with overall footprint of 22 mm × 11 mm

Lumped Element RF Low Pass Filter with 20 dB Improved Suppression at the Center Frequency of an RF Injection Network for Conductive Emission Testing

In this work, a modified low pass filter with 20 dB improved rejection at the center frequency of the RF injection network was developed to monitor the RF rectification of an active implantable medical devices (AIMD) during conductive emission testing. Transfer function of the designed filter was calculated, verified and validated by ADS simulation and S-parameter measurement using RF network analyzer. Analytical calculation, simulation and measurement results of the filter transfer function are in good agreement

Ultra-low frequency magnetic field probe for field measurements in gradient coils during medical device testing

© The Institution of Engineering and Technology 2019. In this Letter ultra-low frequency probes for measuring time-varying magnetic field produced in gradient coils were designed, fabricated, and tested. A 1 cm radius induction loop antenna followed by an instrumentational amplifier with a gain of 40 dB and a cutoff frequency of 30 KHz was designed to measure the magnetic field. The probe was fabricated on a single-sided PCB, FR4 of thickness 1.57 mm and copper of thickness 35 μm was employed in a dB/dt exposure system within a saline phantom using 3D printed waterproofed cover. Simulation and measurement results of the magnetic field were in good agreements

Measuring Electric Fields Produced by MRI Gradient Coils Using a Patch Antenna Probe

As a part of this work a small patch antenna probe was developed to measure the variation in the electric field produced by gradient coils within an MRI in the presence of any active implantable medical devices (AIMDs). This probe was designed, fabricated, and tested within a gradient coil mimicking dB/dt exposure platform. A 2×1 cm small patch antenna followed by an instrumentational amplifier was chosen to measure the electric fields. Probe was fabricated using a 4-layer PCB. The fabricated probe was used to monitor the electric fields within the phantom in the gradient coil environment. To verify the observed behavior of the probe a simulation study was performed using Sim4Life. This study aims to assess the performance of this probe in a tissue mimicking environment within the coil