Radiofrequency Safety Assessment Research Articles

Multi-echo MR thermometry in the upper leg at 7T using near-harmonic 2D reconstruction for initialization.

The aim of this work is the development of a thermometry method to measure temperature increases in vivo, with a precision and accuracy sufficient for validation against thermal simulations. Such an MR thermometry model would be a valuable tool to get an indication on one of the major safety concerns in MR imaging: the tissue heating occurring due to radiofrequency (RF) exposure. To prevent excessive temperature rise, RF power deposition, expressed as specific absorption rate, cannot exceed predefined thresholds. Using these thresholds, MRI has demonstrated an extensive history of safe usage. Nevertheless, MR thermometry would be a valuable tool to address some of the unmet needs in the area of RF safety assessment, such as validation of specific absorption rate and thermal simulations, investigation of local peak temperatures during scanning, or temperature-based safety guidelines. The harmonic initialized model-based multi-echo approach is proposed. The method combines a previously published model-based multi-echo water/fat separated approach with an also previously published near-harmonic 2D reconstruction method. The method is tested on the human thigh with a multi-transmit array at 7T, in three volunteers, and for several RF shims. Precision and accuracy are improved considerably compared to a previous fat-referenced method (precision: 0.09 vs. 0.19°C). Comparison of measured temperature rise distributions to subject-specific simulated counterparts show good relative agreement for multiple RF shim settings. The high precision shows promising potential for validation purposes and other RF safety applications.

A perturbation approach for ultrafast calculation of RF field enhancements near medical implants in MRI

Patients with medical implants often are deprived of magnetic resonance imaging examination because of safety risks. One specific risk is the enhancement of the radiofrequency fields around the medical implant potentially resulting in significant tissue heating and damage. The assessment of this enhancement is a computationally demanding task, with simulations taking hours or days to converge. Conventionally the source of the radiofrequency fields, patient anatomy, and the medical implant are simulated concurrently. To alleviate the computational burden, we reformulate a fast simulation method that views the medical implant as a small perturbation of the simulation domain without the medical implant and calculates the radiofrequency fields associated with this perturbation. Previously, this method required an extensive offline stage where the result is intractable for large simulation domains. Currently, this offline stage is no longer required and the method is completely online. The proposed method results in comparable radiofrequency fields but is orders of magnitude faster compared to standard simulation technique; the finite-difference time-domain, the finite-sums, and the finite element methods. This acceleration could enable patient-specific and potentially online radiofrequency safety assessment.

32‐Channel self‐grounded bow‐tie transceiver array for cardiac MR at 7.0T

Design, implementation, evaluation, and application of a 32-channel Self-Grounded Bow-Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. The array consists of 32 compact SGBT building blocks. Transmission field ( ) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo efficiency mapping was conducted with actual flip angle imaging. The array's applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n=7). shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5)mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R=4 (meansignal-to-noise ratio (whole heart)≥16.7, meancontrast-to-noise ratio≥13.5) without impairing image quality significantly. The compact SGBT building block facilitates a modular high-density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques.

Modeling Electromagnetic Exposure in Humans Inside a Whole-Body Birdcage Coil Excited by a Two-Channel Parallel Transmitter Operated at 123 MHz

The influence of the accuracy level for whole-body birdcage coil models on electromagnetic exposure estimations was evaluated using three anatomical human body models located at the head landmark position. Both generic and vendor-specific birdcage coil models were created and analyzed using a radio-frequency (RF) circuit and a three-dimensional electromagnetic (EM) co-simulation approach to evaluate EM properties of the coil used in a commercial 3T magnetic resonance imaging (MRI) scanner. The fidelity of the coil geometry and excitation, as well as consideration of permissible variations of the coil electrical components and capacitor losses, had a significant impact on the estimated electric field distributions inside the human models. Therefore, the variety of electric fields generated in humans should be carefully considered for a reliable RF safety assessment of a patient with a passive or active implant undergoing a scan in a modern MRI scanner with dual-channel transmit RF coils.

Accelerating implant RF safety assessment using a low-rank inverse update method.

PurposePatients who have medical metallic implants, e.g. orthopaedic implants and pacemakers, often cannot undergo an MRI exam. One of the largest risks is tissue heating due to the radio frequency (RF) fields. The RF safety assessment of implants is computationally demanding. This is due to the large dimensions of the transmit coil compared to the very detailed geometry of an implant.MethodsIn this work, we explore a faster computational method for the RF safety assessment of implants that exploits the small geometry. The method requires the RF field without an implant as a basis and calculates the perturbation that the implant induces. The inputs for this method are the incident fields and a library matrix that contains the RF field response of every edge an implant can occupy. Through a low‐rank inverse update, using the Sherman–Woodbury–Morrison matrix identity, the EM response of arbitrary implants can be computed within seconds. We compare the solution from full‐wave simulations with the results from the presented method, for two implant geometries.ResultsFrom the comparison, we found that the resulting electric and magnetic fields are numerically equivalent (maximum error of 1.35%). However, the computation was between 171 to 2478 times faster than the corresponding GPU accelerated full‐wave simulation.ConclusionsThe presented method enables for rapid and efficient evaluation of the RF fields near implants and might enable situation‐specific scanning conditions.

Investigations on Tissue-Simulating Medium for MRI RF Safety Assessment for Patients With Active Implantable Medical Devices

In this paper, a full-wave simulation-based method is proposed to determine the optimal tissue-simulating medium for magnetic resonance imaging radio-frequency safety assessment for patients with active implantable medical devices (AIMDs). Compared to the standard medium, the proposed tissue-simulating medium is optimized for each AIMD lead path in human bodies. The reciprocity-based transfer function model is used as a criterion for optimizing the medium parameters. Numerical investigations and validations are performed for a neurostimulator to demonstrate the method.

SAR Simulations & Safety

At ultra-high fields, the assessment of radiofrequency (RF) safety presents several new challenges compared to low-field systems. Multi-channel RF transmit coils in combination with parallel transmit techniques produce time-dependent and spatially varying power loss densities in the tissue. Further, in ultra-high-field systems, localized field effects can be more pronounced due to a transition from the quasi stationary to the electromagnetic field regime. Consequently, local information on the RF field is required for reliable RF safety assessment as well as for monitoring of RF exposure during MR examinations. Numerical RF and thermal simulations for realistic exposure scenarios with anatomical body models are currently the only practical way to obtain the requisite local information on magnetic and electric field distributions as well as tissue temperature.In this article, safety regulations and the fundamental characteristics of RF field distributions in ultra-high-field systems are reviewed. Numerical methods for computation of RF fields as well as typical requirements for the analysis of realistic multi-channel RF exposure scenarios including anatomical body models are highlighted. In recent years, computation of the local tissue temperature has become of increasing interest, since a more accurate safety assessment is expected because temperature is directly related to tissue damage. Regarding thermal simulation, bio-heat transfer models and approaches for taking into account the physiological response of the human body to RF exposure are discussed. In addition, suitable methods are presented to validate calculated RF and thermal results with measurements. Finally, the concept of generalized simulation-based specific absorption rate (SAR) matrix models is discussed. These models can be incorporated into local SAR monitoring in multi-channel MR systems and allow the design of RF pulses under constraints for local SAR.

MRI-based transfer function determination for the assessment of implant safety.

We introduce a new MR-based method to determine the transfer function (TF) for radiofrequency (RF) safety assessment of active implantable medical devices. Transfer functions are implant-specific measures that relate the incident tangential electric field on an (elongated) implant to a scattered electric field at its tip. The proposed method allows for TF determination with a high spatial resolution in relatively fast measurements without requiring dedicated bench setups from MRI images. The principle of reciprocity is used in conjunction with the potential to measure currents with MRI to determine TF. Low-flip angle 3D dual gradient echo MRI data are acquired with an implant as transceive antenna, which requires minimal hardware adaptations. The implant-specific TF is determined from the acquired MRI data, with two different postprocessing methods for comparison. TFs of linear and helical implants can be determined accurately (with a Pearson correlation coefficient R ≥ 0.7 between measurements and simulations, and a difference in field at the tip ΔEtip ≤ 19%) from relatively quick (t < 20 minutes) MRI acquisitions with (several) millimeter spatial resolution. Transfer function determination with MRI for RF safety assessment of implantable medical devices is possible. The proposed MR-based method allows for TF determination in more realistic exposure scenarios and solid media. Magn Reson Med 78:2449-2459, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Heating and Safety Concerns of the Radio-Frequency Field in MRI

Magnetic resonance imaging (MRI) is a widely used and powerful imaging technique for non-invasive clinical diagnosis. The absorbed radiofrequency (RF) energy must be carefully managed, as MRI presents one of the highest RF exposures to humans. Temperature increases in the patient caused by high-level RF exposure is a major safety concern in MRI, potentially causing local thermal tissue damage or systemic overheating. This review article summarizes recent findings in MR safety research, including the clear distinction between exposures of patients with and without implants; evaluates the advantages and limitations of numerical simulations for RF safety assessment in MRI; and discusses the need for additional research at high RF exposure levels and in novel MRI systems.

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

MR thermometry (MRT) is a noninvasive method for measuring temperature that can potentially be used for radio frequency (RF) safety monitoring. This application requires measuring absolute temperature. In this study, a multigradient-echo (mGE) MRT sequence was used for that purpose. A drawback of this sequence, however, is that its accuracy is affected by background gradients. In this article, we present a method to minimize this effect and to improve absolute temperature measurements using MRI. By determining background gradients using a B0 map or by combining data acquired with two opposing readout directions, the error can be removed in a homogenous phantom, thus improving temperature maps. All scans were performed on a 3T system using ethylene glycol-filled phantoms. Background gradients were varied, and one phantom was uniformly heated to validate both compensation approaches. Independent temperature recordings were made with optical probes. Errors correlated closely to the background gradients in all experiments. Temperature distributions showed a much smaller standard deviation when the corrections were applied (0.21°C vs. 0.45°C) and correlated well with thermo-optical probes. The corrections offer the possibility to measure RF heating in phantoms more precisely. This allows mGE MRT to become a valuable tool in RF safety assessment.

Radiofrequency heating induced by 7T head MRI: Thermal assessment using discrete vasculature or pennes' bioheat equation

To evaluate and compare the maximum temperature (T(max) ) in the head after exposure to a 300 MHz radiofrequency (RF) field induced by a magnetic resonance imaging (MRI) coil using two thermal simulation methods: Pennes' bioheat equation (PBHE) and discrete vasculature (DIVA). The electromagnetic field induced in the head by a 7T birdcage coil was simulated using finite-difference time-domain (FDTD) and validated by MRI. The specific absorption rate (SAR) distributions normalized to the 10-gram maximum or the whole-head average were used for PBHE and DIVA simulations. For all cases, the T(max) in PBHE was slightly higher than in DIVA. The T(max) was 37.9-38.4°C, depending on the simulation method or perfusion rate. In some situations, RF exposure limited to SAR(max,10g) led to a T(max) higher than allowed by International Electrotechnical Commission (IEC) regulations. Therefore, it is advisable to use thermal simulations to evaluate RF safety of MRI. The simulation method used only slightly influenced the observed maximum temperature; the observed temperature with PBHE was higher in all situations. So PBHE is an appropriate method for RF safety assessment of MRI in the head. Using DIVA simulations, it was found unlikely that the body temperature increases significantly due to energy deposited by a head coil under normal circumstances.

B Phase mapping at 7 T and its application for in vivo electrical conductivity mapping

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B1(+) phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B1(+) phase (instead of the complex B1(+) field) are addressed. Furthermore it was found that the B1(+) phase can be derived directly from the measurable transceive phase arg(B1(+)B1(-)) in the head. Validation for a human head excited by a 7 T-birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B1(+) phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and -13% respectively from literature values. The proposed method for B1(+) phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches.