10-g Averaged Specific Absorption Rate Research Articles

Effect of subject-specific head morphometry on specific absorption rate estimates in parallel-transmit MRI at 7 T.

To assess the accuracy of morphing an established reference electromagnetic head model to a subject-specific morphometry for the estimation of specific absorption rate (SAR) in 7T parallel-transmit (pTx) MRI. Synthetic T1 -weighted MR images were created from three high-resolution open-source electromagnetic head voxel models. The accuracy of morphing a "reference" (multimodal image-based detailed anatomical [MIDA]) electromagnetic model into a different subject's native space (Duke and Ella) was compared. Both linear and nonlinear registration methods were evaluated. Maximum 10-g averaged SAR was estimated for circularly polarized mode and for 5000 random RF shim sets in an eight-channel transmit head coil, and comparison made between the morphed MIDA electromagnetic models and the native Duke and Ella electromagnetic models, respectively. The averaged error in maximum 10-g averaged SAR estimation across pTx MRI shim sets between the MIDA and the Duke target model was reduced from 17.5% with only rigid-body registration, to 11.8% when affine linear registration was used, and further reduced to 10.7% when nonlinear registration was used. The corresponding figures for the Ella model were 16.7%, 11.2%, and 10.1%. We found that morphometry accounts for up to half of the subject-specific differences in pTx SAR. Both linear and nonlinear morphing of an electromagnetic model into a target subject improved SAR agreement by better matching head size, morphometry, and position. However, differences remained, likely arising from details in tissue composition estimation. Thus, the uncertainty of the head morphometry and tissue composition may need to be considered separately to achieve personalized SAR estimation.

Proposed Safety Guidelines for Patient Assistants in an Open MRI Environment.

The wide-open side of an open magnetic resonance imaging (MRI) system allows a patient to easily contact the patient assistant during MRI scans. A wide-open-shaped magnet is highly effective when interventional procedures are necessary. Patient assistants can provide comfort by holding a part of the patient's body. Because current regulations or guidelines are concerned with only patient radio frequency (RF) safety, investigations on the safety of patient assistants exposed to high-magnetic field MRI (up to 1.2 T) are required. In this study, five different poses of patient assistants were numerically simulated at a 1.2 T open MRI system to determine the impact of poses on the RF exposure level. The 10-g averaged specific absorption rate (SAR) levels were analyzed for the poses of each patient assistant wearing gloves. Compared with the patient, up to 29.8% of the patient SAR was observed in the patient assistant. When the patient assistant wore latex gloves, a 63.7% reduction in the 10-g averaged SAR level was observed, which could be a remedy to minimize possible RF hazards. To prevent possible RF hazards during MRI scans, certain clauses regarding the patient assistant's poses or wearing gloves must be added to the existing MRI screening forms.

Numerical Evaluation of Human Body Near Field Exposure to a Vehicular Antenna for Military Applications.

BackgroundThe use of electromagnetic (EM) technologies for military applications is gaining increasing interest to satisfy different operational needs, such as improving battlefield communications or jamming counterpart's signals. This is achieved by the use of high-power EM waves in several frequency bands (e.g., HF, VHF, and UHF). When considering military vehicles, several antennas are present in close proximity to the crew personnel, which are thus potentially exposed to high EM fields.MethodsA typical exposure scenario was reproduced numerically to evaluate the EM exposure of the human body in the presence of an HF vehicular antenna (2–30 MHz). The antenna was modeled as a monopole connected to a 3D polygonal structure representing the vehicle. Both the EM field levels in the absence and in the presence of the human body and also the specific absorption rate (SAR) values were calculated. The presence of the operator, partially standing outside the vehicle, was simulated with the virtual human body model Duke (Virtual Population, V.3). Several exposure scenarios were considered. The presence of a protective helmet was modeled as well.ResultsIn the area usually occupied by the personnel, E-field intensity radiated by the antenna can reach values above the limits settled by international safety guidelines. Nevertheless, local SAR values induced inside the human body reached a maximum value of 14 mW/kg, leading to whole-body averaged and 10-g averaged SAR values well below the corresponding limits.ConclusionA complex and realistic near-field exposure scenario of the crew of a military vehicle was simulated. The obtained E-field values radiated in the free space by a HF vehicular antenna may reach values above the safety guidelines reference levels. Such values are not necessarily meaningful for the exposed subject. Indeed, SAR and E-field values induced inside the body remain well below safety limits.

Performance Enhancement of an MTL Coil Loaded With High-Permittivity Dielectric Liner for 7 T Brain MRI

In a multi-element microstrip transmission line (MTL) transmit array coil, the transmit field ( $B_{1}^{+}$ ) distribution is inhomogeneous due to its standing-wave nature, and the interference effects can severely degrade the $B_{1}^{+}$ and imaging. Therefore, to improve the homogeneity and strength of $B_{1}^{+}$ , this study focuses on the development of a multi-element MTL transmit array coil integrated with a dielectric liner (DL) material. Furthermore, the transmission efficiency ( $T_{x,eff}$ ) is improved in the head region. An eight-element MTL transmits array head coil is investigated using thinner DLs, and the optimized dimensions of the DL are found from its resonant mode at 7 Tesla (T). Simulations and measurements are performed with an MTL transmit array coil at 7 T, and the performance is analyzed for the DL positions and dimensions. Remarkably, the proposed DLs-integrated transmit array coil system offered significant improvements in the $T_{x,eff}$ at different DLs positions. Compared to the case without the DL, the $T_{x,eff}$ is improved by 9% (DL close to the head model), 15% (DL at center), and 39% (DL close to the RF coil). Interestingly, the DL acts as an efficiency tuner element exhibiting a 9% to 39% $T_{x,eff}$ tuning range of improvement. In addition, the RF-shimming technique improves the $B_{1}^{+}$ homogeneity and $T_{x,eff}$ of the coil with DLs by 21% and 42%, respectively, compared to the case without the DL. Moreover, the specific absorption rate (SAR) analysis of the MTL transmit array coil with the DLs is performed. The peak 10g-averaged SAR is reduced from 3.1 W/kg to 2.13 W/kg in the head using DLs and RF-shimming technique. Finally, we used the bench measurement setup to obtain the measured magnetic field. The proposed work exhibits salient features with a smaller DL size and multi-element MTL transmit array coil than other proposed works.

7T MR Thermometry technique for validation of system-predicted SAR with a home-built radiofrequency wrist coil.

A 7T magnetic resonance thermometry (MRT) technique was developed to validate the conversion factor between the system-measured transmitted radiofrequency (RF) power into a home-built RF wrist coil with the system-predicted SAR value. The conversion factor for a new RF coil developed for ultra high magnetic field MRI systems is used to ensure that regulatory limits on RF energy deposition in tissue, specifically the local 10g-averaged specific absorption rate (SAR10g ), are not exceeded. MRT can be used to validate this factor by ensuring that MRT-measured SAR values do not exceed those predicted by the system. A 14-cm diameter high-pass birdcage RF coil was built to image the wrist at 7T. A high spatial and temporal resolution dual-echo gradient echo MRT technique, incorporating quasi-simultaneous RF-induced heating and temperature change measurements using the proton resonance frequency method, was developed. The technique allowed for high-temperature resolution measurements (~±0.1°C) to be performed every 20s over a 4-min heating period, with high spatial resolution (2.56mm3 voxel size) and avoiding phase discontinuities arising from severe magnetic susceptibility-induced B0 inhomogeneities. Magnetic resonance thermometry was performed on a phantom made from polyvinylpyrrolidone to mimic the dielectric properties of muscle tissue at 297.2MHz. Temperature changes measured with MRT and four fiber optic temperature sensors embedded in the phantom were compared. Electromagnetic simulations of the coil and phantom were developed and validated via comparison of simulated and measured B1 + maps in the phantom. The position of maximum SAR within the coil was determined from simulations, and MRT was performed within a wrist-sized piece of meat positioned at that SAR hotspot location. MRT-measured and system-predicted SAR values for the phantom and meat were compared. Temperature change measurements from MRT matched closely to those from the fiber optic temperature sensors. The simulations were validated via close correlation between the simulated and MRT-measured B1 + and SAR maps. Using a coil conversion factor of 2kg-1 , MRT-measured point-SAR values did not exceed the system-predicted SAR10g in either the uniform phantom or in the piece of meat mimicking the wrist located at the SAR hotspot location. A highly accurate MRT technique with high spatial and temporal resolution was developed. This technique can be used to ensure that system-predicted SAR values are not exceeded in practice, thereby providing independent validation of SAR levels delivered by a newly built RF wrist coil. The MRT technique is readily generalizable to perform safety evaluations for other RF coils at 7T.

Improving radiofrequency power and specific absorption rate management with bumped transmit elements in ultra-high field MRI.

In this study, we investigate a strategy to reduce the local specific absorption rate (SAR) while keeping constant inside the region of interest (ROI) at the ultra-high field (B0 ≥ 7T) MRI. Locally raising the resonance structure under the discontinuity (i.e., creating a bump) increases the distance between the accumulated charges and the tissue. As a result, it reduces the electric field and local SAR generated by these charges inside the tissue. The at a point that is sufficiently far from the coil, however, is not affected by this modification. In this study, three different resonant elements (i.e., loop coil, snake antenna, and fractionated dipole [FD]) are investigated. For experimental validation, a bumped FD is further investigated at 10.5T. After the validation, the transmit performances of eight-channel arrays of each element are compared through electromagnetic (EM) simulations. Introducing a bump reduced the peak 10g-averaged SAR by 21, 26, 23% for the loop and snake antenna at 7T, and FD at 10.5T, respectively. In addition, eight-channel bumped FD array at 10.5T had a 27% lower peak 10g-averaged SAR in a realistic human body simulation (i.e., prostate imaging) compared to an eight-channel FD array. In this study, we investigated a simple design strategy based on adding bumps to a resonant element to reduce the local SAR while maintaining inside an ROI. As an example, we modified an FD and performed EM simulations and phantom experiments with a 10.5T scanner. Results show that the peak 10g-averaged SAR can be reduced more than 25%.

A new extrapolation expression for exposure evaluation inside a human-equivalent liquid phantom in the vicinity of wireless power transfer systems

To ensure human safety against electromagnetic exposures from wireless power transfer (WPT) systems operating at MHz-band frequencies, a compliance test must be conducted using either numerical simulations or measurements. In practice, measurements of the specific absorption rate (SAR) inside a human-equivalent liquid phantom are required for exposure evaluation. However, accurate measurements of SAR are difficult, especially at a location close to the phantom boundary, owing to the boundary effect, caused by the field disturbance due to the SAR probe itself. Hence, it is necessary to use an extrapolation method to determine the internal electric field or SAR. In this study, we propose a new extrapolation expression based on the characteristic of electromagnetic fields inside the phantom in order to estimate internal electric field strength or SAR. The validity of the proposed expression was demonstrated both numerically and experimentally. From the results, we confirmed that the difference in the internal electric field strength inside a liquid phantom was less than 3% between our proposed extrapolation expression and the method of moments (MoM). For SAR measurements using two types of actual WPT systems, i.e., solenoid and flat-spiral types, we found that the proposed expression can be used to evaluate the spatial peak 10-g average SAR with a difference of less than 30% compared with those numerically obtained by MoM.

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.

Improved image quality and reduced power deposition in the spine at 3 T using extremely high permittivity materials.

PurposeTo explore the effect of using extremely high permittivity (εr∼1,000) materials on image quality and power requirements of spine imaging at 3 T.Theory and MethodsA linear array of high permittivity dielectric blocks made of lead zirconate titanate (PZT) was designed and characterized by electromagnetic simulations and experiments. Their effect on the transmit efficiency, receive sensitivity, power deposition, and diagnostic image quality was analyzed in vivo in 10 healthy volunteers.ResultsSimulation results showed that for quadrature mode excitation, the PZT blocks improve the transmit efficiency by 75% while reducing the maximum 10g average specific absorption rate (SAR10) by 20%. In vivo experiments in 10 healthy volunteers showed statistically significant improvements for the transmit efficiency, and image quality. Compared to active radiofrequency shimming, image quality was similar, but the required system input power was significantly lower for quadrature excitation using the PZT blocks.ConclusionFor single‐channel transmit systems, using high permittivity PZT blocks offer a way to improve transmit efficiency and image quality in the spine. Results show that the effect, and therefore optimal design, is body mass index and sex specific. Magn Reson Med 79:1192–1199, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

RF induced energy for partially implanted catheters: a computational study.

Magnetic Resonance Imaging (MRI) is a radiological imaging technique widely used in clinical practice. MRI has been proposed to guide the catheters for interventional procedures, such as cardiac ablation. However, there are risks associated with this procedure, such as RF-induced heating of tissue near the catheters. The aim of this study is to develop a quantitative RF-safety method for patients with partially implanted leads at 64 MHz. RF-induced heating is related to the electric field incident along the catheter, which in turns depends on several variables, including the position of the RF feeding sources and the orientation of the polarization, which are however often unknown. This study evaluates the electric field profile along the lead trajectory using simulations with an anatomical human model landmarked at the heart. The energy absorbed in the volume near the tip of ageneric partially implanted lead was computed for all source positions and field orientation. The results showed that varying source positions and field orientation may result in changes of up to 18% for the E-field magnitude and up to 60% for the 10g-averaged specific absorption rate (SAR) in the volume surrounding the tip of the lead.

Resonant Inductive Link for Remote Powering of Pacemakers

This paper presents a wireless power link for powering implantable medical devices. The proposed link operates at 403 MHz and exploits magnetic coupling between an external resonator and an implanted resonator to wirelessly provide power to implantable devices. Experimental results for remote powering of pacemakers are reported and the compliance with safety guidelines is discussed. It is demonstrated that the proposed wireless link is able to deliver up to 1 mW with an induced 10-g average specific absorption rate lower than 1.08 W/kg. This value is significantly below the 2-W/kg recommended limit, thus proving the suitability of the proposed system to be used to energize modern pacemakers.

On the safety margin of using simplified human head models for local SAR simulations of B1-shimming at 7 Tesla

PurposesUsing simplified human models significantly alleviates the difficulty of rendering human models for subject-specific local specific absorption rate (SAR) simulations. Although its accuracy has been demonstrated with the birdcage mode combination of RF transmitters, its accuracy in general B1-shimming, where numerous phase and magnitude combinations can take place, is yet unknown. MethodsThe electromagnetic fields of a 7-Tesla eight-channel brain imaging array were simulated by using four detailed human models from the Virtual Family and their two-, three-, and four-tissue simplifications. The 10-g averaged local SAR was computed for each case with 1,000 sets of uniformly distributed random B1-shimming parameters. Linear regression was applied to relate the local SAR obtained by using detailed and simplified human models. The 99% confidence prediction interval was determined as the safety margin in order to cover the largest local SAR variability introduced by using simplified human models. ResultsThe local SAR computed by using three- and four-tissue simplifications are strongly correlated with those computed by using detailed models. Safety margins of 0.38 and 0.45W/kg/W were found appropriate for each case being considered. ConclusionsThe proposed procedure can be applied to evaluate the safety margin of the local SAR simulated by using simplified human models. However, discretion needs to be exercised since the safety margins in some cases may represent more than 50% overestimation.

A method for safety testing of radiofrequency/microwave-emitting devices using MRI.

Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10-g average specific absorption rate (SAR) computation for safety evaluation of RF-emitting devices. Quantification of the RF power deposition was shown for an MRI-compatible dipole antenna and a non-MRI-compatible mobile phone via phantom temperature change measurements. Validation of the MR temperature mapping method was demonstrated by comparison with physical temperature measurements and electromagnetic field simulations. MR temperature measurements alongside physical property measurements were used to reconstruct 10-g average SAR. The maximum temperature change for a dipole antenna and the maximum 10-g average SAR were 1.83°C and 12.4 W/kg, respectively, for simulations and 1.73°C and 11.9 W/kg, respectively, for experiments. The difference between MR and probe thermometry was <0.15°C. The maximum temperature change and the maximum 10-g average SAR for a cell phone radiating at maximum output for 15 min was 1.7°C and 0.54 W/kg, respectively. Information acquired using MR temperature mapping and thermal property measurements can assess RF/microwave safety with high resolution and fidelity.

Feasibility ofT2-weighted turbo spin echo imaging of the human prostate at 7 tesla

To demonstrate that high quality T2 -weighted (T2w) turbo spin-echo (TSE) imaging of the complete prostate can be achieved routinely and within safety limits at 7 T, using an external transceive body array coil only. Nine healthy volunteers and 12 prostate cancer patients were scanned on a 7 T whole-body system. Preparation consisted of B0 and radiofrequency shimming and localized flip angle calibration. T1 and T2 relaxation times were measured and used to define the T2w-TSE protocol. T2w imaging was performed using a TSE sequence (pulse repetition time/echo time 3000-3640/71 ms) with prolonged excitation and refocusing pulses to reduce specific absorption rate. High quality T2w TSE imaging was performed in less than 2 min in all subjects. Tumors of patients with gold-standard tumor localization (MR-guided biopsy or prostatectomy) were well visualized on 7 T imaging (n = 3). The number of consecutive slices achievable within a 10-g averaged specific absorption rate limit of 10 W/kg was ≥28 in all subjects, sufficient for full prostate coverage with 3-mm slices in at least one direction. High quality T2w TSE prostate imaging can be performed routinely and within specific absorption rate limits at 7 T with an external transceive body array.

Thermal simulations in the human head for high field MRI using parallel transmission

To investigate, via numerical simulations, the compliance of the specific absorption rate (SAR) versus temperature guidelines for the human head in magnetic resonance imaging procedures utilizing parallel transmission at high field. A combination of finite element and finite-difference time-domain methods was used to calculate the evolution of the temperature distribution in the human head for a large number of parallel transmission scenarios. The computations were performed on a new model containing 20 anatomical structures. Among all the radiofrequency field exposure schemes simulated, the recommended 39°C maximum local temperature was never exceeded when the local 10-g average SAR threshold was reached. On the other hand, the maximum temperature barely complied with its guideline when the global SAR reached 3.2 W/kg. The maximal temperature in the eye could very well rise by more than 1°C in both cases. Considering parallel transmission, the recommended values of local 10-g SAR may remain a relevant metric to ensure that the local temperature inside the human head never exceeds 39°C, although it can lead to rises larger than 1°C in the eye. Monitoring temperature instead of SAR can provide increased flexibility in pulse design for parallel transmission.

Occupational Exposure Assessment on an FM Mast: Electric Field and SAR Values

Electric field strengths normally exceed the reference levels for occupational exposure in close vicinity to large frequency modulation (FM) transmitters. Thus, a detailed investigation on compliance with basic restrictions is needed before any administrative protection measures are applied. We prepared a detailed numerical model of a 20-kW FM transmitter on a 32-m mast. An electrically isolated anatomical human model was placed in 3 different positions inside the mast in the region where the values of the electric field were highest. The electric field strengths in this region were up to 700 V/m. The highest calculated whole-body specific absorption rate (SAR) was 0.48 W/kg, whereas the maximum 10-g average SAR in the head and trunk was 1.66 W/kg. The results show that the reference levels in the FM frequency range are very conservative for near field exposure. SAR values are not exceeded even for fields 10 times stronger than the reference levels.

Evaluation of SAR and Temperature Elevation Using Japanese Anatomical Human Models for Body-Worn Devices

This paper investigates the relationship between averaged SAR (Specific Absorption Rate) over 10g mass and temperature elevation in Japanese numerical anatomical models when devices are mounted on the body. Simplifying the radiation source as a half-wavelength dipole, the generated electrical field and SAR are calculated using the FDTD (Finite-Difference Time-Domain) method. Then the bio-heat equation is solved to obtain the temperature elevation due to the SAR derived using the FDTD method as heat source. Frequencies used in the study are 900MHz and 1950 MHz, which are used for mobile phones. In addition, 3500MHz is considered because this frequency is reserved for IMT-Advanced (International Mobile Telecommunication-Advanced System). Computational results obtained herein show that the 10g-average SAR and the temperature elevation are not proportional to frequency. In addition, it is clear that those at 3500MHz are lower than that at 1950MHz even though the frequency is higher. It is the point to be stressed here is that good correlation between the 10 g-average SAR and the temperature elevation is observed even for the body-worn device.

Influence of Specific Absorption Rate Averaging Schemes on Correlation between Mass-Averaged Specific Absorption Rate and Temperature Rise

The influence of specific absorption rate averaging schemes on the spatial correlation between mass-averaged specific absorption rate and radio-frequency-induced steady-state temperature-rise distributions in the “Visible Human” body model exposed to plane waves in the 30–800 MHz frequency range is investigated through finite-difference time-domain modeling. The averaged specific absorption rate is computed on the basis of the IEEE Std. C95.3-2002 specific absorption rate mass-averaging algorithm, employing 1-g and 10-g averaging tissue masses and several air-inclusion factors. The analysis reveals that the 10-g average specific absorption rate yields larger global correlation with the corresponding radio-frequency-induced temperature-rise distribution for the considered plane-wave exposures, while the dependence on the air-inclusion factor features a distinctive threshold behavior.

A Faster Means to Estimate SAR Values for Mobile Phone Applications

Based on experimental studies, a robust transformation factor is introduced for transforming the far-field energy to the near-field one. A simple formula is proposed that accurately and quickly estimates the peak 1-g or 10-g averaged specific absorption rate (SAR) in a human phantom exposed to a mobile handset. It is demonstrated experimentally that the proposed formula has higher accuracy for both 1-g and 10-g averaged SAR across different systems (e.g., CDMA, GSM, DCS, PCS, and WCDMA) by using 100 mobile handsets in four different types to be the device under test (DUT). The corresponding root mean squared errors of the calculated results are 9.96% and 6.82% for 1-g and 10-g averaged peak SAR, respectively, thereby confirming the performance predictions.

Influence of Thermophysiological Parameters on the Calculations of Temperature Rise in the Head of Mobile Phone Users

This paper examines the uncertainty caused to the temperature calculations in the head of mobile phone users, due to the value of the thermophysiological parameters of tissues. A literature review can easily show that the values used for such parameters exhibit a large variability (especially blood perfusion rates). In the current study, it is found that both the tissue distribution inside the model and the variability of thermophysiological tissue properties are significant. It is also confirmed that the 10-g averaged specific absorption rate in the whole head is not a good proxy for the expected temperature rise inside the brain.