Local Specific Absorption Rate Values Research Articles

Deep learning-based local SAR prediction using B1 maps and structural MRI of the head for parallel transmission at 7 T.

To predict subject-specific local specific absorption rate (SAR) distributions of the human head for parallel transmission (pTx) systems at 7 T. Electromagnetic energy deposition in tissues is nonuniform at 7 T, and interference patterns due to individual channels of pTx systems may result in increased local SAR values, which can only be estimated with very high safety margins. We proposed, designed, and demonstrated a multichannel 3D convolutional neural network (CNN) architecture to predict local SAR maps as well as peak-spatial SAR (ps-SAR) levels. We hypothesized that utilizing a three-channel 3D CNN, in which each channel is fed by a map, a phase-reversed map, and an MR image, would improve prediction accuracies and decrease uncertainties in the predictions. We generated 10 new head-neck body models, along with 389 3D pTx MRI data having different RF shim settings, with their B1 and local SAR maps to support efforts in this field. The proposed three-channel 3D CNN predicted ps-SAR10g levels with an average overestimation error of 20%, which was better than the virtual observation points-based estimation error (i.e., 152% average overestimation). The proposed method decreased prediction uncertainties over 20% (i.e., 22.5%-17.7%) compared to other methods. A safety factor of 1.20 would be enough to avoid underestimations for the dataset generated in this work. Multichannel 3D CNN networks can be promising in predicting local SAR values and perform predictions within a second, making them clinically useful as an alternative to virtual observation points-based methods.

Analyzing the SAR in Human Head Tissues under Different Exposure Scenarios

This paper deals with the assessment of induced specific absorption rate (SAR) in various human models under different exposure scenarios, including both laboratory measurements and simulations. Firstly, SAR values were measured in a standardized SAR laboratory using a phantom for two radiofrequency electromagnetic field (RF-EMF) sources at 900 MHz and 1800 MHz. These laboratory measurements served as a reference for SAR calculations conducted on a specific anthropomorphic mannequin (SAM) using a computer simulation technology (CST) program, thus enabling the determination of antenna location and excitation signal levels for further evaluation. Subsequently, simulations were carried out with CST to evaluate average SAR for the head and for specific head tissues such as the brain, muscles, and fat. Realistic computational human models were also used alongside SAM in CST to explore the influence of gender, age, and tissue type on SAR. Various power levels representing low, moderate, and high RF-EMF exposure were applied to the human models to compare against basic restrictions and reference levels. The simulation results indicate significantly higher SAR values calculated for 1800 MHz compared with 900 MHz. The ratio of the highest SAR values at 1800 MHz to 900 MHz is approximately 1.70 for a baby, 2.59 for a child, and 2.84 for both adult female and adult male. While the SAR values for the brain, fat, muscle, and head are comparable at 900 MHz for the baby, the brain’s SAR value at 1800 MHz stands out significantly from the other tissues. In contrast with the baby, the difference in SAR values between 900 MHz and 1800 MHz is more pronounced for the child, adult female and adult male. The lowest SAR values at 900 MHz and 1800 MHz were obtained for brain tissue in all human models, while the head has the highest SAR value. The maximum SAR change ratio between the brain and the head is calculated to be 4.44 for the male at 1800 MHz. The results reveal that, although the applied electromagnetic field levels were below reference levels for general public local exposure, some local SAR values exceeded the International Commission of Non-Ionizing Radiation Protection’s basic restriction for the general public at certain power levels, particularly at 1800 MHz. The SAR analysis derived from this study is significant in understanding the impact of wireless technologies on health, establishing safety standards, guiding technology advancement, conducting risk assessments, and increasing public awareness.

Safety Analysis of Medical Implants in the Human Head Exposed to a Wireless Power Transfer System

The interactions of medical implants in the human body with electromagnetic fields from newly introduced high-field technologies such as near-field wireless power transfer (WPT) are of great concern. In this study, the effects of implants on the specific absorption rate (SAR) in a head model were computationally evaluated in the immediate vicinity of a WPT system operating at 6.78 MHz with a transferred power of 50 W. The SAR in the head model located 30 mm above the WPT system was evaluated with a skull plate, bone plates (different shapes), miniplate, fixtures, and a deep brain stimulator. The results indicated that even small implants had notable effects on the SAR distribution and corresponding local peak and mass-averaged SAR values. Apart from the size and shape of the implant, the crucial factor influencing the SAR was the implant’s position with respect to the WPT system. Therefore, various position configurations were simulated for the WPT system to determine the worst case scenario for each specific implant. Although the local peak SAR value was increased by a factor of 600 in the worst-case scenario (for the skull plate), the SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{1}g}$</tex-math></inline-formula> and SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{10}g}$</tex-math></inline-formula> were only increased by factors of seven and four, respectively. Compliance with international safety limits was studied, followed by computing the maximum allowable transmit power (MATP). It was found that, without the implant, the MATP satisfying SAR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{\text{1}g}$</tex-math></inline-formula> was 43 W for the designed WPT system, and was reduced to 6.9 W in the presence of the skull plate.

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.

Real-time assessment of potential peak local specific absorption rate value without phase monitoring: Trigonometric maximization method for worst-case local specific absorption rate determination.

PurposeMulti‐transmit MRI systems are typically equipped with dedicated hardware to sample the reflected/lost power in the transmit channels. After extensive calibration, the amplitude and phase of the signal at the feed of each array element can be accurately determined. However, determining the phase is more difficult and monitoring errors can lead to a hazardous peak local specific absorption rate (pSAR10g) underestimation. For this purpose, methods were published for online maximum potential pSAR10g estimation without relying on phase monitoring, but these methods produce considerable overestimation. We present a trigonometric maximization method to determine the actual worst‐case pSAR10g without any overestimation.Theory and MethodThe proposed method takes advantage of the sinusoidal relation between the SAR10g in each voxel and the phases of input signals, to return the maximum achievable SAR10g in a few iterations. The method is applied to determine the worst‐case pSAR10g for three multi‐transmit array configurations at 7T: (1) body array with eight fractionated dipoles; (2) head array with eight fractionated dipoles; (3) head array with eight rectangular loops. The obtained worst‐case pSAR10g values are compared with the pSAR10g values determined with a commonly used method and with a more efficient method based on reference‐phases.ResultsFor each voxel, the maximum achievable SAR10g is determined in less than 0.1 ms. Compared to the reference‐phases‐based method, the proposed method reduces the mean overestimation of the actual pSAR10g up to 52%, while never underestimating the true pSAR10g.ConclusionThe proposed method can widely improve the performance of parallel transmission MRI systems without phase monitoring.

Bent folded-end dipole head array for ultrahigh-field MRI turns "dielectric resonance" from an enemy to a friend.

To provide transmit whole-brain coverage at 9.4 T using an array with only eight elements and improve the specific absorption rate (SAR) performance, a novel dipole array was developed, constructed, and tested. The array consists of eight optimized bent folded-end dipole antennas circumscribing a head. Due to the asymmetrical shape of the dipoles (bending and folding) and the presence of an RF shield near the folded portion, the array simultaneously excites two modes: a circular polarized mode of the array itself, and the TE mode ("dielectric resonance") of the human head. Mode mixing can be controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared with unfolded dipoles. By optimizing the length of the folded portion, we can also minimize the peak local SAR (pSAR) value and decouple adjacent dipole elements. The new array improves the SEE (< >/√pSAR) value by about 50%, as compared with the unfolded bent dipole array. It also provides better whole-brain coverage compared with common single-row eight-element dipole arrays, or even to a more complex double-row 16-element surface loop array. In general, we demonstrate that rather than compensating for the constructive interference effect using additional hardware, we can use the "dielectric resonance" to improve coverage, transmit field homogeneity, and SAR efficiency. Overall, this design approach not only improves the transmit performance in terms of the coverage and SAR, but substantially simplifies the common surface loop array design, making it more robust, and therefore safer.

Proposal of 28 GHz In Vitro Exposure System Based on Field Uniformity for Three-Dimensional Cell Culture Experiments.

This paper proposes a novel in vitro exposure system operating at millimeter-wave (mmWave) 28 GHz, one of the frequency bands under consideration for fifth generation (5G) communication. We employed the field uniformity concept along cross-sectional observation planes at shorter distances from the radiation antenna for better efficiency and a small-size system. A choke-ring antenna was designed for this purpose in consideration of a wider beamwidth (BW) and a symmetric far-field pattern across three principal planes. The permittivity of Dulbecco's modified Eagle's medium solution was measured to examine the specific absorption rate (SAR) of the skin cell layer inside a Petri dish model for a three-dimensional (3D) cell culture in vitro experiment. The best deployment of Petri dishes, taking into account a geometrical field symmetry, was proposed. Local SAR values within the cell layer among the Petri dishes were determined with different polarization angles. It was determined that this polarization effect should be considered when the actual exposure and deployment were conducted. We finally proposed an in vitro exposure system based on the field uniformity including downward exposure from an antenna for 3D cell culture experiments. A small-size chamber system was obtained, and the size was estimated using the planar near-field chamber design rule. Bioelectromagnetics. 2019;40:445-457. © 2019 Bioelectromagnetics Society.

It's the little things: On the complexity of planar electrode heating in MRI

Neurological disorders are increasingly analysed and treated with implantable electrodes, and patients with such electrodes are studied with MRI despite the risk of radio-frequency (RF) induced heating during the MRI exam. Recent clinical research suggests that electrodes with smaller diameters of the electrical interface between implant and tissue are beneficial; however, the influence of this electrode contact diameter on RF-induced heating has not been investigated. In this work, electrode contact diameters between 0.3 and 4 mm of implantable electrodes appropriate for stimulation and electrocorticography were evaluated in a 1.5 T MRI system. In situ temperature measurements adapted from the ASTM standard test method were performed and complemented by simulations of the specific absorption rate (SAR) to assess local SAR values, temperature increase and the distribution of dissipated power. Measurements showed temperature changes between 0.8 K and 53 K for different electrode contact diameters, which is well above the legal limit of 1 K. Systematic errors in the temperature measurements are to be expected, as the temperature sensors may disturb the heating pattern near small electrodes. Compared to large electrodes, simulations suggest that small electrodes are subject to less dissipated power, but more localized power density. Thus, smaller electrodes might be classified as safe in current certification procedures but may be more likely to burn adjacent tissue. To assess these local heating phenomena, smaller temperature sensors or new non-invasive temperature sensing methods are needed.

Intersubject specific absorption rate variability analysis through construction of 23 realistic body models for prostate imaging at 7T.

For ultrahigh field (UHF) MRI, the expected local specific absorption rate (SAR) distribution is usually calculated by numerical simulations using a limited number of generic body models and adding a safety margin to take into account intersubject variability. Assessment of this variability with a large model database would be desirable. In this study, a procedure to create such a database with accurate subject-specific models is presented. Using 23 models, intersubject variability is investigated for prostate imaging at 7T with an 8-channel fractionated dipole antenna array with 16 receive loops. From Dixon images of a volunteer acquired at 1.5T with a mockup array in place, an accurate dielectric model is built. Following this procedure, 23 subject-specific models for local SAR assessment at 7T were created enabling an extensive analysis of the intersubject B1+ and peak local SAR variability. For the investigated setup, the maximum possible peak local SAR ranges from 2.6 to 4.6 W/kg for 8 × 1 W input power. The expected peak local SAR values represent a Gaussian distribution W/kg) with realistic prostate-shimmed phase settings and a gamma distribution Γ(24,0.09) with multidimensional radiofrequency pulses. Prostate-shimmed phase settings are similar for all models. Using 1 generic phase setting, average B1+ reduction is 7%. Using only 1 model, the required safety margin for intersubject variability is 1.6 to 1.8. The presented procedure allows for the creation of a customized model database. The results provide valuable insights into B1+ and local SAR variability. Recommended power thresholds per channel are 3.1 W with phase shimming on prostate or 2.6 W for multidimensional pulses.

RF Shimming and Improved SAR Safety for MRI at 7 T With Combined Eight-Element Stepped Impedance Resonators and Traveling-Wave Antenna

A remote transmission and detection for magnetic resonance imaging (MRI) at 7 T was validated by using a patch antenna that excites the traveling-wave (TW) modes guided by the radio frequency (RF) shield of the MRI system. In this paper, RF simulations were performed at 298 MHz for an eight-element stepped impedance resonators (SIRs) head volume coil combined with a patch antenna. The combined structure is loaded with both a cylindrical phantom and the Duke full-body human voxel model. An optimization routine was developed in MATLAB to provide homogenous ${B}_{1}^{+}$ field distributions with minimal 10 g averaged specific absorption rate (SAR) values within the phantom and the Duke human biological model. Before optimization, the TW approach achieved a more homogenous ${B}_{1}^{+}$ field distribution providing a large field of view along the propagation direction compared to the SIRs head volume coil. However, the corresponding peak local SAR values for the TW approach is about 2.4 times higher than those of the SIRs head volume coil. The ${B}_{1}^{+}$ transmission efficiency for the TW approach is higher than that of the SIRs head volume coil by 10–30% due to the coupling of the TW modes into the volume coil. The RF-shimming technique improves the ${B}_{1}^{+}$ field homogeneity for the combined approach by 42%, 48%, and 47% in the central axial, sagittal, and coronal slices, respectively, and satisfies the constraints of maximum local SAR in the human head for MRI at 7 T.

Risks related to RF fields

One of the main safety concerns in MRI is the interaction between the electric field produced by the RF coil and conductive tissue and any implants within the patient. This results in power deposition and potential heating. International safety guidelines are formulated in terms of specific absorption rate (SAR) rather than rises in temperature, although there are strong indications that future regulations will concentrate on direct thermal effects. The effects of implants can be studied via electromagnetic simulations and phantom experiments. As MRI static magnetic fields increase local and global SAR values also become higher, and asymmetry in their spatial distribution also increases. This talk will concentrate on methods to simulate and measure RF heating, both using MRI and MRI-compatible optical techniques, and suggest some methods for potentially reducing heating using targeted RF fields.

Dynamically applied B1+ shimming solutions for non‐contrast enhanced renal angiography at 7.0 tesla

The purpose of this study was to detail a strategy for performing non-contrast enhanced renal magnetic resonance angiography studies at 7.0 T. It is demonstrated that with proper B1+ management, these studies can be successfully performed at ultrahigh field within local specific absorption rate constraints. An inversion prepared gradient echo acquisition, standard for non-contrast renal magnetic resonance angiography studies, required radiofrequency pulse specific B 1+ shimming solutions to be dynamically applied to address the field dependent increases in both B(0) and B1+ inhomogeneity as well as to accommodate limitation in available power. By using more efficient B1+ shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B1+ distributions and homogeneity for the range of shimming strategies used and allowed the calculation of peak local specific absorption rate values normalized by input power and B1+. Increasing B1+ homogeneity was accompanied by decreasing local specific absorption rate per Watt and increasing maximum local specific absorption rate per [B1+](2), which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual.

Specific absorption rate and temperature distributions in human head subjected to mobile phone radiation at different frequencies

This study presents a numerical analysis of specific absorption rate (SAR) and temperature distributions in the realistic human head model exposed to mobile phone radiation at 900 MHz and 1800 MHz. In the realistic human head model, the effects of operating frequency and gap distance between the mobile phone and the human head on distributions of specific absorption rate and temperature profile within the human head are systematically investigated. This study focuses attention on each tissue in the human head in order to investigate the effects of mobile phone radiation on the human head. The SAR and the temperature distribution in various tissues in human head during exposed to mobile phone radiation, obtained by numerical solution of electromagnetic wave propagation and unsteady bioheat transfer equations, are presented. For both frequencies, the highest SAR values are obtained in the region of the skin near the antenna. It is found that the highest SAR values are 0.823 W/kg and 1.187 W/kg for the frequencies of 900 MHz and 1800 MHz, respectively. The SAR values obtained from this study are well below ICNIRP limit for all cases. In addition, it is found that the temperature distributions are not directly proportional to the local SAR values. Moreover, the experimental validation has been carried out by using the infrared camera in order to complement the simulation results.

Local specific absorption rate in high‐pass birdcage and transverse electromagnetic body coils for multiple human body models in clinical landmark positions at 3T

To use electromagnetic (EM) simulations to study the effects of body type, landmark position, and radiofrequency (RF) body coil type on peak local specific absorption rate (SAR) in 3T magnetic resonance imaging (MRI). Numerically computed peak local SAR for four human body models (HBMs) in three landmark positions (head, heart, pelvic) were compared for a high-pass birdcage and a transverse electromagnetic 3T body coil. Local SAR values were normalized to the IEC whole-body average SAR limit of 2.0 W/kg for normal scan mode. Local SAR distributions were highly variable. Consistent with previous reports, the peak local SAR values generally occurred in the neck-shoulder area, near rungs, or between tissues of greatly differing electrical properties. The HBM type significantly influenced the peak local SAR, with stockier HBMs, extending extremities towards rungs, displaying the highest SAR. There was also a trend for higher peak SAR in the head-centric and heart-centric positions. The impact of the coil types studied was not statistically significant. The large variability in peak local SAR indicates the need to include more than one HBM or landmark position when evaluating safety of body coils. It is recommended that an HBM with arms near the rungs be included to create physically realizable high-SAR scenarios.

Real-Time RF Exposure Setup Based on a Multiple Electrode Array (MEA) for Electrophysiological Recording of Neuronal Networks

Real-time investigations on the effects of mobile-phone exposure on neuronal activity are considered the highest priority in the health risk assessment of RF fields. Therefore, this paper describes a new real-time exposure setup for electrophysiology recordings, combining an open transverse electro-magnetic cell with a multiple electrode array. The system was numerically and experimentally characterized at a frequency of 1.8 GHz, representative of the uplink band of the GSM1800. Finite-difference time-domain simulations were performed, as well as measurements of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> scattering parameter and temperature increases for specific absorption rate (SAR) determination. The local mean SAR value near the solution-electrodes interface was evaluated from the temperature measurements and coincided with the simulated one. The same is true for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> measurements that were consistent with numerical results; hence, confirming the accuracy of the modeling. Nevertheless, nonuniform SAR distributions were observed near the recording electrodes. Good system efficiency was determined from both numerical analysis and experimental data. Thermal behavior was evaluated and a small temperature increment of 0.3°C for a local experimental SAR of 3.2 W/kg was measured.

Postoperative MRI examinations in patients treated by deep brain stimulation using a non-standard protocol

MRI in patients bearing deep brain stimulation (DBS) electrodes may induce cerebral lesions due to electrode heating. To avoid neurological deficits related to MRI, post-operative MRI protocol was installed in our institution. However, our protocol comprised a higher specific absorption rate (SAR) and different positioning of lead excess than the later released electrode manufacturer's guidelines. The objective was to evaluate the safety using this protocol. Between January 2000 and May 2008, post-operative MRI was performed in all patients. In selected patients, additional MRI scans were performed with the implanted generator. MRI was acquired at 1.5 T with a RF transmit/receive head coil comprising a T2-weighted fast spin echo (FSE) and a T1-weighted inversion recovery FSE sequence. Local cranial SAR values measured up to 0.9 W/kg compared to the manufacturer's recommendation of 0.1 W/kg. Initial scans (1-7 days after surgery) were performed with externalized leads, long-term scans (>30 days after surgery) with a connected generator. New neurological deficits were assessed before and after MRI. Additional MRIs were compared to the initial postoperative MRI with emphasis on new lesions. In 211 patients, 243 MRIs were performed, including 212 initial post-operative MRI. In 12% (n = 24), 31 additional MRI examinations for various clinical reasons were achieved. No patients demonstrated new neurological deficits during or after MRI acquisitions. No complications were observed using this MRI protocol in DBS patients. Our results suggest that, within this setting, higher SAR values may be feasible for DBS patients than in the manufacturer's guidelines.

Analysis of the Role of Lead Resistivity in Specific Absorption Rate for Deep Brain Stimulator Leads at 3T MRI

Magnetic resonance imaging (MRI) on patients with implanted deep brain stimulators (DBSs) can be hazardous because of the antenna-effect of leads exposed to the incident radio-frequency field. This study evaluated electromagnetic field and specific absorption rate (SAR) changes as a function of lead resistivity on an anatomically precise head model in a 3T system. The anatomical accuracy of our head model allowed for detailed modeling of the path of DBS leads between epidermis and the outer table. Our electromagnetic finite difference time domain (FDTD) analysis showed significant changes of 1 g and 10 g averaged SAR for the range of lead resistivity modeled, including highly conductive leads up to highly resistive leads. Antenna performance and whole-head SAR were sensitive to the presence of the DBS leads only within 10%, while changes of over one order of magnitude were observed for the peak 10 g averaged SAR, suggesting that local SAR values should be considered in DBS guidelines. With rho(lead) = rho(copper) , and the MRI coil driven to produce a whole-head SAR without leads of 3.2 W/kg, the 1 g averaged SAR was 1080 W/kg and the 10 g averaged SAR 120 W/kg at the tip of the DBS lead. Conversely, in the control case without leads, the 1 g and 10 g averaged SAR were 0.5 W/kg and 0.6 W/kg, respectively, in the same location. The SAR at the tip of lead was similar with electrically homogeneous and electrically heterogeneous models. Our results show that computational models can support the development of novel lead technology, properly balancing the requirements of SAR deposition at the tip of the lead and power dissipation of the system battery.

Estimation of local SAR level using RBFNN in three‐layer cylindrical human model

AbstractIn this study, we present a new approach based on radial basis function neural networks (RBFNN) for estimation of specific absorption rate (SAR) in 2D cylindrical human model with three layers. The simulation results obtained using RBFNN were compared with the FDTD results. The estimated local SAR values using RBFNN has proved to be an impressive approximation to the FDTD results. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1958–1961, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23535

Patient safety concept for multichannel transmit coils

To propose and illustrate a safety concept for multichannel transmit coils in MRI based on finite-differences time-domain (FDTD) simulations and validated by measurements. FDTD simulations of specific absorption rate (SAR) distributions in a cylindrical agarose phantom were carried out for various radio frequency (RF) driving conditions of a four-element coil array. Additionally, maps of transmit amplitude, signal phase, and temperature rise following RF heating were measured by MRI. Quantitative agreement was achieved between simulated and measured field distributions, thus validating the numerical modeling. When applying the same RF power to each element of the coil array but systematically varying the RF phase between its elements, the maximum of the SAR distribution was found to vary by a factor of about 15. Our results demonstrate that current RF safety approaches are inadequate to deal with the new challenge of multichannel transmit coils. We propose a new concept based on a systematic investigation of the parameter space for RF phases and amplitudes. In this way the driving conditions generating the highest local SAR values per unit power can be identified and appropriately considered in the RF safety concept of a given MRI system.

SAR and temperature: simulations and comparison to regulatory limits for MRI.

To present and discuss numerical calculations of the specific absorption rate (SAR) and temperature in comparison to regulatory limits. While it is possible to monitor whole-body or whole-head average SAR and/or core body temperature during MRI in practice, this is not generally true for local SAR values or local temperatures throughout the body. While methods of calculation for SAR and temperature are constantly being refined, methods for interpreting results of these calculations in light of regulatory limits also warrant discussion. Numerical calculations of SAR and temperature for the human head in a volume coil for MRI at several different frequencies are presented. Just as the field pattern changes with the frequency, so do the temperature distribution and the ratio of maximum local SAR (in 1-g or 10-g regions) to whole-head average SAR. In all of the cases studied here this ratio is far greater than that in the regulatory limits, indicating that existing limits on local SAR will be exceeded before limits on whole-body or whole-head average SAR are reached. Calculations indicate that both SAR and temperature distributions vary greatly with B(1) field frequency, that temperature distributions do not always correlate well with SAR distributions, and that regulatory limits on local temperature may not be exceeded as readily as those on local SAR.