Simulated Specific Absorption Rate Research Articles

Inductive Power Transfer Link at 13.56 MHz for Leadless Cardiac Pacemakers

Inductive power links are most viable for the long-term powering of cardiac pacemakers. Designing an inductive power link without surpassing the specific absorption rate (SAR) for modern leadless cardiac pacemakers (LCPs) remains a challenging task because of its size and implantation depth. The inductive power link employed in the conventional works is either designed at a high frequency or based on the size, shape, weight, and implantation depth of conventional cardiac pacemakers. Here, a 3-coil inductive power transfer link with a circular transmitter coil and solenoidal receiver coil is designed at 13.56 MHz to provide uninterrupted power to the modern LCPs. Considering the food and drug administration approved term for implant size of modern LCP, the receiver coil is designed with 6 mm diameter and 6.5 mm length. The performance of the link has been verified through simulations and measurements under perfect alignment, lateral and/or angular misalignments, and distance variation between the coils. At a 50 mm horizontal distance between transmitter and receiver coils, the transmission coefficient is −30.9 dB. The maximum simulated average SAR at heterogeneous phantom is 0.30 W/kg, which is lower than the limit set by the Federal Communications Commission for radiation threshold exposure. Experiments conducted on pork’s heart verified the reliability of the simulated results. At a 50 mm distance between the coils, the measured transmission coefficient is −34 dB, and at an input power of 1 W, the power delivered to the load is 0.7 mW.

Design of a wide bandwidth and high gain wearable antenna based on nonuniform metasurface

AbstractA metasurface (MS) antenna for the 5 GHz wireless local area network (WLAN) band is presented in this paper. The antenna is feed by a modified aperture‐coupled structure with an optimized rectangular nonuniform MS to widen the bandwidth and improve the gain. The proposed antenna has a thickness of 4 mm and occupies an area of 50 mm × 50 mm. The operating frequency range of the antenna is 4.72–6.19 GHz, covering the 5.15–5.825 GHz band used by the 5 GHz WLAN. The measured average gain is 7.23 dBi. The simulated specific absorption rate values under 1 g and 10 g standard are lower than the limitation of U.S. and European. Due to its simple structure, wide bandwidth and small size, the proposed MS antenna has great potential in the wearable applications.

A compact dual-band semi-flexible antenna at 2.45 GHz and 5.8 GHz for wearable applications

In this work, a compact dual-band semi-flexible antenna operating at 2.45 GHz and 5.8 GHz for the industrial, scientific and medical (ISM) band is presented. The antenna is fabricated on a semi-flexible substrate material, Rogers Duroid RO3003™ with a low-profile feature with dimensions of 30×38 mm2 which makes it a good solution for wearable applications. Bending investigation is also performed over a vacuum cylinder and the diameters are varied at 50 mm, 80 mm and 100 mm, that represents the average human arm’s diameter. The bending investigation shows that reflection coefficients for all diameters are almost similar which imply that the antenna will operate at the dual-band resonant frequencies, even in bending condition. The simulated specific absorption rate (SAR) in CST MWS® software shows that the antenna obeys the FCC and ICNIRP guidelines for 1 mW of input power. The SAR limits at 2.45 GHz for 1 g of human tissue is simulated at 0.271 W/kg (FCC standard: 1.6 W/kg) while for 10 g is at 0.0551 W/kg (ICNIRP standard: 2 W/kg. On the other hand, the SAR limits at 5.8 GHz are computed at 0.202 W/kg for 1 g and 0.0532 W/kg for 10 g.

Flexible milimeter-wave microstrip patch antenna array for wearable RF energy harvesting applications

In this paper, a series-fed milimeter-wave microstrip patch antenna array operating at 28 GHz is presented for wearable radio-frequency (RF) energy harvesting applications. The antenna array is made of 4×4 rectangular microstrip elements on a polyethylene terephthalate (PET) substrate to provide conformability when directly attached on human body parts. A 4-way Wilkinson power divider is connected to the array for RF power combining. The overall size of the antenna is 47×28×0.25 mm. The half-power beamwidth (HPBW) of the antenna array can be increased up to 151.9⁰ via structural deformation making it suitable for energy harvesting applications. The performance of the antenna array is investigated in terms of impedance matching, gain and radiation pattern. The average simulated specific absorption rate (SAR) of the antenna is 0.52 W/kg which is well below the safety limit of 1.6 W/kg averaged over 1 g of tissue for 100 mW of input power.

Characterization of displacement forces and image artifacts in the presence of passive medical implants in low-field (

To investigate the displacement forces and image artifacts associated with passive medical implants for recently-developed low-field (<100 mT) MRI systems, and to compare these with values from higher field strengths used for clinical diagnosis. Setups were constructed to measure displacement forces in a permanent magnet-based Halbach array used for in vivo MRI at 50 mT, and results compared with measurements at 7T. Image artifacts were assessed using turbo (fast) spin echo imaging sequences for four different passive medical implants: a septal occluder, iliac stent, pedicle screw and (ferromagnetic) endoscopic clip. Comparisons were made with artifacts produced at 1.5, 3 and 7T. Finally, specific absorption rate (SAR) simulations were performed to determine under what operating conditions the limits might be approached at low-field. Displacement forces at 50 mT on all but the ferromagnetic implant were between 1 and 10 mN. Image artifacts at 50 mT were much less than at clinical field strengths for all passive devices, and with the exception of the ferromagnetic clip. SAR simulations show that very long echo train (>128) turbo spin echo sequences can be run with short inter-pulse times (5-10ms) within SAR limits. This work presents the first evaluation of the effects of passive implants at field strengths less than 100 mT in terms of displacement forces, image artifacts and SAR. The results support previous claims that such systems can be used safely and usefully in challenging enviroments such as the intensive care unit.

Passive UHF RFID-based Knitted Wearable Compression Sensor.

One of the major challenges faced by passive on-body wireless Internet of Things (IoT) sensors is the absorption of radiated power by tissues in the human body. We present a battery-less, wearable knitted Ultra High Frequency (UHF, 902-928 MHz) Radio Frequency Identification (RFID) compression sensor (Bellypatch) antenna and show its applicability as an on-body respiratory monitor. The antenna radiation efficiency is satisfactory in both free-space and on-body operations. We extract RF (Radio Frequency) sheet resistance values of three knitted silver-coated nylon fabric candidates at 913 MHz. The best type of fabric is selected based on the extracted RF sheet resistance. Simulated and measured performance of the antenna confirm suitability for on-body applications. The proposed Bellypatch antenna is used to measure the breathing activity of a programmable infant patient emulator mannequin (SimBaby) and a human subject. The antenna is highly sensitive to respiratory compression and relaxation. Fluctuations in the backscatter power level/Received Signal Strength Indicator (RSSI) in both cases range from 6 dB to 15 dB. The improved on-body read range of the proposed sensor antenna is 5.8 m, about 10 times higher than its predecessor wearable knitted strain sensing Bellyband antenna (0.6 m). The maximum simulated Specific Absorption Rate (SAR) on a human torso model is 0.25 W/kg, lower than the maximum allowable limit of 1.6 W/kg.

Temperature-based MRI safety simulations with a limited number of tissues.

Demonstrate ability to produce reasonable simulations of temperature using numerical models of the human body with a limited number of tissues. For both a male and female human body model, numerical simulations were used to calculate temperature distributions in three different models of the same human body: the original model with 35 tissues for the male model and 76 tissues for the female model, a simplified model having only three tissues (muscle, fat, and lung), and a simplified model having six tissues (muscle, fat, lung, bone, brain, and skin). Although a three-tissue model gave reasonable specific absorption rate estimates in comparison to an original with many more tissues, because of tissue-specific thermal and physiological properties that do not affect specific absorption rate, such as rate of perfusion by blood, the three-tissue model did not provide temperature distributions similar to those of the original model. Inclusion of a few additional tissues, as in the six-tissue model, produced results in much better agreement with those from the original model. Reasonable estimates of temperature can be simulated with a limited number of tissues, although this number is higher than the number of tissues required to produce reasonable simulations of specific absorption rate. For exposures primarily in the head and thorax, six tissues may be adequate for reasonable estimates of temperature.

MRSaiFE: An AI-based Approach Towards the Real-Time Prediction of Specific Absorption Rate.

The purpose of this study is to investigate feasibility of estimating the specific absorption rate (SAR) in MRI in real time. To this goal, SAR maps are predicted from 3T- and 7T-simulated magnetic resonance (MR) images in 10 realistic human body models via a convolutional neural network. Two-dimensional (2-D) U-Net architectures with varying contraction layers and different convolutional filters were designed to estimate the SAR distribution in realistic body models. Sim4Life (ZMT, Switzerland) was used to create simulated anatomical images and SAR maps at 3T and 7T imaging frequencies for Duke, Ella, Charlie, and Pregnant Women (at 3, 7, and 9 month gestational stages) body models. Mean squared error (MSE) was used as the cost function and the structural similarity index (SSIM) was reported. A 2-D U-Net with 4 contracting (and 4 expanding) layers and 64 convolutional filters at the initial stage showed the best compromise to estimate SAR distributions. Adam optimizer outperformed stochastic gradient descent (SGD) for all cases with an average SSIM of 90.5∓3.6 % and an average MSE of 0.7∓0.6% for head images at 7T, and an SSIM of >85.1∓6.2 % and an MSE of 0.4∓0.4% for 3T body imaging. Algorithms estimated the SAR maps for 224×224 slices under 30 ms. The proposed methodology shows promise to predict real-time SAR in clinical imaging settings without using extra mapping techniques or patient-specific calibrations.

Architecture of Multiple Convolutional Neural Networks to Construct a Subject-Specific Knee Model for Estimating Local Specific Absorption Rate

Electromagnetic simulation is a credible way to estimate the local specific absorption rate (SAR), which is a key consideration in high-field magnetic resonance imaging of the knee joint. To construct a subject-specific knee model, which is critical for SAR simulation, we proposed an architecture comprising multiple convolutional neural networks. Knee tissues were segmented by three U-Nets. Each network was responsible for the segmentation of two tissues that have relatively similar volumes and distinct intensity distributions (muscle and fat, cancellous and cortical bone, and cartilage and meniscus). Additionally, a weighted loss function was used to further alleviate the effect of class imbalance of the segmented tissues. The outputs of these three networks were merged and morphological filtering was used as the post-processing to eliminate holes and isolated voxels. This method was compared with three other segmentation methods. Good segmentation performance was demonstrated on the test set, and the proposed method was found to be superior to the other methods according to several quantitative measures. Meanwhile, local SAR in a 3 T coil using models constructed with the proposed method, manual delineation, and the comparison methods were also evaluated on the test set. On the whole, the maximum values of SAR10g of the models constructed by the proposed method were closer to the results of manual delineation. Overall, the proposed method exhibits promising potential for precisely constructing knee models for SAR simulation.

An investigation into the minimum number of tissue groups required for 7T in‐silico parallel transmit electromagnetic safety simulations in the human head

Safety limits for the permitted specific absorption rate (SAR) place restrictions on pulse sequence design, especially at ultrahigh fields (≥ 7 tesla). Due to intersubject variability, the SAR is usually conservatively estimated based on standard human models that include an applied safety margin to ensure safe operation. One approach to reducing the restrictions is to create more accurate subject-specific models from their segmented MR images. This study uses electromagnetic simulations to investigate the minimum number of tissue groups required to accurately determine SAR in the human head. Tissue types from a fully characterized electromagnetic human model with 47 tissue types in the head and neck region were grouped into different tissue clusters based on the conductivities, permittivities, and mass densities of the tissues. Electromagnetic simulations of the head model inside a parallel transmit head coil at 7 tesla were used to determine the minimum number of required tissue clusters to accurately determine the subject-specific SAR. The identified tissue clusters were then evaluated using 2 additional well-characterized electromagnetic human models. A minimum of 4-clusters-plus-air was found to be required for accurate SAR estimation. These tissue clusters are centered around gray matter, fat, cortical bone, and cerebrospinal fluid. For all 3 simulated models, the parallel transmit maximum 10g SAR was consistently determined to within an error of <12% relative to the full 47-tissue model. A minimum of 4-clusters-plus-air are required to produce accurate personalized SAR simulations of the human head when using parallel transmit at 7 tesla.

Miniaturization of a Dual-Band Wearable Antenna for WBAN Applications

A planar, compact, dual-band antenna on a semiflexible substrate is proposed for 2.45/5.85 GHz wireless body-area network applications. The proposed antenna is comprised of an I-shaped monopole embedded with an inverted L-shaped slit and an inductive meander line on the front side of the substrate, which excites the higher resonant mode at 5.85 GHz. On the backside, an inverted U-shaped stripline embedded with another inductive meander line is attached to the ground plane to excite the lower resonant mode at 2.45 GHz. The key to the miniaturization procedure is increasing the inductance and the capacitance by loading the inductive meander lines and slit line. The overall size of the proposed antenna is 0.15λ × 0.1λ × 0.004λ at 2.45 GHz. The measured results of the proposed antenna show the impedance bandwidths of 5.7% (2.4–2.54 GHz) and 3.78% (5.72–5.94 GHz) and the maximum peak gains of 2.1 dBi and 3.5 dBi, respectively. The simulated specific absorption rate values satisfy both the U.S. and EUR standards of under 1 g average and under 10 g average, respectively.

A Combination of Transmission Line Models as Design Instruments for Electromagnetically Coupled Microstrip Patch Antennas in the 2.45 GHz ISM Band

This communication presents an analytical framework that combines transmission line models for the design of electromagnetically coupled microstrip patch antennas for the 2.45 GHz industrial, scientific, and medical band. It provides initial values for all dimensions of the antenna, with measured resonance frequency errors below 6%. The initial design is optimized in two subsequent phases to center the resonance frequency and to increase the impedance bandwidth (BW), obtaining measured resonance frequency errors below 0.6% and BW enhancements of more than 1.2 times the original ones, respectively. The model has been validated with antenna prototypes based on rigid and textile materials, exhibiting excellent free-space measured BW of 4% and 5.12%, maximal measured gains of 4.28 and 7.33 dBi, and radiation efficiencies of 63.4% and 71.8%, respectively. Moreover, very stable on-body performance is obtained, with minimal frequency detuning when deploying the textile antenna on the human body. The measured maximum on-body gain for the textile antenna equals 5.5 dBi, with a simulated specific absorption rate of 0.323 W/kg at 2.45 GHz.

An Evaluation of Electromagnetic Exposure While Using Ultra-High Frequency Radiofrequency Identification (UHF RFID) Guns.

The aim is to evaluate specific absorption rate (SAR) values from exposure near handheld ultra-high frequency radiofrequency identification readers (UHF RFID guns—small electronic devices, or even portable computers with relevant accessories—emitting up to several watts of electromagnetic field (EMF) to search for RFID sensors (tags) attached to marked objects), in order to test the hypothesis that they have an insignificant environmental influence. Simulations of SAR in adult male and female models in seven exposure scenarios (gun near the head, arm, chest, hip/thigh of the operator searching for tags, or near to the chest and arm of the scanned person or a bystander). The results showed EMF exposure compliant with SAR limits for general public exposure (ICNIRP/European Recommendation 1999/519/EC) at emissions up to 1 W (reading range 3.5–11 m, depending on tag sensitivity). In the worst-case scenario, guns with a reading range exceeding 5 m (>2 W emission) may cause an SAR exceeding the general public limits in the palm of the user and the torso of the user, a bystander, or a scanned person; occupational exposure limits may be exceeded when emission >5 W. Users of electronic medical implants and pregnant women should be treated as individuals at particular risk in close proximity to guns, even at emissions of 1 W. Only UHF RFID guns emitting below 1 W may be considered as environmentally insignificant EMF sources.

Ultra-Low Power Wearable Infant Sleep Position Sensor.

Numerous wearable sensors have been developed for a variety of needs in medical/healthcare/wellness/sports applications, but there are still doubts about their usefulness due to uncomfortable fit or frequent battery charging. Because the size or capacity of battery is the major factor affecting the convenience of wearable sensors, power consumption must be reduced. We developed a method that can significantly reduce the power consumption by introducing a signal repeater and a special switch that provides power only when needed. Antenna radiation characteristics are an important factor in wireless wearable sensors, but soft material encapsulation for comfortable fit results in poor wireless performance. We improved the antenna radiation characteristics by a local encapsulation patterning. In particular, ultra-low power operation enables the use of paper battery to achieve a very thin and flexible form factor. Also, we verified the human body safety through specific absorption rate simulations. With these methods, we demonstrated a wearable infant sleep position sensor. Infants are unable to call for help in unsafe situations, and it is not easy for caregivers to observe them all the time. Our wearable sensor detects infants’ sleep positions in real time and automatically alerts the caregivers when needed.

Individual variation in simulated fetal SAR assessed in multiple body models.

We generate 12 models from 4 pregnant individuals to evaluate individual differences in local specific absorption rate (SAR) for differing body habitus and fetal and maternal positions. Structural MR images from 4 pregnant subjects (including supine and left-lateral maternal positions) were manually segmented to create 12 body models by rotating the fetus, modifying the fat content, and altering the maternal arm position in 1 of the subjects. Electromagnetic simulations modeled at 3 Tesla determined the average and peak local SAR in the maternal trunk, fetus, fetal brain, and amniotic fluid. We observed a significant range of fetal and maternal peak local SAR across the models (maternal trunk: 19.14-44.03 watts/kg, fetus: 9.93-18.79 watts/kg, fetal brain 3.36-10.3 watts/kg). We found that maternal body habitus changes introduced a significant variation in the maternal peak local SAR but not the fetal local SAR. However, the maternal position (either rotating the mother to left-lateral position or altering the arm position) introduced changes in fetal peak local SAR (range: 11.9-17.9 watts/kg). Rotating the fetus also introduced variation in the fetal and fetal brain peak local SAR. The observed variation in SAR emphasizes the need for more anatomical models to enable better safety management of individuals during fetal MRI, including a wider range of gestational ages.

A Glasses Frame Antenna for the Applications in Internet of Things

A novel concept of metal glasses frame antenna is proposed and relevant investigations are conducted to estimate its performance. A prototype antenna is designed based on a pair of glasses and then measured for demonstration and verification. It is indicated by the simulations and the measurements that the width of the ring rim has only a small impact on the performance. However, the investigation shows that the shape of the frame and even the material of the lens could impose significant influence on the resonance frequency points and the matching performance. The main lobe would shift away from the direct front if the feeding location is moved on the central pole connecting the two rings or the excitation is put on other points of the spectacles frame. The difference between the semi-rimmed glasses and the one with closed rings primarily lies in the beam width of the main lobe. It can be seen that a fractional bandwidth of 14.5% could be realized at 5.8 GHz, falling into the industrial, scientific, and medical (ISM) band. In addition, the maximum value of the simulated specific absorption rate (SAR) is 1.56 W/kg within this frequency band. The measurement results are well consistent with those of the simulations, indicating that it is a potential candidate serving as an efficient tool for sensing and communications for the applications in Internet of Things, especially body-centric wireless networks.

SAR analysis of directive antenna on anatomically real breast phantoms for microwave holography

AbstractMicrowave holography utilizes antennae for the detection of cancerous tissues in the human breast. The safety assessment of an antenna is required prior to its radiation interaction with biological tissues. In this work, a specially designed and developed Vivaldi antenna with directivity at 45° for microwave holographic imaging has been presented. A detailed simulation of Specific Absorption Rate (SAR) and thermal analysis due to the radiation exposure of the designed antenna on self‐designed anatomically real breast phantoms is performed in CST environment. SAR values for 3D printed breast phantoms are also calculated experimentally. The quantitative results of SAR reveal the variation of ±10% in between the simulation and experimental results. It has been found that SAR due to the radiation of the designed antenna lies in the permissible limit. The thermal analysis in CST shows the rise of 0.096% in temperature after the exposure of 2 hours. Hence, the antenna can be used in microwave holography for breast cancer detection.

MRI Conditional Actively Tracked Metallic Electrophysiology Catheters and Guidewires With Miniature Tethered Radio-Frequency Traps: Theory, Design, and Validation.

Cardiovascular interventional devices typically have long metallic braids or backbones to aid in steerability and pushability. However, electromagnetic coupling of metallic-based cardiovascular interventional devices with the radiofrequency (RF) fields present during Magnetic Resonance Imaging (MRI) can make a device unsafe for use in an MRI scanner. We aimed to develop MRI conditional actively-tracked cardiovascular interventional devices by sufficiently attenuating induced currents on the metallic braid/tube and internal-cabling using miniaturized resonant floating RF traps (MBaluns). MBaluns were designed for placement at multiple locations along a conducting cardiovascular device to prevent the establishment of standing waves and to dissipate RF-induced energy. The MBaluns were constructed with loosely-wound solenoids to be sensitive to transverse magnetic fields created by both surface currents on the device's metallic backbone and common-mode currents on internal cables. Electromagnetic simulations were used to optimize MBalun parameters. Following optimization, two different MBalun designs were applied to MR-actively-tracked metallic guidewires and metallic-braided electrophysiology ablation catheters. Control-devices were constructed without MBaluns. MBalun performance was validated using network-analyzer quantification of current attenuation, electromagnetic Specific-Absorption-Rate (SAR) analysis, thermal tests during high SAR pulse sequences, and MRI-guided cardiovascular navigation in swine. Electromagnetic SAR simulations resulted in ≈20 dB attenuation at the tip of the wire using six successive MBaluns. Network-analyzer tests confirmed ∼17 dB/MBalun surface-current attenuation. Thermal tests indicated temperature decreases of 5.9°C in the MBalun-equipped guidewire tip. Both devices allowed rapid vascular navigation resulting from good torquability and MR-Tracking visibility. MBaluns increased device diameter by 20%, relative to conventional devices, providing a spatially-efficient means to prevent heating during MRI. MBaluns allow use of long metallic components, which improves mechanical performance in active MR-guided interventional devices.

Microstrip monopole antenna with a novel UC‐EBG for 2.4 GHz WBAN applications

A microstrip monopole antenna backed with a novel uniplanar compact electromagnetic bandgap (UC-EBG) structure for 2.4 GHz wireless body area network applications is proposed here. The proposed antenna has a size reduction of 50% when the number of EBG array is optimised from 3 × 4 to 2× 3, while the radiation performance is almost invariable at the same time. The operating frequency band under different conditions has the capability of covering the 2.4–2.4835 GHz industrial, scientific, and medical band. Due to the addition of 2×3 UC-EBG on the back, the radiation patterns become unidirectional, and the measured average gain increases from 2.1 to 5.6 dB. Meanwhile, the cross-polarisation of the monopole antenna can be suppressed prominently by the EBG array. The simulated specific absorption rate values are 0.0536 W/kg under 1 g standard and 0.0296 W/kg under 10 g standard, which are far below the limitation of USA and Europe. These results can manifest the reliable wearable performance of the proposed antenna.

Cardiorenal sodium MRI at 7.0 Tesla using a 4/4 channel 1 H/23 Na radiofrequency antenna array.

Cardiorenal syndrome describes disorders of the heart and the kidneys in which a dysfunction of 1 organ induces a dysfunction in the other. This work describes the design, evaluation, and application of a 4/4-channel hydrogen-1/sodium (1 H/23 Na) RF array tailored for cardiorenal MRI at 7.0 Tesla (T) for a better physiometabolic understanding of cardiorenal syndrome. The dual-frequency RF array is composed of a planar posterior section and a modestly curved anterior section, each section consisting of 2 loop elements tailored for 23 Na MR and 2 loopole-type elements customized for 1 H MR. Numerical electromagnetic field and specific absorption rate simulations were carried out. Transmission field ( ) uniformity was optimized and benchmarked against electromagnetic field simulations. An in vivo feasibility study was performed. The proposed array exhibits sufficient RF characteristics, homogeneity, and penetration depth to perform 23 Na MRI of the heart and kidney at 7.0 T. The mean field for sodium in the heart is 7.7 ± 0.8 µT/√kW and in the kidney is 6.9 ± 2.3 µT/√kW. The suitability of the RF array for 23 Na MRI was demonstrated in healthy subjects (acquisition time for 23 Na MRI: 18 min; nominal isotropic spatial resolution: 5 mm [kidney] and 6 mm [heart]). This work provides encouragement for further explorations into densely packed multichannel transceiver arrays tailored for 23 Na MRI of the heart and kidney. Equipped with this technology, the ability to probe sodium concentration in the heart and kidney in vivo using 23 Na MRI stands to make a critical contribution to deciphering the complex interactions between both organs.