Simulated Specific Absorption Rate Research Articles

Miniaturized 3-D Cross-Type Receiver for Wirelessly Powered Capsule Endoscopy

This paper presents the modeling, implementation, and testing of a cross-type receiver (RX) used for wireless power transfer (WPT) into an endoscopic capsule. A 3-D configuration for the receiver is proposed to increase the wireless link power transfer efficiency (WL-PTE) and enhance its robustness to angular variations of the capsule. The diameter and thickness of the flexible transmitter coils are 200 mm and $185~\mu \text{m}$ , respectively. The diameter of the miniaturized RX is 8 mm, which is appropriate for integration within current commercially available capsules. Experimental results demonstrate a WL-PTE of 1.3% and complete system efficiency of 0.9% inside a suitable tissue phantom of the relative permittivity of 300 at 5 MHz. A maximum variation of only 8.4% of the WL-PTE across 90° orientation of the RX was also measured. The minimum WL-PTE of 1% is the recorder for 7-cm translational displacement inside the phantom. A simulated specific absorption rate of 0.86 W/Kg was calculated, which is well below the limit allowed for medical applications. A maximum temperature variation of 2.2 °C was also measured for an operating duration of 10 h, which is typical of a capsule journey inside the gastrointestinal tract. The proposed system represents a viable proposition for WPT in capsule endoscopy.

Brain SAR of average male Korean child to adult models for mobile phone exposure assessment

This paper aims to implement average head models of Korean males and investigate age-related differences in the brain for exposure from radiation from mobile phones. Four male head models composed of a total of 69 structures were developed through a statistical investigation of the anatomical morphology for the age groups of 6, 9, 15 and 20–24 years in age, which are named KR-6, KR-9, KR-15, and KR-22 herein. Three numerical bar phone models with a dual-band built-in antenna were applied to calculate the specific absorption rate (SAR) in the brain; the body lengths of models Mavg and Mlong have the mean value and upper 5th percentile value of commercial bar phone models, respectively, with an antenna at the bottom, whereas Mrev has an antenna on top of the phone body, which is the same as in Mavg but rotated 180°. The cheek and tilt positions were employed for SAR simulations. As a result, a higher peak spatial-average SAR (psSAR) was observed in the brain for the child groups of KR-6 and KR-9 than for the adult groups of KR-15 and KR-22. In most configurations, the position-averaged psSAR10 g in the child brain was 62% (Mlong, 835 MHz), 61% (Mavg, 835 MHz), 102% (Mlong, 1850 MHz), 108% (Mavg, 1850 MHz), and 125% (Mrev, 1850 MHz) higher than in the adult brain. The higher frequency of 1850 MHz showed a wider difference in the brain psSAR between the child and adult groups owing to the shorter penetration depth. When a long phone with an antenna at the bottom operates at a higher frequency, it significantly reduces the brain exposure.

Investigation on Simulation-Based Specific Absorption Rate in Ultra-Wideband Antenna for Breast Cancer Detection

The electrical properties of the cancer affected cell or malignant tumor cell are completely different from normal healthiest breast tissue. Specific absorption rate (SAR) is used as one of the best parameter to identify the malignant tissue in breast with the help of electrical properties of the human breast tissue, for example, permittivity and conductivity. The design of modified octagonal shaped ultra-wideband monopole microstrip antenna with size of $27\times 29\times 1.6$ mm3, fabricated with FR-4 material, is found suitable for ultra-wideband applications and used to detect the cancer. The antenna impedance bandwidth of simulated and measured results are good (S11 < −10 dB) and its operating frequency ranges from 3 to 15 GHz with voltage standing wave ratio ≤ 2. A 3-D breast model is designed with 60 mm radius, and simulated SAR values are tabulated for skin, fatty tissue, glandular tissue, and tumor. As glandular tissue is one of the softest tissues in human female breast, it has the possibility of developing tumor in lobules and ducts. The tumors of different size are incorporated in glandular tissue. It is found that the SAR value varies based on the tumor size in glandular tissue.

Gain Enhancement of a Compact Implantable Dipole for Biomedical Applications

A new compact implantable planar dipole with an enhanced gain performance and a relatively low specific absorption rate (SAR) is proposed. In the proposed dipole, a part of each arm is shaped as a folded meander-line structure, especially designed to have a minimal size and cover the medical device radio communications service band (MedRadio, 401–406 MHz). It is shown that using this technique, the gain of the antenna can be increased without compromising miniaturization. The antenna has a peak realized gain of −23.7 dBi and its total volume is 20 mm3. The antenna has a relatively low SAR, so that the maximum transmitting power satisfying the regulatory limitation is 7.7 dBm. The SAR simulation results are confirmed experimentally.

Evaluating the effects of receive-only arrays in specific absorption rate simulations at 3 and 7 T

Specific absorption rate (SAR) simulations are performed for most clinical and research transmit coil configurations. Such simulations allow the determination of limits in transmit power for patient safety. Different human models and coil configurations have been previously investigated using these simulations. However, only a few works have accounted for the effect of the receive (Rx) arrays in the SAR calculations and they have used very specialized setups or simplified detuning modeling of the Rx elements. In this work, we performed electromagnetic simulations using a clinical alike setup for whole-body scans at 3 T and head scans at 7 T. SAR simulations are performed for both setups with and without Rx arrays. A difference below 10% percent was found for max SAR. The maximum difference for the mean SAR values of the 3 T setups remained within 8% and within 15% of the 7 T setup.

An LCC-C Compensated Wireless Charging System for Implantable Cardiac Pacemakers: Theory, Experiment, and Safety Evaluation

The technique of wireless power transfer (WPT) via magnetic coupling resonance provides a way to transfer power wirelessly to cardiac pacemakers from external source. For wireless charging system to be used in clinical environment, this paper gives priority consideration to three technical difficulties, i.e., implantation, efficiency, and safety. The LCC-C compensation topology for pacemaker wireless charger is presented, where only one secondary side resonance capacitor needs to be implanted, and resonance compensation parameters are derived. The compensation network can make the WPT system operate at stable resonance and high efficiency. The prototype of the wireless charging system for implantable cardiac pacemaker is developed, where a thin flexible receiving unit was designed to effectively shield eddy currents in the pacemaker shell. The experiments of wireless charging through pork tissues reveal that 3.072-W power can be received from a 3.919-W power source at 300 kHz, reaching a rather high WPT efficiency of 78.4%, such that the charging is fast, i.e., the 1050 mA·h, 4.2-V Li-ion battery voltage increases from 3.98 (80% residual capacity) to 4.2 V within only 27 min, and only 3.3 °C maximum temperature rise of the pork is in safe limits. The feasibility and safety of the charging system were further evaluated by simulations of specific absorption rate and temperature rise in human tissues and electromagnetic fields in the pacemaker case.

Dual-Antenna Concept With Complementary Radiation Patterns for Eyewear Applications

A dual-antenna concept to be integrated in a smart eyewear device is presented. The two antennas are printed on FR4 substrate and placed inside an Acrylonitrile butadiene styrene dielectric eyewear frame manufactured using 3-D printing technology. One of the two antennas is a monopole situated at the back of the eyewear with a specifically shaped ground plane underneath. The second antenna is a balanced-fed loop antenna placed in front of the eyewear with an integrated balun circuit. Both antennas are designed to operate in the 2.4 GHz band of the 802.11b/g/n standard. When compared to a reference single monopole antenna usually used in commercial eyewear, higher gain and complementary radiation patterns are obtained in several useful directions using our dual-antenna solution: 1) in front of the eyewear to communicate with a laptop; 2) in the side direction of the eyewear to communicate with a fixed hotspot access point; and 3) in downward direction of the eyewear to communicate with a peripheral smartphone in the pocket of the user. S-parameters and radiation pattern measurements are given with the eyewear placed over a specific anthropomorphic mannequin head phantom to validate the advantages of this novel dual-antenna concept. Specific absorption rate simulation results are presented showing the proposed antennas stay within the authorized limits.

Wireless Power Link Based on Inductive Coupling for Brain Implantable Medical Devices

An inductively powered brain implantable medical device (BIMD) in lieu of a battery-assisted BIMD has the ability to provide a reliable power supply and avoid surgical replacement of a discharged battery. In this letter, we propose a three-dimensional (3-D) bowtie implantable antenna embedded in the cerebral spinal fluid (CSF) coupled with an exterior loop antenna. The power transfer efficiency is maximized by designing a matching circuit for the exterior and implantable antennas to reduce the high intrinsic reactive part of the input impedance. To ensure safety for long-term implantation, a rigorous specific absorption rate simulation was conducted to abide with the safety standard. A liquid CSF phantom was developed to experimentally test the power transfer efficiency between the exterior and implantable antenna. Measurement of the fabricated 0.9 $\text{mm}^{3}$ implantable antenna prototype in the CSF phantom resulted in $-\text{30.12 dB}$ wireless power link, which outperforms the current work in the literature in terms of the size and link efficiency for 3-D implantable antennas.

Selective proton-observed, carbon-edited (selPOCE) MRS method for measurement of glutamate and glutamine 13 C-labeling in the human frontal cortex.

13 C magnetic resonance spectroscopy (MRS) in combination with infusion of 13 C-labeled substrates has led to unique insights into human brain metabolism and neurotransmitter cycling. However, the low sensitivity of direct 13 C MRS and high radiofrequency power requirements has limited 13 C MRS studies to predominantly data acquisition in large volumes of the occipital cortex. The purpose of this study is to develop an MRS technique for localized detection of 13 C-labeling of glutamate and glutamine in the human frontal lobe. We used an indirect (1 H-[13 C]), proton-observed, carbon-edited MRS sequence (selPOCE) for detection of 13 C-labeled metabolites in relatively small volumes located in the frontal lobe at 4 T. The SelPOCE method allows for selective and separate detection of glutamate and glutamine resonances, which significantly overlap at magnetic field strengths used for clinical MRI. Phantom data illustrate how selPOCE can be tuned to selectively detect 13 C labeling in different metabolites. Three-dimensional specific absorption rate simulations of radiofrequency power deposition show that the selPOCE method operates comfortably within the global and local Food and Drug Administration specific absorption rate guidelines. In vivo selPOCE data are presented, which were acquired from a 45-mL volume in the frontal lobe of healthy subjects. The in vivo data show the time-dependent 13 C-labeling of glutamate and glutamine during intravenous infusion of [1-13 C]-glucose. Metrics describing spectral fitting quality of the glutamate and glutamine resonances are reported. The SelPOCE sequence allows the detection of 13 C-labeling in glutamate and glutamine from a relatively small volume in the human frontal lobe at low radiofrequency power requirements. Magn Reson Med 80:11-20, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A Novel Intracranial Pressure Readout Circuit for Passive Wireless LC Sensor.

We present a wide frequency range, low cost, wireless intracranial pressure monitoring system, which includes an implantable passive sensor and an external reader. The passive sensor consists of two spiral coils and transduces the pressure change to a resonant frequency shift. The external portable reader reads out the sensor's resonant frequency over a wide frequency range (35MHz-2.7 GHz). We propose a novel circuit topology, which tracks the system's impedance and phase change at a high frequency with low-cost components. This circuit is very simple and reliable. A prototype has been developed, and measurement results demonstrate that the device achieves a suitable measurement distance (>2 cm), sufficient sample frequency (>6 Hz), fine resolution, and good measurement accuracy for medical practice. Responsivity of this prototype is 0.92MHz/mmHg and resolution is 0.028mmHg. COMSOL specific absorption rate simulation proves that this system is safe. Considerations to improve the device performance have been discussed, which include the size of antenna, the power radiation, the Analog-to-digital converter (ADC) choice, and the signal processing algorithm.

Influence of dissolved gases in coupling waterbolus on superficial hyperthermia and evaluation of a water conditioning system with inline degassing

Purpose. A water conditioning system with inline degassing is presented for real time removal of the dissolved gases to improve heat delivery during superficial hyperthermia. Methodology. Coupled electromagnetic (EM) and thermal models are used to study the influence of air bubbles on the tissue specific absorption rate (SAR) and temperature distributions for a 915 MHz spiral antenna. The design and characterization of the water conditioning system with inline degassing, and evaluation on tissue phantoms are presented. Results. Perturbations in the simulated tissue SAR and temperature distributions for small air volume fractions in the bolus (1%–2%) were confirmed with phantom measurements. The water conditioning system maintained the bolus thickness and temperature within ±0.5% at a flow rate of 1 l min−1 and ±0.2 °C respectively, and degassed the circulating water at an exponential rate with respect to time. The power coupled during phantom heating was more than 90% using the proposed system, and fluctuated between 31% and 90% for circulation of the pre-degassed water. Phantom heating experiments for the 39 °C bolus recorded a 3 °C rise in temperature at 5 mm depth with inline degassing, and 1.9 °C–2.4 °C without inline degassing. Conclusion. The inline bolus water degassing system is simple and cost effective, has shorter treatment pre-preparation time and could improve superficial hyperthermia treatment.

Development and validation of a MRgHIFU non-invasive tissue acoustic property estimation technique

MR-guided high-intensity focussed ultrasound (MRgHIFU) non-invasive ablative surgeries have advanced into clinical trials for treating many pathologies and cancers. A remaining challenge of these surgeries is accurately planning and monitoring tissue heating in the face of patient-specific and dynamic acoustic properties of tissues. Currently, non-invasive measurements of acoustic properties have not been implemented in MRgHIFU treatment planning and monitoring procedures. This methods-driven study presents a technique using MR temperature imaging (MRTI) during low-temperature HIFU sonications to non-invasively estimate sample-specific acoustic absorption and speed of sound values in tissue-mimicking phantoms. Using measured thermal properties, specific absorption rate (SAR) patterns are calculated from the MRTI data and compared to simulated SAR patterns iteratively generated via the Hybrid Angular Spectrum (HAS) method. Once the error between the simulated and measured patterns is minimised, the estimated acoustic property values are compared to the true phantom values obtained via an independent technique. The estimated values are then used to simulate temperature profiles in the phantoms, and compared to experimental temperature profiles. This study demonstrates that trends in acoustic absorption and speed of sound can be non-invasively estimated with average errors of 21% and 1%, respectively. Additionally, temperature predictions using the estimated properties on average match within 1.2 °C of the experimental peak temperature rises in the phantoms. The positive results achieved in tissue-mimicking phantoms presented in this study indicate that this technique may be extended to in vivo applications, improving HIFU sonication temperature rise predictions and treatment assessment.

4G antennas for wireless eyewear devices and related SAR

In this paper, we first present a feasibility study to design 4G antennas (700–960 MHz and 1.7–2.7 GHz) for eyewear devices. Those eyewear devices should be connected to the last generation cellular networks, Wireless Local Area Networks or wireless hotspots. Three coupling element type antennas with their matching networks are evaluated in terms of reflection coefficient and total radiation efficiency when the eyewear is placed on the user's head. We also present Specific Absorption Rate (SAR) simulations when the eyewear is positioned over a homogeneous SAM phantom and over a heterogeneous VH (Visible Human) phantom: the SAR levels are compared to international limit values. In a second step, we present experimental results obtained with 3D printed eyewear and coupling elements etched on a classical PCB substrate where the matching circuits are optimized close to the feeding point of the coupling element. Simulated and measured values are in very good agreement: 7 to 16% and 9 to 35% total efficiency are respectively obtained for the low- and high-frequency bands. However, simulated SAR values are somewhat higher than authorized levels with preoccupant high electromagnetic field distribution close to the eye of the user. Dans cet article, nous présentons tout d'abord une étude de faisabilité d'antennes 4G (700–960 MHz et 1,7–2,7 GHz) intégrables dans une paire de lunettes connectées. Cet objet est destiné à être connecté aux réseaux de communication cellulaires de dernière génération ainsi qu'aux réseaux personnels de type WLAN. Le concept antennaire d'élément de couplage associé à un circuit d'adaptation est ainsi évalué en termes de coefficient de réflexion et d'efficacité rayonnée lorsque les lunettes sont positionnées sur la tête de l'utilisateur. Nous présentons également l'analyse dosimétrique des prototypes optimisés pour le fantôme homogène SAM et le fantôme évolué hétérogène Visible Human (VH) : les niveaux de DAS dans ces deux fantômes sont simulés et comparés aux normes internationales. Dans un second temps, nous présentons les premiers résultats expérimentaux obtenus à partir de prototypes fabriqués par une imprimante 3D pour la monture des lunettes et des procédés de réalisation classique pour les PCB qui accueillent les éléments de couplage antennaire et leur circuit d'adaptation. Les valeurs théoriques et expérimentales concernant les coefficients de réflexion et les efficacités totales des antennes sont en très bon accord, attestant ainsi une efficacité rayonnée variant de 10 à 12% en bande basse et de 20 à 35% en bande haute. Cependant, les simulations dosimétriques révèlent des niveaux de DAS qui peuvent dépasser les normes du fait des fortes efficacités rayonnées obtenues avec les antennes conçues, notamment avec des pics préoccupants observés au niveau de l'œil.

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.

Safety testing and operational procedures for self-developed radiofrequency coils.

The development of novel radiofrequency (RF) coils for human ultrahigh-field (≥7 T), non-proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor-suggested test specifications for third party coils and custom-developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh-field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR-related safety and electrical performance at ultrahigh-field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright © 2015 John Wiley & Sons, Ltd.

Wearable Fabric Reconfigurable Beam-Steering Antenna for On/Off-Body Communication System

This paper presents a comparison of on-body performances between omnidirectional (loop antenna) and reconfigurable beam-steering antennas. Both omnidirectional and reconfigurable antennas were manufactured on the same fabric substrate and operated at the frequency band of the WLAN 802.11a (5.725–5.85 GHz). The reconfigurable antenna was designed to steer the beam directions. In order to implement the beam-steering capability, the antenna used two PIN diodes. The maximum beam directions of three states (states 0, 1, and 2) were steerable in theYZ-plane (h=2°, 28°, and 326°, resp.). The measured peak gains were 5.9–6.6 dBi and the overall half power beam width (HPBW) was 102°. The measured results of total radiated power (TRP) and total isotropic sensitivity (TIS) indicated that the communication efficiency of the reconfigurable beam steering antenna was better than that of the loop antenna. When the input power was 0.04 W (16 dBm), the simulated specific absorption rate (SAR) values of the reconfigurable beam steering antenna on the body were less than 0.979 W/kg (1 g tissue) in all states, satisfying the SAR criteria of the US.

THEORETICAL AND SIMULATION STUDIES ON WATER-LOADED METAL DIAGONAL HORN ANTENNA FOR HYPERTHERMIA APPLICATION

This paper is a continuation of our previous published work in which a water-loaded metal diagonal horn antenna has been designed at 2450 MHz for hyperthermia application and simulated results are compared with those measured. In the present study, theoretical investigations of Specific Absorption Rate (SAR) distribution in a homogeneous biological phantom (muscle) due to direct contact water-loaded metal diagonal horn antenna at 915 and 2450 MHz for hyperthermia application is presented. It is estimated theoretically that, at both the operating frequencies, a reasonable impedance matching is achieved at the interface between the antenna aperture and the biological phantom, where a computation of aperture admittance and reflection coefficient has been performed. Furthermore, it is confirmed through theoretical and simulation studies that the proposed horn antenna gives circularly symmetric SAR distribution in transverse plane in the biological phantom at 915 and 2450 MHz. The simulated and theoretical SAR distributions at 2450 MHz are compared with those determined at 915 MHz. In addition, thermal simulation results based on Pennes' Bio-heat equation (BHE) are applied to the realistic muscle model at 915 and 2450 MHz. The reduction of blood flow rate on temperature distribution is also studied.

An internal octa‐band printed antenna for mobile phones LTE applications

ABSTRACTA novel compact internal antenna combining a monopole and a parasitic planar inverted‐F antenna type of structure is presented. Based on voltage standing wave ratio &lt;3:1, the operation bandwidths are 42% from 704 to 1083 MHz and 57.7% from 1660 to 3000 MHz to cover LTE700/GSM850/900/DCS1800/PCS1900/UMTS/LTE2300/LTE2500 for mobile phones. The proposed antenna is designed and printed on printed circuit board with compact size of 60 × 15 × 1.2 mm3. Measured average low band efficiency is about 60% and high band efficiency is about 70%. The simulated specific absorption rate at required frequency is less than 1.0 mW/g. Both antenna size and performance showed that this antenna has great potential for slim phone applications. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:247–252, 2015

Enhancing the properties of beam forming bolus in hyperthermia: numerical simulation and empirical verification.

In this paper we present a simulation study of the induced specific absorption rate (SAR) within the phantom produced by radiofrequency radiation from a 8MHz capacitive applicator. The main focus of the current study is on demonstrating the beam shaping properties of the bolus system as well as its effect on controlling the therapeutic area. Different electrical conductivities and geometries of the bolus were considered in the simulation of induced SAR distributions in a muscle-equivalent model with uniform dielectric properties. To validate the presented model, we carried out a comparison between the SAR simulation results and the temperature measurements in an agar split-phantom and an excellent agreement was observed.

Incorporating specific absorption rate constraints into wireless signal design

Portable wireless devices used in close proximity to the human body have transmitter power constraints that are dictated in part by regulatory limits on a form of electromagnetic exposure called specific absorption rate (SAR). SAR, measured in Watts per kilogram, is a measure of electromagnetic energy absorption by the body, which can cause the heating of tissue. Some portable wireless devices sold today operate near the accepted human SAR limit. This article examines how SAR constraints can be incorporated into wireless signal design for multiple transmitters. A SAR constraint, although related to a power constraint, differs sufficiently that signals designed specifically for the constraint are needed. Although it is well known that multiple transmitters subject only to a power constraint can improve the link performance of a wireless system compared to a single transmitter, it is not clear when the device is also subject to a SAR constraint: how to model the constraint; whether performance advantages still exist; and how to realize any advantages. In this article, we introduce SAR constraints and discuss the SAR measurement, simulation, and modeling process. We then show how to incorporate the SAR constraint into system performance analysis and code design. We show how SAR codes use multiple transmit antennas to get good combined farfield error performance and near-field SAR performance, improving handset transmitter performance in the critical uplink of a communication system.