Specific Absorption Rate Safety Research Articles

Vivaldi Tapered Slot Antenna for Microwave Imaging in Medical Applications

Microwave imaging has become an active research area in recent years, owing primarily to advancements in detecting the early stages of cancer. The study aimed to create a high-gain compact Vivaldi Tapered Slot antenna (VTSA) for microwave imaging in medical applications and also aims to address several challenges in the development of microwave imaging (MWI) technology for medical applications. These challenges include the ability to detect and identify abnormalities in human tissue and considering safe Specific Absorption Rate (SAR) limits for patients, the approach of balancing of penetration and resolution can be done on the design. The antenna operates at frequencies ranging from 1.7 to 3.1 GHz and is built on a low-cost Flame Retardant-4 (FR-4) substrate with a thickness of 1.6 mm. A compact exponential VTSA is initially presented while designing the proposed antenna for broad impedance bandwidth performances. The simulation used a back-to-back linear array of antennas with or without a phantom, specifically a without phantom (only antennas), a water phantom (cube shape), and an anomaly inside the water phantom. The results revealed a significant shift in the signal graph between the three results, indicating a difference in values between the three simulations. A transient domain solver calculation was used in the simulation. The designed antenna improved a gain of 6.09 dBi and a SAR of 0.326 W/kg by maximizing the edges of the exponential in the tapered section and the feedline slot area. The antenna exhibits differences in scattering parameters on each simulation of anomalies across the required frequency range. The result finds suitability of the experiment and simulation in assessing the microwave imaging capabilities. With the data presented, simulated antennas can be used for microwave imaging. The next study should aim on making a suitable imaging system with dimensions that supported in the antenna range and specifications.

What can we gain from subpopulation universal pulses? A simulation-based study.

The aim of the study was to explore a novel methodology for designing universal pulses (UPs) that balances the benefits of a calibration-free approach with subject-specific online pulse design. The proposed method involves segmenting the population into subpopulations with variability in anatomical shapes and positions reduced to 75%, 50%, and 25% of their original values while keeping the mean values unchanged. An additional 25% extreme case with a large volume of interest and shifted position was included. For each group, a 5kT-points universal inversion pulse was designed and assessed by the normalized root mean square error (NRMSE) on the target longitudinal magnetization profile. The performance was compared to the conventional one-size-fits-all approach. A total of 132 electromagnetic simulations were executed to generate representative anatomies and specific absorption rate (SAR) distributions in a three-dimensional parameter space comprised of head breadth, head length, and Y-shift. The 99.9th percentile on the peak local SAR distribution was utilized to establish an intersubject variability safety margin. UPs designed for subpopulations with decreased head shape and position variability reduced the anatomical safety margin by up to 20%. Furthermore, when a head was significantly different to the average case, the proposed approach improved the inversion homogeneity by up to 24%, compared to the conventional one-size-fits-all approach. Subpopulation UPs present an opportunity to improve the homogeneity and reduce anatomical SAR safety margins at 7T without additional acquisition time for calibration.

Complementary analysis of specific absorption rate safety for an 8Tx/16Rx array with central symmetry of elements for magnetic resonance imaging of the human heart and abdominopelvic organs at 7 T.

A complementary safety assessment of the specific absorption rate (SAR) of the electromagnetic energy was performed in a prototype 8Tx/16Rx RF array for cardiac magnetic resonance imaging (MRI) at 7 T. The study aimed to address two critical aspects of 7-T SAR safety not always explicitly examined by coil vendors: (i) the influence of an RF-array position on a peak SAR value, and (ii) the risk of exceeding the permitted maximal SAR in the tissue surrounding conductive passive implants. The full-wave 3D electromagnetic simulations for the thorax with shifted array position and the whole-body volume in the presence of a dental retainer, an intrauterine contraceptive device (IUD), and a hip joint implant, were performed for two human voxel models. The effect of the array displacement on the SAR was simulated for seven array locations on the thorax shifted from the central position in different directions on 50 mm. The peak SAR values for both models were analyzed for the three phase-only transmit vectors optimized for B1 + homogeneity and transmit efficiency. Peak SAR values due to the shifts of the array position increase up to ≈50%. The worst-case peak SAR value for a dental retainer was found to be in the range of 10% of the maximal SAR in the tissue within the array's borders. For the IUD and artificial hip joint implants the effect was found to be negligible (peak SAR < 1% of the SAR within array borders). In addition to simulations for cardiac MRI, we performed a preliminary B1 + shimming and SAR-safety analysis for the same RF-array at various positions lower on the body trunk to assess a potential application in imaging abdominopelvic organs (prostate, kidney, and liver). The most promising target for an ad hoc alternative application of the array was found to be the prostate.

Compact Ultra-­Wideband Planar Inverted F Antenna on Laminated Paper­-Based Substrate with Reduced Specific Absorption Rate

This paper presents a laminated paper-based Planar Inverted F Antenna (PIFA) for wireless body area network (WBAN) applications. The major challenges like providing adequate thickness, uniformity of surface, and water wet ability associated with paper substrate have been transcended in this antenna design by laminating the substrate with a transparent sheet. The simulated and measured reflection coefficients of the fabricated antenna in free space and on human arm are remarkably good, validating the prototype’s utility. It can operate effectively in GSM, WMTS, GPS, DCS, ISM, WiMAX, HIPERLAN, WLAN, and UWB frequency bands with S11 less than -10 dB and the gain ranging from 0.94 to 9.53 dB (free space) and 1.03 to 6.07 dB (on-body). The electrical size of the proposed antenna is 0.16 λg x 0.12 λg at 0.7 GHz with a group delay below 2.3 ns over the desired frequency bands. The performance characteristics are also investigated on-body and it encompasses most of the desired frequency bands for the body area network. This paper also analyses the specific absorption rate (SAR) of the designed antenna by placing it on a tissue-layered human arm model, which is 0.64 Watt/kg, complying with the SAR safety guidelines.

Slotted Monopole Patch Antenna for Microwave-Based Head Imaging Applications

A modified monopole patch antenna for microwave-based hemorrhagic or ischemic stroke recognition is presented in this article. The designed antenna is fabricated on a cost-effective FR-4 lossy material with a 0.02 loss tangent and 4.4 dielectric constant. Its overall dimensions are 0.32 λ × 0.28 λ × 0.007 λ, where λ is the lower bandwidth 1.3 GHz frequency wavelength. An inset feeding approach is utilized to feed the antenna to reduce the input impedance (z = voltage/current). A total bandwidth (below −10 dB) of 2.4 GHz (1.3–3.7 GHz) is achieved with an effective peak gain of over 6 dBi and an efficiency of over 90%. A time-domain analysis confirms that the antenna produces minimal signal distortion. Simulated and experimental findings share a lot of similarities. Brain tissue is penetrated by the antenna to a satisfactory degree, while still exhibiting a safe specific absorption rate (SAR). The maximum SAR value measured for the head model is constrained to be equal to or below 0.1409 W/kg over the entire usable frequency band. Evaluation of theoretical and experimental evidence indicates the intended antenna is appropriate for Microwave Imaging (MWI) applications.

Design of Low-SAR Mobile Phone Antenna: Theory and Applications

This article focuses on the design of a low specific absorption rate (SAR) mobile phone antennas. First, the interaction mechanism of the radiation fields of mobile phone antennas with nearby human tissues is analyzed by using boundary conditions of electromagnetics. The relations between SAR and the electric field are constructed. Then, a concept of reverse current is proposed to decrease the electric field radiated by the antenna at the air-tissue interface and, consequently, reduce the SAR of human tissues. Finally, a metal-rimmed mobile phone antenna is designed according to the proposed theory. The ground clearance of the designed antenna is 2 mm. Their measured normalized peak head and body SAR values are lower than 1.0 W/kg, much less than the SAR safety limit of 1.6 W/kg according to the guidelines of IEEE and FCC or 2.0 W/kg according to the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The measured -6 dB impedance bandwidths are 830-1040 and 1670-2730 MHz, respectively. The measured efficiencies are higher than -5 dB at the lower band and -4 dB at the higher band. The measured results prove that the proposed theory can provide guidance for the design of low-SAR mobile phone antennas.

Dual‐polarized textile‐based two/four element MIMO antenna with improved isolation for dual wideband application

This article presents the designs of dual-polarized dual wideband textile-based two and four elements multiple-input multiple-output (MIMO) antennas for WLAN (IEEE 802.11a/b/g/c/n) and WiMAX (IEEE 802.16d) applications. These MIMO antennas cover the frequency spectra from 1.5 to 3.8 GHz (87% bandwidth) and 4.1 to 6.1 GHz (40% bandwidth). The characterization of the textile jeans substrate is determined experimentally using a vector network analyzer and dielectric assessment kit. These antennas provide near about 70% radiation efficiency with around 4 dBi peak gain in desired frequency ranges. The diversity performance is improved noticeably by printing meandered line structures on both planes. The proposed MIMO structure has a very low envelop correlation coefficient (ECC) <0.1 and high diversity gain (DG) >9.9. The Medium effective gain (MEG) also lies within a satisfactory value of ±3 dB. The two elements MIMO Antennas provide linear polarization at all desired frequency band while the four-element antenna provides circular polarization at 2.4 GHz and linear polarization at 5.2 and 5.8 GHz application bands. The antenna also depicts good performance in wearable condition with safe specific absorption rate < 1.6 W/kg in all desired frequencies.

A Grounded Coplanar Waveguide-Based Slotted Inverted Delta-Shaped Wideband Antenna for Microwave Head Imaging

A wideband grounded coplanar waveguide (GCPW) based patch antenna for head imaging is discussed in this paper. The proposed antenna is integrated with a slotted inverted delta-shaped main radiating patch, coplanar waveguide elements, and slotted partial ground. The prime objective of usages of coplanar waveguide elements is enhancing antenna effectiveness. In the prototype, examined different categories of slots in the ground and particularly rectangular and elliptical-shaped slots are used to improve the radiation directivity, gain, and efficiency. The optimized dimension of the antenna is $50\times 44\times 1.524$ mm3 by using the Rogers RO4350B substrate. The measured and simulated results demonstrate that the proposed prototype has a bandwidth of 2.01 GHz (1.70–3.71 GHz) with nearly directional radiation characteristics. The highest gain of the prototype is 5.65 dBi, and the maximum 93% radiation efficiency over the frequency band. The antenna has a lower group delay, higher fidelity factor (>95%), which are significant factors for microwave head imaging. Various design structures are performed to achieve the most satisfactory result. The proposed prototype is designed and analyzed by 3D CST 2019 simulator software. Later, the antenna is fabricated and measured to examine the performance. Thereafter, a single antenna and twelve-antennas array element are configured and placed surrounding the 3D realistic shaped Hugo head model to validate the effectiveness of the single antenna as well as the designed antennas array. The antenna has satisfactory field penetration into the human head tissues with a safe SAR (specific absorption rate). Less than 0.00233 W/kg SAR is attained over the operating frequency range, which is lower than the reported antennas. The Iteratively Corrected Delay Multiply and Sum (IC-DMAS) imaging algorithm will be applied for imaging purposes. The investigation through the literature of the statistical and measured data evident that the proposed antenna is apposite for the wideband microwave head imaging applications.

Towards reduction of SAR in scaling up in vivo pulsed EPR imaging to larger objects.

An excessive RF power requirement is one of the main obstacles in the clinical translation of EPR imaging. The radio frequency (RF) pulses used in EPR imaging to excite electron spins must be very short to match their fast relaxation. With traditional pulse schemes and ninety degree flip angles, this can lead to either unsafe specific absorption rate (SAR) levels or unfeasibly long repetition times. In spectroscopy experiments, it has been shown that stochastic excitation and correlation detection can reduce the power while maintaining sensitivity but have yet to be applied to imaging experiments. Stochastic excitation is implemented using a pseudo-random phase modulation of the input stimulus. Using a crossed coil resonator assembly comprised of an outer saddle coil and an inner surface coil, it was possible to obtain a minimum isolation of ~50 dB across a 12 MHz bandwidth. An incident peak RF power of 5 mW was used to excite the system. The low background signal obtained from this resonator allowed us to generate images with 32 dB (>1000:1) signal-to-noise ratio (SNR) while exciting with a traditional pulse sequence in a phantom containing the solid paramagnetic probe NMP-TCNQ (N-methyl pyridinium tetracyanoquinodimethane). Using two different stochastic excitation schemes, we were able to achieve a greater than 4-fold increase in SNR at the same peak power and number of averages, compared to single pulse excitation. This procedure allowed imaging at significantly lower RF power levels than used in conventional EPR imaging system configurations. Similar techniques may enable clinical applications for EPR imaging by facilitating the use of larger RF coils while maintaining a safe SAR level.

Wearable Electromagnetic Head Imaging System Using Flexible Wideband Antenna Array Based on Polymer Technology for Brain Stroke Diagnosis.

Given the increased interest in a fast, portable, and on-spot medical diagnostic tool that enables early diagnosis for patients with brain stroke, a new approach of a wearable electromagnetic head imaging system based on the polymer material is proposed. A flexible low-profile, wideband, and unidirectional antenna array with electromagnetic band gap (EBG) and metamaterial (MTM) unit cells reflector is utilized. The designed antenna consists of a 4 × 4 radiating patch loaded with symmetrical extended open-ended U-slots and fed by combination of series and corporate transmission lines. A mushroom-like 10-EBG unit cell arrays are arranged around the feeding network to reduce surface waves, whereas 4 × 4 MTM unit cells are placed on the back-side of the antenna to enable unidirectional radiation. The antenna is designed and embedded on a multilayer low cost, low loss, transparent, and robust polymer poly-di-methyl-siloxane (PDMS) substrate and optimized to operate in contact with the human head. The simulated and measured results show that the antenna has a fractional bandwidth of 53.8% (1.16-1.94GHz), more than 80% of radiation efficiency, and satisfactory field penetration in the head tissues with a safe specific absorption rate. An eight-element array is then configured on 300 × 360 × 4.1 mm3 PDMS material covering an average human head size and used as a worn part of the imaging system. A realistic-shaped 3-D specific anthropomorphic mannequin (SAM) head phantom is used to verify the performance of the designed array. The imaging results indicate the possibility of using the designed conformal array to detect a bleeding inside the brain using a confocal image algorithm.

Near-Field Wireless Power Transfer to Stent-Based Biomedical Implants

Safe wireless power transfer is an essential requirement for biomedical implants. Emerging technologies are becoming smaller, less invasive, and consume less power. Moreover, stent-based devices are being recognized as a minimally invasive alternative to traditional surgery. Hence, the idea of using the body of the stent as the power receiving element is becoming increasingly attractive. The objective of this paper is to analyze two near-field wireless power transfer methods to stent-based devices, viz., inductive and capacitive coupling. The methods used are lumped element modeling, ac circuit theory, finite-element analysis, experiments with excised bovine muscle tissue, and a live ovine vascular model. Capacitive coupling is proposed as an alternate method due to the transmitter design that can be worn anywhere on the body. It achieves power transfer efficiencies of 2.6% and 1% when placed at depths in muscle tissue of 15 mm and 30 mm, respectively. Safety requirements are also met. The capacitive link can accept an input power of 53 mW before exceeding the safe specific absorption rate limit of 1.6 W/kg averaged over 1 g of tissue.

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.

Probabilistic analysis of the specific absorption rate intersubject variability safety factor in parallel transmission MRI.

Specific absorption rate (SAR) calculations in parallel transmission are commonly performed by using electromagnetic simulations on generic models. In this study, we propose a probabilistic analysis to study the safety factor employed to account for SAR intersubject variability versus risk relationship in head imaging at 7T. Thirty-three finite-element electromagnetic simulations were conducted to sample the four-dimensional parameter space consisting of the head length, head breadth, and shifts in Z and Y random variables. Based on the SAR matrices for each configuration, a multivariate second-order polynomial of the SAR versus the different parameters was reconstructed for different types of radiofrequency pulses. A Monte Carlo calculation was then performed to compute the probability of occurrence of a given SAR value. By testing a large number of radiofrequency excitation pulses, the SAR calculated for the average model amplified by a safety margin of 1.5 was found to return a probability of less than 1% to be exceeded across the adult Caucasian population given the investigated parameters. The proposed method to study SAR intersubject variability uses a reasonable number of electromagnetic simulations. Look-ahead SAR safety margins can be deduced based on risk/benefit ratio assessments. Magn Reson Med 78:1217-1223, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Hair Loss due to Electromagnetic Radiation from Overuse of Cell Phone

Mobile phone usage is nearing 90% of the world population. Harmful effects of Electromagnetic (EMG) radiation have been a concern for regulation. Safe Specific Absorption Rate (SAR) as watt/kg is defined for thermal effects, but EMG has non thermal effects as well. EMG can affect genes, neural tissue, endocrine regulation and sperms. Research workers have pointed out the mechanism of EMG radiation to be acting through DNA breakage and formation of Reactive Oxygen Species (ROS). We are reporting a case of hair loss over the temporal region from repeated, prolonged conversations on mobile phone. Biopsy is negative for any other pathology of hair loss. Use of antioxidants, vitamins, minerals, can aid in recovery of the damaged cells. Low dose 2% minoxidil application combined with nutritional therapy, limiting the duration of mobile phone usage and having hands free chord to keep the mobile phone instrument away from the ear, was seen to help in regrowth of the hair.

Antenna development, link budget analysis and specific absorption rate evaluation in ultra‐wideband implant communications

In this study, the authors developed an ultra-wideband (UWB) planar loop antenna for implant communication, and confirmed its usefulness by comparing its designed and measured reflection and transmission characteristics. With the planar loop antenna as the implant transmitting antenna, then they calculated the implant path loss and specific absorption rate (SAR) using the finite difference time domain method together with an anatomical human body numerical model. Moreover, through a link budget analysis based on the calculated path loss with selection diversity, they derived the transmitting power required to achieve a specific bit error rate performance, and clarified the corresponding local SAR with respect to the international safety limits. As a result, the UWB implant communication is found to be feasible at a data rate as high as 40 Mbps under the SAR safety limits.

SAR Study of Mobile Phones as a function of Antenna Q

High-Q tunable antennas are good alternatives to low-Q passive antennas because the antenna size can be smaller while covering the required long-term evolution (LTE) frequency bands. However, among other things, specific absorption rate (SAR) can become a challenge due to the relative high current density associated with high-Q antennas. The higher energy stored in the electric and magnetic near-field components can result in higher SAR. Hence, SAR study of high-Q antennas is necessary which, if not addressed, might not comply with the SAR safety guidelines. In this paper, SAR as a function of antenna Q is investigated numerically as well as experimentally at two distinct frequencies (720 and 1700 MHz). It is found that SAR increases as a function of antenna Q when no losses are considered. But when losses are included, there is no more a clear trend between SAR and antenna Q. Thus, the final SAR values depend on total losses of the mock-up (MU).

Parallel transmission pulse design with explicit control for the specific absorption rate in the presence of radiofrequency errors.

A new framework for the design of parallel transmit (pTx) pulses is presented introducing constraints for local and global specific absorption rate (SAR) in the presence of errors in the radiofrequency (RF) transmit chain. The first step is the design of a pTx RF pulse with explicit constraints for global and local SAR. Then, the worst possible SAR associated with that pulse due to RF transmission errors ("worst-case SAR") is calculated. Finally, this information is used to re-calculate the pulse with lower SAR constraints, iterating this procedure until its worst-case SAR is within safety limits. Analysis of an actual pTx RF transmit chain revealed amplitude errors as high as 8% (20%) and phase errors above 3° (15°) for spokes (spiral) pulses. Simulations show that using the proposed framework, pulses can be designed with controlled "worst-case SAR" in the presence of errors of this magnitude at minor cost of the excitation profile quality. Our worst-case SAR-constrained pTx design strategy yields pulses with local and global SAR within the safety limits even in the presence of RF transmission errors. This strategy is a natural way to incorporate SAR safety factors in the design of pTx pulses. Magn Reson Med 75:2493-2504, 2016. © 2015 Wiley Periodicals, Inc.

Gradient-modulated SWIFT.

Methods designed to image fast-relaxing spins, such as sweep imaging with Fourier transformation (SWIFT), often utilize high excitation bandwidth and duty cycle, and in some applications the optimal flip angle cannot be used without exceeding safe specific absorption rate (SAR) levels. The aim is to reduce SAR and increase the flexibility of SWIFT by applying time-varying gradient-modulation (GM). The modified sequence is called GM-SWIFT. The method known as gradient-modulated offset independent adiabaticity was used to modulate the radiofrequency (RF) pulse and gradients. An expanded correlation algorithm was developed for GM-SWIFT to correct the phase and scale effects. Simulations and phantom and in vivo human experiments were performed to verify the correlation algorithm and to evaluate imaging performance. GM-SWIFT reduces SAR, RF amplitude, and acquisition time by up to 90%, 70%, and 45%, respectively, while maintaining image quality. The choice of GM parameter influences the lower limit of short T2 (*) sensitivity, which can be exploited to suppress unwanted image haze from unresolvable ultrashort T2 (*) signals originating from plastic materials in the coil housing and fixatives. GM-SWIFT reduces peak and total RF power requirements and provides additional flexibility for optimizing SAR, RF amplitude, scan time, and image quality.

Local specific absorption rate in brain tumors at 7 tesla.

MR safety at 7 Tesla relies on accurate numerical simulations of transmit electromagnetic fields to fully assess local specific absorption rate (SAR) safety. Numerical simulations for SAR safety are currently performed using models of healthy patients. These simulations might not be useful for estimating SAR in patients who have large lesions with potentially abnormal dielectric properties, e.g., brain tumors. In this study, brain tumor patient models are constructed based on scans of four patients with high grade brain tumors. Dielectric properties for the modeled tumors are assigned based on electrical properties tomography data for the same patients. Simulations were performed to determine SAR. Local SAR increases in the tumors by as much as 30%. However, the location of the maximum 10-gram averaged SAR typically occurs outside of the tumor, and thus does not increase. In the worst case, if the tumor model is moved to the location of maximum electric field intensity, then we do observe an increase in the estimated peak 10-gram SAR directly related to the tumor. Peak local SAR estimation made on the results of a healthy patient model simulation may underestimate the true peak local SAR in a brain tumor patient.

Design of parallel transmission pulses for simultaneous multislice with explicit control for peak power and local specific absorption rate.

To design parallel transmit (pTx) simultaneous multislice (SMS) spokes pulses with explicit control for peak power and local and global specific absorption rate (SAR). We design SMS pTx least-squares and magnitude least squares spokes pulses while constraining local SAR using the virtual observation points (VOPs) compression of SAR matrices. We evaluate our approach in simulations of a head (7T) and a body (3T) coil with eight channels arranged in two z-rows. For many of our simulations, control of average power by Tikhonov regularization of the SMS pTx spokes pulse design yielded pulses that violated hardware and SAR safety limits. On the other hand, control of peak power alone yielded pulses that violated local SAR limits. Pulses optimized with control of both local SAR and peak power satisfied all constraints and therefore had the best excitation performance under limited power and SAR constraints. These results extend our previous results for single slice pTx excitations but are more pronounced because of the large power demands and SAR of SMS pulses. Explicit control of local SAR and peak power is required to generate optimal SMS pTx excitations satisfying both the system's hardware limits and regulatory safety limits.