Specific Absorption Rate Calculations Research Articles

Preparation of stable colloidal dispersion of surface modified Fe3O4 nanoparticles for magnetic heating applications

The effect of surface modification on enhancing the magnetic heating behavior of magnetic nano fluids were investigated, for this purpose Fe3O4 nanoparticles were synthesized using co-precipitation method and surface modification was done using citric acid, ascorbic acid, tetraethyl orthosilicate (TEOS), polyvinyl alcohol (PVA) and polyethylene glycol (PEG). Experimental heating tests using AC magnetic field were done in the frequency of 100 kHz and different magnetic field (H) intensities. Theoretically the specific absorption rate (SAR) in magnetic nano fluids is independent of nanoparticles concentration but the experimental results showed different behavior. The theoretical SAR value @ H = 12kA.m–1 for Nano fluids containing bare Fe3O4 nanoparticles was 11.5 W/g but in experimental tests the obtained value was 9.72 W/g for nano fluid containing 20,000 ppm of dispersed nanoparticles. The experimental SAR calculation was repeated for sample containing 10,000 ppm of nanoparticles and the results showed increase in experimental SAR that is an evidence of nanoparticles agglomeration in higher concentrations. The surface modification has improved the dispersion ability of the nanoparticles. The Ratio of SAR, experimental, 20000ppm to SAR, experimental, 10000ppm was 0.85 for bare Fe3O4 nanoparticles dispersion but in case of surface modified nanoparticles this ratio has increased up to 0.98 that shows lower agglomeration of nanoparticles as a result of surface modification, although on the other hand the surface modification agents were magnetically passive and so it is expected that in constant concentration the SAR for bare Fe3O4 nanoparticles to be higher than this variable for surface modified nanoparticles. At lower concentrations the dispersions containing bare Fe3O4 nanoparticles showed higher SAR values but at higher concentrations the surface modified Fe3O4 nanoparticles showed better results although the active agent amount was lower at them. Finally, it should be noted that the nanoparticles that were surface modified using polymeric agents showed the highest decrease in experimental SAR amounts comparing theoretical results that was because of the large molecules of polymers comparing other implemented surface modification agents.

Terahertz Microstrip Patch Antenna for Breast Tumour Detection

Breast cancer is one of the most common cancers among Malaysian women. It is critical to discover strategies to detect the tumour early on. Terahertz (THz) frequency provides excellent qualities for detecting tumours such as low photon energy and non-ionising radiation as compared to prior methods such as mammography, ultrasound, and magnetic resonance imaging (MRI) that use optical to X-ray frequencies. The purpose of this work is to analyse and locate a breast tumour as well as to compute the maximum specific absorption rate (SAR) value. It was designed a THz rectangular microstrip patch antenna with an inset feed. To improve the antenna's performance, graphene was used for the patch and polyimide for the substrate. This antenna covered a bandwidth of 31.6 GHz and worked in the frequency range of 0.283-0.599 THz. To identify the location of a tumour, compute the SAR value, and localise the tumour, SAR simulation was used. The maximum SAR shifted to the tumor's position due to greater absorption rate around its tissue due to higher dielectric constant features. It was calculated that 1e-05g of average mass is required to be less than total tissue mass, which is 2.0063e-05g. SAR study revealed a maximum SAR value of 2.49391e+06 W/kg, which was not more than the overall absorption rate for human body safety. The SAR calculation result revealed that the tumour is within the range of the tumor's initial location.

Assessment of the Variability of Human Exposure to Radiofrequency Electromagnetic Fields Arising from 5.9 GHz Vehicular Communication in Urban Environments.

This paper assessed the variability of radiofrequency exposure among road users in urban settings due to vehicle-to-vehicle (V2V) communication operating at 5.9 GHz. The study evaluated the absorbed dose of radiofrequencies using whole-body specific absorption rate (SAR) in human models spanning different age groups, from children to adults. To overcome limitations of previous studies, we developed a novel hybrid procedure that combines deterministic and stochastic approaches, enabling assessment across multiple urban layouts. Real urban conditions and varying propagation scenarios were considered in SAR calculations. By varying the road user's position within 1.5-300 m from transmitting cars, the SAR distribution was determined. Median SAR remained consistently low, around 0.70 mW/kg, even with multiple transmitting cars and multiple emitting antennas, using maximum power allowed in US (44.8 dBm). The 99th percentile of SAR distribution varied based on body mass, decreasing for heavier models (typically adults) and increasing with the number of transmitting cars and antennas. The highest absorbed dose (73 mW/kg) occurred in a child model. The SAR consistently remained below the 80 mW/kg limit for whole-body exposure to electromagnetic fields in the 100 kHz-300 GHz range.

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.

Specific absorption rate (SAR) simulations for low-field (< 0.1 T) MRI systems

ObjectiveTo simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.MethodsElectromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.ResultsSimulations of RF coil characteristics and B1+ transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180° refocusing pulses of duration ~ 10 ms are used for TSE sequences does SAR need to be carefully considered.ConclusionThis work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T1ρ, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed.

FDTD-based SAR calculation of a wearable antenna for wireless body area network devices

AbstractWireless-connected wearable electronics are finding extensive usage for diagnostic and therapeutic purposes after the globally spread pandemic disease of COVID-19. Although they are undoubtedly helpful for keeping physical distance, their health effects are still under investigation from different aspects and are still a concern for the end-users. In this study, a custom M-shaped wearable antenna covering the wireless body area network and wireless local area network frequencies is designed, built, and measured. A beret cap made from a 2 mm thick textile is used as a substrate. The specific absorption rate (SAR) in a realistic human-head model due to electromagnetic energy produced by the antenna is evaluated using the finite-difference time-domain method. The SAR distributions for 1-g and 10-g tissues are calculated at 2.4 and 5.8 GHz. It is shown that the obtained maximum SAR values for 1-g and 10-g tissues at each frequency of interest were less than the limits determined by IEEE RF exposure guidelines and standards.

An investigation into the dependence of virtual observation point‐based specific absorption rate calculation complexity on number of channels

An investigation into the dependence of virtual observation point‐based specific absorption rate calculation complexity on number of channels

An investigation into the dependence of virtual observation point-based specific absorption rate calculation complexity on number of channels.

This study aims to find a relation between the number of channels and the computational burden for specific absorption rate (SAR) calculation using virtual observation point-based SAR compression. Eleven different arrays of rectangular loops covering a cylinder of fixed size around the head of an anatomically correct voxel model were simulated. The resulting Q-matrices were compressed with 2 different compression algorithms, with the overestimation fixed to a certain fraction of worst-case SAR, median SAR, or minimum SAR. The latter 2 were calculated from 1e6 normalized random excitation vectors. The number of virtual observation points increased with the number of channels to the power of 2.3-3.7, depending on the compression algorithm when holding the relative error fixed. Together with the increase in the size of the Q-matrices (and therefore the size of the virtual observation points), the total increase in computational burden with the number of channels was to the power of 4.3-5.7. The computational cost emphasizes the need to use the best possible compression algorithms when moving to high channel counts.

The impact of respiratory motion on electromagnetic fields and specific absorption rate in cardiac imaging at 7T.

To present electromagnetic simulation setups for detailed analyses of respiration's impact on and E-fields, local specific absorption rate (SAR) and associated safety-limits for 7T cardiac imaging. Finite-difference time-domain electromagnetic field simulations were performed at five respiratory states using a breathing body model and a 16-element 7T body transceiver RF-coil array. and SAR are analyzed for fixed and moving coil configurations. SAR variations are investigated using phase/amplitude shimming considering (i) a local SAR-controlled mode (here SAR calculations consider RF amplitudes and phases) and (ii) a channel-wise power-controlled mode (SAR boundary calculation is independent of the channels' phases, only dependent on the channels' maximum amplitude). Respiration-induced variations of both amplitude and phase are observed. The flip angle homogeneity depends on the respiratory state used for shimming; best results were achieved for shimming on inhale and exhale simultaneously ( ). The results reflect that respiration impacts position and amplitude of the local SAR maximum. With the local-SAR-control mode, a safety factor of up to 1.4 is needed to accommodate for respiratory variations while the power control mode appears respiration-robust when the coil moves with respiration (SAR peak decrease: 9% exhale→inhale). Instead, a spatially fixed coil setup yields higher SAR variations with respiration. Respiratory motion does not only affect the distribution and hence the image contrast, but also location and magnitude of the peak spatial SAR. Therefore, respiration effects may need to be included in safety analyses of RF coils applied to the human thorax.

Bench to bore ramifications of inter-subject head differences on RF shimming and specific absorption rates at 7T

PurposeThis study shows how inter-subject variation over a dataset of 72 head models results in specific absorption rate (SAR) and B1+ field homogeneity differences using common shim scenarios. MethodsMR-CT datasets were used to segment 71 head models into 10 tissue compartments. These head models were affixed to the shoulders and neck of the virtual family Duke model and placed within an 8 channel transmit surface-loop array to simulate the electromagnetic fields of a 7T imaging experiment. Radio frequency (RF) shimming using the Gerchberg-Saxton algorithm and Circularly Polarized shim weights over the entire brain and select slices of each model was simulated. Various SAR metrics and B1+ maps were calculated to demonstrate the contribution of head variation to transmit inhomogeneity and SAR variability. ResultsWith varying head geometries the loading for each transmit loop changes as evidenced by changes in S-parameters. The varying shim conditions and head geometries are shown to affect excitation uniformity, spatial distributions of local SAR, and SAR averaging over different pulse sequences. The Gerchberg-Saxton RF shimming algorithm outperforms circularly polarized shimming for all head models. Peak local SAR within the coil most often occurs nearest the coil on the periphery of the body. Shim conditions vary the spatial distribution of SAR. ConclusionThe work gives further support to the need for fast and more subject specific SAR calculations to maintain safety. Local SAR10g is shown to vary spatially given shim conditions, subject geometry and composition, and position within the coil.

Measurement and Calculation of SAR Due to 900 MHz Electromagnetic Waves

Purpose: The widespread use of mobile phones and Base Transceiver Stations (BTSs) has generated public concern about exposure to Electromagnetic (EM) waves. In this study, the electric field intensity and Specific Absorption Rate (SAR) in the emergency, general hospitalization, radiology, and laboratory departments of four hospitals in Arak (Iran) are reported.&#x0D; Materials and Methods: Electric field strength in the 900 MHz frequency band was obtained using a TES 592 radiometer. Then, SAR induced in the brain, skin, fat and bone tissues were calculated based on equations and the obtained values were compared with the thresholds recommended by the International Commissions.&#x0D; Results: The obtained results showed that the electric field’s mean value was 1.334 V/m which is almost 2.7% of the threshold introduced by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and 2.6% of the threshold adopted by the Institute of Electrical and Electronics Engineers (IEEE). The highest SAR value was 1.6 W/kg for the skin, which is lower than the threshold values presented by ICNIRP (2 W/kg) and IEEE (1.6 W/kg).&#x0D; Conclusion: The findings of the present work show that for both quantities in Arak hospitals the SAR values are less than the thresholds announced by IEEE and ICNIRP committees. To deal with the concerns of the community that is generally caused by a lack of awareness, the executions of educational and public awareness programs are recommended.

Design analysis and validation of coaxial probe for tissue dielectric properties evaluation used in specific absorption rate measurement

ABSTRACT This study distinguishes the design and analysis of a coaxial probe for measurement of biological body dielectric properties, in this measurement estimating the human tissue-equivalent liquid (TEL) permittivity and conductivity, to monitor and maintain the international standards for specific absorption rate (SAR) evaluation over the frequency band of 800 MHz-5 GHz. In addition, deionized (DI) water and ethanediol dielectric properties have been evaluated and the designed probe results compared to the commercial Dielectric Assessment kit (DAK) 3.5 probe. The obtained results are in good agreement with each other, moreover, the SAR calculation and each source of uncertainty budget analysis are estimated. Therefore, this fabricated probe may be suitable for liquid dielectric properties measurement.

Carbothermal treated iron oxide nanoparticles with improving magnetic heating efficiency for hyperthermia

Iron oxide nanoparticles with high magnetic heating efficiency is highly desired for magnetic hyperthermia applications. In this paper, carbothermal treatments with graphene are used for improving the magnetic heating efficiency of iron oxide nanoparticles. With the increasing treatment temperature from 400 °C to 700 °C, the specific absorption rate (SAR) increases 2–7 folds. Oxygen vacancies resulted from carbon reduction process, along with the rapid grain growth (from 5.1 nm to 225.6 nm), improve eddy current loss. SAR calculations suggest that eddy current loss accounts up to 23.9%-78.5% of the total SAR after carbothermal treatments, which is the main factor for SAR improvements. This work may provide a methodological reference of preparing iron oxide nanoparticles for magnetic hyperthermia.

Local SAR compression algorithm with improved compression, speed, and flexibility.

Local specific absorption rate (SAR) compression algorithms are essential for enabling online SAR monitoring in parallel transmission. A better compression resulting in a lower number of virtual observation points improves speed of SAR calculation for online supervision and pulse design. An iterative expansion of an existing algorithm presented by Lee et al is proposed in this work. The original algorithm is used within a loop, making use of the virtual observation points from the previous iteration as the starting subvolume, while decreasing the overestimation with each iteration. This algorithm is evaluated on the SAR matrices of three different simulated arrays. The number of virtual observation points is approximately halved with the new algorithm, while at the same time the compression time is reduced with speed-up factors of up to 2.5. The new algorithm improves the original algorithm in terms of compression rate and speed.

UWB Sierpinski Fractal Antenna for Breast Tumour Detection Using SAR

Breast tumour is the most common tumour in the world and one in eight women is likely to develop breast tumours. Survival rate is higher only when the tumour is detected at the early stages. The various methodologies currently available to detect breast tumour are ultrasound, MRI, X-ray mammography etc. But these methods are not cost effective and have other reliability issues. In this paper, tumour is detected by passing microwave signals into the homogeneous breast model by designing an Ultra-Wideband (UWB) Sierpinski fractal micro strip patch antenna (MSA) with reduced ground. SAR (Specific Absorption Rate) calculation helps to find the presence of tumour. The antenna characteristics are studied using CST studio suite 2017 software. Sierpinski fractals and reduced ground are used to achieve UWB and enhance bandwidth. The preferred antenna provides a bandwidth width of 9.71 GHz (2.79 - 12GHz).

Carbothermal Treated Iron Oxide Nanoparticles with Improving Magnetic Heating Efficiency for Hyperthermia

Iron oxide nanoparticles with high magnetic heating efficiency is highly desired for magnetic hyperthermia applications. In this paper, carbothermal treatments with graphene were used for improving the magnetic heating efficiency of iron oxide nanoparticles. With the increasing treatment temperature from 400°C to 700°C, the specific absorption rate (SAR) increased 2-7 folds. Oxygen vacancies resulted from carbon reduction process, along with the rapid grain growth (from 5.1 nm to 225.6 nm), improved eddy current loss. SAR calculations suggested that eddy current loss accounted up to 23.9%-78.5% of the total SAR after carbothermal treatments, which was the main factor for SAR improvements. This work may provide a methodological reference of preparing iron oxide nanoparticles for magnetic hyperthermia.

Individualized SAR calculations using computer vision-based MR segmentation and a fast electromagnetic solver.

We propose a fast, patient-specific workflow for on-line specific absorption rate (SAR) supervision. An individualized electromagnetic model is created while the subject is on the table, followed by rapid SAR estimates for that individual. Our goal is an improved correspondence between the patient and model, reducing reliance on general anatomical body models. A 3D fat-water 3T acquisition (~2minutes) is automatically segmented using a computer vision algorithm (~1minute) into what we found to be the most important electromagnetic tissue classes: air, bone, fat, and soft tissues. We then compute the individual's EM field exposure and global and local SAR matrices using a fast electromagnetic integral equation solver. We assess the approach in 10 volunteers and compare to the SAR seen in a standard generic body model (Duke). The on-the-table workflow averaged 7'44″. Simulation of the simplified Duke models confirmed that only air, bone, fat, and soft tissue classes are needed to estimate global and local SAR with an error of 6.7% and 2.7%, respectively, compared to the full model. In contrast, our volunteers showed a 16.0% and 20.3% population variability in global and local SAR, respectively, which was mostly underestimated by the Duke model. Timely construction and deployment of a patient-specific model is computationally feasible. The benefit of resolving the population heterogeneity compared favorably to the modest modeling error incurred. This suggests that individualized SAR estimates can improve electromagnetic safety in MRI and possibly reduce conservative safety margins that account for patient-model mismatch, especially in non-standard patients.

Staked T Lines Integrated Circular Microstrip Tri-band Wearable Antenna for GSM, ISM and NATO Applications

Antenna designed with fabrics as dielectric substrate find applications in health monitoring systems, fire-fighting, rescue operations, space, navigation and military personal communications. The aim of this paper is to design a flexible, microstrip antennas that will be mechanically robust and light weight. This type of antenna will find applications in Global System for Mobile communication (GSM), Wireless Fidelity (Wi-Fi) and North Atlantic Treaty Organization (NATO) bands. A tri – band microstrip antenna is designed and constructed with woolen felt as substrate. The dielectric constant of woolen substrate is 1.45, The thickness of the substrate is 1.588 mm. The gain and axial ratio are stable when bent and in presence of moisture. Also the operating band is compliant with specifications. The operating frequencies are 1800MHz (GSM), 3.5GHz (Wi-Fi) and 4-5GHz (NATO). The simulated result of S11 is measured as for all three bands are -13.399dB, -35.114dB and -36.254dB respectively. The tri-band wearable antenna is verified and validated using network analyzer. The testing of triband antenna also involves the calculation of Specific Absorption Rate (SAR) under the standards of IEEE and ICNIRP. The antennas with fabric as substrate allow hands free operation, comfortable and are also economically feasible.

Magnetic fluid hyperthermia simulations in evaluation of SAR calculation methods

PurposeThe purpose of this study is to employ magnetic fluid hyperthermia simulations in the precise computation of Specific Absorption Rate functions -SAR(T)-, and in the evaluation of the predictive capacity of different SAR calculation methods. MethodsMagnetic fluid hyperthermia experiments were carried out using magnetite-based nanofluids. The respective SAR values were estimated through four different calculation methods including the initial slope method, the Box-Lucas method, the corrected slope method and the incremental analysis method (INCAM). A novel numerical model combining the heat transfer equations and the Navier-Stokes equations was developed to reproduce the experimental heating process. To address variations in heating efficiency with temperature, the expression of the power dissipation as a Gaussian function of temperature was introduced and the Levenberg-Marquardt optimization algorithm was employed to compute the function parameters and determine the function’s effective branch within each measurement’s temperature range. The power dissipation function was then reduced to the respective SAR function. ResultsThe INCAM exhibited the lowest relative errors ranging between 0.62 and 15.03% with respect to the simulations. SAR(T) functions exhibited significant variations, up to 45%, within the MFH-relevant temperature range. ConclusionsThe examined calculation methods are not suitable to accurately quantify the heating efficiency of a magnetic fluid. Numerical models can be exploited to effectively compute SAR(T) and contribute to the development of robust hyperthermia treatment planning applications.

Impact of RF Shimming on RF-Induced Heating Near Implantable Medical Electrodes in a 3T MRI Coil

Radio frequency (RF) shimming is a widely used method to improve the MRI quality by adjusting the magnitude and phase of the transmit elements in the RF coil and eliminating the field inhomogeneity in 3T MRI system. This process alters the field distribution inside the MRI borehole and changes both the level and location of the absorbed RF energy in patients. It is necessary to assess the impact of RF shimming on the risk of RF-induced heating for patients especially those who have implantable longitude medical electrodes in place during the MRI procedure. This paper investigates the impacts of RF shimming on the RF-induced heating of a generic pacemaker electrode using a transfer function method. Two different shimming schemes were studied. The first excitation scheme was designed to obtain a homogeneous ${B_{1 + }}$ field in the image region-chest region. The second condition maximized the temperature rise around the generic pacemaker electrode with different implant trajectories. All the results were normalized to 2 W/kg average whole-body specific absorption rate (SAR) and a Q-matrices method was used to improve the SAR calculation efficiency. The average temperature rise at the electrode tip under quadrature excitation was 3.7 °C while under optimized excitation was 2.3 °C. The worst-case temperature rise for a selected trajectory was found to be 33.17 °C, which was almost 6 times of the temperature rise under quadrature excitation. Therefore, the RF-shimming condition can significantly change the RF-induced heating of electrodes. To ensure patient safety under MRI, RF-shimming condition should be considered and thoroughly integrated into the RF heating assessment.