Human Head Model Research Articles

Data-driven uncertainty quantification in computational human head models

Computational models of the human head are promising tools for estimating the impact-induced response of the brain, and thus play an important role in the prediction of traumatic brain injury. The basic constituents of these models (i.e., model geometry, material properties, and boundary conditions) are often associated with significant uncertainty and variability. As a result, uncertainty quantification (UQ), which involves quantification of the effect of this uncertainty and variability on the simulated response, becomes critical to ensure reliability of model predictions. Modern biofidelic head model simulations are associated with very high computational cost and high-dimensional inputs and outputs, which limits the applicability of traditional UQ methods on these systems. In this study, a two-stage, data-driven manifold learning-based framework is proposed for UQ of computational head models. This framework is demonstrated on a 2D subject-specific head model, where the goal is to quantify uncertainty in the simulated strain fields (i.e., output), given variability in the material properties of different brain substructures (i.e., input). In the first stage, a data-driven method based on multi-dimensional Gaussian kernel-density estimation and diffusion maps is used to generate realizations of the input random vector directly from the available data. Computational simulations of a small number of realizations provide input–output pairs for training data-driven surrogate models in the second stage. The surrogate models employ nonlinear dimensionality reduction using Grassmannian diffusion maps, Gaussian process regression to create a low-cost mapping between the input random vector and the reduced solution space, and geometric harmonics models for mapping between the reduced space and the Grassmann manifold. It is demonstrated that the surrogate models provide highly accurate approximations of the computational model while significantly reducing the computational cost. Monte Carlo simulations of the surrogate models are used for uncertainty propagation. UQ of the strain fields highlights significant spatial variation in model uncertainty, and reveals key differences in uncertainty among commonly used strain-based brain injury predictor variables.

Guidelines for Burr Hole Surgery in Combination With Tumor Treating Fields for Glioblastoma: A Computational Study on Dose Optimization and Array Layout Planning.

Tumor treating fields (TTFields) is an anti-cancer technology increasingly used for the treatment of glioblastoma. Recently, cranial burr holes have been used experimentally to enhance the intensity (dose) of TTFields in the underlying tumor region. In the present study, we used computational finite element methods to systematically characterize the impact of the burr hole position and the TTFields transducer array layout on the TTFields distribution calculated in a realistic human head model. We investigated a multitude of burr hole positions and layouts to illustrate the basic principles of optimal treatment planning. The goal of the paper was to provide simple rules of thumb for physicians to use when planning the TTFields in combination with skull remodeling surgery. Our study suggests a number of key findings, namely that (1) burr holes should be placed directly above the region of interest, (2) field enhancement occurs mainly underneath the holes, (3) the ipsilateral array should directly overlap the holes and the contralateral array should be placed directly opposite, (4) arrays in a pair should be placed at far distance and not close to each other to avoid current shunting, and finally (5) rotation arrays around their central normal axis can be done without diminishing the enhancing effect of the burr holes. Minor deviations and adjustments (<3 cm) of arrays reduces the enhancement to some extent although the procedure is still effective in these settings. In conclusion, our study provides simple guiding principles for implementation of dose-enhanced TTFields in combination with burr-holes. Future studies are required to validate our findings in additional models at the patient specific level.

The significance of nanoparticles in brain cancer diagnosis and treatment: modeling and simulation

A numerical analysis of specific absorption rate (SAR) and temperature distributions in a realistic human head model is presented in this study. The key challenge is to rise cancer temperature to an optimal temperature without heating nearby healthy tissues. The model’s uniqueness is that it captures the effect of nanoparticles on both brain cancer diagnosis and treatment. A realistic human head model with a cancerous brain segmented from 2D magnetic resonance imaging (MRI) gained from an actual patient using 3D Slicer, modeled, and simulated using CST-Microwave Studio, and illuminated by Archimedes spiral antenna. At frequencies of 2450 MHz and 915 MHz, the model simulated the absence and presence of various nanoparticles. The obtained results suggest that when using nanoparticles, it is possible to achieve sufficient energy deposition and temperature rise to therapeutic values (greater than 42 °C) in brain cancers using the proposed noninvasive hyperthermia system at 915 MHz frequency, especially for gold nanoparticles, without harming surrounding healthy tissue. Our research might pave the way for a clinical applicator prototype that can heat brain cancer.

Influence of the model parameters for the finite element simulation of bone conduction in the human head

Bone conduction (BC) describes the transmission of vibrations via osseous, cartilage, and soft tissue pathways that contribute to the hearing sensation. It is important, e.g., in the context of BC hearing aids as well as for the occlusion effect and one’s own voice perception when using hearing protection or hearing devices. Finite element (FE) simulations of BC allow to overcome practical and ethical limitations of measurements with real heads and help to gain insights into underlying mechanisms. However, FE models inevitably require substantial simplifications due to the assignment and definition of volumetric domains and corresponding materials. Additionally, the material parameters are often only known within certain ranges. In this presentation, an overview on FE models of human heads used for BC research is given, and similarities and differences are discussed. The influence of the FE-inherent simplifications and the challenges in determining the model parameters are illustrated by varying the geometry and the material properties of a simple ellipsoidal model. The variability of the model predictions is compared to simulation results obtained with more realistic geometries and to the inter-subject variability observed in measurements with real human heads reported in the literature.

Angle-tuned coils: attractive building blocks for TMS with improved depth-spread performance

Objective. A novel angle-tuned ring coil is proposed for improving the depth-spread performance of transcranial magnetic stimulation (TMS) coils and serve as the building blocks for high-performance composite coils and multisite TMS systems. Approach. Improving depth-spread performance by reducing field divergence through creating a more elliptical emitted field distribution from the coil. To accomplish that, instead of enriching the Fourier components along the planarized (x-y) directions, which requires different arrays to occupy large brain surface areas, we worked along the radial (z) direction by using tilted coil angles and stacking coil numbers to reduce the divergence of the emitted near field without occupying large head surface areas. The emitted electric field distributions were theoretically simulated in spherical and real human head models to analyze the depth-spread performance of proposed coils and compare with existing figure-8 coils. The results were then experimentally validated with field probes and in-vivo animal tests. Main results. The proposed ‘angle-tuning’ concept improves the depth-spread performance of individual coils with a significantly smaller footprint than existing and proposed coils. For composite structures, using the proposed coils as basic building blocks simplifies the design and manufacturing process and helps accomplish a leading depth-spread performance. In addition, the footprint of the proposed system is intrinsically small, making them suitable for multisite stimulations of inter and intra-hemispheric brain regions with an improved spread and less electric field divergence. Significance. Few brain functions are operated by isolated single brain regions but rather by coordinated networks involving multiple brain regions. Simultaneous or sequential multisite stimulations may provide tools for mechanistic studies of brain functions and the treatment of neuropsychiatric disorders. The proposed AT coil goes beyond the traditional depth-spread tradeoff rule of TMS coils, which provides the possibility of building new composite structures and new multisite TMS tools.

Supine dynamic simulation and latex pillow design for Chinese women based on finite element method

The ergonomics on body pressure distribution of latex product design is the future trend in bedding product design. The numerical simulation of latex pillow compression performance is critical for the optimal design of bedding products. This paper reports the use of the finite element method to simulate the compression performance of latex foam and the design of latex pillows based on the pressure distribution and displacement changes of the human pillow system. The compression performance, shear performance and tensile performance of the latex foam are tested, and the stress and strain images of three mechanical experiments are obtained. Combined with the experimental results, three methods of ‘hyper foam test data, hyper foam model parameter method and low-density foam method’ were used to perform uniaxial compression simulation on bedding latex foam using the finite element software ABAQUS. The simulation results and experimental values were fitted and analyzed. The analysis found that the low-density foam simulation method has the largest R2, which is 0.9871, and the simulation result is the most accurate. The optimal simulation method was selected to simulate the dynamic pressure changes and cervical displacement of the human head and latex pillow models, as well as the simulation experiments with latex pillows of different heights. The curve is fitted to obtain the second-order polynomial equations of pressure and cervical socket displacement. In contrast, it can be concluded that latex pillow products with a neck height of 100 mm are better for the supine position of younger adult women in China.

Protective Performance of Helmets and Goggles in Mitigating Brain Biomechanical Response to Primary Blast Exposure

The current combat helmets are primarily designed to mitigate blunt impacts and ballistic loadings. Their protection against primary blast wave is not well studied. In this paper, we comprehensively assessed the protective capabilities of the advanced combat helmet and goggles against blast waves with different intensity and directions. Using a high-fidelity human head model, we compared the intracranial pressure (ICP), cerebrospinal fluid (CSF) cavitation, and brain strain and strain rate predicted from bare head, helmet-head and helmet-goggles-head simulations. The helmet was found to be effective in mitigating the positive ICP (24–57%) and strain rate (5–34%) in all blast scenarios. Goggles were found to be effective in mitigating the positive ICP in frontal (6–16%) and lateral (5–7%) blast exposures. However, the helmet and goggles had minimal effects on mitigating CSF cavitation and even increased brain strain. Further investigation showed that wearing a helmet leads to higher risk of cavitation. In addition, their presence increased the head kinetic energy, leading to larger strains in the brain. Our findings can improve our understanding of the protective effects of helmets and goggles and guide the design of helmet pads to mitigate brain responses to blast.

Numerical investigation of the energy distribution of Low-intensity transcranial focused ultrasound neuromodulation for hippocampus

ObjectiveUltrasonic neuromodulation as a safe and non-invasive brain stimulation method that delivers a low-intensity, focused ultrasound to nervous system tissue in a targeted area of the brain. The objective of this study is to numerically investigate the ultrasound wave propagation and the energy distribution within the brain tissues using customized single element focused ultrasound transducers (SEFT), targeting the hippocampus. MethodsA high resolution detailed human head model with seven tissue types was constructed from magnetic resonance imaging (MRI). A full-wave finite-difference time-domain simulation platform, Sim4life, was then used to simulate a 3D non-linear ultrasound wave equation to the specific region of interest, the hippocampus. Three customized SEFT were used to test the effect of transducer positions, and another customized transducer was used to compare the sensitivity effect on heterogeneous and homogeneous brain models. Finally, the sensitivity and performance of low intensity focusing ultrasound stimulation were evaluated. ResultsAn optimized application of SEFT was customized to deliver 100 W/m2 intensity of energy deposition at the hippocampus region. About 85.65% of the generated volume beam was delivered to the targeted hippocampus region and the beam overlap parameter was affected by different transducer positions. Deflection angle changes of SEFT at the range of ± 5% did not have a significant effect on energy delivery and position displacement. Only 0.5% of peak pressure change was observed between heterogeneous and homogeneous brain models. The sensitivity analysis also showed that the sound speed is the most influential acoustic parameter. SignificanceThis study demonstrated that ultrasound neuromodulation targeting the depth brain tissue of the hippocampus could be a potential and promising alternative method to some non-acoustic brain stimulation modalities. In the numerical study of ultrasound brain stimulations, ultrasound parameters and the brain model need to be properly determined to simulate the ultrasonic neuromodulations.

Genetic Algorithm for TMS Coil Position Optimization in Stroke Treatment.

Transcranial magnetic stimulation (TMS), a non-invasive technique to stimulate human brain, has been widely used in stroke treatment for its capability of regulating synaptic plasticity and promoting cortical functional reconstruction. As shown in previous studies, the high electric field (E-field) intensity around the lesion helps in the recovery of brain function, thus the spatial location and angle of coil truly matter for the significant correlation with therapeutic effect of TMS. But, the error caused by coil placement in current clinical setting is still non-negligible and a more precise coil positioning method needs to be proposed. In this study, two kinds of real brain stroke models of ischemic stroke and hemorrhagic stroke were established by inserting relative lesions into three human head models. A coil position optimization algorithm, based on the genetic algorithm (GA), was developed to search the spatial location and rotation angle of the coil in four 4 × 4 cm search domains around the lesion. It maximized the average intensity of the E-field in the voxel of interest (VOI). In this way, maximum 17.48% higher E-field intensity than that of clinical TMS stimulation was obtained. Besides, our method also shows the potential to avoid unnecessary exposure to the non-target regions. The proposed algorithm was verified to provide an optimal position after nine iterations and displayed good robustness for coil location optimization between different stroke models. To conclude, the optimized spatial location and rotation angle of the coil for TMS stroke treatment could be obtained through our algorithm, reducing the intensity and duration of human electromagnetic exposure and presenting a significant therapeutic potential of TMS for stroke.

A Comparative Study of Birdcage RF Coil Configurations for Ultra-High Field Magnetic Resonance Imaging.

Improvements in transmission and reception sensitivities of radiofrequency (RF) coils used in ultra-high field (UHF) magnetic resonance imaging (MRI) are needed to reduce specific absorption rates (SAR) and RF power deposition, albeit without applying high-power RF. Here, we propose a method to simultaneously improve transmission efficiency and reception sensitivity of a band-pass birdcage RF coil (BP-BC RF coil) by combining a multi-channel wireless RF element (MCWE) with a high permittivity material (HPM) in a 7.0 T MRI. Electromagnetic field (EM-field) simulations, performed using two types of phantoms, viz., a cylindrical phantom filled with oil and a human head model, were used to compare the effects of MCWE and HPM on BP-BC RF coils. EM-fields were calculated using the finite difference time-domain (FDTD) method and analyzed using Matlab software. Next, to improve RF transmission efficiency, we compared two HPM structures, namely, a hollow cylinder shape HPM (hcHPM) and segmented cylinder shape HPM (scHPM). The scHPM and MCWE model comprised 16 elements (16-rad BP-BC RF coil) and this coil configuration demonstrated superior RF transmission efficiency and reception sensitivity along with an acceptable SAR. We expect wider clinical application of this combination in 7.0 T MRIs, which were recently approved by the United States Food and Drug Administration.

Key factors in the cortical response to transcranial electrical Stimulations—A multi-scale modeling study

Transcranial electrode stimulation (tES), one of the techniques used to apply non-invasive brain stimulation (NIBS), modulates cortical activities by delivering weak electric currents through scalp-attached electrodes. This emerging technique has gained increasing attention recently; however, the results of tES vary greatly depending upon subjects and the stimulation paradigm, and its cellular mechanism remains unclear. In particular, there is a controversy over the factors that determine the cortical response to tES. Some studies have reported that the electric field's (EF) orientation is the determining factor, while others have demonstrated that the EF magnitude itself is the crucial factor. In this work, we conducted an in-depth investigation of cortical activity in two types of electrode montages used widely—the conventional (C)-tES and high-definition (HD)-tES—as well as two stimulation waveforms—direct current (DC) and alternating current (AC). To do so, we constructed a multi-scale model by coupling an anatomically realistic human head model and morphologically realistic multi-compartmental models of three types of cortical neurons (layer 2/3 pyramidal neuron, layer 4 basket cell, layer 5 pyramidal neuron). Then, we quantified the neuronal response to the C-/HD-tDCS/tACS and explored the relation between the electric field (EF) and the radial field's (RF: radial component of EF) magnitude and the cortical neurons' threshold. The EF tES induced depended upon the electrode montage, and the neuronal responses were correlated with the EF rather than the RF's magnitude. The electrode montages and stimulation waveforms caused a small difference in threshold, but the higher correlation between the EF's magnitude and the threshold was consistent. Further, we observed that the neurons' morphological features affected the degree of the correlation highly. Thus, the EF magnitude was a key factor in the responses of neurons with arborized axons. Our results demonstrate that the crucial factor in neuronal excitability depends upon the neuron models' morphological and biophysical properties. Hence, to predict the cellular targets of NIBS precisely, it is necessary to adopt more advanced neuron models that mimic realistic morphological and biophysical features of actual human cells.

Design and analysis of circular microstrip patch probe array for precise specific absorption rate measurement at quad-band

AbstractThis paper distinguishes the design and analysis of 3 × 2 circular microstrip patch probe array for specific absorption rate (SAR) measurement at quad-band. This novel approach consists of FR4 substrate radial dimension of 88 mm and it has six circular array elements with a radius of 17 mm; out of these six elements, three array elements are having rectangular slots with dimensions of 1.8 × 1.5 × 1.5 mm3. The array elements are coupled with six probes which have a dimension of 100 mm with a 2.5 mm tip radius; these six probes are embedded into a tissue-equivalent liquid-filled human head mannequin. In this mannequin, instantaneous SAR at any six positions has been investigated. The proposed design resonates at 1.8, 2.1, 2.4, and 2.5 GHz with a return loss of −11.53, −15.90, −15.73, and −25.49 dB. The circular microstrip probe array is fed by a 50 Ohm coaxial feed. In addition, the 3D human head model analysis is also presented. The precisely estimated SAR values are 0.135, 0.108, 0.167, and 0.244 W/kg at 1 g of tissue. For the traceable measurements, each source of uncertainty budget has been estimated.

Comparison of Transcranial Focused Ultrasound and Transcranial Pulse Stimulation for Neuromodulation: A Computational Study

ObjectiveThe objective of the study was to investigate transcranial wave propagation through two low-intensity focused ultrasound (LIFU)–based brain stimulation techniques—transcranial focused ultrasound stimulation (tFUS) and transcranial pulse stimulation (TPS). Although tFUS involves delivering long trains of acoustic pulses, the newly introduced TPS delivers ultrashort (∼3 μs) pulses repeated at 4 Hz. Accordingly, only a single simulation study with limited geometry currently exists for TPS. We considered a high-resolution three-dimensional (3D) whole human head model in addition to water bath simulations. We anticipate that the results of this study will help researchers investigating LIFU have a better understanding of the effects of the two different techniques. ApproachWith an objective to first reproduce previous computational results, we considered two spherical tFUS transducers that were previously modeled. We assumed identical parameters (geometry, position, and imaging data set) to demonstrate differences, purely because of the waveform considered. For simulations with a 3D head data set, we also considered a parabolic transducer that has been used for TPS delivery. ResultsOur initial results successfully verified previous modeling workflow. The tFUS distribution was characterized by the typical elliptical profile, with its major axis perpendicular to the face of the transducer. The TPS distribution resembled two mirrored meniscus profiles, with its widest diameter oriented parallel to the face of the transducer. The observed intensity value differences were theoretical because the two waveforms differ in both intensity and time. The consideration of a realistic 3D human head model resulted in only a minor distortion of the two waveforms. SignificanceThis study simulated TPS administration using a 3D realistic image-derived data set. Although our comparison results are strictly limited to the model parameters and assumptions made, we were able to elucidate some clear differences between the two approaches. We hope this initial study will pave the way for systematic comparison between the two approaches in the future.

Evaluation of Blast Simulation Methods for Modeling Blast Wave Interaction With Human Head.

Blast-induced traumatic brain injury (bTBI) research is crucial in asymmetric warfare. The finite element analysis is an attractive option to simulate the blast wave interaction with the head. The popular blast simulation methods are ConWep-based pure Lagrangian, Arbitrary-Lagrangian-Eulerian, and coupling method. This study examines the accuracy and efficiency of ConWep and coupling methods in predicting the biomechanical response of the head. The simplified cylindrical, spherical surrogates and biofidelic human head models are subjected to field-relevant blast loads using these methods. The reflected overpressures at the surface and pressures inside the brain from the head models are qualitatively and quantitatively evaluated against the available experiments. Both methods capture the overall trends of experiments. Our results suggest that the accuracy of the ConWep method is mainly governed by the radius of curvature of the surrogate head. For the relatively smaller radius of curvature, such as cylindrical or spherical head surrogate, ConWep does not accurately capture decay of reflected blast overpressures and brain pressures. For the larger radius of curvature, such as the biofidelic human head, the predictions from ConWep match reasonably well with the experiment. For all the head surrogates considered, the reflected overpressure-time histories predicted by the coupling method match reasonably well with the experiment. Coupling method uniquely captures the shadowing and union of shock waves governed by the geometry-driven flow dynamics around the head. Overall, these findings will assist the bTBI modeling community to judiciously select an objective-driven modeling methodology.

Magneto- and electrophosphene thresholds in the retina: a dosimetry modeling study

Objective. Sensations of flickering light produced by time-varying magnetic fields or electric currents are called magneto- or electrophosphenes. Phosphene thresholds have been used in international guidelines and standards as an estimate of the thresholds of exposure that produce effects in the central nervous system (CNS). However, the estimated threshold values have a large range of uncertainty. Approach. Phosphene thresholds were approximated by simulating five phosphene threshold experiments. Retinal electric fields and currents induced by electric and magnetic stimulation were calculated using the finite element method and 14 anatomically realistic computational models of human heads. Main results. The radial component of retinal current density was determined to be in the range of 6.0–20.6 mA m−2. This study produces more accurate estimates for threshold current density in the retina using detailed anatomical models and the estimates had a reduced range of uncertainty compared to earlier studies. Significance. The results are useful for studying the mechanisms of retinal phosphenes and for the development of exposure limits for the CNS.

Fast and numerically stable Mie solution of EM near field and absorption for stratified spheres

PurposeThe purpose of this paper is the development of an analytic computational model for electromagnetic (EM) wave scattering from spherical objects. The main application field is the modeling of electrically large objects, where the standard numerical techniques require huge computational resources. An example is full-wave modeling of the human head in the millimeter-wave regime. Hence, an approximate model or analytical approach is used.Design/methodology/approachThe Mie–Debye theorem is used for calculating the EM scattering from a layered dielectric sphere. The evaluation of the analytical expressions involved in the infinite sum has several numerical instabilities, which makes the precise calculation a challenge. The model is validated through an application example with comparing results to numerical calculations (finite element method). The human head model is used with the approximation of a two-layer sphere, where the brain tissues and the cranial bones are represented by homogeneous materials.FindingsA significant improvement is introduced for the stable calculation of the Mie coefficients of a core–shell stratified sphere illuminated by a linearly polarized EM plane wave. Using this technique, a semi-analytical expression is derived for the power loss in the sphere resulting in quick and accurate calculations.Originality/valueTwo methods are introduced in this work with the main objective of estimating the final precision of the results. This is an important aspect for potentially unstable calculations, and the existing implementations have not included this feature so far.

Modal analysis of a human head model with finite element method

Cerebral concussion caused by a contact in American football games is a serious problem today, since its mechanism remains unclear so far. From some prior researches head vibration would induce the cerebral concussion. In this study we had carried out a modal analysis for a human head model consisting of skull, falx cerebri, cerebellar tentorium, cerebrum, cerebral cortex, corpus callosum, cerebellum, brainstem, septum pellucidum, cervical cord, cervical vertebrae, and lateral ventricles (cerebrospinal fluid) with mechanical properties that we referred to some papers. The results show that the 1st, the 2nd and the 3rd modes are sagittal rotation (eigenfrequency: 7.29 Hz), coronal rotation (eigenfrequency: 7.65 Hz), and axial rotation (eigenfrequency: 11.24 Hz), respectively. The maximum of 1st principal stress, 3rd principal stress, equivalent stress and 1st shear stress for these three modes were located in the cervical cord, cervical vertebrae and skull, not inside the brain. On the other hand, the maximum and minimum principal strain were located inside the brain for these three modes. Nucleating higher strain inside the brain might be caused to occur cerebral concussion when the head vibrated due to the contact.

Investigation of photon doses reaching healthy tissues in the use of different neutron energies in boron neutron capture therapy

Boron neutron capture therapy is a unique treatment method that aims to kill the tumor cells with the help of heavy particles. Particles resulting from the interaction of the tumor region containing 10B atoms with thermal or epithermal neutrons have the most important role in this treatment method. In this study, gamma radiation reaching healthy tissues, which is the result of 10B(n,a)7Li reaction, was investigated. A simulation suitable for boron neutron capture therapy treatment, including the human head model, was created by the Monte Carlo N-Particle (MCNP) program. By using five different neutron energies, the gamma radiations resulting from the 10B(n,a)7Li reaction in the determined regions, close to the tumor tissue, were investigated. It was observed that the healthy tissue between the tumor area and the surface is exposed to the highest gamma flux and the highest gamma radiation absorption. It was also observed that these values increase as neutron energy decreases. It was found that the gamma doses received by some regions outside the neutron irradiation area could be significant. It has been understood that the change in neutron energy may cause significant changes in gamma radiation values reaching healthy tissues, especially in regions close to the surface. In boron neutron capture therapy treatments, the neutrons sent to the tumor should be selected depending on the location of the tumor and the size of the tumor area. This study contains significant data about photon doses in healthy tissues around the brain region treated using different neutron energies with the boron neutron capture therapy technique.

Numerical Analysis of RF-Induced Heating While Wearing Face Mask at Magnetic Resonance Imaging

In response to the unprecedented COVID-19 pandemic, wearing face masks in public places and common facilities has been strongly recommended to help prevent the further spread of the virus. However, conductive components of the face mask carry the potential risk of radio-frequency (RF)-induced heating when exposed to an RF electromagnetic field, particularly during magnetic resonance imaging (MRI). In this study, a realistic human head model wearing a face mask exposed to a strong RF electromagnetic field in 1.5 T, 3 T, and 7 T MRI was simulated. A nose wire in contact with the skin and a mask sheet with relatively high electrical conductivity, emulating a silver nanoparticle-coated face mask, was modeled to investigate the worst case of RF-induced heating that could occur during the MRI scan. 24 scenarios were simulated by using finite-difference time-domain (FDTD)-based electromagnetic solver and thermal-transient solver from Sim4Life commercial simulation software. According to the results, a notable enhancement in the specific absorption rate (SAR) and temperature rise was observed in the local region of the skin where the wire contacted the skin around the edge of the high-conductive mask sheet. In particular, a maximum of a 12-fold increase in mass-averaged SAR and a temperature increase of more than 8.0&#x00B0;C occurred because of the conductive face mask at 3 T, compared to the normal polymer-based face mask with low conductivity. Our results confirmed that the degree of RF-induced heating due to the face mask could be completely different depending on the RF frequency of the MRI, location where the nose wire contacted the skin, and conductivity of the mask sheet. To the best of our knowledge, this is the first study to assess the face mask as a factor for RF-induced heating during MRI. These findings are important for providing a safety guide that allows patients to safely undergo MRI while wearing a face mask during the COVID-19 pandemic.

Variability in Specific Absorption Rate From Variation in Tissue Properties

This study evaluates the variance of specific absorption rate (SAR) due to expected variance in the dielectric properties of tissues in a 3D anatomical human head model exposed to a half-wave dipole antenna at 835 and 1900 MHz. Stochastic finite difference time domain (S-FDTD) is applied to calculate variations in the local SAR, and the 1- and 10-gram averaged SAR values. These are also compared at 835 MHz to variations found from Monte Carlo FDTD. It is found that for both frequencies dielectric property variance results in a variance of peak 1- or 10-gram SAR of approximately 30% to 55% of the mean SAR, depending on the frequency. These results show that to reach 95% confidence with the calculated SAR values for evaluating exposure guidelines, statistical variations in tissue electrical properties must be taken into account.