Inhomogeneous Head Research Articles

Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

Non-invasive human brain functional imaging with millisecond resolution can be achieved only with magnetoencephalography (MEG) and electroencephalography (EEG). MEG has better spatial resolution than EEG because signal distortion due to inhomogeneous head conductivity is negligible in MEG but serious in EEG. However, this advantage has been practically limited by the necessary setback distances between the sensors and scalp, because the Dewar vessel containing liquid helium for superconducting quantum interference devices (SQUIDs) requires a thick vacuum wall. Latest developments of high critical temperature (high-Tc) SQUIDs or optically pumped magnetometers have allowed closer placement of MEG sensors to the scalp. Here we introduce the use of tunnel magneto-resistive (TMR) sensors for scalp-attached MEG. Improvement of TMR sensitivity with magnetic flux concentrators enabled scalp-tangential MEG at 2.6 mm above the scalp, to target the largest signal component produced by the neural current below. In a healthy subject, our single-channel TMR-MEG system clearly demonstrated the N20m, the initial cortical component of the somatosensory evoked response after median nerve stimulation. Multisite measurement confirmed a spatially and temporally steep peak of N20m, immediately above the source at a latency around 20 ms, indicating a new approach to non-invasive functional brain imaging with millimeter and millisecond resolutions.

Microwave power penetration enhancement inside an inhomogeneous human head

The penetration of microwave power inside a human head model is improved by employing a dielectric loaded rectangular waveguide as the transmission source. A multi-layer reflection model is investigated to evaluate the combined material characteristics of different lossy human head tissues at 2.45 GHz. A waveguide loaded with a calculated permittivity of 3.62 is shown to maximise the microwave power penetration at the desired frequency. A Quartz (SiO2) loaded rectangular waveguide fed by a microstrip antenna is designed to validate the power penetration improvement inside an inhomogeneous human head phantom. A measured 1.33 dB power penetration increment is observed for the dielectric loaded waveguide over a standard rectangular waveguide at 50 mm inside the head, with an 81.9% reduction in the size of the transmission source.

A Simulation-Based Methodology of Developing 3D Printed Anthropomorphic Phantoms for Microwave Imaging Systems.

This work is devoted to the development and manufacturing of realistic benchmark phantoms to evaluate the performance of microwave imaging devices. The 3D (3 dimensional) printed phantoms contain several cavities, designed to be filled with liquid solutions that mimic biological tissues in terms of complex permittivity over a wide frequency range. Numerical versions (stereolithography (STL) format files) of these phantoms were used to perform simulations to investigate experimental parameters. The purpose of this paper is two-fold. First, a general methodology for the development of a biological phantom is presented. Second, this approach is applied to the particular case of the experimental device developed by the Department of Electronics and Telecommunications at Politecnico di Torino (POLITO) that currently uses a homogeneous version of the head phantom considered in this paper. Numerical versions of the introduced inhomogeneous head phantoms were used to evaluate the effect of various parameters related to their development, such as the permittivity of the equivalent biological tissue, coupling medium, thickness and nature of the phantom walls, and number of compartments. To shed light on the effects of blood circulation on the recognition of a randomly shaped stroke, a numerical brain model including blood vessels was considered.

Effect of A Superstrate on On-Head Matched Antennas for Biomedical Applications

The effect of using a superstrate dielectric layer on an on-head matched antenna for biomedical diagnosis applications is investigated. Two on-head matched antennas are considered with different length meandered lines ensuring operation around 0.9 GHz frequency. The first antenna’s conductive radiating structure is in direct contact with the head phantom, whereas the second one utilises a 0.5 mm thick superstrate layer on top of the conducting layer as a buffer. The lateral dimensions of both antennas are held constant at 30 × 30 mm2. The electric and magnetic field distribution is analysed and the power penetration, 50 mm inside the head phantom, is derived from the electromagnetic field surrounding the antennas. Both homogeneous and inhomogeneous head phantoms are considered while evaluating the antennas in terms of their reflection coefficient, current distribution, electric field, magnetic field, specific absorption rate (SAR) and power penetration inside the head. The antennas are fabricated and measured utilizing an inhomogeneous phantom to validate the proposed performance improvement using a superstrate. It is shown that the superstrate antenna achieves a ~8 dB increase in power penetration inside the head phantom along with a 0.0731 W/kg decrease in SAR compared to the antenna without a superstrate.

Evaluation of Delta4DVH Anatomy in 3D Patient-Specific IMRT Quality Assurance.

Purpose:To evaluate the performance of Delta4DVH Anatomy in patient-specific intensity-modulated radiotherapy quality assurance.Materials and Methods:Dose comparisons were performed between Anatomy doses calculated with treatment plan dose measured modification and pencil beam algorithms, treatment planning system doses, film doses, and ion chamber measured doses in homogeneous and inhomogeneous geometries. The sensitivity of Anatomy doses to machine errors and output calibration errors was also investigated.Results:For a Volumetric Modulated Arc Therapy (VMAT) plan evaluated on the Delta4 geometry, the conventional gamma passing rate was 99.6%. For a water-equivalent slab geometry, good agreements were found between dose profiles in film, treatment planning system, and Anatomy treatment plan dose measured modification and pencil beam calculations. Gamma passing rate for Anatomy treatment plan dose measured modification and pencil beam doses versus treatment planning system doses was 100%. However, gamma passing rate dropped to 97.2% and 96% for treatment plan dose measured modification and pencil beam calculations in inhomogeneous head & neck phantom, respectively. For the 10 patients’ quality assurance plans, good agreements were found between ion chamber measured doses and the planned ones (deviation: 0.09% ± 1.17%). The averaged gamma passing rate for conventional and Anatomy treatment plan dose measured modification and pencil beam gamma analyses in Delta4 geometry was 99.6% ± 0.89%, 98.54% ± 1.60%, and 98.95% ± 1.27%, respectively, higher than averaged gamma passing rate of 97.75% ± 1.23% and 93.04% ± 2.69% for treatment plan dose measured modification and pencil beam in patients’ geometries, respectively. Anatomy treatment plan dose measured modification dose profiles agreed well with those in treatment planning system for both Delta4 and patients’ geometries, while pencil beam doses demonstrated substantial disagreement in patients’ geometries when compared to treatment planning system doses. Both treatment planning system doses are sensitive to multileaf collimator and monitor unit (MU) errors for high and medium dose metrics but not sensitive to the gantry and collimator rotation error smaller than 3°.Conclusions:The new Delta4DVH Anatomy with treatment plan dose measured modification algorithm is a useful tool for the anatomy-based patient-specific quality assurance. Cautions should be taken when using pencil beam algorithm due to its limitations in handling heterogeneity and in high-dose gradient regions.

Evaluation of Full scatter convolution algorithm based Treatment Planning System performance in the presence of inhomogeneities using three-dimensional film dosimetry

Introduction: Inclusion of inhomogeneities such as air-filled cavities in the head and neck treatment fields may result in potential dosimetric disagreement which was caused by electronic disequilibrium. Most of treatments planning systems (TPS) are not able to predict dose distribution of inhomogeneous regions accurately. EBT2 films are used frequently in radiotherapy quality assurance testing to evaluate the performance of dose delivery systems and TPS. The development and usage of 3D film dosimetry method may be useful. Materials and Methods: 9 pieces of EBT2 films were embedded in a rectangular inhomogeneous head and neck phantom. Films were exposed with 6 MV X-ray photons with parallel-opposed fields and three approximate small field sizes were examined. A homemade computer code was developed in MATLAB for creation of 3D dose map of irradiated films and calculation of enclosed volumes which were surrounded by isodose lines of films. Calculated dose volumes should be compared with Dose Volume Histogram (DVH) which was derived from TPS. Results: our results showed significant differences between the results of the film dosimetry and TPS. Maximum difference of calculated and measured values was observed in volumes surrounded by 95% isodose curves and 3x3cm2 field size (P=0.035) and minimum difference was observed in volumes surrounded by 95% isodose curves and 5x5cm2 field size (P=0.047). Conclusion: The results of this study demonstrated overestimated results in volumetric calculation of TRGRT dose calculation algorithm in the presence of inhomogeneities and approxi small fields sizes. Also, these results confirmed that utilization of an accurate and comprehen method such as 3D film dosimetry can be useful for TPS verification.

High spatial resolution identification of hematoma in inhomogeneous head phantom using broadband fNIR system

This paper presents a novel method for early detection of hematomas using highly sensitive optical fNIR imaging methods based on broadband photon migration. The NIR experimental measurements of inhomogeneous multi-layer phantoms representing human head are compared to 3D numerical modeling over broadband frequencies of 30–1000 MHz. A finite element method (FEM) simulation of the head phantom are compared to measurements of insertion loss and phase using custom-designed broadband free space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 670 nm, 795 nm, 850 nm, though results of 670 nm are discussed here. Standard error is used to compute error between 3D FEM modeling and experimental measurements by fitting experimental data to the asqrt {frequency} + b . Error results are shown at narrowband and broadband frequency modulation in order to have confidence in 3D numerical modeling. A novel method is established here to identify presence of hematoma based on first and second derivatives of changes in insertion loss and phase (∆IL and ∆IP), where frequency modulated photons sensitive to different sizes of hematoma is identified for wavelength of 670 nm. The high accuracy of this comparison provides confidence in optical bio-imaging and its eventual application to TBI detection.

A regularised boundary element formulation for contactless SAR evaluations within homogeneous and inhomogeneous head phantoms

This work presents a Boundary Element Method (BEM) formulation for contactless electromagnetic field assessments. The new scheme is based on a regularized BEM approach that requires the use of electric measurements only. The regularization is obtained by leveraging on an extension of Calderon techniques to rectangular systems leading to well-conditioned problems independent of the discretization density. This enables the use of highly discretized Huygens surfaces that can be consequently placed very near to the radiating source. In addition, the new regularized scheme is hybridized with both surfacic homogeneous and volumetric inhomogeneous forward BEM solvers accelerated with fast matrix-vector multiplication schemes. This allows for rapid and effective dosimetric assessments and permits the use of inhomogeneous and realistic head phantoms. Numerical results corroborate the theory and confirms the practical effectiveness of all newly proposed formulations.

Distribution of energy absorption in an inhomogeneous head model at 900 MHz

An inhomogeneous model of the human head, which consists of a number of tissues, is compared with a homogeneous model, commonly used for the evaluation of electromagnetic (EM) field inside the head in the 900-MHz frequency band. For both models, the level of exposure in the head is evaluated by means of numerical simulation of the Specific Absorption Rate (SAR) in the whole head and in separated areas. It is shown that the homogeneous and inhomogeneous models lead approximately to the same total SAR, but the magnitudes of the maximal point SAR and the local SAR in the inhomogeneous model are significantly higher than in the homogeneous one.

3D Numerical modeling and its experimental verifications for an inhomogeneous head phantom using broadband fNIR system

Modeling behavior of broadband (30-1000 MHz) frequency modulated near infrared photons through a multilayer phantom is of interest to optical bio-imaging research. Photon dynamics in phantom are predicted using three-dimension (3D) finite element numerical simulation and are related to the measured insertion loss and phase for a given human head geometry in this paper based on three layers of phantom each with distinct optical parameter properties. Simulation and experimental results are achieved for single, two, and three layers solid phantoms using COMSOL (COMSOL AB, Tegnérgatan 23, SE-111 40, Stockholm, Sweden) (for FEM) simulation and custom-designed broadband free space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 680, 795, and 850 nm. Standard error is used to compute error between two-dimension and 3D FE modeling along with experimental results by fitting experimental data to the functional form of afrequency+b. Error results are shown at narrowband and broadband frequency modulation. Confidence in numerical modeling of the photonic behavior using 3D FEM for human head has been established here by comparing the reflection mode's experimental results with the predictions made by COMSOL for known commercial solid brain phantoms.

The definite non-uniqueness results for deterministic EEG and MEG data

The solvability of the inverse problems of electroencephalography and magnetoencephalography has been studied extensively in the literature using a variety of models, including spherical and non-spherical geometries, homogeneous and inhomogeneous head models, and neuronal excitations involving the discrete and continuous distribution of dipoles. Among the important methods used are the methods based on spectral decompositions, physical arguments and integral representation techniques. Regarding the uniqueness of these inverse problems, a general result, independent of the geometry and the homogeneity of the conducting medium, has been obtained recently by the second author. This paper summarizes this result which appears to be mathematically definitive, in the sense that the geometry is arbitrary, the brain is surrounded by shells of varying conductivities, the neuronal current is arbitrary, the data are complete and the proofs are analytical. Furthermore, this paper includes a summary of the main steps of the proofs leading to the above result. In addition, it demonstrates the consistency of this general uniqueness result with all earlier results known in the literature.

Uncertainty and sensitivity analysis for anisotropic inhomogeneous head tissue conductivity in human head modelling

The accuracy of an electroencephalography (EEG) forward problem partially depends on the head tissue conductivities. These conductivities are anisotropic and inhomogeneous in nature. This paper investigates the effects of conductivity uncertainty and analyses its sensitivity on an EEG forward problem for a spherical and a realistic head models. We estimate the uncertain conductivities using an efficient constraint based on an optimization method and perturb it by means of the volume and directional constraints. Assigning the uncertain conductivities, we construct spherical and realistic head models by means of a stochastic finite element method for fixed dipolar sources. We also compute EEG based on the constructed head models. We use a probabilistic sensitivity analysis method to determine the sensitivity indexes. These indexes characterize the conductivities with the most or the least effects on the computed outputs. These results demonstrate that conductivity uncertainty has significant effects on EEG. These results also show that the uncertain conductivities of the scalp, the radial direction of the skull and transversal direction in the white matter are more sensible.

A systematic study of head tissue inhomogeneity and anisotropy on EEG forward problem computing

In this study, we propose a stochastic method to analyze the effects of inhomogeneous anisotropic tissue conductivity on electroencephalogram (EEG) in forward computation. We apply this method to an inhomogeneous and anisotropic spherical human head model. We apply stochastic finite element method based on Legendre polynomials, Karhunen-Loeve expansion and stochastic Galerkin methods. We apply Volume and Wang's constraints to restrict the anisotropic conductivities for both the white matter (WM) and the skull tissue compartments. The EEGs resulting from deterministic and stochastic FEMs are compared using statistical measurement techniques. Based on these comparisons, we find that EEGs generated by incorporating WM and skull inhomogeneous anisotropic tissue properties individually result in an average of 56.5 and 57.5% relative errors, respectively. Incorporating these tissue properties for both layers together generate 43.5% average relative error. Inhomogeneous scalp tissue causes 27% average relative error and a full inhomogeneous anisotropic model brings in an average of 45.5% relative error. The study results demonstrate that the effects of inhomogeneous anisotropic tissue conductivity are significant on EEG.

散乱X線補正による歯科用コーンビームCTにおける再構成像の精度向上

A method for correcting X-ray scattering to improve the CT-number accuracy of dental cone-beam CT images is proposed. The proposed method is based on an approximate function; that is, X-ray scatter components are approximately calculated by using the mean pixel value of a measured transmittance image and a scatter parameter. The scatter parameter is defined as the ratio of scatter-generation and attenuation coefficients. When the correction method was applied to an inhomogeneous head phantom, the CT numbers were almost completely corrected and the artifacts in the corrected axial and sagittal reconstructed images were reduced. The absolute error from the true CT number for the mandible was reduced from 549 to 30 HU by the correction method. Volume-rendering images with few artifacts were also acquired using the corrected reconstructed images. These results demonstrate that the proposed method is effective for improving the accuracy of hardness and shape measurements of a mandible in cone-beam CT images. The method enables the use of a model based on such corrected cone-beam CT images for diagnostic support and surgical planning.

SU-FF-T-187: Dosimetric Accuracy Analysis of Dose Calculation Algorithms Using Inhomogeneity Phantom for IMRT Treatment Plan

Purpose: We evaluated the dosimetric accuracy of dose calculation with a convolution/superposition (C/S) and a pencil‐beam algorithm with the use of the multiple inhomogeneous head and neck phantom for intensity modulated radiation therapy(IMRT)treatment plan. Method and Materials: The inhomogeneous head and neck phantom for IMRTquality assurance (QA) was housed in a custom‐designed package for efficient evaluation of the measured doses using the different materials and various detectors. Comparisons of measured and deconvolved beam profile obtained from a Gaussian fitting approaches were performed using the real penumbra width to remove ionization chamber size effect. The relative dosimetry obtained with the single‐beam dose calculation algorithm was also compared to the MapCheck measurements for a head and neck IMRTtreatment plans. The accuracy of IMRTdose calculations with the C/S and pencil‐beam algorithms were also investigated with respect to measurements used in head and neck phantom in the presence of inhomogeneities. Results: The deconvolved penumbra was 2.2 mm and calculated value was 2.6 mm with pencil‐beam algorithm. The differences between calculated dose and measured dose with the MapCheck were less than 4.0% for the horizontal profile of IMRT head and neck treatment plan. The differences for the C/S and pencil‐beam dose calculation algorithms were within 5.0% in average target doses.Ionization chamber experiments showed approximately 2.5% better agreement than the glass rod detector. Conclusion: Our results show that accurate measurements of the penumbral region with consideration of inhomogeneities improve the accuracy of the dose calculation algorithms predicted by the treatment planning system. Therefore, it is important to choose an appropriate detector

Estimation of in vivo brain-to-skull conductivity ratio in humans

The electrical conductivity value of the human skull is important for biophysics research of the brain. In the present study, the human brain-to-skull conductivity ratio was estimated through in vivo experiments utilizing intra-cranial electrical stimulation in two epilepsy patients. A realistic geometry inhomogeneous head model including the implanted silastic grids was constructed with the aid of the finite element method, and used to estimate the conductivity ratio. Averaging over 49 sets of measurements, the mean value and standard deviation of the brain-to-skull conductivity ratio were found to be 18.7 and 2.1, respectively.

Temperature Increase in Human Heads Due to Different Models of Cellular Phones

The temperature increase in a realistic human head model exposed to five different cellular phone models is investigated by a bioheat equation and the finite-difference time-domain (FDTD) method at 900 and 1800 MHz, respectively. These five cellular phone models are greatly differ from each other in structure. The realistic human head model is used to investigate the temperature increase in the head affected by varying factors including; electric properties of human head tissues, hand absorption power effect, cellular phone models, homogeneous and inhomogeneous head models, cellular phone material, and the distance between the cellular phone and the head. Computed results show that the temperature increase in the head reaches the steady-state value for exposure time of about 20 minutes. For worst-case, the maximum temperature increase in the head is found to be 0.19° C at 900 MHz and 0.055° C at 1800 MHz for the inhomogeneous human head with six tissues exposed to a metallic handset held by a hand, with antenna output power of 0.6 W and 0.125 W, respectively. The maximum temperature increase in the head is far less than the threshold temperature increase of 4.5° C which can cause neuron damage.

Spatio-temporal EEG source localization using a three-dimensional subspace FINE approach in a realistic geometry inhomogeneous head model

The subspace source localization approach, i.e., first principle vectors (FINE), is able to enhance the spatial resolvability and localization accuracy for closely-spaced neural sources from EEG and MEG measurements. Computer simulations were conducted to evaluate the performance of the FINE algorithm in an inhomogeneous realistic geometry head model under a variety of conditions. The source localization abilities of FINE were examined at different cortical regions and at different depths. The present computer simulation results indicate that FINE has enhanced source localization capability, as compared with MUSIC and RAP-MUSIC, when sources are closely spaced, highly noise-contaminated, or inter-correlated. The source localization accuracy of FINE is better, for closely-spaced sources, than MUSIC at various noise levels, i.e., signal-to-noise ratio (SNR) from 6 dB to 16 dB, and RAP-MUSIC at relatively low noise levels, i.e., 6 dB to 12 dB. The FINE approach has been further applied to localize brain sources of motor potentials, obtained during the finger tapping tasks in a human subject. The experimental results suggest that the detailed neural activity distribution could be revealed by FINE. The present study suggests that FINE provides enhanced performance in localizing multiple closely spaced, and inter-correlated sources under low SNR, and may become an important alternative to brain source localization from EEG or MEG.

SU‐FF‐T‐412: The Development of Multiple Heterogeneous Head and Neck Phantoms for Quality Assurance of Inhomogeneity Correction Algorithm in the Intensity Modulated Radiotherapy and Conformal Dynamic Arc Radiotherapy

Purpose:To quantify the differences between heterogeneous dose estimates from the calculation algorithm of 3‐D treatment planning systems and dosimetry measurements, radiation was delivered in accordance with IMRT and conformal dynamic arc radiotherapy (CDART) QC procedures using multiple heterogeneous head & neck phantoms. Method and Materials: The multiple inhomogeneous head and neck phantoms were constructed by various materials such as water equivalent homogeneous (acryl), air equivalent (cavity), bone equivalent (teflon) materials. The absolute and relative dosimetry were done using pinpoint ionization chamber, film and TLD (Harshaw 100). With custom‐written software modules, the measured and calculated dose distributions were superimposed and compared. Results: The point dose measured at the interface between water and air material region shows about 7 % mismatch with the treatment planning system in IMRT. This dose difference is even increased at the interface between bone and water material region reaches up to 18 % under‐dose comparative to planned value. However, the point dose error at the interface between water and bone material region is relatively small showing about 3.2% over‐dose. In CDART, the distribution verification result shows that there is a mismatch of about 1.5 % of whole points revealing more than 5% of the dose difference between measured values and planning value when the inhomoheneious phantoms are used. Our results suggest that the effectiveness of the imhomogeneity corrections used in IMRT and CDART planning should be evaluated in order to ensure meaningful quality assurance and delivery. Conclusion: The robustness of inhomogeneity correction used in IMRT and CDART treatment planning may be a significant factor in assuring predictable, accurate delivery of IMRT.

SAR affected by shapes and electrical properties of the human head exposed to a cellular phone

AbstractThe local maximum specific absorption rate (SAR) induced in spherical, cubical, coarse, and realistic human‐head models exposed to a cellular phone is investigated using the finite‐difference time‐domain (FDTD) method at 900 and 1800 MHz, respectively. These four human‐head models each have greatly different shapes but almost the same volume. Obtained local maximum SARs induced in these four human‐head models for homogeneous cases are found to have an average differences of about 15% and 12% at 900 and 1800 MHz, respectively. A comparison of SARs induced in the realistic human‐head model for homogeneous and inhomogeneous cases is also made. It is found that the local maximum SAR induced in the homogeneous human‐head model is larger than that induced in the inhomogeneous human head model. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 1–4, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20189