Layered Tissue Model Research Articles

A fluoroscopic imaging-guided computational analyses to inform internal tissue loads within fat pad of the diabetic foot during gait

To accurately predict internal tissue loads for early diagnostics of diabetic foot ulcerations, a novel data-driven computational analysis was conducted. A dedicated dual fluoroscopic system was combined with a pressure mat to simultaneously characterize foot motions and soft tissue’s material properties during gait. Finite element (FE) models of the heel pad of a diabetic patient were constructed with 3D trajectories of the calcaneus applied as boundary conditions to simulate gait events. The tensile and compressive stresses occurring in the plantar tissue were computed. Predictions of the layered tissue FE model with anatomically-accurate heel pad structures (i.e., fat and skin) were compared with those of the traditional lumped tissue (i.e., homogeneous) models. The influence of different material properties (patient-specific versus generic) on internal tissue stresses was also investigated. The results showed the peak tensile stresses in the layered tissue model were predominantly found in the skin and distributed towards the circumferential regions of the heel, while peak compressive stresses in the fat tissue-bone interface were up to 51.4% lower than those seen in the lumped models. Performing FE analyses at four different phases of walking revealed that ignorance of layered tissue structures resulted in an unphysiological increase of peak-to-peak value of stress fluctuation in the fat and skin tissue components. Thus, to produce more clinical-relevant predictions, foot FE models are suggested to include layered tissue structures of the plantar tissue for an improved estimation of internal stresses in the diabetic foot in gait.

Efficient computation of the steady-state and time-domain solutions of the photon diffusion equation in layered turbid media

Accurate and efficient forward models of photon migration in heterogeneous geometries are important for many applications of light in medicine because many biological tissues exhibit a layered structure of independent optical properties and thickness. However, closed form analytical solutions are not readily available for layered tissue-models, and often are modeled using computationally expensive numerical techniques or theoretical approximations that limit accuracy and real-time analysis. Here, we develop an open-source accurate, efficient, and stable numerical routine to solve the diffusion equation in the steady-state and time-domain for a layered cylinder tissue model with an arbitrary number of layers and specified thickness and optical coefficients. We show that the steady-state (< 0.1 ms) and time-domain (< 0.5 ms) fluence (for an 8-layer medium) can be calculated with absolute numerical errors approaching machine precision. The numerical implementation increased computation speed by 3 to 4 orders of magnitude compared to previously reported theoretical solutions in layered media. We verify our solutions asymptotically to homogeneous tissue geometries using closed form analytical solutions to assess convergence and numerical accuracy. Approximate solutions to compute the reflected intensity are presented which can decrease the computation time by an additional 2–3 orders of magnitude. We also compare our solutions for 2, 3, and 5 layered media to gold-standard Monte Carlo simulations in layered tissue models of high interest in biomedical optics (e.g. skin/fat/muscle and brain). The presented routine could enable more robust real-time data analysis tools in heterogeneous tissues that are important in many clinical applications such as functional brain imaging and diffuse optical spectroscopy.

Inverse Solution of Thermoacoustic Wave Equation for Cylindrical Layered Media

Thermoacoustic imaging is a crossbred approach taking advantages of electromagnetic and ultrasound disciplines, together. A significant number of current medical imaging strategies are based on reconstruction of source distribution from information collected by sensors over a surface covering the region to be imaged. Reconstruction in thermoacoustic imaging depends on the inverse solution of thermoacoustic wave equation. Homogeneous assumption of tissue to be imaged results in degradation of image quality. In our paper, inverse solution of the thermoacoustic wave equation using layered tissue model consisting of concentric annular layers on a cylindrical cross-section is investigated for cross-sectional thermoacustic imaging of breast and brain. By using Green’s functions and surface integral methods we derive an exact analytic inverse solution of thermoacoustic wave equation in frequency domain. Our inverse solution is an extension of conventional solution to layered cylindrical structures. By carrying out simulations, using numerical test phantoms consisting of thermoacoustic sources distributed in the layered model, our layered medium assumption solution was tested and benchmarked with conventional solutions based on homogeneous medium assumption in frequency domain. In thermoacoustic image reconstruction, where the medium is assumed as homogeneous medium, the solution of nonhomogeneous thermoacoustic wave equation results in geometrical distortions, artifacts and reduced image resolution due to inconvenient medium assumptions.

In‐silico and in‐vitro testing of an implantable superstrate loaded biocompatible antenna for MICS band applications

AbstractA superstrate loaded body implantable microstrip patch antenna working on Medical Implant Communications Service (MICS) (402‐405 MHz) band for biotelemetry applications is proposed. A 0.64‐mm thick high‐dielectric material Rogers RO Duroid 3010 is used both as substrate and superstrate of dielectric constant 10.2. Shorting pin is used to make compact antenna with footprint of 13.3 × 14.6 mm2. Appreciable bandwidth of 40% is covered by antenna with return loss of −30.95. In‐silico testing inside skin, brain and three layered tissue models is done to test functionality of antenna in homogeneous as well as inhomogeneous tissue environment. For validation of results the fabricated antenna is successfully tested inside in‐vitro solution of homogeneous skin mimicking liquid.

Measurement of absorption and reduced scattering coefficients in Asian human epidermis, dermis, and subcutaneous fat tissues in the 400- to 1100-nm wavelength range for optical penetration depth and energy deposition analysis.

.Significance: In laser therapy and diagnosis of skin diseases, the irradiated light distribution, which is determined by the absorption coefficient and reduced scattering coefficient of the epidermis, dermis, and subcutaneous fat, affects the treatment outcome and diagnosis accuracy. Although values for and have been reported, detailed analysis for Asian skin tissues is still lacking.Aim: We present and measurements of Asian skin tissues in the 400- to 1100-nm wavelength range for evaluating optical penetration depth and energy deposition.Approach: The measurements with Asian human skin samples are performed employing a double integrating sphere spectrometric system and an inverse Monte Carlo technique. Using the measured parameters, the optical penetration depth and energy deposition are quantitatively analyzed.Results: The of the epidermis layer varies among different ethnic groups, while the of the other layers and the of all of the layers exhibit almost no differences. The analysis reveals that the optical penetration depth and the energy deposition affect the photodynamic therapy treatment depth and the heat production in skin tissue, respectively.Conclusions: The experimentally measured values of and for Asian skin tissues are presented, and the light behavior in Asian skin tissues is analyzed using a layered tissue model.

Study of Rotation and Bending Effects on a Flexible Hybrid Implanted Power Transfer and Wireless Antenna System.

We present rotational misalignment and bending effects on a hybrid system to transfer power and data wirelessly for an implantable device. The proposed system consists of a high-frequency coil (13.56 MHz) to transfer power and an ultra-high frequency antenna (905 MHz) for data communication. The system performance and the transmitted power were studied under two misalignment conditions: (1) receiver rotation around itself with reference to the transmitter, and (2) bending of the implanted receiver under three different radii. Implanted receiver was printed on a flexible Kapton substrate and placed inside a layered body tissue model at a 30 mm depth. It is shown that the inductive link is stable under rotational misalignment and three bending conditions, whereas the communication data link is suitable to be used if the rotation angle is less than 75° or larger than 150°. The results show that the resonance frequency varies by 1.6%, 11.05%, and 6.62% for the bending radii of 120 mm, 80 mm, and 40 mm, respectively. Moreover, transmission efficiency varies by 4.3% for the bending radius of 120 mm. Decreasing the bending radius has more effects on antenna transmission efficiency that may cause severe losses in the communication link.

Analysis of estimation of optical properties of sub superficial structures in multi layered tissue model using distribution function method

Background and objectiveThis paper is focused on the analysis of a method of estimation of the absorption and scattering coefficients of nonhomogeneous two layered structures in time resolved near infrared spectroscopy using method based on cumulative distributions of time of flight of photons. MethodsThe research is based on the analysis of the superposition of cumulative distributions of time of flight of photons. This approach allows for detailed analysis of small variations in characteristics of time of flight of photons caused by an electromagnetic propagation in highly scattering non-homogeneous media. The method presented, based on the variation of statistical minimum distance estimation, is compared to the method of standard curve fitting. It is analyzed by fitting the results obtained from Monte-Carlo simulations of light propagation in the turbid medium to the data from the simulated measurements. ResultsThe analysis is carried out for a vast range of optical properties of two layered medium in reflectance geometry. ConclusionsThe method allows the estimation of the optical parameters despite the noise in the measured signal, with higher accuracy and generally with smaller number of error function evaluations.

Performance study of a fractal UWB MIMO antenna for on-body WBAN applications

In this paper, a fractal ultrawideband multiple-input–multiple-output antenna is proposed and its characteristics are studied in free space as well as in the proximity of on-body wireless body area network scenario. The performance of the wideband antennas are affected strongly in the proximity of the human body. Koch fractal geometry is used at the outer edges of basic octagonal geometry in the design, which helps to achieve desired miniaturization, wideband and stable radiation pattern. The presented antenna has a compact size of 25 mm × 40 mm. In addition, the effect of body tissues on antenna characteristics is analyzed by a four layered (skin, fat, muscle and bone) human tissue model. The comparative analysis of S21 is performed at a various body parts position such as abdomen, near forehead and chest with different antenna orientations.

A Dual-Band HF and UHF Antenna System for Implanted Neural Recording and Stimulation Devices

We present a dual-band high-frequency (HF) and ultrahigh-frequency (UHF) (13.56 and 902-928 MHz) antenna system for implanted neural recording and/or stimulation devices. This system consists of implant-side and external antennas. These antennas are intended for dual-band systems, where power is supplied to the implant via inductive coupling in the HF band, while data are transferred via UHF near-field coupling. The antenna system consists of an implant-side antenna of 25 mm diameter and 3 mm thickness, as well as an external antenna of 85 mm diameter and 3 mm thickness. We present both timeand frequency-domain simulation results for antenna performance and specific absorption rate in a layered tissue model, as well as experimental confirmation of antenna performance in a saline tissue proxy. With an implant depth of 11 mm and an air gap of 5 mm, the power link has a 17% measured power transfer efficiency in the HF band, inclusive of matching component losses, and the UHF communication link has 38-dB insertion loss over the 902-928-MHz band.

Simulation study on the effect of tissue geometries to fluence composition for non-invasive fetal pulse oximetry.

Transabdominal fetal pulse oximetry is a method to estimate the state of oxygenation of a fetus in-utero, utilizing the principle of reflection pulse oximetry. The extraction of fetal related information from a mixed fetal-maternal signal is elementary. Minimizing the ratio of purely maternal components of the signal at the detector side obviously facilitates signal separation. In this paper we analyze the influence of tissue geometries to the fluence composition at the surface of the abdomen. Monte-Carlo method is used to compute photon propagation in spherical layered tissue models. Spatial fluence distributions at the surface of the models are visualized and discussed. Our results show the characteristic effects of the distance between the fetus and the surface and the radius of the abdomen to the fluence composition at the detector. Further, the simulations indicate suitable source-detector configurations considering various anatomical conditions. We conclude that an adoption of the source-detector configuration to the individual tissue geometry at hand is necessary to achieve a proper signal composition and quality. Utilizing simulations for sensor design enhances the understanding of photon distributions in complex tissue geometries and supports a successful implementation of transabdominal fetal pulse oximetry.

Validity of the semi-infinite tumor model in diffuse reflectance spectroscopy for epithelial cancer diagnosis: a Monte Carlo study

The accurate understanding of optical properties of human tissues plays an important role in the optical diagnosis of early epithelial cancer. Many inverse models used to determine the optical properties of a tumor have assumed that the tumor was semi-infinite, which infers infinite width and length but finite thickness. However, this simplified assumption could lead to large errors for small tumor, especially at the early stages. We used a modified Monte Carlo code, which is able to simulate light transport in a layered tissue model with buried tumor-like targets, to investigate the validity of the semi-infinite tumor assumption in two common epithelial tissue models: a squamous cell carcinoma (SCC) tissue model and a basal cell carcinoma (BCC) tissue model. The SCC tissue model consisted of three layers, i.e. the top epithelium, the middle tumor and the bottom stroma. The BCC tissue model also consisted of three layers, i.e. the top epidermis, the middle tumor and the bottom dermis. Diffuse reflectance was simulated for two common fiber-optic probes. In one probe, both source and detector fibers were perpendicular to the tissue surface; while in the other, both fibers were tilted at 45 degrees relative to the normal axis of the tissue surface. It was demonstrated that the validity of the semi-infinite tumor model depends on both the fiber-optic probe configuration and the tumor dimensions. Two look-up tables, which relate the validity of the semi-infinite tumor model to the tumor width in terms of the source-detector separation, were derived to guide the selection of appropriate tumor models and fiber optic probe configuration for the optical diagnosis of early epithelial cancers.

Modeling the detectability of vesicoureteral reflux using microwave radiometry

We present the modeling efforts on antenna design, frequency selection and receiver sensitivity estimation to detect vesicoureteral reflux (VUR) using microwave (MW) radiometry as warm urine from the bladder maintained at fever range temperature using a MW hyperthermia device reflows into the kidneys. The radiometer center frequency (fc), frequency band (Δf) and aperture radius (ra) of the physical antenna for kidney temperature monitoring are determined using a simplified universal antenna model with a circular aperture. Anatomical information extracted from the computed tomography (CT) images of children aged 4–6 years is used to construct a layered 3D tissue model. Radiometric antenna efficiency is evaluated in terms of the ratio of the power collected from the target at depth to the total power received by the antenna (η). The power ratio of the theoretical antenna is used to design a microstrip log spiral antenna with directional radiation pattern over fc ± Δf/2. Power received by the log spiral from the deep target is enhanced using a thin low-loss dielectric matching layer. A cylindrical metal cup is proposed to shield the antenna from electromagnetic interference (EMI). Transient thermal simulations are carried out to determine the minimum detectable change in the antenna brightness temperature (δTB) for 15–25 mL urine refluxes at 40–42 °C located 35 mm from the skin surface. Theoretical antenna simulations indicate maximum η over 1.1–1.6 GHz for ra = 30–40 mm. Simulations of the 35 mm radius tapered log spiral yielded a higher power ratio over fc ± Δf/2 for the 35–40 mm deep targets in the presence of an optimal matching layer. Radiometric temperature calculations indicate δTB ⩾ 0.1 K for the 15 mL urine at 40 °C and 35 mm depth. Higher η and δTB were observed for the antenna and matching layer inside the metal cup. Reflection measurements of the log spiral in a saline phantom are in agreement with the simulation data. The numerical study suggests that a radiometer with fc = 1.35 GHz, Δf = 500 MHz and detector sensitivity better than 0.1 K would be the appropriate tool to noninvasively detect VUR using the log spiral antenna.

Adaptive exponential approximation of optical density ratio using third-order regression equations for depth profiling of a blood inclusion in a skin tissue model

We propose the use of the third-order multiple regression equations in approximating an optical density ratio with an adaptive exponential function for measuring depth and thickness of a blood inclusion embedded in a skin tissue model. To ensure accuracy of the exponential approximation, we investigated numerically various relations of optical density ratios versus depth and thickness, on the basis of Monte Carlo simulations for a layered skin tissue model. The third-order multiple regression analysis based on the relations above was used to derive regression equations for the determination of depth and thickness. Experiments with skin tissue phantoms were used to assess this approach.

Crosstalk and error analysis of fat layer on continuous wave near-infrared spectroscopy measurements

Accurate estimation of concentration changes in muscles by continuous wave near-IR spectroscopy for muscle measurements suffers from underestimation and crosstalk problems due to the presence of superficial skin and fat layers. Underestimation error is basically caused by a homogeneous medium assumption in the calculations leading to the partial volume effect. The homogeneous medium assumption and wavelength dependence of mean partial path length in the muscle layer cause the crosstalk. We investigate underestimation errors and crosstalk by Monte Carlo simulations with a three layered (skin-fat-muscle) tissue model for a two-wavelength system where the choice of first wavelength is in the 675- to 775-nm range and the second wavelength is in the 825- to 900-nm range. Means of absolute underestimation errors and crosstalk over the considered wavelength pairs are found to be higher for greater fat thicknesses. Estimation errors of concentration changes for Hb and HbO(2) are calculated to be close for an ischemia type protocol where both Hb and HbO(2) are assumed to have equal magnitude but opposite concentration changes. The minimum estimation errors are found for the 700825- and 725825-nm pairs for this protocol.

Worst Case Temperature Rise in a One-Dimensional Tissue Model Exposed to Radiofrequency Radiation

This paper investigates the temperature rise in a 1-D layered tissue model, which is irradiated with nonionizing radiation. Of the numerous tissue configurations that correspond to realistic body trunk and limb representations, only those are examined which maximize the averaged specific absorption rate (SAR). The results show that the old IEEE standard on safety was more conservative in terms of temperature rise than the Non-Ionizing Radiation Protection guidelines. They also indicate that the removal of heat exchange from the skin surface can induce a significant temperature rise in it, which is, however, mostly due to imposing the adiabatic conditions themselves, rather than the electromagnetic energy absorption.

Perturbation and differential Monte Carlo methods for measurement of optical properties in a layered epithelial tissue model

The use of perturbation and differential Monte Carlo (pMC/dMC) methods in conjunction with nonlinear optimization algorithms were proposed recently as a means to solve inverse photon migration problems in regionwise heterogeneous turbid media. We demonstrate the application of pMC/dMC methods for the recovery of optical properties in a two-layer extended epithelial tissue model from experimental measurements of spatially resolved diffuse reflectance. The results demonstrate that pMC/dMC methods provide a rapid and accurate approach to solve two-region inverse photon migration problems in the transport regime, that is, on spatial scales smaller than a transport mean free path and in media where optical scattering need not dominate absorption. The pMC/dMC approach is found to be effective over a broad range of absorption (50 to 400%) and scattering (70 to 130%) perturbations. The recovery of optical properties from spatially resolved diffuse reflectance measurements is examined for different sets of source-detector separation. These results provide some guidance for the design of compact fiber-based probes to determine and isolate optical properties from both epithelial and stromal layers of superficial tissues.

Coupled Forward-Adjoint Monte Carlo Simulations of Radiative Transport for the Study of Optical Probe Design in Heterogeneous Tissues

We introduce a novel Monte Carlo method for the analysis of optical probe design that couples a forward and an adjoint simulation to produce spatial-angular maps of the detected light field within the tissue under investigation. Our technique utilizes a generalized reciprocity theory for radiative transport and is often more efficient than using either forward or adjoint simulations alone. For a given probe con. guration, the technique produces rigorous, transport-based estimates of the joint probability that photons will both visit any specified target susubvolume and be detected. This approach enables the entire tissue region to be subdivided into a collection of target subvolumes to provide a phase-space map of joint probabilities. Such maps are generated efficiently using only one forward and one adjoint simulation for a given probe configuration. These maps are used to identify those probe configurations that best interrogate targeted subvolumes. Inverse solutions in a layered tissue model serve to illustrate and reinforce our analysis.

A Theoretical Study about Parameter Estimation of Space Distributed Local Tissue Impedance

In this paper, a new configuration of the electrodes, called divided electrode, is proposed for a short time measurement of bio-impedance in a cross section of a local tissue. Its capability is examined by computer simulations, where a distributed equivalent circuit is used as a model of the tissue. Estimation of impedance parameters is carried out by use of the Gauss-Newton Method. As a result of them, usefulness of the proposed method is confirmed by computer simulations using a typical layered tissue model.

The dependence of electromagnetic energy absorption upon human head tissue composition in the frequency range of 300-3000 MHz

The requirements for testing compliance of cellular phones with electromagnetic safety limits demand evaluation of the maximum exposure that may occur in the user group under normal operational conditions. Under these conditions, the tissues of the ear region are most exposed, the tissue composition of which is complex and varies considerably from user to user. The objective of this paper is to derive head tissue equivalent dielectric parameters that enable the utilization of one generic homogeneous head for testing compliance for the entire user group, i.e., granting no underestimation, but also not greatly overestimating the actual maximum user exposure. As a primary study, a simple analytical model of an infinite half-space layered tissue model exposed to a plane wave was utilized to investigate the impact of impedance matching standing waves, etc. On the spatial-peak specific absorption rate. The tissue layers were varied in composition and thickness, representing the anatomical variation of the exposed head region covering the user group including adults and children ( 90% percentile). Based on the worst-case tissue layer compositions with respect to absorption at each frequency, head tissue equivalent dielectric parameters for homogeneous modeling were derived, which result in the same spatial-peak absorption. The validity of this approach for near-field exposures was demonstrated by replacing the plane wave by different near-field sources (dipoles and generic phones) and the layered structure with magnetic-resonance-image-based nonhomogeneous human head models.

Influence of tumour depth, blood absorption and autofluorescence on measurements of exogenous fluorophores in tissue

We investigated the influence of tumour depth and differences in blood concentration and autofluorescence between tumour and normal tissue on the fluorescence of a tumour-localising agent. Carotenoporphyrin, CP(Me)3, was injected into rats and nude mice with intradermal tumours. On the tumours an incision was made, uncovering 2 mm2 of the tumour, and fluorescence measurements, including excitation-emission maps and fluorescence ratios, were made on skin, covered and uncovered tumour. The measured fluorescence ratio in the uncovered tumour showed a three- to tenfold increase compared to the covered tumour. We used a one-dimensional layered tissue model to analyse the data. In conclusion, even with a high tumour-selectivity deeper lying tumours cannot always be detected, particularly if the tumour has a high blood concentration or low autofluorescence intensity.