Bioheat Model Research Articles

Estimation of temperature elevation generated by ultrasonic irradiation in biological tissues using the thermal wave method

In most previous models, simulation of the temperature generation in tissue is based on the Pennes bio-heat transfer equation, which implies an instantaneous thermal energy deposition in the medium. Due to the long thermal relaxation time τ (20 s–30 s) in biological tissues, the actual temperature elevation during clinical treatments could be different from the value predicted by the Pennes bioheat equation. The thermal wave model of bio-heat transfer (TWMBT) defines a thermal relaxation time to describe the tissue heating from ultrasound exposure. In this paper, COMSOL Multiphysics 3.5a, a finite element method software package, is used to simulate the temperature response in tissues based on Pennes and TWMBT equations. We further discuss different factors in the bio-heat transfer model on the influence of the temperature rising and it is found that the temperature response in tissue under ultrasound exposure is a rising process with a declining rate. The thermal relaxation time inhibits the temperature elevation at the beginning of ultrasonic heating. Besides, thermal relaxation in TWMBT leads to lower temperature estimation than that based on Pennes equation during the same period of time. The blood flow carrying heat dominates most to the decline of temperature rising rate and the influence increases with temperature rising. On the contrary, heat diffusion, which can be described by thermal conductivity, has little effect on the temperature rising.

Human thermal response with improved AVA modeling of the digits

The arterio-venous anastomoses (AVA) play a major role in the blood circulation in the peripheral body parts. In this work, the segmental bioheat model of Salloum et al. [1] is improved to accurately predict skin blood flow rate in the hands and fingers, and the local and overall human thermal responses in transient environments. The improvements in the model include: 1) extending the artery tree to include the arterial branching to the five fingers; 2) modeling and distribution of the blood flow between the deep and superficial veins in the peripherals; 3) adjusting arteries' radii during dilation and constriction; 4) innovative modeling of AVA of the fingers. The model focus is on the accurate blood flow calculation to the different body segments proposing a better blood control mechanism through relating the arterial tree radii as well as the AVA control mechanism to cardiac output. The skin blood flow and digits' dynamic thermal response predicted by the model were compared with published experimental values on body core and skin temperatures and local skin temperatures of fingers. Good agreement was obtained with experimentally reported values on average skin, core, and finger skin temperature response of subjects exposed to gradual decrease in air temperature from 32.3 °C to 13 °C. The new integrated AVA model of the fingers with the bioheat model is capable of predicting digits' dynamics thermal response with better accuracy than some previous models while also incorporating the complex central and local thermoregulatory functions.

Role of optical coefficients and healthy tissue-sparing characteristics in gold nanorod-assisted thermal therapy

Purpose: This study seeks to define parameters for gold nanorod assisted thermal therapy, to achieve the thermal ablation temperature (50–60°C) in the tumour region and spare healthy tissue surrounding the tumour. Also, a criterion for size selection of gold nanorods is described based on the role of optical coefficients.Theory and methods: In this study a tissue domain (comprising a 3 mm tumour and 7 mm of surrounding healthy tissue) embedded with gold nanorods is irradiated with electromagnetic radiation within the therapeutic wavelength band. Optical interaction is captured using light scattering theory (Mie-electrostatic approach). The resulting temperature field is evaluated using Penne's bioheat model. The effect of key parameters, namely irradiation intensity, irradiation duration and volume fraction, on tissue temperature is also modelled numerically.Results: With increasing nanorod diameter – from 5 nm to 15 nm – the scattering coefficient increases ∼76 times as compared to a 1.7-fold increase in absorption coefficient. Scattering is considerably minimised by having smaller gold nanorods of 5 nm diameter. For this study, gold nanorods of 5 nm diameter and volume fraction 0.001%, irradiated with 50 W/m2-nm for 250 s ablated the tumour as well as spare healthy tissue 2 mm beyond the tumour region.Conclusion: Overall it may be concluded that tumour ablation as well as surrounding healthy tissue-sparing (within millimetres immediately adjacent to the tumour) can be achieved through a combination of specified parameters, namely diameter and volume fraction of gold nanorods, irradiation intensity and duration.

FEM Approach for Transient Heat Transfer in Human Eye

In this paper, a bio-heat transfer model of temperature distribution in human eye is discussed using appropriate boundary conditions for cornea and sclera. Variational finite element method with Crank-Nicolson scheme is used to calculate the transient temperature distribution in normal human eye. The temperature with and without the effect of blood perfusion and metabolism on retina is simulated and compared for various ambient temperatures, evaporation rates and lens thermal conductivities. The obtained results are compared with experimental results and past results found in literatures. The results show that the steady state corneal temperature is achieved in around 31 and 45 minute of exposure at ambient temperatures 10℃ and 50℃ respectively. Steady state eye temperature is achieved earlier at higher evaporation rate. Similar result is achieved for higher lens thermal conductivity and also for lower ambient temperature.

Robust Modeling of Human Thermal Response to Hand and Forearm cooling for Improved Performance and Energy Efficiency

A modeling approach is used to study the effectiveness of the Active Cooling (AC) method of hand and forearm immersion in cold water to reduce core temperature following heavy physical activity. A transient multi-node segmental bioheat model based on physiology and accurate mathematical modeling of thermoregulatory functions are used to predict human segmental core and skin temperatures, and arterial blood flow for given metabolic rate and environmental conditions. The validity of the model is confirmed by comparison with published experimental data on core temperature during and after immersion of forearms and hands in cold water. The validated model is used in a case study to enhance understanding of associated body thermal changes and arterial blood flow and AVA mechanisms during AC interventions that alleviate thermal stress in hot environment. The time needed for the core temperature to drop from 38.0°C to 37.0°C is found to be 33 minutes when subject is at rest and is exposed to air cooling at 21°C compared to 15 minutes when hands and forearms are immersed in water at 10°C. The average sensible heat loss during the cooling period associated with immersion of forearms and hands in water at 10°C was found to be 106.2 W compared to 75.9 W for passive air cooling at 21°C. The active cooling was found to be an effective method for accelerating reduction on core temperature and can be used with efficient, localized, and portable cooling devices.

Numerical study of heat transfer and blood flow in two-layered porous liver tissue during microwave ablation process using single and double slot antenna

Microwave ablation (MWA) is a process that uses the heat from microwave energy to kill cancer cells without damaging the surrounding tissue. MWA is emerging as an attractive modality for thermal therapy of large soft tissue targets in short periods of time, making it particularly suitable for ablation of liver cancer. The effectiveness of this technique is related to the temperature achieved during the process, as well as the input microwave power and heating time of treatment. The models of heat and blood flow transport have been used extensively in the study of MWA process and a strong tool for predicting the temperature profile, blood velocity profile and coagulation zone. These models play important role in the optimization of devices and economical way to evaluate new hypothetical designs. Furthermore, they can be used as a guideline for the practical treatment. In this work, the interstitial MWA in two-layered porous liver by single slot microwave coaxial antenna (MCA) and double slot MCA is carried out. A complete mathematical model of MWA of the porous media approach is proposed, which uses transient momentum equations (Brinkman model extended Darcy model) and energy equation coupled with electromagnetic wave propagation equation to describe the specific absorption rate (SAR) profile, temperature profile and blood velocity profile within the porous liver. The coupled nonlinear set of these equations is solved using the axisymmetric finite element method (FEM). The role of transport phenomena in two layers porous media (layers of tumor and normal tissue) for advancing the progress in biomedical applications is investigated numerically. The study aims to understand of the influences of antenna type on the SAR profile, temperature profile and blood velocity profile. In particular, the results calculated from a porous media model are compared with the results calculated from a bioheat model as well as the experimental results from previous work in order to show the validity of the numerical results. The results show that the maximum SAR, temperature and blood velocity appears within the porous liver when using a single slot MCA which is higher than when using a double slot antenna. However, the volumetric SAR, temperature and blood velocity profile within the porous liver when using a double slot MCA provides a wider region in porous liver around the slot MCA and has two hot spot zones which occurs in the vicinity of these double slots.

Thermal comfort and energy performance of a low-mixing ceiling-mounted personalized ventilator system

This paper studies the performance of an integrated ceiling diffuser and personalized ventilator coaxial nozzle system to localize the air conditioning and fresh air needs around the occupants. The coaxial nozzle minimizes air entrainment between the fresh air stream and the room air and allows effective delivery of clean air. A detailed 3D CFD model is coupled to a bioheat model to improve prediction of the microenvironment conditions around the human and the associated local and overall thermal comfort. Subjective assessments were carried out with 10 human participants who were given the chance to undergo three different experimental runs individually and cast their votes about thermal comfort. The 30 samples obtained experimentally were within ±20% of the predicted numerical values especially for the upper body parts. Extensive simulations were performed to assess the effect of nozzle supply temperature and flow rate on the performance of the cooling system and on occupant comfort. The localized air conditioning system reduced the energy consumption by up to 34% when compared with conventional mixing systems providing the same level of thermal comfort.

Desk fans for the control of the convection flow around occupants using ceiling mounted personalized ventilation

This study aims at studying by modeling and experimentation the performance enhancement of ceiling-mounted personalized ventilation (PV) nozzle when assisted by small desk-mounted fans to reduce the effect of thermal plume generated by the occupant. Detailed computational fluid dynamics CFD simulations were performed to study the flow, thermal, and CO2 concentration fields around an occupant using a single jet ceiling-mounted PV nozzle in the conditioned space while the fans are on. The CFD model was integrated with a bioheat model to determine the corresponding human body segmental heat fluxes and convective plumes for predicting the occupant segmental and overall comfort. The CFD model was validated with experiments using a cylindrical heat source and comparing measured velocity, temperature and CO2 concentrations in the vicinity of the nozzle and the cylinder. Good agreement is found between the measured and the numerically predicted values. The performance of the PV nozzle assisted with desk fans is assessed by evaluation of ventilation effectiveness in the occupant microclimate and comfort conditions and results are compared with single jet PV and co-axial jet PV, both without the use of desk fans. The desk-mounted fans were able to reduce the convection plumes around the occupant and improved the performance of the single jet PV nozzle by doubling the ventilation effectiveness and improving comfort. They permitted also to achieve a reduced energy saving by up to 13% when compared with conventional mixing ventilation systems.

The Energy Saving Potential and the Associated Thermal Comfort of Displacement Ventilation Systems Assisted by Personalised Ventilation

This study examined the effect of assisting displacement ventilation ( DV) systems with personalised ventilation modules on the segmental and overall comfort of the human body during transient load variations and the associated energy saving of the combined system. A transient thermal space model has been developed for the DV system aided by the personalised task ventilator. The model was coupled with a human bioheat model to predict the segmental skin and core temperature and to assess segmental and overall thermal comfort. A case study of a typical office including six occupants was considered to assess the energy savings associated with operation of the DV system with and without personalised ventilation for the same comfort level. Higher room air temperatures in the occupied zone of the order of 26℃ (78.8°F) were found acceptable for the combined system resulting in a thermal comfort of 1.4 on a scale of −4 (very uncomfortable) to +4 (very comfortable). The DV system aided by the personalised ventilator used 27% less energy over nineteen hours of daily operation when compared to a standalone DV system providing the same level of comfort. Higher room temperatures exceeding the limit of 26℃ were found not to be beneficial in terms of energy saving.

Numerical Simulation of Thermal Damage to Living Biological Tissues Induced by Laser Irradiation Based on a Generalized Dual Phase Lag Model

A generalized dual phase lag (DPL) bioheat model based on the nonequilibrium heat transfer in living biological tissues is applied to investigate thermal damage induced by laser irradiation. Comparisons of the temperature responses and thermal damages between the generalized and classical DPL bioheat model, derived from the constitutive DPL model and Pennes bioheat equation, are carried out in this study. It is shown that the generalized DPL model could predict significantly different temperature and thermal damage from the classical DPL model and Pennes bioheat conduction model. The generalized DPL equation can reduce to the classical Pennes heat conduction equation only when the phase lag times of temperature gradient (τ T ) and heat flux vector (τ q ) are both zero. The effects of laser parameters such as laser exposure time, laser irradiance, and coupling factor on the thermal damage are also studied.

Procedure to Estimate Thermophysical and Geometrical Parameters of Embedded Cancerous Lesions Using Thermography

Based on the fact that malignant cancerous lesions (neoplasms) develop high metabolism and use more blood supply than normal tissue, infrared thermography (IR) has become a reliable clinical technique used to indicate noninvasively the presence of cancerous diseases, e.g., skin and breast cancer. However, to diagnose cancerous diseases by IR, the technique requires procedures that explore the relationship between the neoplasm characteristics (size, blood perfusion rate and heat generated) and the resulting temperature distribution on the skin surface. In this research work the dual reciprocity boundary element method (DRBEM) has been coupled with the simulated annealing technique (SA) in a new inverse procedure, which coupled to the IR technique, is capable of estimating simultaneously geometrical and thermophysical parameters of the neoplasm. The method is of an evolutionary type, requiring random initial values for the unknown parameters and no calculations of sensitivities or search directions. In addition, the DRBEM does not require any re-meshing at each proposed solution to solve the bioheat model. The inverse procedure has been tested considering input data for simulated neoplasms of different sizes and positions in relation to the skin surface. The successful estimation of unknown neoplasm parameters validates the idea of using the SA technique and the DRBEM in the estimation of parameters. Other estimation techniques, based on genetic algorithms or sensitivity coefficients, have not been capable of obtaining a solution because the skin surface temperature difference is very small.

Kalman Filtered MR Temperature Imaging for Laser Induced Thermal Therapies

The feasibility of using a stochastic form of Pennes bioheat model within a 3-D finite element based Kalman filter (KF) algorithm is critically evaluated for the ability to provide temperature field estimates in the event of magnetic resonance temperature imaging (MRTI) data loss during laser induced thermal therapy (LITT). The ability to recover missing MRTI data was analyzed by systematically removing spatiotemporal information from a clinical MR-guided LITT procedure in human brain and comparing predictions in these regions to the original measurements. Performance was quantitatively evaluated in terms of a dimensionless L(2) (RMS) norm of the temperature error weighted by acquisition uncertainty. During periods of no data corruption, observed error histories demonstrate that the Kalman algorithm does not alter the high quality temperature measurement provided by MR thermal imaging. The KF-MRTI implementation considered is seen to predict the bioheat transfer with RMS error < 4 for a short period of time, ∆t < 10 s, until the data corruption subsides. In its present form, the KF-MRTI method currently fails to compensate for consecutive for consecutive time periods of data loss ∆t > 10 sec.

The influence of wind on outdoor thermal comfort in the city of Beirut: A theoretical and field study

This article studies the outdoor comfort in the city of Beirut to improve the quality of open spaces and better explain the people's tolerance and acceptance of outdoor environmental conditions. Field experiments were conducted to develop statistical correlations for thermal sensation and thermal comfort of people in the outdoors to environmental parameters. A transient bioheat model was developed to study the effect of wind speed and frequency in the physiological responses of the human body and its effect on the overall body thermal sensation and comfort state. The model was experimentally validated and simulated for three different wind frequencies of 0.15, 0.25, and 0.35 Hz, representing a range of wind frequencies encountered during an average summer day. For each of these wind frequencies, simulations were performed for two air velocity ranges: V1 = 0.5 m/s to 2.8 m/s (1.64 ft/s to 9.18 ft/s) and V2 = 0.5 m/s to 1.8 m/s (1.64 ft/s to 5.9 ft/s) at air temperatures of 30°C and 34°C (86°F and 93.2°F) and relative humidity of 40% and 70%. The numerical results showed that for velocity range V1, the overall comfort improved from –1.15 to –0.82 with the increase of wind frequency, while at velocity range V2, comfort improved from –1.27 to –0.99 with wind frequency for the same air temperature and relative humidity. It is concluded that the positive effect of wind frequency and velocity amplitude in making people more tolerant of outdoor conditions decreases with the increase in air temperature and relative humidity.

Chilled ceiling and displacement ventilation aided with personalized evaporative cooler

This paper aims at studying the energy impact of a chilled ceiling displacement ventilation CC/ DV system aided with a personalized evaporative cooler ( PEC) directed towards the occupant trunk and face. A simulation model is developed for integrating the personalized cooler with the ascending thermal plume. The thermal model of the conditioned room air around the person is integrated with a segmental bioheat and thermal comfort model to predict the human thermal comfort. The model is validated with experimental data on the vertical temperature distribution in the room, and the recorded overall comfort perceived by surveyed subjects. Experimental results agreed well with predicted values of temperature and comfort level. When using personalized cooling, the DV supply air temperature can be as high as 24 °C while the PEC at flow rates of 3–10 l/s achieved similar comfort with a DV system at supply temperature of 21 °C. At equal thermal comfort level, the integrated CC/ DV system, PEC model resulted in up to 17.5% energy savings compared to the CC/ DV system without a PEC. When mixed air is used in the CC/ DV system additional 25% savings in energy is realized when compared with energy used for the 100% fresh air without the PEC.

Assessing thermal comfort of active people in transitional spaces in presence of air movement

The aim of this study is to develop a modeling methodology to assess thermal comfort and sensation of active people in transitional spaces and consider how comfort can be achieved by air movement while changing upper body clothing properties. The modeling is based on a bioheat model, capable of predicting segmental skin and core temperature from locally ventilated clothed body parts. The bioheat model is integrated with thermal comfort and sensation models to predict comfort in presence of air movement.The model accuracy in predicting comfort was validated by and agreed with the results of a survey administered to subjects wearing typical clothing at different activity levels to record their overall and local thermal sensation and comfort in a transitional space at Beirut summer climate. The transitional space temperature monitored during the experiments ranged between 27°C and 30°C.A parametric study is performed to assess thermal comfort in transitional spaces for different air movement levels and for three clothing designs. The high permeable clothing at 1.5m/s and indoor temperature of 30°C improved the Predicted Mean Vote to values less than 0.5 compared to 1.01 attained with typical low permeable clothing.

Three-dimensional bio-heat transfer simulation of sequential and simultaneous retinal laser irradiation

Transient simulation of retinal laser irradiation is performed for a square array comprising 3 × 3 uniformly distributed spots under sequential and simultaneous mode. A finite volume formulation of the Pennes bio-heat transfer model is employed as the governing equation to generate the temperature distribution within the eye domain. Temperature distribution developed due to laser power of 0.2 W/spot and time duration of 100 ms/spot is used to compare the merits of sequential and simultaneous irradiation of spots. Simultaneous mode is preferred when the time of surgery is limited, with possible excess heating in the inter-spot retinal regions. The inter-spot spacing D is also varied between one diameter and one and half diameter of the spot size (diameter) to evaluate the safe placement of consecutive spots. D ≥ 0.75 mm can be considered as the optimum spot distance for non-interference due to excess heating. Effect of pulsation of incident laser power is observed to favorably reduce the average temperature of the irradiated regions of the retina toward photocoagulation temperature.

Radiofrequency heating in porcine models with a "large" 32 cm internal diameter, 7 T (296 MHz) head coil.

Temperatures were measured in vivo in four pigs (mean animal weight = 110.75 kg and standard deviation = 6.13 kg) due to a continuous wave radiofrequency (RF) power irradiation with a 31.75 cm internal diameter and a 15.24 cm long, 7 T (296 MHz), eight channel, transverse electromagnetic head coil. The temperatures were measured in the subcutaneous layer of the scalp, 5, 10, 15, and 20 mm deep in the brain, and rectum using fluoroptic temperature probes. The RF power was delivered to the pig's head for ∼3 h (mean deposition time = 3.14 h and standard deviation = 0.06 h) at the whole head average specific absorption rate of ∼3 W kg(-1) (mean average specific absorption rate = 3.08 W kg(-1) and standard deviation = 0.09 W kg(-1)). Next, simple bioheat transfer models were used to simulate the RF power induced temperature changes. Results show that the RF power produced uniform temperature changes in the pigs' heads (mean temperature change = 1.68°C and standard deviation = 0.13°C) with no plateau achieved during the heating. No thermoregulatory alterations were detected due to the heating because the temperature responses of the pre-RF and post-RF epochs were not statistically significantly different. Simple, validated bioheat models may provide accurate temperature changes.

Modelling millimetre wave propagation and absorption in a high resolution skin model: the effect of sweat glands

The aim of this work was to investigate the potential effect of sweat gland ducts (SGD) on specific absorption rate (SAR) and temperature distributions during mm-wave irradiation. High resolution electromagnetic and bio-heat transfer models of human skin with SGD were developed using a commercially available simulation software package (SEMCAD X™). The skin model consisted of a 30 µm stratum corneum, 350 µm epidermis and papillary dermis (EPD) and 1000 µm dermis. Five SGD of 60 µm radius and 300 µm height were embedded linearly with 370 µm separation. A WR-10 waveguide positioned 20 µm from the skin surface and delivering 94 GHz electromagnetic radiation was included in the model. Saline conductivity was assigned inside SGD. SAR and temperatures were computed with and without SGD. Despite their small scale, SAR was significantly higher within SGD than in the EPD without SGD. Without SGD, SAR and temperature maxima were in the dermis near EPD. With SGD, SAR maximum was inside SGD while temperature maximum moved to the EPD/stratum-corneum junction. Since the EPD participates actively in perception, the effect of SGD should be taken into account in nociceptive studies involving mm-waves. This research represents a significant step towards higher spatial resolution numerical modelling of the skin and shows that microstructures can play a significant role in mm-wave absorption and induced temperature distributions.

A two-equation coupled system for determination of liver tissue temperature during thermal ablation

Ablation techniques have become a widespread choice for the treatment of cancerous tumours, where surgical resection techniques have poor prognosis. However, the extent and the shape of the ablation zone can be significantly affected by heat loss due to blood perfusion, which is difficult both to measure and to model accurately. A two-equation coupled bio-heat model is thus presented here to model the heat exchange between blood flow and its surrounding biological tissue, by considering the vasculature as a porous medium. The cooling effects of different generations of the vasculature are examined separately in the model. It is shown both analytically and computationally that the model behaviour is dependent on a non-dimensional number, γ, so termed here the thermal significance coefficient: for vessels in the highest generation of the vasculature such as big arteries and veins, γ is found to be much larger than unity, indicating that the blood in these large vessels holds a constant temperature, in which case the two-equation coupled bio-heat model can be simplified into the Pennes model; on the other hand, for small vessels in the bottom generation of the vasculature such as arterioles, capillaries, venules, γ is far smaller than unity, suggesting that the blood in small vessels is in continuous equilibrium with tissue temperature, in which case the model is equivalent to the Klinger model. However, most of the temperature equilibration occurs as the blood travels via the middle generations of the vasculature such as terminal artery branches, for which γ is of order unity and hence the two-equation coupled model cannot be further simplified. The implications of this for practical implementation of the bio-heat model are discussed.

In vivo characterization of tissue thermal properties of the kidney during local hyperthermia induced by MR‐guided high‐intensity focused ultrasound

The purpose of this study was to evaluate quantitatively in vivo the tissue thermal properties during high-intensity focused ultrasound (HIFU) heating. For this purpose, a total of 52 localized sonications were performed in the kidneys of six pigs with HIFU monitored in real time by volumetric MR thermometry. The kidney perfusion was modified by modulation of the flow in the aorta by insertion of an inflatable angioplasty balloon. The resulting temperature data were analyzed using the bio-heat transfer model in order to validate the model under in vivo conditions and to estimate quantitatively the absorption (α), thermal diffusivity (D) and perfusion (w(b)) of renal tissue. An excellent correspondence was observed between the bio-heat transfer model and the experimental data. The absorption and thermal diffusivity were independent of the flow, with mean values (± standard deviation) of 20.7 ± 5.1 mm(3) K J(-1) and 0.23 ± 0.11 mm(2) s(-1), respectively, whereas the perfusion decreased significantly by 84% (p < 0.01) with arterial flow (mean values of w(b) of 0.06 ± 0.02 and 0.008 ± 0.007 mL(-1) mL s(-1)), as predicted by the model. The quantitative analysis of the volumetric temperature distribution during nondestructive HIFU sonication allows the determination of the thermal parameters, and may therefore improve the quality of the planning of noninvasive therapy with MR-guided HIFU.