Transient changes during microwave ablation simulation : a comparative shape analysis

Microwave ablation therapy is a hyperthermic treatment for killing cancerous tumours whereby microwave energy is dispersed into a target tissue region. Modelling can provide a prediction for the outcome of ablation, this paper explores changes in size and shape of temperature and Specific absorption rate fields throughout the course of simulated treatment with different probe concepts. Here, an axisymmetric geometry of a probe embedded within a tissue material is created, solving coupled electromagnetic and bioheat equations using the finite element method, utilizing hp discretisation with the NGSolve library. Results show dynamic changes across all metrics, with different responses from different probe concepts. The sleeve probe yielded the most circular specific absorption rate pattern with circularity of 0.81 initially but suffered the largest reduction throughout ablation. Similarly, reflection coefficients differ drastically from their initial values, with the sleeve probe again experiencing the largest change, suggesting that it is the most sensitive the changes in the tissue dielectric properties in these select probe designs. These collective characteristic observations highlight the need to consider dielectric property changes and probe specific responses during the design cycle.

A study of fractional order dual-phase-lag bioheat transfer model

In this study, we have established a space-time fractional DPL bioheat transfer model in the presence of temperature-dependent metabolic and space-time dependent electromagnetic heat sources. Applying the Legendre wavelet collocation method, the fractional order partial differential equation is reduced into the system of algebraic equations, which has been solved using the Newton iteration method. The error bound as well as stability analysis and numerical scheme validation are provided. The time to achieve for the position of hyperthermia is discussed in three cases: the DPL model, the time-fractional DPL model, and the space-time-fractional DPL model. The effect of variability of time and space fractional derivative orders (α and β), transmitted power (P) and lagging times on the temperature profile in biological tissue at a different time are discussed in detail. We conclude that a suitable value of α, β, τ_T, τ_q, and P provides a desirable temperature at a particular time in thermal therapies. Such knowledge will be very useful in the clinical therapeutic application.

Adding a recyclable amendment to facilitate sewage sludge biodrying and reduce costs

Finding an economical amendment, available in a steady supply, is needed to support the biodrying industrialization. This research developed a recyclable biodrying amendment (RBA) to condition the biodrying of sewage sludge. The pilot-scale treatment (T_R), which included the addition of equivalent weights of RBA and sawdust as amendments, resulted in a higher pile temperature and longer thermophilic phase compared to the control (T_C), which used only sawdust as an amendment. The final moisture content levels were below 50% with both T_R and T_C. The heat use efficiency for water evaporation was 72.2% and 73.0% in T_R and T_C, respectively. The activity of α-amylase and cellulose 1,4-β-cellobiosidase increased during the thermophilic phase, while the activity of endo-1,4-β-glucanase and endo-1,4-β-xylanase decreased during the thermophilic phase with both T_R and T_C. The fourier-transform infrared spectra indicated that adding the RBA resulted in good biodegradability of the lipids, proteins, and polysaccharides. The humic acid to fulvic acid ratio in T_R and T_C increased from 0.33 (T_R) and 0.35 (T_C) on day 0-0.46 (T_R) and 0.45 (T_C) on day 21, indicating the humification process. The RBA recovery rate was 95.6% and can be reused. These findings highlight that adding RBA showed satisfactory biodrying performance, reduced the amendment cost, and the biodrying product could be incinerated without energy deficit.

Computational Modelling of the Bioheat Transfer Process in Human Skin Subjected to Direct Heating and/or Cooling Sources: A Systematic Review

The purpose of this systematic review is to analyze characteristics and methodologies utilized in bioheat transfer models of human skin to provide state-of-the-art knowledge on the topic. This review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines. PubMed, EMBASE and Web of Science databases were searched up to May 30th, 2019 for bioheat transfer models focusing on direct contact between skin and temperature (heat and/or cold) source. Ten studies were included. A 16-item checklist was used to assess their methodological quality. Four studies analyzed healthy skin and six included pathological conditions. All determined skin's thermal behavior, and studies including pathological conditions also analyzed burn damage. Studies did not present a wide variety of mathematical formulation, emphasizing on modelling equations of well-established models from the literature, such as the Pennes' bioheat transfer equation, and the Henriques and Moritz model to quantify skin damage. Reporting of modelling characteristics and formulation of the computational models is not standardized and there is shortage of implementation of validation procedures, hindering representative conclusions. The lack of validation procedures led to low methodological quality. However, all studies provided strategies and parameters as starting points for future developments in this research area.

Identifying heat source intensity in treatment of cancerous tumor using therapy based on local hyperthermia - The Trefftz method approachs

The presented study considers the equation of hyperbolic conduction of heat with perfusion in order to identify such intensity of spatial heat source that will lead to hyperthermia of a cancerous tumor placed in healthy tissue. The tumor is assumed to be in the form of a sphere with a small radius. In order that the determined intensity of the heat source does not damage healthy tissue, different temperature distributions as a function of time are anticipated at the tumor's border. The mathematical tools used are based on the Trefftz method. The results are presented in the form of numbers and graphs illustrating the intensity of the identified heat source and matching the obtained temperature distributions in the tumor to the predicted ones.

Simulation of high-intensity focused ultrasound lesions in presence of boiling

Background

The lesions induced by high-intensity focused ultrasound (HIFU) thermal ablations are particularly difficult to simulate due to the complexity of the involved phenomena. In particular, boiling has a strong influence on the lesion shape. Thus, it must be accounted for if it happens during the pulses to be modeled. However, no acoustic model enables the simulation of the resulting wave scattering. Therefore, we propose an equivalent model for the heat deposition pattern in the presence of boiling.

Methods

Firstly, the acoustic field is simulated with k-Wave and the heat source term is calculated. Then, a thermal model is designed, including the equivalent model for boiling. It is rigorously calibrated and validated through the use of diversified ex vivo and in vivo data, including usually unexploited data types related to the bubble clouds.

Results

The proposed model enabled to efficiently simulate unitary pulses properties, including the sizes of the lesions, their morphology, the boiling onset time, and the influence of the boiling onset time on the lesions sizes.

Conclusions

In this article, the whole procedure of model design, calibration, and validation is discussed. In addition to depicting the creative use of data, our modeling approach focuses on the understanding of the mechanisms influencing the shape of the lesion. Further work is required to study the influence of the remaining bubble clouds in the context of pulse groups.

A new method for temperature-field reconstruction during ultrasound-monitored cryosurgery using potential-field analogy

The current study aims at developing computational tools in order to gain information about the thermal history in areas invisible to ultrasound imaging during cryosurgery. This invisibility results from the high absorption rate of the ultrasound energy by the frozen region, which leads to an apparent opacity in the cryotreated area and a shadow behind it. A proof-of-concept for freezing-front estimation is demonstrated in the current study, using the new potential-field analogy method (PFAM). This method is further integrated with a recently developed temperature-field reconstruction method (TFRM) to estimate the temperature distribution within the frozen region. This study uses prostate cryosurgery as a developmental model and trans-rectal ultrasound imaging as a choice of practice. Results of this study indicate that the proposed PFAM is a viable and computationally inexpensive solution to estimate the extent of freezing in the acoustic shadow region. Comparison of PFAM estimations and experimental data shows an average mismatch of less than 2 mm in freezing-front location, which is comparable to the uncertainty in ultrasound imaging. Comparison of the integrated PFAM + TFRM scheme with a full-scale finite-elements analysis (FEA) indicates an average mismatch of 0.9 mm for the freezing front location and 0.1 mm for the lethal temperature isotherm of -45 °C. Comparison of the integrated PFAM + TFRM scheme with experimental temperature measurements show a difference in the range of 2 °C and 6 °C for selected points of measurement. Results of this study demonstrate the integrated PFAM + TFRM scheme as a viable and computationally inexpensive means to gain information about the thermal history in the frozen region during ultrasound-monitored cryosurgery.