Joakim Wren

A hybrid equation for simulation of perfused tissue during thermal treatment

Bio-heat equations (BHEs) are necessary for predicting tissue temperature during thermal treatment. For some applications, however, existing BHEs describe the convective heat transfer by the blood perfusion in an unsatisfactory way. The two most frequently used equations, the BHE of Pennes and the k eff equation, use for instance either a heat sink or an increased thermal conductivity in order to account for the blood perfusion. Both these methods introduce modelling inaccuracies when applied to an ordinary tissue continuum with a variety of vessel sizes. In this study, a hybrid equation that includes both an increased thermal conductivity and a heat sink is proposed. The equation relies on the different thermal characteristics associated with small, intermediate and large sized vessels together with the possibilities of modelling these vessels using an effective thermal conductivity in combination with a heat sink. The relative importance of these two terms is accounted for by a coefficient g . For g = 0 and g = 1, the hybrid equation coincides with the BHE of Pennes and the k eff equation, respectively. The hybrid equation is used here in order to simulate temperature fields for two tissue models. The temperature field is greatly affected by g , and the effect is dependent on, e.g. the boundary conditions and the power supply. Since the BHE of Pennes and the k eff equation are included in the hybrid equation, this equation can also be useful for evaluation of the included equations. Both these heat transfer modes are included in the proposed equation, which enables implementation in standard thermal simulation programmes.Bio-heat equations (BHEs) are necessary for predicting tissue temperature during thermal treatment. For some applications, however, existing BHEs describe the convective heat transfer by the blood perfusion in an unsatisfactory way. The two most frequently used equations, the BHE of Pennes and the k(eff) equation, use for instance either a heat sink or an increased thermal conductivity in order to account for the blood perfusion. Both these methods introduce modelling inaccuracies when applied to an ordinary tissue continuum with a variety of vessel sizes. In this study, a hybrid equation that includes both an increased thermal conductivity and a heat sink is proposed. The equation relies on the different thermal characteristics associated with small, intermediate and large sized vessels together with the possibilities of modelling these vessels using an effective thermal conductivity in combination with a heat sink. The relative importance of these two terms is accounted for by a coefficient beta. For beta = 0 and beta = 1, the hybrid equation coincides with the BHE of Pennes and the k(eff) equation, respectively. The hybrid equation is used here in order to simulate temperature fields for two tissue models. The temperature field is greatly affected by beta, and the effect is dependent on, e.g. the boundary conditions and the power supply. Since the BHE of Pennes and the k(eff) equation are included in the hybrid equation, this equation can also be useful for evaluation of the included equations. Both these heat transfer modes are included in the proposed equation, which enables implementation in standard thermal simulation programmes.

Analysis of temperature measurement for monitoring radio-frequency brain lesioning

During ablative neurosurgery of movement disorders, for instance therapy of Parkinsons disease, temperature monitoring is crucial. This study aims at a quantitative comparison of measurement deviations between the maximum temperature located outside the lesioning electrode and two possible thermocouple locations inside the electrode. In order to obtain the detailed temperature field necessary for the analysis, four finite element models associated with different surroundings and with different power supplies are studied. The results from the simulations show that both the power level and the power density as well as the surrounding medium affect the temperature measurement and the temperature field in general. Since the maximum temperature is located outside the electrode there will always be a deviation in time and level between the measured and the maximum temperature. The deviation is usually 2–7 s and 3–12°C, depending on, for example, the thermocouple location and surrounding medium. Therefore, not only the measured temperature but also the relation between measured and maximum temperature must be accounted for during therapy and device design.

Evaluation of three temperature measurement methods used during microwave thermotherapy of prostatic enlargement

Three temperature measurement methods used during microwave thermotherapy of prostatic enlargement are analysed and evaluated using a phantom model. A commercial transurethral microwave thermotherapy (TUMT) system that uses a radiometric thermometer for temperature control was used to heat the phantom. The transient temperature distribution was obtained by using both fibreoptic (which is considered as gold standard) and thermocouple measurements. Both methods are subject to potential measurement errors caused by electromagnetic and/or thermal interference. The error sources are analysed and the measurement methods evaluated. The radiometric temperature and especially its relation to the transient temperature distribution was evaluated based on the fibreoptic and thermocouple measurements. These measurements in principle gave equivalent temperature distributions, and thermal interference was concluded to be the largest source of measurement error. The radiometric measurement method gave qualitative rather than quantitative readings of the temperature, and an underestimation of more than 10°C was obtained for some parts of the heated area. The area that gives most of the radiometric signal was relatively close to the catheter in contrast to previously published results.

A comparison betweenin vitro studies of protein lesions generated by brain electrodes and finite element model simulations

The aim of this study was to develop a finite element model for simulation of the thermal characteristics of brain electrodes and to compare its performances with an in vitro experimental albumin model. Ten lesions were created in albumin using a monopolar electrode connected to a Leksell Neuro Generator and a computer-assisted video system was used to determine the size of the generated lesions. A finite element model was set up of the in vitro experiments using the same thermal properties. With a very simple heat source applied to the finite element model in the proximity of the upper part of the tip, a good agreement (no deviations in width and distance from tip but a deviation in length of −1.6 mm) with the in vitro experiments (width 4.6±0.1 mm and length 7.4±0.1 mm) was achieved when comparing the outline of the lesion. In addition, a gelatinous albumin-model was set up and compared to computer simulations resulting in deviations in width of −0.4 mm, length of −2.2 mm and distance from the tip of −0.1 mm. Hence, the utilisation of finite element model simulations may be a useful complement to in-vitro experiments.

Investigation of medical thermal treatment using a hybrid bio-heat model

Bio-heat transfer, - heat transfer affecting living organism under the influence of blood perfusion -, is given great and increasing attention in medicine today. One reason is the increasing use of thermal treatment methods in for example heart- and neuro-surgery. Analysis and modelling of the thermal aspects is frequently carried out at every stage of device and method development, as it exhibits unique possibilities to understand the complex interactions present. This work investigates the use of a hybrid bio-heat model/equation, which is subsequently used to analyse temperature measurement during thermal treatment of the prostate.

Comparison between a detailed and a simplified finite element model of radio-frequency lesioning in the brain

A detailed and a simplified model of a lesioning electrode was made using the finite element method. 15 simulations of the lesioning procedure were performed for each model and the resulting lesion volumes were compared in order to investigate if the simplified model is adequate. The simplified model resulted in a very slight overestimation of the volume compared to the detailed model. It was thus concluded that the simplified model is adequate for simulations.

Towards Efficient CFD-Simulations of Engine LikeTurbine Guide Vane Film Cooling

It is well known that the efficiency of a gas turbine can be increased by using higher combustion temperatures and that this demands improved cooling. This study focuses on strategies to decrease t ...

Fan Shaped and Cylindrical Holes Studied in Vane Film Cooling Test Rig

In order to optimize the vane film cooling and thereby increase the efficiency of a gas turbine, different film cooling configurations were experimentally investigated. Dynamic similarity was obtai ...

Microwave Thermotherapy of Prostatic Enlargement—Analysis of Radiometric Thermometry Using a Hybrid Bio-heat Equation

The radiometric temperature measurement included in a commercial device for transurethral microwave thermotherapy (TUMT) of the prostate was investigated utilizing both phantom experiments and computer simulations. Two finite element (FE) models were developed. One is in part based on the experimental results, and serves as a complement to the experiments, while the other describes a perfused tissue situation for which the hybrid bio-heat equation was used to model the thermal effects of blood perfusion. The aim of the study was to investigate how the radiometric thermometer is affected by the temperature close to the antenna, and to analyze the relation between blood perfusion, temperature distribution and radiometric temperature measurement. It was found that the radiometric temperature was affected to a greater extent by the temperature very close to the antenna, in contrast to what has been expected in previous studies. The blood perfusion was found to mainly affect the temperature distribution outside the maximum temperature (located 2–3 mm outside the cooled catheter). Thus, the relation between the radiometric temperature and the temperature in the treated area is relatively weak.

Simulation of temperature measurement during transurethral hyperthermia treatment of the prostate

Microwave treatment of patients with prostatic enlargement is currently under development. For the safety of the patient it is of utmost importance to determine the intraprostatic temperature as well as the temperature in the surrounding tissues.