Microdialysis of the basal ganglia was used in parallel to deep brain stimulation (DBS) for patients with Parkinson’s disease. The aim of this study was to patientspecifically simulate and visualize the maximum tissue volume of influence (TVI_max) for each microdialysis catheter and the electric field generated around each DBS electrode. The finite element method (FEM) was used for the simulations. The method allowed mapping of the anatomical origin of the microdialysis data and the electric stimulation for each patient. It  was seen that the sampling and stimulation targets differed among the patients, and the results will therefore be used in the future interpretation of the biochemical data.

Microdialysis can be used in parallel to deep brain stimulation (DBS) to relate biochemical changes to the clinical outcome. The aim of the study was to use the finite element method to predict the tissue volume of influence (TVI(max)) and its cross-sectional radius (r (TVImax)) when using brain microdialysis, and visualize the TVI(max) in relation to patient anatomy. An equation based on Fick's law was used to simulate the TVI(max). Factorial design and regression analysis were used to investigate the impact of the diffusion coefficient, tortuosity and loss rate on the r (TVImax). A calf brain tissue experiment was performed to further evaluate these parameters. The model was implemented with pre-(MRI) and post-(CT) operative patient images for simulation of the TVI(max) for four patients undergoing microdialysis in parallel to DBS. Using physiologically relevant parameter values, the r (TVImax) for analytes with a diffusion coefficient D = 7.5 × 10(-6) cm(2)/s was estimated to 0.85 ± 0.25 mm. The simulations showed agreement with experimental data. Due to an implanted gold thread, the catheter positions were visible in the post-operative images. The TVI(max) was visualized for each catheter. The biochemical changes could thereby be related to their anatomical origin, facilitating interpretation of results.

Aluminium is a widely used material, which is found in a number of products e.g. thin aluminium bands that is the base material in many heat exchangers. Rolling processes are used to produce these thin aluminium bands, in order to get the right properties and to get the aluminium easier to roll, heat treatment is needed. This heat treatment of aluminium ingots prior to the rolling is in focus in this work, where computational fluid dynamics and computational heat transfer techniques is used to predict the heating process in a hot air pre-heating furnace. The used approach includes steady state computational fluid dynamics simulations combined with transient computational heat transfer simulations. The simulation results in form of spatial and temporal distributed aluminium ingot temperature was compared with temperature measurement in a thermocouple prepared ingot in the actual pre-heating furnace. Simulation results correspond well with the measurements and there are small differences. Results of the described simulation approach open the possibility to predict spatial and temporal temperature distribution in these kinds of pre-heating processes.

Atherosclerosis is one of the main reasons for cardivascular disease which cause many deaths every year especially in the Western world. The development of atherosclerosis is strongly believed to be influenced by hemodynamic forces in the arteries e.g. wall shear stress (WSS). Estimations of WSS are therefore very important. By combining magnetic resonance imaging (MRI), image processing and computational fluid dynamic (CFD) simulations, it is possible to estimate subject specific WSS in the human arteries. The framework for performing such work includes i.e. using inlet boundary conditions which, however, will influence the final result i.e. the WSS distribution. This paper aims to investigate the influence of the inflow boundary condition in the human aorta with comparing two settings for the inflow: 1) subject specific inlet profile measured with MRI and 2) uniform profile with the subject specific mass flow rate. The analysis of WSS will be performed both on spatial location along the artery as well as on the temporal location in the cardiac cycle. Subject specific data have been used for geometry, inflow velocity profile and blood viscosity. The recommendation due to our findings from nine healthy subjects, is that a measured subject specific inlet boundary condition must be used in order to get a subject specific WSS distribution; the difference in WSS is 8-34% compared to using a mass-flow correct uniform profile. Temporal variations were clearly seen in the WSS differences due to the different inflow velocity profiles used. The lowest influence of the inlet boundary condition was found at peak velocity in the cardiac cycle. The aortic geometry does not form the flow in such extent (compared to the influence by inlet boundary condition) to obtain a more correct WSS distribution further away from the inlet at the systolic parts of the cardiac cycle. The shape of the vessel has only a significant influence at low velocities i.e. the diastolic phase of the cardiac cycle.

Radiofrequency lesioning of nuclei in the thalamus or the basal ganglia can be used to reduce symptoms caused by e.g. movement disorders such as Parkinson's disease. Enlarged cavities containing cerebrospinal fluid (CSF) are commonly present in the basal ganglia and tend to increase in size and number with age. Since the cavities have different electrical and thermal properties compared with brain tissue, it is likely that they can affect the lesioning process and thereby the treatment outcome. Computer simulations using the finite element method and in vitro experiments have been used to investigate the impact of cysts on lesions' size and shape. Simulations of the electric current and temperature distributions as well as convective movements have been conducted for various sizes, shapes and locations of the cysts as well as different target temperatures. Circulation of the CSF caused by the heating was found to spread heat effectively and the higher electric conductivity of the CSF increased heating of the cyst. These two effects were together able to greatly alter the resulting lesion size and shape when the cyst was in contact with the electrode tip. Similar results were obtained for the experiments.

Experimental exploration of equipment for stereotactic functional neurosurgery based on heating induced by radio-frequency current is most often carried out prior to surgery in order to secure a correct function of the equipment. The experiments are normally conducted in an experimental model including an albumin solution in which the treatment electrode is submerged, followed by a heating session during which a protein clot is generated around the electrode tip. The clot is believed to reflect the lesion generated in the brain during treatment. It is thereby presupposed that both the thermal and electric properties of the model are similar to brain tissue. This study investigates the presence of convective movements in the albumin solution using laser Doppler velocimetry. The result clearly shows that convective movements that depend on the time dependent heating characteristics of the equipment arise in the solution upon heating. The convective movements detected show a clear discrepancy compared with the in vivo situation that the experimental model tries to mimic, both the velocity (maximum velocity of about 5 mm/s) and mass flux are greater in this experimental setting. Furthermore the flow geometry is completely different since only a small fraction of the tissue surrounding the electrode in vivo consists of moving blood, whereas the entire surrounding given by the albumin solution in the experimental model is moving. Earlier investigations by our group (Eriksson et al., 1999, Med. Biol. Eng. Comput. 37, pp. 737-741, Wren, 2001, Ph.D. thesis, and Wren et al., 2001, Med. Biol. Eng. Comput. 39, pp. 255-262) indicate that the heat flux is an essential parameter for the lesion growth and final size, and that presence of convective movements in the model might substantially increase the heat flux. Thus, convective movements of the magnitude presented here will very likely underestimate the size of the brain lesion, a finding that definitely should be taken into consideration when using the model prior to patient treatment. Copyright © 2007 by ASME.

Patient specific modelling of the blood flow through the human aorta is performed using computational fluid dynamics (CFD) and magnetic resonance imaging (MRI). Velocity patterns are compared between computer simulations and measurements. The workflow includes several steps: MRI measurement to obtain both geometry and velocity, an automatic levelset segmentation followed by meshing of the geometrical model and CFD setup to perform the simulations follwed by the actual simulations. The computational results agree well with the measured data.

Radio-frequency brain lesioning is a method for reducing e.g. symptoms of movement disorders. A small electrode is used to thermally coagulate malfunctioning tissue. Influence on lesion size from thermal and electric conductivity of the tissue, microvascular perfusion and preset electrode temperature was investigated using a finite-element model. Perfusion was modelled as an increased thermal conductivity in non-coagulated tissue. The parameters were analysed using a 2⁴-factorial design (n = 16) and quadratic regression analysis (n = 47). Increased thermal conductivity of the tissue increased lesion volume, while increased perfusion decreased it since coagulation creates a thermally insulating layer due to the cessation of blood perfusion. These effects were strengthened with increased preset temperature. The electric conductivity had negligible effect. Simulations were found realistic compared to in vivo experimental lesions.

Errors in body temperature measurement might seriously influence the evaluation of an individual's health condition. We studied individual variation, measurement technique and the equipment used when assessing body temperature. In the first part of the study, three volunteers performed repeated measurements for five mornings. In the second part, the morning rectal, oral, ear and axillary temperatures were measured once in 84 men and women (19–59 years). The repeated measurements showed a daily temperature difference of 0.1–0.4°C in rectal and oral temperatures, 0.2°C−1.7°C in the ear and 0.1–0.9°C in the axillary temperatures. In the sample of 84 subjects, men and postmenopausal women had a lower mean body temperature compared to premenopausal women. The mean deviation between rectal temperature, and oral, ear and axillary temperatures, respectively, was > 0.5°C, with a large individual variation. In conclusion, in order to improve the evaluation of body temperature, the assessment should be based on the individual variation, the same site of measurement and no adjustment of oral, ear or axillary temperatures to the rectal site.

Radio-frequency (RF) lesioning in the

brain was simulated using the finite element method

(FEM). Heating for 60 s with temperature control in

order to keep the tip at 80 °C was simulated. Length,

L, (2 – 4 mm) and diameter, D, (0.5 – 2.5 mm) of the

electrode tip were varied and the resulting lesion

volumes were used to calculate a regression model:

Lesion Volume = – 13.1D + 15.7L⋅D + 13.1D2 mm3.

The results can be useful for electrode design and

prediction of lesion size.

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.

This study aims at investigating the radial dimensional changes, as a result of an applied intraluminal pressure for an elastic tube with non-uniform cross section. The study is related to a method for non-invasive pressure measurement using the extracorporeal tube as part of the sensor. The intended application is for monitoring of pressure in the blood and dialysate tubes during haemodialysis. The intention is to find a tube cross-section geometry that results in an expansion of the tube so that it is suitable to use as a component in a pressure sensor. The tube should have high radial compliance and expand in a well-defined manner to be able to transfer the intraluminal pressure to a transducer element sensing the radial force. Radial expansion was studied experimentally for tubes with different cross-section geometries. For small tube expansions the resolution in the experimental measurements was not sufficient to study the radial expansion. In this case, numerical simulation was performed. We conclude that a tube with essentially elliptic outer surface and circular inner surface, with a relation of 1:2 between the size of the thin and thick wall, results in a radial expansion upon application of pressure indicating that this tube is suitable for use as part of a sensor.

Background: In aortic prosthetic valves, both the Doppler-estimated gradients and orifice areas are misleading in the assessment of hemodynamic performance. The parameter of major interest is the net pressure gradient after pressure recovery (PR). We, therefore, investigated, in vitro, our ability to predict the net pressure gradient and applied the formulas in a representative patient population with 2 different valve designs. Methods: We studied the St Jude Medical (SJM) standard valve (size 19-27) and SJM Biocor (size 21-27) in an in vitro steady-flow model with simultaneous Doppler-estimated pressure and catheter pressure measurements. Using echocardiography, we also studied patients who received the SJM (n = 66) and SJM Biocor (n = 45). Results: In the SJM, we observed PR both within the prosthesis and aorta, whereas in the SJM Biocor, PR was only present in the aorta. We estimated the PR within the valve and within the aorta separately from echocardiographic in vitro data, combining a regression equation (valve) with an equation on the basis of fluid mechanics theory (aorta). The difference between estimated and catheter-obtained net gradients (mean ± SD) was 0.6 ± 1.6 mm Hg in the SJM and - 0.2 ± 1.9 mm Hg in the SJM Biocor. When these equations were applied in vivo, we found that PR had an overall value of 57 ± 7% of the peak Doppler gradient in the SJM and 33 ± 9% in the SJM Biocor. Conclusions: The in vitro results indicate that it is possible to predict the net pressure gradient by Doppler in bileaflet and stented biologic valves. Our data indicate that important PR is also present in stented biologic valves.

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 keff 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 ▀. For ▀ = 0 and ▀ = 1, the hybrid equation coincides with the BHE of Pennes and the keff 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 ▀, and the effect is dependent on, e.g. the boundary conditions and the power supply. Since the BHE of Pennes and the keff 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.

During ablative neurosurgery of movement disorders, for instance therapy of Parkinson's 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.

To clinically measure blood pressure at extra corporeal bloodlines involves a hazard due to the infection risk and a risk for thrombosis formation. The aim was to design a non-invasive pressure sensor, measuring directly on a tube section. A modified tube cross-section was used to improve sensitivity. Using the developed sensing principle, a consistent relation (r=0.999) was obtained between pressure and output signal. The output was stable and an acceptable drift within the temperature-range. The method shows great promise for applications in monitoring of the dialysis process.

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.