Biomedical subjects

J de Bree

Publications and source records attributed to J de Bree.

4 recordsLinked to original sources

PubMed1996

A description of discrete vessel segments in thermal modelling of tissues.

In hyperthermia treatment planning vessels with a diameter larger than 0.5 mm must be treated individually. Such vessels can be described as 3D curves with associated diameters. The temperature profile along the vessel is discretized one dimensionally. Separately the tissue is discretized three dimensionally on a regular grid of voxels. The vessel as well as the tissue are positioned in one global space. Methods are supplied to describe the tissue-vessel interaction, the shift of the blood temperature profile describing the flow of blood along the vessel and the calculation of the vessel wall temperature. The calculation of the interaction is based on tissue temperature samples and the blood temperature together with the distance between the centre of the vessel and the tissue temperature sample. An analytical expression for a vessel inside a coaxial tissue cylinder is then used for the calculation of the heat flow rate across the vessel wall. The basic test system is a vessel segment embedded inside a coaxial tissue cylinder. All the tests use this setup while the following simulation parameters are varied: position and orientation of the vessel relative to the tissue grid, vessel radius, sample density of the blood temperature and power deposition inside the tissue cylinder. The blood temperature profile is examined by calculation of the local estimate of the equilibration length. All tests show excellent agreement with the theory.

Biophysical Phenomena↗

PubMed1996

A 3-D SAR model for current source interstitial hyperthermia.

A three-dimensional (3-D) model is presented for the calculation of the specific absorption rate (SAR) in human tissue during current source interstitial hyperthermia. The model is capable of millimeter resolution and can cope with irregular implants in heterogeneous tissue. The SAR distribution is calculated from the electrical potential. The potential distribution is determined by the dielectric properties of the tissue and by the electrode configuration. The dielectric properties and the current injection of the electrodes are represented on a 3-D uniform grid. The calculated potential at an electrode current injection point is not the actual electrode potential at that point. To estimate this potential a grid independent representation of an electrode together with an analytical solution in the neighborhood of the electrode are used. The calculated potential on the electrode surface is used to estimate the electrode impedance. The tissue implementation is validated by comparing calculated distributions with analytical solutions. The electrode implementation is verified by comparing different discretizations of an electrode configuration and by comparing numerically calculated electrode impedances with analytically calculated impedances.

Absorption↗

PubMedDate not supplied

Numerical analysis of capacitively coupled electrodes for interstitial hyperthermia.

Multi electrode current source interstitial hyperthermia (MECS-IHT) employs individually controlled, 27 MHz radiofrequency electrodes inserted into plastic brachytherapy catheters. In order to get a firm understanding of the physical behaviour of the electrodes and to verify the current source approximation in our hyperthermia treatment planning system we have investigated (1) the electrical properties of the electrode-catheter-tissue system, and (2) the impact of inhomogeneity of the electrical properties of the tissue in the vicinity of the electrodes. The results validate the use of the ideal current source approximation in the treatment planning SAR model. The models predict the presence of a significant heat source inside the electrode wall when lossy catheter materials are used, producing a conductive heating component in addition to the SAR in the tissue. For a given catheter spacing this conductive component will produce a more heterogeneous temperature distribution. Thus, low-loss catheter materials like polyethylene and Teflon are recommended. The SAR is highly localized near the catheter. Calculations concerning a fat-muscle interface show that the SAR is higher in the fatty tissue than in the muscle tissue; 3D SAR control by individually controlled electrode segments is essential in such a situation.

Animals↗

PubMedDate not supplied

The influence of vasculature on temperature distributions in MECS interstitial hyperthermia: importance of longitudinal control.

The quality of temperature distributions that can be generated with the Multi Electrode Current Source (MECS) interstitial hyperthermia (IHT) system, which allows 3D control of the temperature distribution, has been investigated. For the investigations, computer models of idealised anatomies containing discrete vessels, were used. A 7-catheter hexagonal implant geometry with a nearest neighbour distance of 15 mm was used. In each interstitial catheter with a diameter of 2.1 mm a number of 1 up to 4 electrodes were placed along an 'active section' with a length of 50 mm. The electrode segments had lengths of 50, 20, 12 and 9 mm respectively. Both single vessel and vessel network situations were analysed. This study shows that even in situations with discrete vasculature and perfusion heterogeneity it remains possible to obtain satisfactory temperature distributions with the MECS IHT system. Due to its 3D spatial control the temperature homogeneity in the implant can be made quite satisfactory.

Blood Vessels↗