Biomedical subjects

J J Lagendijk

Publications and source records attributed to J J Lagendijk.

At least 37 records · Page 2Linked to original sources

PubMed1999

Calculation of change in brain temperatures due to exposure to a mobile phone.

In this study we evaluated for a realistic head model the 3D temperature rise induced by a mobile phone. This was done numerically with the consecutive use of an FDTD model to predict the absorbed electromagnetic power distribution, and a thermal model describing bioheat transfer both by conduction and by blood flow. We calculated a maximum rise in brain temperature of 0.11 degrees C for an antenna with an average emitted power of 0.25 W, the maximum value in common mobile phones, and indefinite exposure. Maximum temperature rise is at the skin. The power distributions were characterized by a maximum averaged SAR over an arbitrarily shaped 10 g volume of approximately 1.6 W kg(-1). Although these power distributions are not in compliance with all proposed safety standards, temperature rises are far too small to have lasting effects. We verified our simulations by measuring the skin temperature rise experimentally. Our simulation method can be instrumental in further development of safety standards.

Body Temperature↗

PubMed1998

Modelling tissue heating with ferromagnetic seeds.

Interstitial hyperthermia using ferromagnetic seeds demands accurate treatment planning: the seed characteristics and implant geometry must be determined prior to the treatment. A new, finite difference based, seed modelling method is presented. The seed, together with all its surrounding (non-tissue) layers is described as one unit, independent of the tissue grid. The calculation of the seed-tissue interaction is based on the local seed temperature and several tissue temperature samples in the direct vicinity. All the layers between the seed and the surrounding tissue are taken into account in this interaction calculation. The presented implementation describes the analytical solution of the modelled steady-state configurations very accurately. The separation between tissue and seed allows easy assessment of the resulting seed temperature profile which is essential to the optimization of the seed characteristics in treatment planning. The thermal effect due to blood flow in the modelled tissue volume surrounding the seed can be accounted for by inclusion of a heat sink term as well as by inclusion of realistic discrete vasculature.

Algorithms↗

PubMed1998

Dose uniformity of ferromagnetic seed implants in tissue with discrete vasculature: a numerical study on the impact of seed characteristics and implantation techniques.

The results from simulations with a new three-dimensional treatment planning system for interstitial hyperthermia with ferromagnetic seeds are presented in this study. The thermal model incorporates discrete vessel structures as well as a heat sink and enhanced thermal conductivity. Both the discrete vessels and the ferroseeds are described parametrically in separate calculation spaces. This parametric description has the advantage of an arbitrary orientation of the structures within the tissue grid, easy manipulation of the structures and independence from the resolution of the tissue voxels (tissue calculation space). The power absorption of the self-regulating seeds is according to empirical data. The thermal effects of an unlimited number of thin layers surrounding the seed (coatings, catheters) can be modelled. The initial calculations have been performed for an array of 12 identical ferromagnetic seeds in a tissue volume with a computer generated artificial vessel network spanning four vessel generations in both the arterial and venous tree. The heterogeneously distributed large isolated vessels impair the temperature distribution significantly, indicating the limited accuracy of continuum models. Simulations with different types of ferromagnetic seeds have confirmed that the efforts of previous studies to optimize the self-regulating temperature control and the implantation techniques of the ferroseeds will improve the homogeneity of the temperature distribution in the target volume. Multifilament seeds implanted in brachytherapy needles and tubular seeds appear to be the most favourable configurations. The division of long seeds into shorter segments with the appropriate Curie temperature will further improve the homogeneity of the temperature distribution without increasing the average temperature in the volume of interest. Given the proper thermal tissue data, the model presented in this study will prove to be a useful tool in making choices for the implant geometry, seed spacing and Curie temperature.

Biophysical Phenomena↗

PubMed1998

Thermal properties of capacitively coupled electrodes in interstitial hyperthermia.

The multi-electrode current source (MECS) interstitial hyperthermia system which is used for treatment of cancer, employs segmented electrodes inserted in plastic tubes implanted in the treatment volume. The mean power deposition of the individual electrodes is controlled by varying the duty cycle of the RF signal applied to the electrodes, using thermocouples inside the electrodes for thermometry. A non-zero loss angle results in self-heating of the catheter. The thermal influence of self-heating was investigated and an analysis of the measurement of temperatures inside the catheter during and after heating is presented. Analytical models and a high-resolution numerical model were used for the calculation of steady state and transient distributions, respectively. The model results are compared with experimental data obtained in a muscle equivalent phantom. Results indicate that there is no difference between temperature inside and outside the catheter when using lossless catheter materials (e.g. PE and PTFE). Self-heating in the catheter wall has an adverse effect on the uniformity of the stationary temperature distribution and the reliability of temperature measurement with internal thermometry. These problems remain within acceptable limits for mildly lossy materials; the difference between the temperature inside and outside is only 6% when using low-loss Nylon. Analysis of the thermal decay after power-off shows that low-loss materials allow more time to obtain an accurate estimate of the tissue temperature at the catheter wall during power-on. This effect is enhanced by the presence of minute air layers in the applicator. Distortion of temperature gradients along the catheter was also investigated. Key factors are the thermal conduction across the catheter wall, and especially the presence of minute layers of air between consecutive layers of the probe. The distortion extends less than two millimetres, which is acceptable. The simulation results are compatible with measurements in phantoms and show that, if the proper choice of materials is made, the MECS applicator answers our expectations and that the temperature measurement inside the catheter can be used for direct feedback treatment control.

Biophysical Phenomena↗

PubMed1998

Comparison of temperature distributions in interstitial hyperthermia: experiments in bovine tongues versus generic simulations.

Temperature distributions resulting from hyperthermia treatments on isolated perfused bovine tongues were compared with simulations by a treatment planning system. The aim was to test whether the discrete vessel model used for the treatment planning is able to predict correct generic temperature distributions. Tongues were heated with the multielectrode current source interstitial hyperthermia treatment (MECS IHT) system, while the steady-state temperature distribution was mapped by scanning 10 thermocouples along paths perpendicular to the interstitial implant. For simulations a tongue was defined with generic discrete vasculature and an electrode implant analogue to the experiments. To model vascular generations not described discretely, a local heatsink was implemented at the end of each terminating branch. The discretely modelled vasculature showed itself on the temperature distributions in two ways. Individual vessels caused very local, sharp wells in the tracked temperature profiles. In the presence of large vessels a collective behaviour was also seen, i.e. a regional lowering of temperature. Both phenomena can be recognized in the experimentally obtained temperature distributions too. Predicting correct generic temperature distributions is feasible with the discrete vessel model used.

Animals↗

PubMed1998

A flexible algorithm for construction of 3-D vessel networks for use in thermal modeling.

A new algorithm for the construction of artificial blood vessel networks is presented. The algorithm produces three-dimensional (3-D) geometrical representations of both arterial and venous networks. The key ingredient of the algorithm is a 3-D potential function defined in the tissue volume. This potential function controls the paths by which points are connected to existing vessels, thereby producing new vessel segments. The potential function has no physiological interpretation, but, by adjustment of parameters governing the potential, it is possible to produce networks that have physiologically meaningful geometrical properties. If desired, the veins can be generated counter current to the arteries. Furthermore, the potential function allows fashioning of the networks to the presence of bone or air cavities. The resulting networks can be used for thermal simulations of hyperthermia treatment.

Algorithms↗

PubMed1998

Treatment planning of brain implants using vascular information and a new template technique.

A new template technique has been developed for implanting hyperthermia catheters in the treatment of brain tumors. The technique utilizes an imaging template and a drill template which can be rigidly secured to the head with three skull screws. The anatomic and vascular information needed for hyperthermia treatment planning may be assessed with three-dimensional magnetic resonance (MR) imaging and angiography acquisitions which use a surface coil. In the companioning treatment planning system the catheter positions and lengths and the electrodes in the catheter can be interactively manipulated relative to the anatomy and vasculature. The visualization of the blood vessels relative to the template allows the minimization of the risk on intracranial hemorrhages. This template technique is useful for any brain tumor implants, especially when a large number of catheters are involved. A phantom test has shown that this procedure has an accuracy in the order of 1 mm provided that the MR-related geometry distortions are minimized.

Brain Neoplasms↗

PubMed1997

Spatial temperature control with a 27 MHz current source interstitial hyperthermia system.

PURPOSE: This article gives an overview of the properties of a 27 MHz current source interstitial hyperthermia system, affecting temperature uniformity. METHODS AND MATERIALS: Applicators can be inserted in standard flexible afterloading catheters. Maximum temperatures are measured with seven-point constantan-manganin thermocouple probes inside each applicator. Temperature can be controlled automatically using a simple control algorithm. Three-dimensional power absorption and thermal models for inhomogeneous tissues are available to optimize applicator geometry and phase configuration. Properties of the interstitial heating system have been verified both in phantom experiments and in in vivo treatments of rhabdomyosarcomas implanted in the flank of a rat. RESULTS: An experiment with four electrodes in one catheter proves that longitudinal control of the specific absorption rate (SAR) is feasible. Local cooling applied by cold water circulation through a catheter perpendicular to the afterloading catheter could be compensated by independent control of electrode power. Furthermore, comparison of two different phase configurations using four dual electrode applicators shows that the SAR distribution can be manipulated significantly, utilizing the phase of the electrodes. Finally, the temperature can be controlled safely and model calculations are in fair agreement with the measurements. CONCLUSIONS: The features of the 27 MHz current source interstitial hyperthermia system enable spatial temperature control at approximately 1.5 cm.

Animals↗

PubMed1997

Implications of using thermocouple thermometry in 27 MHz capacitively coupled interstitial hyperthermia.

The 27 MHz Multi Electrode Current Source (MECS) interstitial hyperthermia system uses segmented electrodes, 10-20 mm long, to steer the 3D power deposition. This power control at a scale of 1-2 cm requires detailed and accurate temperature feedback data. To this end seven-point thermocouples are integrated into the probes. The aim of this work was to evaluate the feasibility and reliability of integrated thermometry in the 27 MHz MECS system, with special attention to the interference between electrode and thermometry and its effect on system performance. We investigated the impact of a seven-sensor thermocouple probe (outer diameter 150 microns) on the apparent impedance and power output of a 20 mm dual electrode (O.D. 1.5 mm) in a polyethylene catheter in a muscle equivalent medium (sigma 1 = 0.6 S m-1). The cross coupling between electrode and thermocouple was found to be small (1-2 pF) and to cause no problems in the dual-electrode mode, and only minimal problems in the single-electrode mode. Power loss into the thermometry system can be prevented using simple filters. The temperature readings are reliable and representative of the actual tissue temperature around the electrode. Self-heating effects, occurring in some catheter materials, are eliminated by sampling the temperature after a short power-off interval. We conclude that integrated thermocouple thermometry is compatible with 27 MHz capacitively coupled interstitial hyperthermia. The performance of the system is not affected and the temperatures measured are a reliable indication of the maximum tissue temperatures.

Electrodes↗

PubMed1997

Design of applicators for a 27 MHz multielectrode current source interstitial hyperthermia system; impedance matching and effective power.

In interstitial heating one of the main requirements for achieving a certain elevated temperature in a tumour is that the effective power per applicator (Peff), i.e. the power which is actually deposited in the tissue, is sufficiently high. In this paper this requirement is discussed for the applicators of the 27 MHz multielectrode current source (MECS) interstitial hyperthermia (IHT) system. To minimize power reflection, the applicator impedance was matched with the generator impedance by adjusting the length of the coaxial cable in between. Transmission line losses, applicator efficiency and subsequently Peff were computed for several applicator types. The actual Peff per electrode was obtained from calorimetric measurements. Experiments with RC loads, which can be seen as perfect applicators, were performed to investigate the effect of mismatching on Peff. Applicator losses were measured for clinically used applicators, both single- and dual-electrode, utilizing saline phantoms. A simple spherical tumour model, using the effective heat conductivity (keff) to account for heat transport, was used to estimate Peff for a given tumour size, implant size and applicator density. Computations of Peff of various MECS-IHT electrodes were in close agreement with the phantom measurements. Most of the initial generator power was absorbed in the transmission line (60-65%). The efficiency of the applicators was about 65%. For both single- and dual-electrode applicators the effective electrode power was found to be about 1 W. Model calculations show that Peff of 1 W is sufficient to reach a minimum tumour temperature of 43 degrees C in well perfused tumours (keff = 3 W m-1 degree C-1), using a typical implant with 2 cm electrodes and 1.5 cm spacing. Mismatching can considerably affect Peff. Both a reduction to almost zero and a two-fold increase are possible. However, because the matching theory is well understood, mismatching is not a serious problem in clinical practice and can even be used to increase Peff if necessary. We conclude that the applicator design and the impedance matching method chosen in the MECS system allow heating to temperatures in the therapeutic range with implants used in clinical practice.

Biophysical Phenomena↗

PubMed1997

Accuracy of geometrical modelling of heat transfer from tissue to blood vessels.

We have developed a thermal model in which blood vessels are described as geometrical objects, 3D curves with associated diameters. Here the behaviour of the model is examined for low resolutions compared with the vessel diameter and for strongly curved vessels. The tests include a single straight vessel and vessels describing the path of a helix embedded in square tissue blocks. The tests show the excellent behaviour of our discrete vessel implementation.

Blood Vessels↗

PubMed1997

Tests of the geometrical description of blood vessels in a thermal model using counter-current geometries.

We have developed a thermal model, for use in hyperthermia treatment planning, in which blood vessels are described as geometrical objects; 3D curves with associated diameters. For the calculation of the heat exchange with the tissue an analytic result is used. To arrive at this result some assumptions were made. One of these assumptions is a cylindrically symmetric temperature distribution. In this paper the behaviour of the model is examined for counter-current vessel geometries for which this assumption is not valid. Counter-current vessel pairs intersecting a circular tissue slice are tested. For these 2D geometries vessel spacing, tissue radius and resolution are varied, as well as the position of the vessel pair with respect to the discretized tissue grid. The simulation results are evaluated by comparison of the different heat flow rates with analytical predictions. The tests show that for a fixed vessel configuration the accuracy is not a simple decreasing function of the voxel dimensions, but is also sensitive to the position of the configuration with respect to the discretized tissue grid.

Blood Vessels↗

PubMed1996

Dose uniformity in MECS interstitial hyperthermia: the impact of longitudinal control in model anatomies.

The quality of temperature distributions that can be generated with the multi-electrode current source (MECS) interstitial hyperthermia system, which allows 3D control of the spatial SAR distribution, has been investigated. For the investigations, computer models of idealized anatomies were used. These anatomical models did not contain discrete vessels. Binary-media anatomies, containing media interfaces oriented parallel, perpendicular or oblique with respect to the long axis of the implant, represent simple anatomies which can be encountered in the clinic. The implant volume was about 40 cm3. A seven-catheter hexagonal implant geometry with a nearest-neighbor distance of 15 mm was used. In each interstitial probe between one and four electrodes with a diameter of 2.1 mm were placed along an "active section' with a length of 50 mm. The electrode segments had lengths of 50, 20, 12 and 9 mm. This study shows that even with high contrasts in electrical and thermal conductivity in the implant it remains possible to obtain satisfactory temperature distributions with the MECS system. Due to its 3D spatial control the temperature homogeneity in the implant can be made quite satisfactory, with T10-T90 of the order of 2-3 K. Treatment planning must ensure that the placement of the current source electrodes is compatible with the media configuration.

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↗

PubMed1994

The influence of respiration induced motion of the kidneys on the accuracy of radiotherapy treatment planning, a magnetic resonance imaging study.

An MRI study has been performed to determine the respiration induced motion of the kidneys. Under normal respiration conditions displacements of the left and right kidney varied from 2 to 24 mm and 4 to 35 mm, respectively. Under forced respiration conditions displacements were larger and ranged from 10 to 66 mm for the left kidney and 10 to 86 mm for the right kidney. The influence of kidney motion on the radiation dose was determined for patients irradiated on the total abdomen for ovarian cancer with shielding of the kidneys during part of the treatment. The kidney motion resulted in a larger fraction of the kidney volume receiving a dose between 20 and 22 Gy.

Adult↗

PubMed1994

Numerical analysis of the magnetic field for arbitrary magnetic susceptibility distributions in 3D.

This paper demonstrates a method to calculate the magnetic field distribution in and around a 3D object when it is magnetized by a strong homogeneous magnetic field. The numerical technique is based on the explicit finite difference method. The calculation method is validated against analytical solutions for a sphere. As an application cylinders with different ratios of lengths and diameter are studied.

Magnetic Resonance Imaging↗

PubMed1994

The theoretical and experimental evaluation of the heat balance in perfused tissue.

Accurate treatment planning is necessary for the successful application of hyperthermia in the clinic. The validity of four different bioheat models or combinations of models is evaluated: the conventional bioheat transfer equation, the limited effective conductivity model, a mixed heat sink-effective conductivity model and a discrete vessel model. The heat balance for the heated volume, and especially the ratio between conductive heat removal and heat escape through the veins, is different for each of these models. Model predictions were compared with results from experiments on isolated perfused bovine tongues. Tongues were suspended in a water-filled container and heated by conduction. The steady state temperature distribution and heat balance were determined at various blood flow rates. Increased blood flow was found to lower the mean tissue temperature and to enhance both conductive and venous heat removal. This result agrees only with the mixed heat sink-effective conductivity and the discrete vessel model predictions. At low flow rates a modified heat sink term should be used because the venous efflux temperature was significantly lower than the mean tissue temperature.

Animals↗

PubMed1992

Numerical analysis of the magnetic field for arbitrary magnetic susceptibility distributions in 2D.

We describe a numerical technique for calculating the 2D magnetic field in arbitrary magnetic susceptibility distribution. The technique we used is the explicit finite difference method with an addition of the Du Fort-Frankle algorithm. The proposed algorithm is unconditionally stable and has excellent convergence properties. For simple geometries, numerical results were compared against analytical solutions and appeared to be in excellent agreement.

Algorithms↗