Biomedical subjects

J D van Dijk

Publications and source records attributed to J D van Dijk.

22 records · Page 2Linked to original sources

PubMedDate not supplied

Quality assurance in various radiative hyperthermia systems applying a phantom with LED matrix.

The Amsterdam phantom with LED-matrix is applied as an instrument in testing the performance of four types of radiative deep-body hyperthermia systems, which are in clinical use in Germany and The Netherlands. The devices tested were Essen's BSD-1000, Berlin's BSD-2000, Utrecht's Coaxial TEM applicator and Amsterdam's Four-waveguide-array. Photographs were taken of the matrix of dipoles loaded with light-emitting diodes (LED) to visualize the distribution of the RF power deposition or specific absorption rate (SAR) in the aperture midplane. The utility of the phantom with LED matrix for various types of radiative hyperthermia systems is demonstrated. Within this preliminary study, the influence of important parameters on the SAR-pattern in the aperture midplane was demonstrated. After corrections on the phase relation of the applicators a central focus in the SAR distribution could be realized in all systems and could also be moved in any direction. The patterns of the central focus changed in its absolute values and its proportions depending on the relative relations of phase and amplitude of the lateral applicators with respect to the top and bottom applicator. Frequency dependency was recognized for the central focus of the BSD-1000 as well as for the irradiation pattern of a single applicator for the BSD-2000. In the Coaxial TEM applicator it was demonstrated that the dimension of the open water bolus influenced the absolute value of SAR in the aperture midplane.

Biophysical Phenomena↗

PubMedDate not supplied

The effect of catheters and coatings on the performance of palladium-nickel thermoseeds: evaluation and design of implantation techniques.

In the development of materials for self-regulating thermoseeds much effort is put in improvement of the self-regulating temperature control mechanism of the seeds. The catheters and coatings which are needed to implant the seeds or to guarantee biocompatibility, generally impair the optimized performance of the ferromagnetic seeds. The influence of various coatings on the performance of PdNi seeds has been investigated by means of one-dimensional modelling and calorimetric experiments. Implantation using thin walled catheters is acceptable provided that the catheters are filled with water to assure good thermal coupling. Air layers inside catheters should be avoided as they reduce the sharp gradient of the heat production at the Curie temperature significantly. An alternative for the application of catheters is to insert the seeds into metallic needles. The effect of shielding by the metal needle can be minimized by driving the seed into its saturated state using a high magnetic field strength. The thermal interaction between the seed and surrounding tissue can also be enhanced by placing PdNi, e.g. tubular, on the outside of the catheter or brachytherapy needle. An additional advantage of this new design is an increase in the heat production and the quality of temperature control due to an increase in the amount of PdNi. For permanent implantation seeds can be coated with an inert metal, ceramics or plastic. The performance of the seeds is not affected by any of the coatings if certain conditions are met. For plastic coatings the thickness of the coating has to be very thin, preferably < or = 20 microns, to avoid thermal isolation.

Biocompatible Materials↗

PubMedDate not supplied

Three-dimensional temperature control of palladium-nickel thermoseeds: a computer aided and experimental evaluation.

The capability of self-regulating thermoseeds to compensate for nonuniform cooling along their longitudinal axis has been investigated in this study. For this purpose a quasi three-dimensional computer model has been developed. Calculations of the temperature profile in tissue with nonuniform heat loss demonstrated a clear improvement in the longitudinal temperature control of PdNi seeds compared to constant power seeds. Further, two strategies for improved control of nonuniform cooling along the longitudinal axis of ferromagnetic seeds have been investigated: (1) application of a 'normal' undivided seed; and (2) division of a long seed in smaller segments of which each segment is able to respond more directly to local variations in the temperature distribution. Calculations with the quasi three-dimensional model showed that the loose segments do respond more directly to their close proximity. However, the equilibrium temperature of a segment in an area with high local blood flow will be relatively low due the limited heat production of PdNi thermoseeds. In the undivided seeds the high thermal conductivity of PdNi causes some levelling of the longitudinal temperature gradient in the seed. In addition, calorimetric experiments have shown that the heat production of a segmented seed is less effective because of a demagnetizing field. Also, the absence of PdNi between the segments reduces the heat production of the seed.

Animals↗

PubMedDate not supplied

The measurement of fringing fields in a radio-frequency hyperthermia array with emphasis on bolus size.

The limited aperture size through which the em-field of the applicator is emanated and the constraining of this em-field near the bolus' edge is related to the appearance of superficial 'hot spot' phenomena in radiative hyperthermia. Regarding systems based on the concept of the annular phased array two questions arise: (1) what is the relative strength of the radial component present in the incident field of the radiators, and (2) in what way are fringing fields related to the bolus size? To address both of the above questions, the spatial distribution of the em-field emanated through the aperture of an applicator of the Amsterdam four waveguide-array system has been investigated for a long bolus and a short bolus. The em vector field emanated by the applicator has been characterized in two perpendicular planes, i.e. the aperture midplane and the sagittal midplane. It should be noted that this distribution depends on the propagation conditions throughout the coupling bolus, the phantom and other volumes attached, such as other applicators. Therefore two sets of propagation conditions have been measured: (1) the minimum number of parameters determining the propagation of the em-field namely one single waveguide, one bolus and a homogeneous phantom, and (2) the propagation conditions as for the clinical setting. It is stressed that the study concerns one specific radiative hyperthermia system, namely the AMC four-waveguide array, but that, based on the similarities discussed above, results may be extrapolated towards other radiative hyperthermia systems. According to the current study, bolus prolongation might lead to a clear clinical improvement, which is due to a decrease of the fringing field amplitude compared to the field amplitude in the centre of the aperture midplane. Bolus prolongation will lead to an extended heating area, the field lines being more aligned to the patient's main axis.

Electromagnetic Fields↗