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J Crezee

Publications and source records attributed to J Crezee.

25 records · Page 2Linked to original sources
PubMed1987

Generation of a fast and homogeneous temperature step.

A method is described for the fast and homogeneous increase of the temperature of a biological specimen with a volume of 1 mm3 or less. Heating is obtained by joule dissipation from a 30 MHz alternating current. The step quality is determined by measuring the temporal and spatial behaviour of the temperature after a power pulse in and around the biological preparation. With the described equipment a temperature step of 3 degrees C can be obtained in 8 ms. The temperature step inhomogeneity is less than 15%. The compatibility of the method with electrophysiological and mechanical measurement and instrumentation is demonstrated by the detection of transients in transmembrane potential and force of a papillary muscle from the rabbit heart.

Action Potentials↗
PubMedDate not supplied

Spatial steering with quadruple electrodes in 27 MHz capacitively coupled interstitial hyperthermia.

PURPOSE: The 27 MHz Multi Electrode Current Source (MECS) interstitial hyperthermia system uses probes consisting of multiple independent electrodes, 10-20 mm long, to steer the 3-D power deposition. Seven point thermocouples integrated into the probes provide matching 3-D temperature feedback data. To improve spatial steering the number of independent segments was increased; the feasibility and reliability of four independent electrodes integrated into a single probe were evaluated, with special attention to efficiency and to interference between separate electrodes. METHODS: The contribution of secondary coupling on the apparent electrode impedance and the dependence of cross coupling on the distance between leads, thermocouple and electrodes are computed using simple analytical models. The effect of this secondary coupling was assessed experimentally by comparing power delivery by dual and quadruple electrodes, and by quadruple electrodes in different electrode configurations (segment length 10 or 20 mm) in a nylon catheter in a muscle equivalent medium. RESULTS: Cross coupling with the thermocouple and other electrodes was computed to be of the same magnitude as the primary coupling for a quadruple electrode. Fortunately, this does not affect operation of the electrode, there was no difference in performance between quadruple and dual electrodes, and the output power was effectively independent of the electrode configuration. CONCLUSION: Quadruple MECS electrodes for improved 3-D power control are feasible.

Electrodes↗
PubMedDate not supplied

Temperature and SAR measurements in deep-body hyperthermia with thermocouple thermometry.

Multisensor (7-14) thermocouple thermometry is used at our department for temperature measurement with our 'Coaxial TEM' regional hyperthermia system. A special design of the thermometry system with high resolution (0.005 degrees C) and fast data-acquisition (all channels within 320 ms) together with a pulsed power technique allows assessment of specific absorption rate (SAR) information in patients along catheter tracks. A disadvantage of thermocouple thermometry, EM interference, is almost entirely eliminated by application of absorbing ferrite beads around the probe leads. We investigated the effect of remaining disturbance on the temperature decay after power-off, both experimentally in phantoms and in the clinic, and with numerical simulations. Probe and tissue characteristics influence the response time tau dist of the decay of the disturbance. In our clinical practice a normal pulse sequence is 50 s power-on, 10 s power-off: a response time longer than the power-off time results in a deflection of the temperature course at the start. Based on analysis of temperature decays correction of temperature is possible. A double-pulse technique is introduced to provide an initial correction of temperature, and fast information about accuracy. Sometimes disturbance with a relatively long response time occurs, probably due to a bad contact between probe, catheter and/or tissue. Thermocouple thermometry proved to be suitable to measure the SAR along a catheter track. This is used to optimize the SAR distribution by patient positioning before treatment. A clinical example illustrates this.

Body Temperature↗
PubMedDate not supplied

Dose uniformity in scanned focused ultrasound hyperthermia.

Scanned focused ultrasound (SFUS) is unique amongst noninvasive methods of inducing hyperthermia in that the absorbed power (SAR) distribution may be controlled at a scale of 0.5 cm or better. This high degree of spatial control of SAR implies that differences in local tissue cooling due to heterogeneity in perfusion, variations in the density of discrete thermally significant vessels and even local cooling around single large vessels may be compensated during SFUS treatments. In this paper we calculate thermal dose distributions arising from three different SFUS techniques: (1) the high-temperature short-duration (HTSD) ultrasound technique; (2) conventional systems employing a fixed scan and fixed SAR simulated by a uniform SAR; and (3) we optimize the SAR distribution using information from angiography and thermal models. These techniques are tested in the same anatomy having discrete vessels, a non-uniform vessel density and with little, if any, preheating of the incoming blood (i.e. the worst-case situation). The application of a uniform SAR to this volume resulted in a highly non-uniform thermal dose distribution. The situation is clearly improved in HTSD hyperthermia; however, the desired accuracy of positioning within the tumour is high, and to minimize background heating the focus must be stepped slowly and preferably in a semi-random pattern through the target volume. The third technique which employs spatial control over SAR resulted in the greatest uniformity in thermal dose. However, the use of this technique requires the input of the complete three-dimensional discrete vessel network and the availability of a tested three-dimensional discrete vessel thermal model.

Biophysical Phenomena↗
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↗