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A A De Leeuw

Publications and source records attributed to A A De Leeuw.

14 recordsLinked to original sources
PubMed2001

High-resolution SAR modelling for regional hyperthermia: testing quasistatic zooming at 10 MHz.

Present-day regional hyperthermia treatment planning systems are limited to centimetre resolution. To obtain CT-resolution SAR distributions, a method called quasistatic zooming has been developed: using the centimetre-resolution E-field distribution and the CT-resolution tomogram, the CT-resolution SAR distribution is obtained. For a low frequency of 10 MHz this method has been validated sucessfully using CT-resolution SAR computations. It appears that these CT-resolution SAR distributions are completely different from centimetre-resolution SAR distributions, indicating the necessity for high-resolution SAR modelling. Using the presented zooming technique, reliable CT-resolution SAR modelling is now possible with relatively short computation times. So far, the zooming method has only been validated for low frequencies, but clinically relevant frequencies appear to be possible.

Algorithms↗
PubMed2001

Determination and validation of the actual 3D temperature distribution during interstitial hyperthermia of prostate carcinoma.

To determine the thermal dose of a hyperthermia treatment, knowledge of the three-dimensional (3D) temperature distribution is mandatory. The aim of this paper is to validate an interstitial hyperthermia treatment planning system with which the full 3D temperature distribution can be obtained in individual patients. Within a phase I study, 12 patients with prostate cancer were treated with interstitial hyperthermia using our multi electrode current source interstitial hyperthermia treatment (MECS IHT) system. The temperature distribution was measured from within the heating devices and by additional thermometry. The perfusion level was estimated and the heating implant reconstructed. The steady-state temperature distribution was calculated using our interstitial hyperthermia treatment planning system. The simulated temperature distribution was validated by individually comparing the measured and simulated thermo-sensors, both for the thermometry integrated with the heating applicators and the additional thermometry. The entire procedure was also performed on a no-flow agar-agar phantom. It was shown that the calculated temperature distribution of an individual patient during MECS interstitial hyperthermia is very heterogeneous. The validation indicates that the calculated temperature elevations match the measurements within approximately 1 degrees C. Possible improvements are more precise reconstruction, incorporation of discrete vasculature and using a temperature-dependent, heterogeneous perfusion distribution. Further technical improvements of the MECS-IHT system may also result in better temperature calculations.

Agar↗
PubMed2001

Quasistatic zooming for regional hyperthermia treatment planning.

Due to current computer limitations, specific absorption rate (SAR) distributions in regional hyperthermia treatment planning (HTP) are limited to centimetre resolution. However, since patient anatomy is highly structured on a millimetre scale, millimetre-resolution SAR modelling is required. A method called quasistatic zooming has been developed to obtain a high-resolution SAR distribution within a volume of interest (VOI): using the low-resolution E-field distribution and the high-resolution patient anatomy, the high-resolution SAR distribution is computed within a small zoom volume Q (small compared with the wavelength in water (lambda(w))). Repeating this procedure yields the zoomed-resolution SAR distribution in an arbitrary VOI. To validate this method for a VOI that is not small compared with lambda(w), high-resolution finite-difference time-domain (FDTD) modelling is needed. Since this is impractical for a clinical applicator, a computer model of a small applicator has been created. A partial patient anatomy is inserted into the applicator and both high- and low-resolution SAR distributions are computed for this geometry. For the same geometry, zoomed-resolution SAR distributions are computed with different sizes of Q. To compare the low- and zoomed-resolution SAR distributions with the high-resolution one, the correlation and averaged absolute difference are computed. These numbers are improved considerably using zooming (correlation 58% to 92%; averaged absolute difference 43% to 20%). These results appear to be independent of the size of Q, up to 0.3 lambda(w). Quasistatic zooming is a valuable tool in high-resolution regional HTP.

Computer Simulation↗
PubMed2001

Quasistatic zooming of FDTD E-field computations: the impact of down-scaling techniques.

Due to current computer limitations, regional hyperthermia treatment planning (HTP) is practically limited to a resolution of 1 cm, whereas a millimetre resolution is desired. Using the centimetre resolution E-field distribution, computed with, for example, the finite-difference time-domain (FDTD) method and the millimetre resolution patient anatomy it is possible to obtain a millimetre resolution SAR distribution in a volume of interest (VOI) by means of quasistatic zooming. To compute the required low-resolution E-field distribution, a low-resolution dielectric geometry is needed which is constructed by down-scaling the millimetre resolution dielectric geometry. In this study we have investigated which down-scaling technique results in a dielectric geometry that yields the best low-resolution E-field distribution as input for quasistatic zooming. A segmented 2 mm resolution CT data set of a patient has been down-scaled to 1 cm resolution using three different techniques: 'winner-takes-all', 'volumetric averaging' and 'anisotropic volumetric averaging'. The E-field distributions computed for those low-resolution dielectric geometries have been used as input for quasistatic zooming. The resulting zoomed-resolution SAR distributions were compared with a reference: the 2 mm resolution SAR distribution computed with the FDTD method. The E-field distribution for both a simple phantom and the complex partial patient geometry down-scaled using 'anisotropic volumetric averaging' resulted in zoomed-resolution SAR distributions that best approximate the corresponding high-resolution SAR distribution (correlation 97, 96% and absolute averaged difference 6, 14% respectively).

Anisotropy↗
PubMed2001

Regional hyperthermia applicator design using FDTD modelling.

Recently published results confirm the positive effect of regional hyperthermia combined with external radiotherapy on pelvic tumours. Several studies have been published on the improvement of RF annular array applicator systems with dipoles and a closed water bolus. This study investigates the performance of a next-generation applicator system for regional hyperthermia with a multi-ring annular array of antennas and an open water bolus. A cavity slot antenna is introduced to enhance the directivity and reduce mutual coupling between the antennas. Several design parameters, i.e. dimensions, number of antennas and operating frequency, have been evaluated using several patient models. Performance indices have been defined to evaluate the effect of parameter variation on the specific absorption rate (SAR) distribution. The performance of the new applicator type is compared with the Coaxial TEM. Operating frequency appears to be the main parameter with a positive influence on the performance. A SAR increase in tumour of 1.7 relative to the Coaxial TEM system can be obtained with a three-ring, six-antenna per ring cavity slot applicator operating at 150 MHz.

Female↗
PubMedDate not supplied

Development of a regional hyperthermia treatment planning system.

A flexible and fast regional hyperthermia treatment planning system for the Coaxial TEM System has been devised and is presented. Using Hounsfield Unit based thresholding and manually outlining of the tumour, a 40 cm CT data set (slice thickness 5 mm) is segmented and down scaled to a resolution of 1 cm, requiring only 30 min. The SAR model is based on the finite-difference time-domain (FDTD) method. The number of time steps to achieve numerical stability has been determined and was found to be 7000. Various optimizations of the SAR model have been applied, resulting in a relatively short computation time of 3.7 h (memory requirements 121 MB) on a Pentium III, 450 MHz standard personal computer, running GNU/Linux. The model has been validated using absolute value(Ez) measurements in a standard phantom inserted in the Coaxial TEM Applicator under different conditions and a good agreement was found. Hyperthermia treatment planning in combination with the homemade visualization tools have provided much insight in the regional hyperthermia treatment with the Coaxial TEM Applicator.

Electromagnetic Fields↗
PubMedDate not supplied

The use of absorbing structures during regional hyperthermia treatment.

Local pain is the main factor that limits regional hyperthermia treatment. Using the SAR model of the regional hyperthermia treatment planning system, the capability of absorbing blocks to reduce peripheral hot spots was investigated. The effect of rectangular absorbers of various size and salinity on an elliptical phantom in the Coaxial TEM was evaluated. The computed results were compared with SAR values measured in the phantom. Absorbers of 9 x 9 x 4 cm3 and a salinity of 18 gram l(-1) provide a SAR reduction in the muscle equivalent material, centrally under the absorber of at least 50% at a depth of up to 3 cm. The effect on the central (i.e. tumour) region is less than 20%. Larger absorbers have a more global effect and cause more attenuation in the central region. The attenuating effect depends strongly on the thickness of the fat layer between muscle and absorber. More than 2 cm fat limits the effective use of absorbers. Absorbers can induce a significant increase of SAR in muscle and fat near their edges. This effect also depends on absorber size and salinity and the thickness of the fat layer. The effect of an absorber was also evaluated with a patient anatomy, yielding results in agreement with the phantom experiments.

Humans↗
PubMedDate not supplied

The significance of accurate dielectric tissue data for hyperthermia treatment planning.

For hyperthermia treatment planning, dielectric properties of several tissue types are required. Since it is difficult to perform patient specific dielectric imaging, default values based on literature data are used. However, these show a large spread (approximately 50%). Consequently, it is important to know what limit this spread imposes on the accuracy of the SAR and subsequently on the temperature distributions. Hyperthermia treatment plans performed with different values for the dielectric properties were compared. This showed that a spread of 50% resulted in the average absolute difference of approximately 20% in both SAR and temperature distributions (heat sink approach) for regional hyperthermia. For interstitial hyperthermia, a spread of 25% resulted in the averaged absolute difference of approximately 10% in the SAR distributions and 5% in the temperature distributions (heat sink approach). Considering other problems that hamper hyperthermia treatment planning, it can be concluded that default values for the dielectric properties suffice.

Brain Neoplasms↗
PubMedDate not supplied

Design of a clinical deep-body hyperthermia system based on the 'coaxial TEM' applicator.

The design of a clinical deep-body hyperthermia system based on the 'coaxial TEM' applicator is presented. The 'coaxial TEM' applicator produces a radiative circumferential RF field, and the adjustable aperture width of this applicator allows the optimization of field dimensions and penetration depth. A new type of open water bolus, in which the patient is almost freely floating, avoids physical stresses on the patient, provides an optimal coupling of the EM energy into the patient, and may limit aberrant heating. A convenient patient position is possible with this system and discomfort is expected to be minimal. Preliminary phantom experiments with the clinical applicator resulted in typical SAR distributions of a radiative circumferential field applicator with the central interference at maximum.

Electromagnetic Fields↗
PubMedDate not supplied

Specific absorption rate steering by patient positioning in the 'Coaxial TEM' system: phantom investigation.

Cylindrical and elliptical homogenous phantoms were used to investigate, both theoretically and experimentally, the effect of phantom positioning on the specific absorption rate (SAR) distribution of the 'Coaxial TEM' system. Theoretical predictions indicated that the maximum of the SAR distribution was stationary around the central axis of the applicator system, irrespective of the position of the phantom. Therefore the maximum SAR can be located at different phantom sites as required. Although two-dimensional models give a qualitative insight in the phenomena studied, they lack quantitive agreement. The steering capacity has been confirmed experimentally and is now clinically applied by positioning the tumour mass according to these insights.

Evaluation Studies as Topic↗
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

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↗