**5.2 Evaluation of waveguide applicator**

To evaluate this applicator from technical point of view we made a series of experiments, see e.g. Fig. 20, where you can see example of measurement of temperature distribution by IR camera.

Fig. 20. Temperature distribution obtained on surface of a model of mouse

Here you can see temperature distribution obtained on surface of a model of mouse made from agar – with a simulated tumour on mouse back. Experiment has been done by heating phantom during 2 minutes delivering a power of 10 W. Maximum of temperature increase has been found approximately 10 ºC. Similar results with different increase in temperature we have got also in other technical experiments on phantom or live mouse when power or heating time was changed.

Next Fig. 21. gives example of temperature increase on the surface of the phantom and 1 cm in that phantom. Temperature was measured by our 4-chanel thermometer. In this case with two thermo probes. Heating here is scheduled to 9 times repeated 30 s of heating and 30 s pause. Difference in temperature on the surface and under it is on the level of 1 ºC. That means very good homogeneity of temperature distribution in the treated area during planned biological experiments.

### **5.3 Array applicator**

The main goal of the planned biological experiment is a hyperthermia treatment of the experimentally induced pedicle tumours of the rat to verify the feasibility of ultrasound diagnostics and magnetic resonance imaging respectively to map the temperature distribution in the target area of the treatment. That means to heat effective volume of approximately cylindrical shape (diameter approx. 2 cm, height approx. 3 cm). Temperature to be reached is 41 ºC or more (i.e. temperature increase of at least 4 ºC from starting point 37 ºC), time period of heating is 45 minutes.

Aperture of this waveguide is 4.8 x 2.4 cm and it is excited at frequency 2.45 GHz. Effective heating is in the middle of the real aperture – its size is approximately 2.4 x 2.4 cm. Waveguide is filled by teflon to reduce its cut-off frequency. Power from generator is possible to control from 10 to 180 W, in these experiments we work between 10 and 20 W

To evaluate this applicator from technical point of view we made a series of experiments, see e.g. Fig. 20, where you can see example of measurement of temperature distribution by

Fig. 20. Temperature distribution obtained on surface of a model of mouse

Here you can see temperature distribution obtained on surface of a model of mouse made from agar – with a simulated tumour on mouse back. Experiment has been done by heating phantom during 2 minutes delivering a power of 10 W. Maximum of temperature increase has been found approximately 10 ºC. Similar results with different increase in temperature we have got also in other technical experiments on phantom or live mouse when power or

Next Fig. 21. gives example of temperature increase on the surface of the phantom and 1 cm in that phantom. Temperature was measured by our 4-chanel thermometer. In this case with two thermo probes. Heating here is scheduled to 9 times repeated 30 s of heating and 30 s pause. Difference in temperature on the surface and under it is on the level of 1 ºC. That means very good homogeneity of temperature distribution in the treated area during

The main goal of the planned biological experiment is a hyperthermia treatment of the experimentally induced pedicle tumours of the rat to verify the feasibility of ultrasound diagnostics and magnetic resonance imaging respectively to map the temperature distribution in the target area of the treatment. That means to heat effective volume of approximately cylindrical shape (diameter approx. 2 cm, height approx. 3 cm). Temperature to be reached is 41 ºC or more (i.e. temperature increase of at least 4 ºC from starting point

mostly.

IR camera.

heating time was changed.

planned biological experiments.

37 ºC), time period of heating is 45 minutes.

**5.3 Array applicator** 

**5.2 Evaluation of waveguide applicator** 

Fig. 21. Temperatures during experiments

Considering the necessary effective heating depth for the planned experiments, we have found 915 MHz to be suitable frequency. As an excellent compatibility of the applicator with non-invasive temperature measurement system (ultrasound or NMR) is a fundamental condition for our project, we should have to use non-magnetic metallic sheets of minimised dimensions to create the conductive elements of the applicator. Therefore the applicator itself (see Fig. 22.) is created by two inductive loops tuned to resonance by capacitive elements (Vrba, 1993). Dimensions of these resonant loops were designed by our software, developed for this purpose. Coupling between coaxial feeder and resonant loops (not shown in Fig. 22.) as well as a mutual coupling between resonating loops could be adjusted to optimum by microwave network analyser.

Fig. 22. a) Arrangement of discussed microwave hyperthermia applicator, b) Photograph of the discussed applicator

The position of the loops is fixed by perspex holder. There is a special cylindrical space for experimental animal in lower part of this perspex holder. As the heated tissue has a high dielectric losses, both loops are very well separated and so no significant resonance in heated area can occur. From this follows, that either the position of the loops with respect to heated area or the distance between the loops is not very critical.

First measurements to evaluate the basic properties of the discussed applicator were done on agar phantom of muscle tissue:


Prospective Applications of Microwaves in Medicine 525

In Fig. 25. we can see temperature vs. time measurement in the case of agar phantom inserted in the studied applicator. Next Fig. 26. shows experimental setup of applicator and simple exposure chambers installed at Medical Faculty of Charles University in Pilsen. 3D

Fig. 26. Exposure system for research of electromagnetic field and biological system

distribution of SAR distribution was verified numerically (Fig. 24).

Fig. 24. Numerical SAR analysis.

Fig. 25. Temperature measurements

interactions


Exact tuning of the resonant loops to frequency 915 MHz has been easy and we could optimise the coupling between the coaxial feeders and resonant loops as well, reflection coefficient less than 0.1. We have tested the power to be delivered to the applicator to obtain sufficient temperature increase (approximately 4 ºC in less than 5 minutes is required). With power 10 W delivered to each loop for period of 2 minutes we succeeded to obtain the temperature increase of approximately 7 ºC. To keep the increased temperature for a long time, 2 W in each loops were sufficient. Similar values were obtained during first experiments on rats also. Even with higher level of delivered microwave power we did not observe the change of resonant frequency (caused by increased temperature of the loops). This applicator has been developed for German Cancer Research Institute in Heidelberg. And it is being used there for a series of animal experiments to study effect of hyperthermia on tumours and possibility to combine hyperthermia with chemotherapy etc.

Compatibility of this applicator with a Magnetic resonance unit (MR) has been studied and it has been demonstrated. We have tested the influence of the applicators on US diagnostics and NMR imaging and the result of this evaluation shows very good compatibility. Only a negligible deterioration of the US images has been observed when the incident power was kept under 100 W.

Details about influence of microwave power on MR imaging are given in Fig. 23. We can see here a sequence of images of the discussed applicator made by MR unit for four different cases. First case (upper left) is image for the case without power excitation of the applicator. Second case (left down) a power of 10 W has been delivered to each loop. We can see quite clear configuration of the applicator set-up. Third case (upper right) gives situation when 20 W has been delivered to each loop. Slight noise but still quite a clear configuration of the applicator set-up can be observed. Fourths case (right down) gives situation when 40 W has been delivered to each loop. In this case noise disturbed the possibility to observe the configuration of the applicator.

Fig. 23. MR images of the discussed applicator


Exact tuning of the resonant loops to frequency 915 MHz has been easy and we could optimise the coupling between the coaxial feeders and resonant loops as well, reflection coefficient less than 0.1. We have tested the power to be delivered to the applicator to obtain sufficient temperature increase (approximately 4 ºC in less than 5 minutes is required). With power 10 W delivered to each loop for period of 2 minutes we succeeded to obtain the temperature increase of approximately 7 ºC. To keep the increased temperature for a long time, 2 W in each loops were sufficient. Similar values were obtained during first experiments on rats also. Even with higher level of delivered microwave power we did not observe the change of resonant frequency (caused by increased temperature of the loops). This applicator has been developed for German Cancer Research Institute in Heidelberg. And it is being used there for a series of animal experiments to study effect of hyperthermia

Compatibility of this applicator with a Magnetic resonance unit (MR) has been studied and it has been demonstrated. We have tested the influence of the applicators on US diagnostics and NMR imaging and the result of this evaluation shows very good compatibility. Only a negligible deterioration of the US images has been observed when the incident power was

Details about influence of microwave power on MR imaging are given in Fig. 23. We can see here a sequence of images of the discussed applicator made by MR unit for four different cases. First case (upper left) is image for the case without power excitation of the applicator. Second case (left down) a power of 10 W has been delivered to each loop. We can see quite clear configuration of the applicator set-up. Third case (upper right) gives situation when 20 W has been delivered to each loop. Slight noise but still quite a clear configuration of the applicator set-up can be observed. Fourths case (right down) gives situation when 40 W has been delivered to each loop. In this case noise disturbed the possibility to observe the

on tumours and possibility to combine hyperthermia with chemotherapy etc.


homogeneity.

kept under 100 W.

configuration of the applicator.

Fig. 23. MR images of the discussed applicator

In Fig. 25. we can see temperature vs. time measurement in the case of agar phantom inserted in the studied applicator. Next Fig. 26. shows experimental setup of applicator and simple exposure chambers installed at Medical Faculty of Charles University in Pilsen. 3D distribution of SAR distribution was verified numerically (Fig. 24).

Fig. 24. Numerical SAR analysis.

Fig. 25. Temperature measurements

Fig. 26. Exposure system for research of electromagnetic field and biological system interactions

Prospective Applications of Microwaves in Medicine 527

method is used as narrowband. The method of measurement in free space has its limitations in the demand for high-loss dielectrics measured. Electromagnetic wave through the material must be attenuated by at least 10 dB. Otherwise, standing waves will be created, which contribute significantly to the inaccuracy of this method. The method of measurement in cavity resonators gives us results with good accuracy. On the other hand, it is difficult to produce precise machining of the resonator and the measured material inserted inside (Ramachandraiah et al., 1975). Latest often used method for measuring complex permittivity measurement method is the open end of the waveguide, in our case, the coaxial cable. This method can be considered as very accurate. Moreover, we can achieve a good repetition of the results when we maintain the phase stability of the measuring coaxial cable (Tanaba & Joines, 1976). In this work, this application was selected for the main method for measuring complex permittivity of biological tissues

The reflection method represents measurement of reflection coefficient on the interface between two materials, on the open end of the coaxial line and the material under test. It is a well-known method for determining the dielectric parameters (Tanaba & Joines, 1976). This method is based on the fact that the reflection coefficient of an open-ended coaxial line depends on the dielectric parameters of material under test which is attached to it. For calculating the complex permittivity from the measured reflection coefficient, it is useful to use an equivalent circuit of an open-ended coaxial line. The probe translates changes in the permittivity of a material under test into changes of the input reflection coefficient of the probe. The surface of the sample of material under test must be in perfect contact with the probe. The thickness of a measured sample must be at least twice of equivalent penetration depth of the electromagnetic wave. This assures that the waves reflected from the material

For measurement probes, we have adapted the standard N and SMA RF connectors from which the parts for connecting to a panel were removed. The measurement probes can be described by the equivalent circuit consisting of the coupling capacitance between the inner and outer conductor out of the coaxial structure and radiating conductance which represents propagation losses (Popovic el al., 2005). These capacitance and conductance are

Probe for measuring the complex permittivity of biological tissues has been adapted from a standard RF connector for coaxial cable N-type connector, see Fig. 27. The original proposed range of application was fBW = 1 GHz, which together with the development of microwave technology is extended until today's fBW = 12 GHz. N connector used in the construction of the probe is standard N 50Ω connector that can be used up to frequency f = 5 GHz. The measurements were kept with a high degree of the accuracy and repeatability. The measurement probe based on N connectors had significantly less bandwidth, and

because of its non-invasive nature.

**6.3.1 Principle of reflection method** 

**6.4 Measurement probes** 

f1 = 100 MHz and f2 = 1 GHz.

**6.4.1 N probe** 

frequency and permittivity dependent.

under test interface are attenuated (Stuchly et al., 1994).
