**3. Applicators for microwave thermotherapy**

Simulations of SAR distribution obtained by array of microwave applicators radiating at frequency 434 MHz will be described here. We will compare here the SAR distribution in a homogeneous agar phantom (which has similar dielectric characteristics to muscle tissue) and in an anatomical based biological model, which has been developed from a Computer Tomography (CT) scan.

Prospective Applications of Microwaves in Medicine 511

Microwave stripline applicator with TEM (Transversal Electromagnetic Mode) is being described in the following. The TEM wave propagation depends on dielectric parameters, such as permittivity ε and permeability µ. The upper and bottom part of applicator consists of high conductivity material (copper). Transversal dimensions of the described applicator are 50 x 30 mm. Distance of exciting probe from short circuited section is 15 mm. Lateral sides of applicator are made from dielectric material, from acrylic glass, which is 2 mm thick. The second part of applicator consists of stripline horn. Dimensions of the horn aperture are 120 x 80 mm and length of applicator equals to two wavelengths. Wavelength depends on permittivity of environment (in our case εH20 = 78). Whereas 70% of human body consists of water, applicator is filled with water. Advantage of this is better transfer of EM energy into human body and smaller dimensions of the applicator. This applicator was designed by the Finite Difference Time Domain (FDTD) simulator EM Field simulator (e.g.

Waveguide type applicators are very suitable for the deep local thermotherapy treatment based either on the principle of dielectric filled waveguide or on the principle of evanescent modes (i.e. waveguides excited below its cut-off frequency) - which is our specific solution and original contribution to the theory of microwave hyperthermia applicators. Technology of evanescent mode applicators enable us to design applicators with as small aperture as necessary also for relatively low frequencies e.g. from 10 to 100 MHz, needed for deep penetration into the biological tissue (i.e. up to 10 centimetres under the body surface). In theoretical and experimental evaluation, the grade of homogeneity of the temperature distribution in the target area has been tested, see the Fig. 4. Our mathematical approach is based on idea of waveguide TM01 (Transverse Magnetic) mode excited in the agar phantom under the given conditions (see the dashed lines). Measurement of SAR (full lines) has been

Fig. 4. Normalized SAR distribution (both calculated (full line) and measured (dashed line))

60 <sup>60</sup> <sup>60</sup>

80

80 80 100

measurement calculation

h

Goal of this work is the development of the new applicators with higher treatment effects. Methodological approach for the solution of the problem will be theoretical and experimental study of the new types of regional applicators. Our aim is to improve the present theoretical model to optimize the temperature distribution in the treated area.

**3.2 Microwave stripline TEM applicators** 

**3.3 Deep local heating applicators** 

done on agar phantom of the muscle tissue.

**3.4 New principles for the regional applicators** 

in the agar phantom.

SEMCAD X).

### **3.1 Waveguide applicators**

Wave guide applicators are based on microwave waveguide technology. Major advantage of waveguide applicator is that EM field in theirs aperture is roughly at 50% of its area very near to the case of EM plane wave, which is the wave with the best homogeneity and gives us the deepest penetration into the treated area.

Mostly rectangular and circular waveguides are being used to build microwave thermotherapy applicator. Examples of mentioned applicators are given in the Fig. 2. Typically they are operating at 434 MHz (depth of efficient treatment is between 4 and 6 cm), in case of deeper treatment lower frequency must be selected . Waveguide applicators displayed in Fig. 2a. are filled by dielectric material in order to decrease their cut off frequency. Air filled Evanescent mode applicator (Fig. 2b.) is excited under its cut-off frequency.

Fig. 2. a) Dielectric filled waveguide applicator, b) air filled Evanescent mode applicator.

On the next picture (Fig. 3.) there is a sketch of evanescent waveguide mode applicator working at 70 MHz, including its SAR distribution. There is a water bolus between aperture of the applicator and the agar phantom mimicking the biological tissue.

Interesting possibility how to achieve relatively sharp beam from external applicator is to use a focusing principle. The aperture of a standard rectangular waveguide applicator is then divided into 3 or 5 sectors with shifted excitation (i.e. different amplitude and phase). Another way how to focus microwave power is to use an array of external applicators, where amplitudes and phases of excitation of single applicators can be used for electronic steering of the focus area.

Fig. 3. Evanescent mode applicator and its calculated SAR pattern

#### **3.2 Microwave stripline TEM applicators**

510 Advances in Cancer Therapy

Wave guide applicators are based on microwave waveguide technology. Major advantage of waveguide applicator is that EM field in theirs aperture is roughly at 50% of its area very near to the case of EM plane wave, which is the wave with the best homogeneity and gives

Mostly rectangular and circular waveguides are being used to build microwave thermotherapy applicator. Examples of mentioned applicators are given in the Fig. 2. Typically they are operating at 434 MHz (depth of efficient treatment is between 4 and 6 cm), in case of deeper treatment lower frequency must be selected . Waveguide applicators displayed in Fig. 2a. are filled by dielectric material in order to decrease their cut off frequency. Air filled

a) b)

On the next picture (Fig. 3.) there is a sketch of evanescent waveguide mode applicator working at 70 MHz, including its SAR distribution. There is a water bolus between aperture

Interesting possibility how to achieve relatively sharp beam from external applicator is to use a focusing principle. The aperture of a standard rectangular waveguide applicator is then divided into 3 or 5 sectors with shifted excitation (i.e. different amplitude and phase). Another way how to focus microwave power is to use an array of external applicators, where amplitudes and phases of excitation of single applicators can be used for electronic

Fig. 2. a) Dielectric filled waveguide applicator, b) air filled Evanescent mode applicator.

of the applicator and the agar phantom mimicking the biological tissue.

Fig. 3. Evanescent mode applicator and its calculated SAR pattern

Evanescent mode applicator (Fig. 2b.) is excited under its cut-off frequency.

**3.1 Waveguide applicators** 

steering of the focus area.

us the deepest penetration into the treated area.

Microwave stripline applicator with TEM (Transversal Electromagnetic Mode) is being described in the following. The TEM wave propagation depends on dielectric parameters, such as permittivity ε and permeability µ. The upper and bottom part of applicator consists of high conductivity material (copper). Transversal dimensions of the described applicator are 50 x 30 mm. Distance of exciting probe from short circuited section is 15 mm. Lateral sides of applicator are made from dielectric material, from acrylic glass, which is 2 mm thick. The second part of applicator consists of stripline horn. Dimensions of the horn aperture are 120 x 80 mm and length of applicator equals to two wavelengths. Wavelength depends on permittivity of environment (in our case εH20 = 78). Whereas 70% of human body consists of water, applicator is filled with water. Advantage of this is better transfer of EM energy into human body and smaller dimensions of the applicator. This applicator was designed by the Finite Difference Time Domain (FDTD) simulator EM Field simulator (e.g. SEMCAD X).

#### **3.3 Deep local heating applicators**

Waveguide type applicators are very suitable for the deep local thermotherapy treatment based either on the principle of dielectric filled waveguide or on the principle of evanescent modes (i.e. waveguides excited below its cut-off frequency) - which is our specific solution and original contribution to the theory of microwave hyperthermia applicators. Technology of evanescent mode applicators enable us to design applicators with as small aperture as necessary also for relatively low frequencies e.g. from 10 to 100 MHz, needed for deep penetration into the biological tissue (i.e. up to 10 centimetres under the body surface). In theoretical and experimental evaluation, the grade of homogeneity of the temperature distribution in the target area has been tested, see the Fig. 4. Our mathematical approach is based on idea of waveguide TM01 (Transverse Magnetic) mode excited in the agar phantom under the given conditions (see the dashed lines). Measurement of SAR (full lines) has been done on agar phantom of the muscle tissue.

Fig. 4. Normalized SAR distribution (both calculated (full line) and measured (dashed line)) in the agar phantom.

#### **3.4 New principles for the regional applicators**

Goal of this work is the development of the new applicators with higher treatment effects. Methodological approach for the solution of the problem will be theoretical and experimental study of the new types of regional applicators. Our aim is to improve the present theoretical model to optimize the temperature distribution in the treated area.

Prospective Applications of Microwaves in Medicine 513

Each voxel was assigned to one of 3 different types of biological tissue, such as bone, muscle

Name Conductivity [S/m] Relative Permittivity [-]

EM behaviour of arrays of 2, 3 and 4 stripline type applicators coupled to homogeneous cylindrical agar phantom and consequently on 3D anatomical model of woman's calf. Cylindrical agar phantom represents muscle tissue of human limb. In our case we simulated muscle tissue of woman's calf with diameter of 8 cm. The 3D anatomical model consists of 3 various types of tissue of woman's calf created from CT scans. All of here discussed arrays

Case of array of 2 applicators on cylindrical agar phantom coupled to 3D anatomical model of woman's calf is shown on following picture (Fig. 7.). From both normalized SAR distributions (Fig. 8.) it follows that this array of 2 applicators is suitable for treatment of

The other possible configuration is array of 3 applicators coupled to homogeneous cylindrical agar phantom (Fig. 9a.). Consequently, we studied simulations of 3 applicators

Another interesting case is shown on Fig. 10. –in this picture there are displayed interesting shapes of SAR distribution by change of phases of applicators and change of amplitude of

Agar 0.80 54.00 Bone Cortical 0.09 13.07 Muscle 0.80 56.86 Fat 0.04 5.56

Fig. 6. 3D anatomical model of woman's calf

and fat with dielectric parameter shown in table (Table 1.).

Table 2. Dielectric properties at frequency 434 MHz

**3.6.1 Array of 2 microwave stripline applicators** 

**3.6.2 Array of 3 microwave stripline applicators** 

coupled to the 3D anatomical model (Fig. 9b.).

of applicators were simulated by FTDT program SEMCAD X.

tumor, which cover larger area near to surface of human limb.

one type of applicator located in the middle of all applicators.

**3.6 Array of applicators** 

We can design the regional applicators on the basis of the analytical description of the excited electromagnetic field for the case of simplified homogeneous model of the treated biological tissue. For a more realistic case of the non-homogeneous dielectric composition of the biological tissue the analytical calculation does not guarantee enough exact results. Therefore we would need to build the equipment for experimental tests of the applicators and we will study the possibilities of its numerical mathematical modelling.

evanescent mode applicator

Fig. 5. Set up of proposed regional applicator (4 evanescent mode waveguides applicators)

Applicators must have perfect impedance matching for good protection of microwave generator against reflected power. For measurement of impedance matching a reflection parameter s11 is mostly used. This parameter shows, how much electromagnetic power is reflected back to generator. Electromagnetic field is inside discussed applicator excited at frequency of 434 MHz.

#### **3.5 Anatomical model**

To verify basic functionality of created applicator we can use a homogeneous agar phantom. This homogeneous phantom represents only one type of tissue in human body – in our case it represents muscle tissue. For further discussion we will use cylindrical agar phantom with diameter 8 cm, which can basically represent a human limb. In reality human body is very inhomogeneous, and therefore it is for hyperthermia treatment planning necessary to use anatomical precise 3D models with all types of biological tissues taken into account. 3D model thus can be created from segmentation of series of several scans from CT or MRI. Such scans are 2 dimensional transversal gray – scale cuts. After then is created 3D model imported to the program, which is used for hyperthermia treatment planning. This program must both cooperate well with a segmentation program and in the same moment to enable calculation of SAR distribution.

We can choose e.g. woman's calf (Fig. 6.) as an example to demonstrate EM simulations potential. Given anatomical model was created by segmentation program 3D – DOCTOR (vector-based 3D imaging, modelling and measurement software (Able Software Corp., 2011)). We should have available DICOM scans from CT (MRRF, 2010). DICOM scans hold the original quality of data. Model of woman's calf has resolution of 1 mm and mean voxel size is 1 x 1 x 1 mm.

We can design the regional applicators on the basis of the analytical description of the excited electromagnetic field for the case of simplified homogeneous model of the treated biological tissue. For a more realistic case of the non-homogeneous dielectric composition of the biological tissue the analytical calculation does not guarantee enough exact results. Therefore we would need to build the equipment for experimental tests of the applicators

**phantom** <sup>35</sup>

Fig. 5. Set up of proposed regional applicator (4 evanescent mode waveguides applicators) Applicators must have perfect impedance matching for good protection of microwave generator against reflected power. For measurement of impedance matching a reflection parameter s11 is mostly used. This parameter shows, how much electromagnetic power is reflected back to generator. Electromagnetic field is inside discussed applicator excited at

To verify basic functionality of created applicator we can use a homogeneous agar phantom. This homogeneous phantom represents only one type of tissue in human body – in our case it represents muscle tissue. For further discussion we will use cylindrical agar phantom with diameter 8 cm, which can basically represent a human limb. In reality human body is very inhomogeneous, and therefore it is for hyperthermia treatment planning necessary to use anatomical precise 3D models with all types of biological tissues taken into account. 3D model thus can be created from segmentation of series of several scans from CT or MRI. Such scans are 2 dimensional transversal gray – scale cuts. After then is created 3D model imported to the program, which is used for hyperthermia treatment planning. This program must both cooperate well with a segmentation program and in the same moment to enable

We can choose e.g. woman's calf (Fig. 6.) as an example to demonstrate EM simulations potential. Given anatomical model was created by segmentation program 3D – DOCTOR (vector-based 3D imaging, modelling and measurement software (Able Software Corp., 2011)). We should have available DICOM scans from CT (MRRF, 2010). DICOM scans hold the original quality of data. Model of woman's calf has resolution of 1 mm and mean voxel

44

and we will study the possibilities of its numerical mathematical modelling.

35

evanescent mode applicator

frequency of 434 MHz.

**3.5 Anatomical model** 

calculation of SAR distribution.

size is 1 x 1 x 1 mm.

Fig. 6. 3D anatomical model of woman's calf

Each voxel was assigned to one of 3 different types of biological tissue, such as bone, muscle and fat with dielectric parameter shown in table (Table 1.).


Table 2. Dielectric properties at frequency 434 MHz

### **3.6 Array of applicators**

EM behaviour of arrays of 2, 3 and 4 stripline type applicators coupled to homogeneous cylindrical agar phantom and consequently on 3D anatomical model of woman's calf. Cylindrical agar phantom represents muscle tissue of human limb. In our case we simulated muscle tissue of woman's calf with diameter of 8 cm. The 3D anatomical model consists of 3 various types of tissue of woman's calf created from CT scans. All of here discussed arrays of applicators were simulated by FTDT program SEMCAD X.

#### **3.6.1 Array of 2 microwave stripline applicators**

Case of array of 2 applicators on cylindrical agar phantom coupled to 3D anatomical model of woman's calf is shown on following picture (Fig. 7.). From both normalized SAR distributions (Fig. 8.) it follows that this array of 2 applicators is suitable for treatment of tumor, which cover larger area near to surface of human limb.
