A 2450 Mhz Slab-Loaded Direct Contact Applicator with Choke

A Teflon-slab-loaded, direct contact microwave diathermy applicator has been developed. It produces minimal leakage radiation during effective heating of simulated planar tissue models. Because of TEM mode excitation, heating patterns are relatively uniform.

STUDIES [1]- [3] of slab-loaded rectangular waveguides with flanges ( Fig. 1), used as microwave diathermy applicators at 2450 MHz, have shown that their heating patterns induced in phantoms are highly uniform in the central region. These direct contact applicators were not designed to minimize leakage radiation. To reduce leakage radiation, the wavgeuide flanges have been replaced with a microwave choke. The purpose of this paper is to report on the design and performance of a slab-loaded rectangular waveguide that produces minimal leakage radiation while delivering a thermally effective absorbed dose to simulated tissue models.

II. DESIGN
The waveguide, shown in Fig. 2, is a WR-430 guide with a standard waveguide to coaxial adapter. Its flanges were removed to surround the remaining aperture with a microwave choke. By loading the waveguide with two Teflon slabs, an inhomogeneously filled applicator with improved performance is obtained. A cross section of this design is shown in Fig. 3.

A. Slab-Loading
The length of the two slabs along the axis of propagation is 7.6 cm. As is indicated in Fig. 3, they have a thickness of 3.2 cm and a height of 5.4cm. The purpose of the slabs is to provide for a more uniform heating pattern in the central region of the aperture by exciting the TEM mode in the air space between the slabs. The following equation from [4]    guide theory [4], higher order modes will not be excited in the waveguide.

B. Choke
In accordance with standard design procedure, the microwave choke is one-quarter-wavelength long at 2450 MHz or 3.1 cm. As indicated in Fig. 3 characteristics of the slab-loaded applicator with choke, which will be discussed below, are summarized in Table I.

A. Heating Patterns
The experimental setup for measuring heating patterns is shown in Fig. 4. It is described in detail in previously published papers [2], [5]. Briefly, for internal heating, the applicator is placed symmetrically on top of a planar phantom with a simulated fat layer (either 1 or 2cm thick) above simulated muscle tissue. (The configuration of this phantom is shown in Fig. 4.) Before heating the phantom, the ambient temperature profile in the simulated muscle tissue is recorded. After heating the phantom, one-half of the phantom is quickly removed (to minimize thermal diffusion) so that a thermographic camera can view the heating pattern.
A typical thermogram with a selected profile is shown in    The width w of the temperature profile is 6.4 cm (w is defined [2] as the width of the trace where the temperature rise above ambient level is half the maximum temperature rise). If the slab-loaded applicator with flanges is used [2], the heating pattern is highly uniform; w is 7.3 cm instead of 6.4 cm. This suggests that in the presence of the choke, the uniformity of the electric field distribution between the two slabs is reduced. Yet the heating pattern of a slab-loaded applicator with choke is still significantly broader than of an empty waveguide with choke; its width is 6.4 cm instead of  From Fig. 6, for the slab-loaded applicator with choke, the depth of penetration d in simulated muscle tissue is equal to 1.3 cm (d is defined [2] as the distance between the fat-muscle interface and the depth at which there is a 50-percent falloff from the maximum temperature rise).
The above value of d remains the same for direct contact and l-cm spacing for both phantoms (1-and 2-cm fat layers).
To determine the required net power for a thermally effective absorbed dose in the simulated muscle tissue of the planar phantom, the width profiles, such as shown in Fig. 5, were analyzed. The data in Fig. 5 were obtained by heating the phantom with the 1-cm layer for ten seconds with a net power of 125 W delivered to the applicator. A specific absorption rate (SAR) calculation [3] using these data indicated that a net power of 22.3 W is needed to deliver a maximum SAR of 235 W/kg to the simulated muscle tissue of the phantom. For a 1-cm spacing between phantom and applicator, a net power of 27.3 W must be delivered to the applicator to produce 235 W/kg in the phantom.
With the 2-cm fat layer planar phantom, the values of the net power for direct contact and l-cm spacing are essentially the same, namely 23.3 and 23.2 W, respectively.
The VSWR values associated with the above test conditions of phantom and applicator combinations are listed in Table I for a l-cm spacing between the applicator and the top of the 2-cm fat layer of the phantom, while a minimum value of 1.4 is achieved for the same spacing between the applicator and the 1-cm fat layer phantom.
Surface heating patterns were obtained by placing the applicator directly on top of a planar phantom consisting only of simulated muscle tissue. As in Fig. 2, the applica-1421 tor is positioned on top of the phantom with the apertme length long the x-axis and its width along the y-axis. After the rapid microwave heating is completed, the external surface of the phantom is viewed with a thermographic camera. Fig. 8 shows a surface pattern and a parallel temperature profile. This profile along the x-direction is obtained by setting the white scan line in the region of maximum heating. The parallel profile is quite uniform.
The thermogram of Fig. 9 shows the same heating pattern except that the phantom has been rotated 90°so that a normal profile, which is a profile along they-axis, can be obtained. As in Fig. 8, the temperature profile is also quite uniform.

B. Leakage Measurements
The measurement techniques for leakage were discussed previously [5].  [7]. This design is a viable candidate for use in hyperthermia treatments of cancer. It could immediately be applied to animal [8] and clinical hyperthermia studies of cancer [9] that presently use the slab-loaded design without the choke.

ACKNOWLEDGMENT
The mechanical design and the fabrication of the microwave choke were developed at the Bureau of Radiological Health model shop under the able direction of J. E. Duff.