Size reduction and radiation pattern shaping of multi-fed DCC slot antennas used in conformal microwave array hyperthermia applicators

Abstract

The use of conformal antenna array in the treatment of superficial diseases can significantly increase patient comfort while enhancing the local control of large treatment area with irregular shapes. Originally a regular square multi-fed slot antenna (Dual Concentric Conductor - DCC) was proposed as basic unit cell of the array. The square DCC works well when the outline of the treatment area is rectangular such as in the main chest or back area but is not suitable to outline diseases spreading along the armpit and neck area. In addition as the area of the patch increases, the overall power density decreases affecting the efficiency and thus the ability to deliver the necessary thermal dose with medium power amplifier (<50W). A large number of small efficient antennas is preferable as the disease is more accurately contoured and the lower power requirement for the amplifiers correlates with system reliability, durability, linearity and overall reduced cost. For such reason we developed a set of design rules for multi-fed slot antennas with irregular contours and we implemented a design that reduce the area while increasing the perimeter of the slot, thus increasing the antenna efficiency and the power density. The simulation performed with several commercial packages (Ansoft HFSS, Imst Empire, SemcadX and CST Microwave Studio) show that the size reducing method can be applied to several shapes and for different frequencies. The SAR measurements of several DCCs are performed using an in-house high resolution scanning system with tumor equivalent liquid phantom both at 915 MHz for superficial hyperthermia systems in US) and 433 MHz (Europe). The experimental results are compared with the expected theoretical predictions and both simulated and measured patterns of single antennas of various size and shapes are then summed in various combinations using Matlab to show possible treatment irregular contours of complex diseases. The local control is expected to significantly improve while maintaining the patient comfort.

Conformal microwave array (CMA) applicators for hyperthermia of diffuse chest wall recurrence

“…Several multiple element array applicators have been reported in the literature, but have not been characterized for heating properties over contoured anatomy. For this effort, electromagnetic (EM) and thermal simulations were performed using commercial software and 3D simulation approaches that have been described previously for EM (63, 64, 66) and thermal (34, 35, 68, 73) modeling of planar applicators. EM simulations were performed with HFSS (Ansoft Corp, Pittsburg PA) for a 42×27cm L-shape CMA to assess the ability to deliver localized heating to diffuse chestwall disease that extends from the sternum over a mastectomy scar and around the side to the back.…”

“…Shown in Figure 4c is the steady state temperature distribution in one cross sectional plane of the heating array for a 42°C waterbolus. Blood perfusion rates of 2 and 1 kg/m 3 /s were assumed inside the target and surrounding muscle tissue for the tissue properties reported in Maccarini et al (66). The temperature distribution in Figure 4c shows thermal elevation into the hyperthermic temperature range (40–44°C) everywhere within the 15 mm deep target tissue under the perimeter of the active heating elements.…”

“…(63–65) demonstrated the ability to produce similar peripherally enhanced heating patterns that extend out to the perimeter of triangular or other polygon shapes as well as square apertures. Furthermore, the microstrip feedlines that distribute power from RF connectors on one edge of the PCB were optimized to include tuning stubs at each DCC aperture feedpoint and the original open microstrip design was replaced with coplanar waveguide buried between two ground planes to reduce losses in the PCB structure and eliminate radiation into air behind the antenna array (66). These enhancements improved matching and power efficiency of the PCB slot apertures without changing the SAR distribution in tissue.…”

Vesicoureteral Reflux in Children: A Phantom Study of Microwave Heating and Radiometric Thermometry of Pediatric Bladder

“…The current DCC antenna design is based on experience in hyperthermia of recurrent breast cancer where mostly muscle tissue is expected, and where lower reflection coefficients have been observed [23]. The introduction of a fat layer in our VUR application does increase the resonance frequency of this DCC antenna, but an optimized antenna can clearly be obtained through small modifications of this design.…”

“…The DCC antenna consists of a front surface copper foil sheet with a central 25 mm square patch surrounded by a 2.5 mm gap to the surrounding ground plane while the back surface consists of microstrip feed lines connecting to the center of each side of the square patch through four vias [21]–[23]. This particular design includes a 180° phase delay in two adjoining sides of the square, resulting in an antenna with better penetration depth compared to the standard DCC due to phase addition of electric fields centrally from opposing sides of the slot radiator.…”