# Comparative Study of Minimally Invasive Microwave Ablation Applicators
## 3 Results
### Performance of the Homogeneous Model for All Designs
#### 3.1.2 Temperature Distribution of the Homogeneous Model for All Designs  
We examine the temperature distribution in Figure 4 of all the above applicators using the same homogeneous model. Figure 4: Temperature distribution of the different applicators embedded in a homogeneous tissue with \(\varepsilon_{\tau}\) = 46.8, \(\sigma\) = 0.861 at 915 MHz. Monopole antenna (**a**), capacitive capped monopole antenna (**b**), multi-slot coaxial antenna (**c**), dual-slot antenna (**d**), triaxial antenna (**e**), choke dipole antenna (**f**), balun-free base-fed monopole (**g**), floating sleeve dipole antenna (FSD) (**h**), balun-free helical dipole antenna (**i**), and helical open waveguide structure (**j**). The blue, purple, red, and yellow colors indicate temperature contours of 38 \({}^{\circ}\)C, 42 \({}^{\circ}\)C, 46 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C, respectively. We present the temperature contours of 38, 42, 46, and 50 degrees Celsius. We examine especially these contours on the grounds that irreversible changes start at 38 \({}^{\circ}\)C and beyond, the possibility of cell necrosis starts at 42 \({}^{\circ}\)C, the cancer cells are destroyed into few hours at 46 \({}^{\circ}\)C and into few minutes at 50 \({}^{\circ}\)C. The shapes of the temperature equi-level lines follow that of the SAR. The desired spherical or rectangular shapes, assumed more adaptable to tumor ones, are supported again by capacitive capped (b), multi-slot coaxial (c), balun-free (g), and open helical (j) cases. However, only the last one is promising to control-restrict the area above 42 \({}^{\circ}\)C inside the tumor. As we can see from Figure 4, the results of our simulations for the homogeneous model are close to what depicted in the corresponding papers [21; 41; 43; 44; 45; 46; 47; 48]. However, as shown next in Section 3.3.2, the behavior of temperature distribution changes when we are using a more realistic two-compartment model for the tumor. Temperature Distribution of Helical Open Waveguide Structure vs. Monopole for the Two-Compartment Model  
The helical open waveguide structure [41] is discussed here, with simulation results presented to demonstrate its flexibility in producing various ablation zone shapes. Figure 5 presents two designs in which the spirals are arranged in a suboptimal configuration, reducing their effectiveness in containing the electromagnetic field within the tumor. This arrangement results in unintended exposure of surrounding healthy tissue to potentially harmful levels of electromagnetic energy. Figure 5a depicts a helical open waveguide structure consisting of three spirals, while Figure 5b shows a structure with two spirals. Both configurations share a height of 12 mm and a monopole length of 7.61 mm. The adjustment of the number of spirals, in combination with their opening (e.g., different openings for tumors with diameters of 3 cm and 2 cm) and the dimensions of the coaxial cable, play a crucial role in mitigating the issue of backward heating. This issue can be effectively addressed by increasing the number of spirals, as demonstrated in Figure 6. Figure 6 illustrates the ablation zones produced under different configurations. These results demonstrate that by adjusting the power of the generator and altering the monopole length and spiral height, it is possible to produce various ablation zone shapes. In the first two images (a, b), the ablation zone assumes a spherical shape. In images (c, d), simulations present an ellipsoid-shaped ablation zone. In images (e-g), the simulations depict a bell-shaped ablation zone. These simulations were conducted on a large intestine model with a tumor diameter of 3 cm and power source 25 W, with the properties of both the large intestine and tumor taken from [41], as large intestine \(\epsilon_{\rm r}\) = 57, \(\sigma\) = 1.09, and tumor \(\epsilon_{\rm r}\)t = 60, \(\sigma\)t = 0.67.