# Comparative Study of Minimally Invasive Microwave Ablation Applicators  
[PERSON]  
address this issue, modified antennas have been proposed through the incorporation of chokes or sleeves [6; 7; 12; 13; 14; 15; 16; 17; 18]. These modifications improve the radiation pattern and reduce the backward heating effect, thereby minimizing the tail of the comet-shaped ablation zones. Initially, chokes and sleeves were added externally to the antenna, but more recent designs have integrated these components within the antenna structure to reduce invasiveness [6; 7; 19; 20]. Additionally, backward heating has been mitigated by incorporating more slots into the antenna design [6; 7; 21; 22; 23; 24; 25; 26] or by employing cooling systems (water-cooled antennas [27; 28; 29; 30; 31], gas-cooled antennas [32; 33; 34]). Alternative designs include helix and spiral antennas [7; 35; 36; 37; 38; 39; 40; 41] and flexible antennas as well [42]. Finally, in case it is not possible to achieve the desired efficiency when optimizing the structural/electrical parameters of the antenna alone, an impedance matching network can be introduced into the design to further enhance the antenna's performance [6; 35]. Another solution to achieve impedance matching and improve the antenna's performance is a balun (balanced-to-unbalanced transformer). This is a critical component in antenna systems to transition between balanced and unbalanced circuits while preserving the desired signal characteristics. Balanced systems, such as dipole antennas, have two symmetrical conductors with equal and opposite currents, while unbalanced systems, such as coaxial cables, consist of a single conductor with a return path through a ground plane. Without a balun, the unbalanced nature of the feed line can induce undesired currents on the outer surface of the coaxial cable, leading to distortions in the radiation pattern, increased reflections, and unwanted heating along the feed line. Various balun designs have been developed to address these issues, including \(\lambda/4\) sleeves, choke baluns, and tapered-slot configurations. These designs introduce high-impedance points or suppress surface currents, ensuring effective impedance matching and minimizing interference. The choice of balun modification depends on the specific application, bandwidth requirements, and physical constraints, making it a versatile solution for achieving efficient and reliable antenna performance in diverse scenarios. Therefore, the choice of applicator plays a crucial role in achieving optimal therapeutic outcomes. Factors such as the geometry of the applicator, operating frequency and cooling mechanisms can significantly affect the precision of energy delivery and the extent of the ablation zone. The designed applicators include the monopole (initial design) [43], capacitive capped monopole antenna [44], dual-slot antenna [45], multi-slot antenna [21], floating sleeve dipole (FSD) antenna [46], choke dipole antenna [47], triaxial antenna [47], balun-free base-fed monopole (balun-free helical antenna) [47], balun-free helical dipole antenna [48], and a helical open waveguide structure (featuring a helix with an increasing radius starting at the end of the outer conductor of the coaxial cable) [41]. The selection of these designs is deliberate. The basic monopole design was initially implemented to observe the comet-shaped ablation zone, followed by the dual-slot antenna, a modified design intended to address the backward heating issue. The triaxial and choke dipole antennas, along with the FSD dipole antenna, were incorporated because they are utilized in commercial MWA systems approved by the U.S. Food and Drug Administration (FDA) [47]. According to [21; 44], multi-slot antenna and capacitive capped monopole antenna provide more spherical ablation zones than others. Moreover, the FSD antenna was implemented on the grounds that it provides similar results to the balun-free helical antenna. Finally, all helical designs were included to compare them with our recent work [41].