雙導管中絕緣同軸槽狀天線應用於微波熱療時之輸入阻抗及加熱特性之設計及分析

Abstract

絕緣同軸槽狀天線 (ICSA) 和絕緣偶極天線 (IDA) 在微波熱療上的 應用已經是非常廣泛地了。在本篇論文中，我們推導並延伸King等人[12] 所討論過的在單導管中均勻絕緣偶極天線的波數和特性阻抗模型 (IDA)成 對於絕緣同軸槽狀天線 (ICSA) 和絕緣偶極天線 (IDA)在雙層導管中亦能 使用的模型。與絕緣偶極天線相比較時，同時在絕緣同軸槽狀天線的第一 臂和第二臂間的裂隙的兩端開口，將會多出兩段多餘電流路徑而得以縮短 絕緣同軸槽狀天線的實際長度。除此之外，同軸槽狀天線的輸入阻抗是由 插入深度所決定，不同的插入深度將導致不同的反射係數 (S11)。為了解 決這個阻抗匹配的問題，我們在同軸槽狀天線的饋伺端加入一段四分之一 波長的扼流槽以避免電流的回流。有效並且有系統的最佳化絕緣同軸槽狀 天線設計亦被提出。因此，在相當好的阻抗匹配下，我們即可輕易地獲得 所想要的沿著天線軸的電流分佈和徑向的比吸收率 (SAR)分佈圖。經由這 有效並且有系統的最佳化天線設計之助，我們就可以針對不同種類、體積 的腫瘤控制天線的加熱範圍以避免因為不必要的加熱而造成健康組織的破 壞。氣冷系統[19]亦可被使用於雙層導管中以減少天線表面的溫度並且得 以獲得更好的輻射效率。當然，這亦可減少患者的疼痛和避免因為天線表 面溫度過熱而造成天線的損壞。 Insulated coaxial slot antennas (ICSA) and Insulated dipole antennas (IDA) are widely used in interstitial microwavehyperthermia. In this paper, wavenumber and characteristicimpedance models of insulated dipole antenna (IDA) in a single-layer catheter discussed by King et al. [12] are extended to the cases of IDA and ICSA in a two-layer catheter. Openings at both sides of the feed gap between arm one and arm two of theuniform ICSA are used to obtain two access current paths to shorten the physical length of the ICSA when compared with IDA. Moreover, input impedance of the ICSA depends upon the insertion depth, different insertion depths will lead to different return loss (S11). In order to solve the impedance matching problem, a quarter-wavelength choke is also imposed at the feed of the ICSA to prevent the current from flowing backward to the generator. Besides, an efficient and systematic designoptimization method is proposed for the design of ICSA to obtainthe desired current distribution along the axis of the antenna and thespatial specific absorption rate (SAR) pattern under good impedancematching. By the help of the efficient and systematic designoptimization of ICSA, we are capable of controlling the heating volume to fit into the specific shape and size of each individual tumor, and to prevent from corrupting the healthy tissues due to unnecessaryheating. Moreover, an air cooling system [19] can also be applied inthe two-layer catheter to take away the heat on the surface of the antenna and gain more radiation efficiency for the antenna. Of course, it will also reduce the pain of the patient and prevent fromdamaging the antenna caused by the over heating temperature on the antenna surface.