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A review of antenna designs for percutaneous microwave ablation

Published:March 24, 2021DOI:https://doi.org/10.1016/j.ejmp.2021.03.010

      Highlights

      • We review the state-of-the-art of antenna designs in percutaneous MWA.
      • We propose the three design criteria for percutaneous MW antennas.
      • We review the current percutaneous MW antennas regarding the three design criteria.
      • We evaluate the current solutions for achieving the three design criteria.

      Abstract

      Microwave (MW) antenna is a key element in microwave ablation (MWA) treatments as the means that energy is delivered in a focused manner to the tumor and its surrounding area. The energy delivered results in a rise in temperature to a lethal level, resulting in cell death in the ablation zone. The delivery of energy and hence the success of MWA is closely dependent on the structure of the antennas. Therefore, three design criteria, such as expected ablation zone pattern, efficiency of energy delivery, and minimization of the diameter of the antennas have been the focus along the evolution of the MW antenna. To further improve the performance of MWA in the treatment of various tumors through inventing novel antennas, this article reviews the state-of-the-art and summarizes the development of MW antenna designs regarding the three design criteria.

      Keywords

      Introduction

      Microwave ablation (MWA) is one of the energy-based minimally invasive modalities in the treatment of various tumors, such as liver tumors [
      • Izzo F.
      • Granata V.
      • Grassi R.
      • et al.
      Radiofrequency Ablation and Microwave Ablation in Liver Tumors: An Update.
      ], lung tumors [
      • Prud'homme C.
      • Deschamps F.
      • Moulin B.
      • et al.
      Image-guided lung metastasis ablation: a literature review.
      ], renal tumors [
      • Anglickis M.
      • Anglickiene G.
      • Andreikaite G.
      • et al.
      Microwave thermal ablation versus open partial nephrectomy for the treatment of small renal tumors in patients over 70 years old.
      ], and bone tumors [
      • Fan Q.Y.
      • Zhou Y.
      • Zhang M.H.
      • et al.
      Microwave Ablation of Primary Malignant Pelvic Bone Tumors.
      ]. In general, tumor ablation in MWA is achieved with one or more MW antennas inserted into the targeted tissue percutaneously under the guidance of an image-guided device [
      • Lu A.
      • Woodrum D.A.
      • Felmlee J.P.
      • et al.
      Improved MR-thermometry during hepatic microwave ablation by correcting for intermittent electromagnetic interference artifacts.
      ,
      • Gorny K.R.
      • Favazza C.P.
      • Lu A.
      • et al.
      Practical implementation of robust MR-thermometry during clinical MR-guided microwave ablations in the liver at 1.5 T.
      ,
      • Paul J.
      • Vogl T.J.
      • Chacko A.
      Dual energy computed tomography thermometry during hepatic microwave ablation in an ex-vivo porcine model.
      ,
      • Hui T.C.H.
      • Brace C.L.
      • Hinshaw J.L.
      • et al.
      Microwave ablation of the liver in a live porcine model: the impact of power, time and total energy on ablation zone size and shape.
      ,
      • Head H.W.
      • Dodd G.D.
      Thermal ablation for hepatocellular carcinoma.
      ,
      • Deshazer G.
      • Prakash P.
      • Merck D.
      • et al.
      Experimental measurement of microwave ablation heating pattern and comparison to computer simulations.
      ]. High-frequency electromagnetic energy (commercially available at 915 MHz or 2.45 GHz) generated by a microwave power generator is transferred through a coaxial cable as an antenna into surrounding tissues to cause a rise in temperature to 60 °C or above, which leads to protein denaturation and coagulative necrosis [
      • Simon C.J.
      • Dupuy D.E.
      • Mayo-Smith W.W.
      Microwave ablation: principles and applications.
      ,
      • Ahmed M.
      • Brace C.L.
      • Lee F.T.
      • et al.
      Principles of and advances in percutaneous ablation.
      ,
      • Gao X.
      • Tian Z.
      • Cheng Y.Y.
      • et al.
      Experimental and numerical study of microwave ablation on ex-vivo porcine lung.
      ,
      • Lopresto V.
      • Pinto R.
      • Lodato R.
      • et al.
      Design and realisation of tissue-equivalent dielectric simulators for dosimetric studies on microwave antennas for interstitial ablation.
      ,
      • Chu K.F.
      • Dupuy D.E.
      Thermal ablation of tumours: biological mechanisms and advances in therapy.
      ,
      • Kim C.
      Understanding the nuances of microwave ablation for more accurate post-treatment assessment.
      ,
      • Tafti B.A.
      • Genshaft S.
      • Suh R.
      • et al.
      Lung ablation: indications and techniques.
      ]. It is widely accepted that the heat energy is due to the agitation of polar molecules (i.e., water molecules) in the tissue caused by the high-frequency electromagnetic energy [
      • Kok H.P.
      • Cressman E.N.K.
      • Ceelen W.
      • et al.
      Heating technology for malignant tumors: a review.
      ,

      Ashour AS, Asran M, Fotiadis DI, Optimal power for microwave slotted probes in ablating different hepatocellular carcinoma sizes. Comput Biol Med. 127 2020 104101 104101.

      ,
      • Lubner M.G.
      • Brace C.L.
      • Ziemlewicz T.J.
      • et al.
      Microwave ablation of hepatic malignancy.
      ].
      The design of a MW antenna plays a critical role in achieving an efficient MW power transmission and hence a successful tumor ablation. At present, the most widely used antenna is based on the semi-rigid coaxial cable, which generally consists of the inner conductor, dielectric, and outer conductor, as shown in Fig. 1. Such a design seems fitting for MWA due to its cylindrical geometry and small diameter.
      Figure thumbnail gr1
      Fig. 1The axial view and transverse view of the semi-rigid coaxial cable (grey: dielectric, black: conductor).
      In the MWA, an ideal antenna would be the one that can achieve the expected ablation zone pattern (size and shape) with a high efficiency in energy transmission and low invasiveness to patients. Therefore, the expected ablation zone pattern, high efficiency, and low invasiveness are three design criteria that are used to evaluate the design of antennas.
      Obtaining an expected ablation zone pattern means the successful ablation of tumor with the minimal damage to surrounding normal tissues. There exist both omni-directional and directional radiation pattern approaches for delivery of the microwave energy in literature. As for omni-directional antennas, energy can be transferred into tissues in all directions around the antenna, which was usually used to eradicate tumors in a regular shape (e.g., spherical or elliptical) [
      • Gao H.J.
      • Wang X.R.
      • Wu S.C.
      • et al.
      Conformal coverage of liver tumors by the thermal coagulation zone in 2450-MHz microwave ablation.
      ,
      • Tehrani M.H.H.
      • Soltani M.
      • Kashkooli F.M.
      • et al.
      Use of microwave ablation for thermal treatment of solid tumors with different shapes and sizes-A computational approach.
      ,
      • Shamekhi L.
      • Sayehvand H.-O.
      • Karami H.
      Tumour shape-dependent microwave hyperthermia using a novel coaxial micro-cut slot antenna.
      ]. On the other hand, a directional antenna that affords spatial control of the energy deposition may be a better choice when tumors are in proximity to critical structures, such as blood vessels or bile ducts.
      The transmission efficiency of the antenna represents how much energy is transferred into tissues compared with the total energy. A high efficiency is preferred in antenna design, as this means less energy loss (i.e., energy reflected to the power generator) inside the antenna. This energy reflection is usually caused by the impedance mismatching. It can increase the heating of the antenna itself and the tissues in very close proximity, including the proximally up the shaft which can lead to injury of healthy tissues.
      Furthermore, with the concept of minimally invasive ablation, an antenna with a small size is more desirable when considering the comfort and safety of patients during MWA procedures.
      To date, a plethora of research efforts have been directed towards the design of MW antennas for achieving the three above-mentioned objectives (expected ablation zone pattern, high efficiency, and low invasiveness). The dipole [
      • Hurter W.
      • Reinbold F.
      • Lorenz W.J.
      A dipole antenna for interstitial microwave hyperthermia.
      ], monopole [
      • Labonte S.
      • Ali H.O.
      • Roy L.
      Monopoles for microwave catheter ablation of heart tissue.
      ], and slot antennas [
      • Wu M.S.
      • Ito K.
      • Kasai H.
      Analysis of current and electric field distributions of coaxial-slot antenna for interstitial microwave hyperthermia.
      ] are three basic designs. However, the ablation zone patterns of those antennas are negatively affected by the backward heating along the shaft of the antennas caused by leaking currents because of the unbalanced antenna structure [
      • Mohtashami Y.
      • Luyen H.
      • Hagness S.C.
      • et al.
      Non-coaxial-based microwave ablation antennas for creating symmetric and asymmetric coagulation zones.
      ,

      Acikgoz H, Mittra R, Ieee. Microwave Coaxial Antenna for Cancer Treatment: Reducing the Backward Heating Using a Double Choke. ISAP. 2015:1-4.

      ,
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ]. As a result, “comet-shaped” ablation zones are usually generated in MWA, which is obviously not an optimal ablation zone pattern for sphere- or ellipsoid-like tumors. The tail of the comet inevitably damages the normal tissues along the shaft of MW antenna. Therefore, modified designs have been proposed to improve radiation properties of antennas by adding a choke or sleeve outside the antennas [
      • Icheln C.
      • Krogerus J.
      • Vainikainen P.
      Use of balun chokes in small-antenna radiation measurements.
      ,
      • Yang D.S.
      • Bertram J.M.
      • Converse M.C.
      • et al.
      A floating sleeve antenna yields localized hepatic microwave ablation.
      ,
      • Maini S.
      Design optimization of tapered cap floating sleeve antenna for interstitial microwave ablation for liver tumor.
      ,
      • Hassan E.
      • Takruri-Rizk H.
      • Hope M.
      Applicator design considerations of microwave tumor ablation.
      ,
      • Neagu V.
      A study of microwave ablation antenna optimization.
      ], which can effectively suppress the reverse heating. Unfortunately, this is at the expense of an increase in the diameter of the antenna. To keep the antenna as small as possible, some researchers have proposed to modify the inner structure of antennas such that the sleeves or chokes are implemented inside the antenna [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ,
      • Wakaki T.
      • Michiyama T.
      • Kuwano S.
      A new interstitial choke embedded antenna for microwave ablation.
      ,
      • Maini S.
      • Shekhawat D.S.
      Analysis of copper tube sleeve coaxial spiral antenna for interstitial hepatic microwave ablation.
      ]. Further studies showed that the backward heating can be also restrained by adding antenna slots or a cooling system, optimizing the outer conductor structure, or changing the frequency of electromagnetic energy [
      • Hung L.Y.
      • Hagness S.C.
      • Behdad N.
      A minimally invasive coax-fed microwave ablation antenna with a tapered balun.
      ,
      • Wang T.
      • Zhao G.
      • Qiu B.S.
      Theoretical evaluation of the treatment effectiveness of a novel coaxial multi-slot antenna for conformal microwave ablation of tumors.
      ,
      • Fallahi H.
      • Clausing D.
      • Shahzad A.
      • et al.
      Microwave antennas for thermal ablation of benign adrenal adenomas.
      ,
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]. The directional ablation zone is another practical ablation zone pattern for MWA in clinical practice. Compared with an antenna that can generate an omni-directional ablation zone pattern, a directional antenna can simplify the treatment and offer protection to normal tissues when tumors are adjacent to vulnerable biological structures [
      • Alnassan H.
      • Kastler A.
      • Wang X.
      • et al.
      Modified dipole antenna for directional microwave ablation using 3D numerical simulation.
      ,
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ]. When the efficiency of the antenna is not satisfactory by just optimizing the antenna structural parameters, an impedance matching network can be implemented in the design of antennas as a means to further improve the efficacy [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ,
      • Xu Y.S.
      • Moser M.A.J.
      • Zhang E.
      • et al.
      Large and round ablation zones with microwave ablation: A preliminary study of an optimal aperiodic tri-slot coaxial antenna with the pi-matching network section.
      ,
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      ].
      There have been several published reviews of MWA, such as [
      • Simon C.J.
      • Dupuy D.E.
      • Mayo-Smith W.W.
      Microwave ablation: principles and applications.
      ,
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ,
      • Brace C.L.
      Microwave tissue ablation: biophysics, technology, and applications.
      ,
      • Bertram J.M.
      • Yang D.
      • Converse M.C.
      • et al.
      A review of coaxial-based interstitial antennas for hepatic microwave ablation.
      ,
      • Tammam E.
      • Said A.M.
      • Ibrahim A.A.
      • et al.
      About the interstitial microwave cancer ablation: principles, advantages and challenges.
      ]. However, considerable progress has been made in MWA in last few years, especially with respect to the design of antennas, which is the primary focus of this paper. It is noteworthy that Fallahi et al. [
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ] also reviewed the antenna designs in MWA, focusing on various antenna designs for site-specific thermal ablation of tissues. However, some new antenna designs reported in recent years (e.g., antennas with double chokes, ten-slot antennas, and tri-slot antennas) were not mentioned in their review. The methods to address the issues in the antenna design were not discussed thoroughly in literature. Therefore, this review paper updates the state-of-the-art of antenna designs in MWA and discusses the problems and solutions in the antenna designs, specifically in solving the three above-mentioned issues of MWA. It should be noted that results from clinical trials were not included in this review. If one is looking for information about the clinical performance and trials related to MWA, several detailed reports, for example [
      • Favi E.
      • Raiteri M.
      • Paone G.
      • et al.
      Microwave ablation of renal cell carcinoma of the transplanted kidney: two cases.
      ,
      • Yamada Y.
      • Shiaishi T.
      • Ueno A.
      • et al.
      Phase I study of cancer lesion-targeted microwave coagulation therapy for localized prostate cancer: A pilot clinical study protocol.
      ,
      • Zhang T.Q.
      • Huang S.M.
      • Gu Y.K.
      • et al.
      Sequential and simultaneous 4-Antenna microwave ablation in an ex vivo bovine liver model.
      ,
      • Dimitri M.
      • Staderini F.
      • Brancadoro M.
      • et al.
      A new microwave applicator for laparoscopic and robotic liver resection.
      ,
      • Iezzi R.
      • Cioni R.
      • Basile D.
      • et al.
      Standardizing percutaneous Microwave Ablation in the treatment of Lung Tumors: a prospective multicenter trial (MALT study).
      ,
      • Scapaticci R.
      • Lopresto V.
      • Pinto R.
      • et al.
      Monitoring thermal ablation via microwave tomography: an ex vivo experimental assessment.
      ,
      • Lubner M.G.
      • Brace C.L.
      • Hinshaw J.L.
      • et al.
      Microwave tumor ablation: mechanism of action, clinical results, and devices.
      ,
      • Zhou W.
      • Chen Y.D.
      • Zhang L.
      • et al.
      Percutaneous microwave ablation of metastatic lymph nodes from papillary thyroid carcinoma: preliminary results.
      ,
      • Hu Y.R.
      • Yang Z.W.
      • Sun Y.S.
      Analysis of the feasibility of microwave ablation for colorectal liver metastases: a preliminary report.
      ], are available.

      Principle of MWA

      MWA is a physical intervention in which polar molecules (e.g., water) are forced to oscillate under an externally applied high frequency electromagnetic field to generate dielectric heating. This heat energy, in turn, raises the temperature of the tissues to a lethal level to cause the coagulative necrosis of cells. Therefore, MWA is a coupled problem that includes the electromagnetic wave conduction and heat transfer in biological tissues.
      The propagation and absorption of electromagnetic wave in tissue can be described by the following Maxwell’s equations [
      • Chiang J.
      • Wang P.
      • Brace C.L.
      Computational modelling of microwave tumour ablations.
      ,
      • Maini S.
      FEM simulation of tapered cap floating sleeve antenna for hepatocellular carcinoma therapy.
      ,
      • Singh S.
      • Melnik R.
      Thermal ablation of biological tissues in disease treatment: A review of computational models and future directions.
      ]:
      ×E=-Bt×H=J+DtB=0D=ρfree
      (1)


      where E (V/m) is the electric field, B (Wb/m2) is the magnetic flux density, H (A/m) is the magnetic field, J (A/m2) is the current density, D (C/m2) is the electric flux density, and ρfree (C/m2) is the free charge density,
      The transmission of electromagnetic energy is determined by the magnetic permeability and dielectric permittivity of the surrounding media. Because the magnetic permeability of biological tissues is approximately equal, the propagation of electromagnetic energy is mainly determined by the dielectric permittivity of tissues. The dielectric permittivity of various tissues is different and contains the real and imaginary components, where the real part represents the ability to store electrical energy relative to vacuum, and the imaginary part represents the loss of microwave energy in the medium.
      The complex relative permittivity of the tissue (εr) is often defined as [
      • Prakash P.
      Theoretical modeling for hepatic microwave ablation.
      ,
      • Lee M.
      • Son T.
      Helical slot antenna for the microwave ablation.
      ]:
      εr=εr-jσcωε0
      (2)


      where εr is the relative permittivity of biologic tissue, σc(S/m) is the electrical conductivity, ω (rad/s) is the angular frequency and ε0 is the permittivity of vacuum. It has been found that the relative permittivity and electrical conductivity of biological tissues are dependent on frequency, temperature, and water content [
      • Ji Z.
      • Brace C.L.
      Expanded modeling of temperature-dependent dielectric properties for microwave thermal ablation.
      ,

      Brace CL, Ieee. Temperature-dependent dielectric properties of liver tissue measured during thermal ablation: Toward an improved numerical model. In: Conf Proc IEEE Eng Med Biol Soc.2008:230–3.

      ,
      • Manago R.
      • Saito K.
      Coagulated region analysis in a microwave surgical device with temperature-dependent physical properties of tissue.
      ]. Therefore, taking these factors into consideration is of critical importance to develop an accurate computer model for MWA.
      The physics of heat transfer in biological tissues can be described by the Pennes’ bio-heat transfer equation, as follows [
      • Pennes H.H.
      Analysis of tissue and arterial blood temperatures in the resting human forearm.
      ]:
      ρCTt=kT-Qb+Qm+Qe
      (3)


      where ρ (kg/m3) is the tissue density, C (J/(kg·K)) is the tissue specific heat, T (K) is the tissue temperature, k (W/(mK)) is the thermal conductivity that is also a temperature-dependent variable [
      • Valvano J.W.
      • Cochran J.R.
      • Diller K.R.
      Thermal conductivity and diffusivity of biomaterials measured with self-heated thermistors.
      ,
      • Sturesson C.
      • Andersson-Engels S.
      A mathematical model for predicting the temperature distribution in laser-induced hyperthermia. Experimental evaluation and applications.
      ], Qb (W/m3) denotes heat loss by blood perfusion, Qm (W/m3) is the metabolic heat generation, and Qe (W/m3) is the external heat source by MWA defined as [
      • Selmi M.
      • Bin Dukhyil A.A.
      • Belmabrouk H.
      Numerical analysis of human cancer therapy using microwave ablation.
      ,
      • Sinha A.
      • Misra J.C.
      Numerical study of flow and heat transfer during oscillatory blood flow in diseased arteries in presence of magnetic fields.
      ]:
      Qe=12σcE2
      (4)


      where E (V/m) is the magnitude of the electrical field.
      There are two widely accepted models to predict the ablation zone, namely the temperature threshold model and the Arrhenius damage model [
      • Prakash P.
      Theoretical modeling for hepatic microwave ablation.
      ,
      • Zhang B.
      • Moser M.A.J.
      • Zhang E.M.
      • et al.
      A review of radiofrequency ablation: Large target tissue necrosis and mathematical modelling.
      ]. The temperature threshold model is the commonly-used method for predicting tissue death in MWA, which is based on the mechanisms of instantaneous cell death due to coagulation necrosis. However, there is no general consensus on the temperature to be used. The temperature thresholds in the range of 50–60 °C have been used as indicators of treatment endpoint in different literature [
      • Prakash P.
      • Diederich C.J.
      Considerations for theoretical modelling of thermal ablation with catheter-based ultrasonic sources: Implications for treatment planning, monitoring and control.
      ], and the ablation time also needs to be considered while the threshold at a relative low temperature is used, like 50–54 °C. Due to the death of biological tissues in MWA is related to not only the treatment temperature and ablation time but also the tissue type, the Arrhenius damage model has also been used in the computer simulation [
      • Pearce J.A.
      Models for thermal damage in tissues: processes and applications.
      ,
      • Pearce J.
      Relationship between Arrhenius models of thermal damage and the CEM 43 thermal dose.
      ]. It is noteworthy that a shrinkage up to about 30% can occur in the ablation zone of liver tissues during MWA owing to the high temperature induced [
      • Farina L.
      • Weiss N.
      • Nissenbaum Y.
      • et al.
      Characterisation of tissue shrinkage during microwave thermal ablation.
      ,
      • Brace C.L.
      • Diaz T.A.
      • Hinshaw J.L.
      • et al.
      Tissue contraction caused by radiofrequency and microwave ablation: a laboratory study in liver and lung.
      ,
      • Rossmann C.
      • Garrett-Mayer E.
      • Rattay F.
      • et al.
      Dynamics of tissue shrinkage during ablative temperature exposures.
      ]. This phenomenon can affect the accurate estimation of the ablation zone, leading to failure in achieving the expected ablation zone patterns. An ex vivo liver tissue shrinkage predictive model has been proposed and validated in literature [
      • Amabile C.
      • Farina L.
      • Lopresto V.
      • et al.
      Tissue shrinkage in microwave ablation of liver: an ex vivo predictive model.
      ,
      • Farina L.
      • Nissenbaum Y.
      • Goldberg S.N.
      • et al.
      Tissue shrinkage in microwave ablation: ex vivo predictive model validation.
      ]. Therefore, taking the tissue contraction during MWA treatment into account is also critical for the accuracy of numerical simulations in the prediction of ablation zone.

      MW antenna properties

      To describe the magnitude of the wavelength that propagates through the tissue, the effective wavelength (λeff) is introduced, which is defined as [
      • Ortega-Palacios R.
      • Trujillo-Romero C.J.
      • Cepeda-Rubio M.F.J.
      • et al.
      Heat transfer study in breast tumor phantom during microwave ablation: modeling and experimental results for three different antennas.
      ,
      • Guerrero Lopez G.D.
      • Jesus Cepeda Rubio M.F.
      • Hernandez Jacquez J.I.
      • et al.
      Computational FEM model, phantom and ex vivo swine breast validation of an optimized double-slot microcoaxial antenna designed for minimally invasive breast tumor ablation: theoretical and experimental comparison of temperature, size of lesion, and SWR, preliminary data.
      ]:
      λeff=cfεr
      (5)


      where c (m/s) is the speed of light in free space, f (Hz) is the operating frequency.
      There are two critical factors commonly used to describe the performance of the designed MW antennas during MWA procedures: the reflection coefficient (S11) and the specific absorption rate (SAR) [
      • Bertram J.M.
      • Yang D.
      • Converse M.C.
      • et al.
      A review of coaxial-based interstitial antennas for hepatic microwave ablation.
      ]. S11 represents how much the electromagnetic power is reflected to the power source compared with the total input power. In general, this factor is used to evaluate the power transmission efficiency of an antenna, which can be given as [
      • Gas P.
      • Szymanik B.
      Shape optimization of the multi-slot coaxial antenna for local hepatic heating during microwave ablation.
      ,
      • Jiang Y.X.
      • Zhao J.Z.
      • Li W.T.
      • et al.
      A coaxial slot antenna with frequency of 433 MHz for microwave ablation therapies: design, simulation, and experimental research.
      ,
      • Gas P.
      Optimization of multi-slot coaxial antennas for microwave thermotherapy based on the S11-parameter analysis.
      ]:
      S11=20log10Γ=10log10PrPin
      (6)


      where Γ is a complex-valued reflection coefficient, which can be expressed in the logarithmic scale by the S11-parameter (dB), Pr (W) and Pin (W) indicate the reflected power and the input power, respectively.
      SAR represents the amount of electromagnetic energy absorbed by per unit mass of tissues. It can be defined as [
      • Keangin P.
      • Rattanadecho P.
      • Wessapan T.
      An analysis of heat transfer in liver tissue during microwave ablation using single and double slot antenna.
      ,
      • Bertram J.M.
      • Yang D.S.
      • Converse M.C.
      • et al.
      Antenna design for microwave hepatic ablation using an axisymmetric electromagnetic model.
      ,
      • Huang C.-F.
      • Li C.-F.
      • Chao H.-Y.
      The study of SAR distribution in microwave ablation for tumours.
      ,
      • Chen D.D.
      • Du Y.X.
      • Chen Z.B.
      • et al.
      Computer modeling and in vitro experimental study of water-cooled microwave ablation array.
      ,
      • Cavagnaro M.
      • Pinto R.
      • Lopresto V.
      Numerical models to evaluate the temperature increase induced by ex vivo microwave thermal ablation.
      ,

      Icnirp. Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz). Health Phys. 2020;118(5):483–524.

      ]:
      SAR=σc2ρE2
      (7)


      Generally, a highly localized SAR pattern around the tip of the antenna is always required in the procedure of tumor ablation. Because the electromagnetic energy turns into thermal energy that heats the tissues to a lethal level of temperature, SAR pattern can also be used as an index of tumor ablation performance of the designed MW antenna.
      To sum up, combining the MWA principles and antenna properties, we can computationally model and simulate the MWA procedure to shorten the amount of time and reduce the cost involved in the development cycle of a novel antenna. The novel antenna prototype can then be manufactured for the ex vivo and/or in vivo experiment validation, facilitating the translation of a newly-designed antenna into clinical practice.

      Three basic MW antenna types

      As shown in Fig. 2, dipole, monopole, and slot antennas are three basic coaxial-based ones, from which most of the commercially available MW antennas were developed [
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ].
      Figure thumbnail gr2
      Fig. 2Three basic MW antenna designs: (A) dipole antenna (reproduced from
      [
      • Hurter W.
      • Reinbold F.
      • Lorenz W.J.
      A dipole antenna for interstitial microwave hyperthermia.
      ]
      ), (B) experimental ablation zone pattern of the dipole antenna in ex vivo porcine muscle, (black arrow indicates the comet-tail ablation pattern) (adapted from
      [
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ]
      with the permission of Begall House). (C) Two common types of monopole antennas (reproduced from [
      • Bertram J.M.
      • Yang D.
      • Converse M.C.
      • et al.
      A review of coaxial-based interstitial antennas for hepatic microwave ablation.
      ,
      • Mohtashami Y.
      • Hagness S.C.
      • Behdad N.
      A hybrid slot/monopole antenna with directional heating patterns for microwave ablation.
      ]), and (D) slot antenna (reproduced from
      [
      • Fallahi H.
      • Prakash P.
      Antenna designs for microwave tissue ablation.
      ]
      ).
      The dipole antenna (Fig. 2A) was proposed by adding a metal block at the distal end of the coaxial cable and electrically connected to the inner conductor. The infinitesimal gap between the coaxial cable and the metal block is the location where the radiation intensity is strongest, acting as the effective source for propagating electromagnetic waves [
      • Bertram J.M.
      • Yang D.
      • Converse M.C.
      • et al.
      A review of coaxial-based interstitial antennas for hepatic microwave ablation.
      ,
      • Hurter W.
      • Reinbold F.
      A dipole antenna for interstitial microwave hyperthermia.
      ]. One arm of the dipole is the metal block in a short axial length, and the other is the outer conductor of the coaxial cable in a relatively long axial length. This unbalanced feature easily makes this antenna with the so-called ‘leaking current’, which travels from the gap to the MW power generator along the outer surface of the outer conductor. The tissues along the shaft of the antenna can be thermally damaged by the leaking current, which has been called ‘the backward heating’ problem [
      • Etoz S.
      • Brace C.L.
      Analysis of microwave ablation antenna optimization techniques.
      ]. As a result, the ablation zone pattern can be affected by the backward heating, leading to the undesired damage to surrounding normal tissues with a ‘comet-tail’ shape of ablation zone pattern, as shown in Fig. 2B. At the same time, the thermal damage area is dependent on the insertion depth of the antenna with the existence of the backward heating.
      Two conventional types of monopole antenna are shown in Fig. 2C. The dielectric wrapped tip monopole antenna (DWT) is created by extending the inner conductor and dielectric of semi-rigid coaxial cable outwards while keeping the length of the outer conductor unchanged. DWT becomes the so-called air wrapped tip (AWT) monopole antenna when only the inner conductor is extended all the way to the tip. In addition, the inner conductor of the AWT can be modified for other purposes. For instance, a helical structure of the inner conductor allows for a shorter antenna [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]. This is because the phase velocity of the electromagnetic wave propagating down the axis of a helical structure occurs more slowly than that along a straight wire. As a result, the wavelength of waveguide becomes shorter and the size of the antenna is greatly reduced.
      On the other hand, the helical structure acts as an inductor, presenting an inductive impedance during the transmission of MW energy, which can help to neutralize the inherent capacitive reactance of the antenna itself. Mazen et al. [
      • Yassin M.M.
      • Tammam E.
      • Ibrahim A.A.
      • et al.
      Dielectric-loaded 5.8 GHz interstitial monopole antenna for spherically-shaped hepatic tumors ablation.
      ] demonstrated that the transmission efficiency of the DWT was better than that of the AWT, because compared to air used in an AWT, the dielectric material, with its higher relative permittivity, can improve the impedance between the antenna and the tissues, which in turn improves the matching between the antenna and the generator [
      • Yassin M.M.
      • Tammam E.
      • Ibrahim A.A.
      • et al.
      Dielectric-loaded 5.8 GHz interstitial monopole antenna for spherically-shaped hepatic tumors ablation.
      ]. Some simulation results showed that both designs can achieve good energy deposition at the active tip of the antennas [
      • Labonte S.
      • Ali H.O.
      • Roy L.
      Monopoles for microwave catheter ablation of heart tissue.
      ,
      • Mohtashami Y.
      • Hagness S.C.
      • Behdad N.
      A hybrid slot/monopole antenna with directional heating patterns for microwave ablation.
      ]. However, they all have the backward heating problem as the dipole antenna does.
      The slot antenna (Fig. 2D) is another basic MW antenna, which is characterized by a soldering connection between the inner and outer conductors at the end of the semi-rigid coaxial cable. A slot far smaller (less than one tenth) than the effective wavelength is cut through the outer conductor near the soldering point to allow electromagnetic wave propagation into the tissues. The slot antenna is usually wrapped with a catheter for protection during the application, but the research has shown that increasing the catheter thickness could reduce the power localization [
      • Hamada L.
      • Wu M.-S.
      • Ito K.
      • et al.
      Basic analysis on SAR distribution of coaxial-slot antenna array for interstitial microwave hyperthermia.
      ]. It is worth mentioning that this slot antenna also has the backward heating problem, like the dipole and monopole antennas [
      • Pongpakpien S.
      • Preechaphonkul W.
      • Rattanadecho P.
      Effects of thermal and electrical properties on porous liver during microwave ablation using microwave coaxial slot antenna.
      ].
      In conclusion, though these three basic antennas can make the electromagnetic energy more localized around the active tip of the antennas and heat up target tissues to a lethal level, they all have the backward heating problem caused by the leaking currents. This backward heating can cause thermal damage to surrounding normal tissues along the axial direction of the antenna, leading to less focused ablation zone patterns.

      Designs to overcome the backward heating

      Various designs have been proposed to date to overcome the backward heating issue of MW antennas in literature. Those designs can mainly be classified into two categories: 1) the optimization of the physical structure and 2) the optimization of the operating frequency of MW antennas. Five state-of-the-art designs in current literature were systematically reviewed on the mechanisms and performances on overcoming the problem of backward heating in this paper.

      Addition of the sleeve, choke, or cap

      As shown in Fig. 3A, Nevels et al. [
      • Nevels R.D.
      • Arndt G.D.
      • Raffoul G.W.
      • et al.
      Microwave catheter design.
      ] found that the leaking current can be restrained by electrically connecting a choke to the outer conductor of monopole MW antennas, leading to more energy localized around the active tip of the antennas [
      • Lara J.E.
      • Vera A.
      • Leija L.
      • et al.
      Modeling of electromagnetic and temperature distributions of an intersticial coaxial-based choked antenna for hepatic tumor microwave ablation.
      ]. Hulusi et al. [

      Acikgoz H, Mittra R, Ieee. Microwave Coaxial Antenna for Cancer Treatment: Reducing the Backward Heating Using a Double Choke. ISAP. 2015:1-4.

      ] demonstrated that with a length of about λeff/4, the choke results in an infinite impedance at its open end to confine the currents on the outer conductor to the region between the choke and the end of the line. The same effect was also found in the work of Yang et al. [
      • Yang D.S.
      • Bertram J.M.
      • Converse M.C.
      • et al.
      A floating sleeve antenna yields localized hepatic microwave ablation.
      ] by adding a metal floating sleeve, which is electrically isolated from the outer conductor of the dipole antenna body, as shown in Fig. 3B. The result of this strategy is that the MW energy propagates into the tissues in two ways. Most of the wave propagates directly into the tissues through the gap at the antenna tip, while minimal power propagates through the Teflon gap between the floating sleeve and the outer conductor. Yang et al. [
      • Yang D.S.
      • Bertram J.M.
      • Converse M.C.
      • et al.
      A floating sleeve antenna yields localized hepatic microwave ablation.
      ] found that these two waves could have a similar amplitude and reversed phase in the area towards the tail of the antenna when the length of the sleeve is roughly half of the effective wavelength by computer simulation. In such a case, the two energy waves can be neutralized and result in the complete attenuation of SAR in the antenna tail. After the leaking current is restrained, the tail of the ablation area can be eliminated to make the shape of the ablation zone become more spherical and suitable for treatment. At present, these two effective methods are still widely used in the clinical setting.
      Figure thumbnail gr3
      Fig. 3Examples of antennas with choke, sleeve and cap: (A) monopole antenna with the choke (reproduced from
      [
      • Nevels R.D.
      • Arndt G.D.
      • Raffoul G.W.
      • et al.
      Microwave catheter design.
      ]
      ), (B) dipole antenna with the sleeve (reproduced from
      [
      • Yang D.S.
      • Bertram J.M.
      • Converse M.C.
      • et al.
      A floating sleeve antenna yields localized hepatic microwave ablation.
      ]
      ), (C) cap-choked antenna (reproduced from
      [
      • Cavagnaro M.
      • Amabile C.
      • Bernardi P.
      • et al.
      A minimally invasive antenna for microwave ablation therapies: design, performances, and experimental assessment.
      ]
      ).
      A metal cap attached to the tip of the MW antenna is another way to limit the leaking current [
      • Nevels R.D.
      • Arndt G.D.
      • Raffoul G.W.
      • et al.
      Microwave catheter design.
      ,
      • Cavagnaro M.
      • Amabile C.
      • Bernardi P.
      • et al.
      A minimally invasive antenna for microwave ablation therapies: design, performances, and experimental assessment.
      ]. It can increase the capacitance and force MW energy to radiate in a forward direction from the tip; this cap can come in different shapes such as ring, disk, and arrow. It should be noted that the cap can also be used in combination with the floating sleeve or choke for a better performance, for example, the cap-choked antenna proposed in [
      • Cavagnaro M.
      • Amabile C.
      • Bernardi P.
      • et al.
      A minimally invasive antenna for microwave ablation therapies: design, performances, and experimental assessment.
      ], as shown in Fig. 3C.
      With success in suppressing backward heating of the antenna with single choke, sleeve, or cap (also called baluns), Hulusi et al. demonstrated that the leaking currents can be further reduced by adding a second choke [

      Acikgoz H, Mittra R, Ieee. Microwave Coaxial Antenna for Cancer Treatment: Reducing the Backward Heating Using a Double Choke. ISAP. 2015:1-4.

      ]. As shown in Fig. 4A, there are two antenna structures that consist of a coaxial slot and double chokes of same lengths. The difference lies in how the chokes are combined and in what order. For example, RW-FW and FW-RW, where the RW and FW are the abbreviations of the “reversed” and “forward”, standing for the choke reversing and facing the slot, respectively. They concluded in a numerical study that 1) the antenna in RW-FW results in less backward heating and larger SAR value near the antenna tip than that in FW-RW combination or with a single choke and 2) the periodic structure of RW-FW did not improve the SAR level along the antenna.
      Figure thumbnail gr4
      Fig. 4(A) Slot antenna with double chokes, RW-FW (left), FW-RW (right) (reproduced from
      [

      Acikgoz H, Mittra R, Ieee. Microwave Coaxial Antenna for Cancer Treatment: Reducing the Backward Heating Using a Double Choke. ISAP. 2015:1-4.

      ]
      ) and (B) monopole antenna with dual rings (reproduced from
      [
      • Yassin M.M.
      • Tammam E.
      • Ibrahim A.A.
      • et al.
      A dual ring interstitial monopole antenna with spherical heating pattern for hepatic tumor microwave ablation.
      ]
      ).
      As shown in Fig. 4B, Yassin et al. proposed a monopole antenna design with dual conductor rings and studied this in a numerical study [
      • Yassin M.M.
      • Tammam E.
      • Ibrahim A.A.
      • et al.
      A dual ring interstitial monopole antenna with spherical heating pattern for hepatic tumor microwave ablation.
      ]. These two additional rings can be used as a capacitive cap to enable the antenna to match at the desired frequency band, leading to further reduction in the axial radiation length along the antenna. They showed that the heating pattern of this modified antenna was spherical and independent of the insertion depth. Compared to the simulation results of the monopole antenna, this modified antenna was able to achieve a larger ablation zone and a more focused heating pattern.
      Studies suggest that these two novel designs have less backward heating and can achieve a larger ablation zone, yet comprehensive experiments with different tissues under a wide range of input power are still required to verify the feasibility and effectiveness of these designs.
      Though the radiation performance of antennas can be effectively improved by the addition of sleeve or choke, there is a shortcoming that the diameter of conventional antennas is inevitably increased; for an antenna to be useful clinically, it must be possible for it to be placed percutaneously. Recently, techniques have been introduced for the reduction of the diameter of conventional antennas by embedding the choke or sleeve within coaxial cables. Specifically, due to the lower permittivity of air, Luyen et al. [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ] found that the diameter of conventional antennas can be reduced by using an impedance-matched air-filled coax section at the end of a Teflon-filled coaxial feed line. Then a choke and sleeve are added to the air-filled section respectively without extending the overall coaxial cable diameter. The two designs are shown in Fig. 5A and B, and the performance comparison between the conventional and modified antennas with the choke or floating sleeve in ex vivo porcine tissues is tabulated in Table 1, where the overall diameter represents the diameter of the antenna.
      Figure thumbnail gr5
      Fig. 5(A) Modified antennas with the embedded choke (reproduced from
      [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ]
      ), (B) Modified antennas with the embedded sleeve (reproduced from
      [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ]
      ).
      Table 1Experiment results of the conventional and modified antennas with the choke and floating sleeve in ex vivo porcine muscle (adapted from 
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ).
      AntennaFrequency

      (GHz)
      Power

      (W)
      Time

      (min)
      Overall diameter

      (mm)
      Ablation zone

      (cm2)
      Aspect ratio
      Conventional choked dipole73053.54.1×2.70.66
      Modified choked dipole73052.54.0×2.80.70
      Conventional floating sleeve dipole73053.54.3×2.80.65
      Modified floating sleeve dipole73052.53.7×2.80.76
      In the study, the aspect ratio (short diameter vs. long diameter [
      • Ibitoye Z.A.
      • Nwoye E.O.
      • Aweda M.A.
      • et al.
      Optimization of dual slot antenna using floating metallic sleeve for microwave ablation.
      ]) was used as a measure of the roundness of ablation area; when this ratio is close to 1, this represents less backward heating. It is notable that although most MWA research and commercial products have focused on operation at the ISM frequencies of 915 MHz and 2.45 GHz [
      • Leung U.
      • Kuk D.
      • D'Angelica M.I.
      • et al.
      Long-term outcomes following microwave ablation for liver malignancies.
      ,
      • Curto S.
      • Taj-Eldin M.
      • Fairchild D.
      • et al.
      Microwave ablation at 915 MHz vs 2.45 GHz: A theoretical and experimental investigation.
      ,
      • Sawicki J.F.
      • Shea J.D.
      • Behdad N.
      • et al.
      The impact of frequency on the performance of microwave ablation.
      ], a few works have systematically analyzed the performance of MWA at higher frequencies [
      • Sawicki J.F.
      • Shea J.D.
      • Behdad N.
      • et al.
      The impact of frequency on the performance of microwave ablation.
      ,
      • Hung L.
      • Gao F.
      • Hagness S.C.
      • et al.
      Microwave ablation at 10.0 GHz Achieves comparable ablation zones to 1.9 GHz in ex vivo bovine liver.
      ]. These works support that it is possible to design shorter and more flexible MW antennas when the higher frequency was used, and potentially realize more spherical thermal lesions all the while producing thermal lesions with volumes comparable to ISM frequencies [
      • Sawicki J.F.
      • Luyen H.
      • Mohtashami Y.
      • et al.
      The performance of higher frequency microwave ablation in the presence of perfusion.
      ]. However, the cable has a tendency to heat up significantly when the higher frequency is used [
      • Sawicki J.F.
      • Shea J.D.
      • Behdad N.
      • et al.
      The impact of frequency on the performance of microwave ablation.
      ], a factor that needs to be considered in the antenna design before it can be used clinically.
      As demonstrated in Table 1, the conventional and modified antennas can achieve a similar ablation zone under the same condition, while the aspect ratio of the modified antennas is better than that of the conventional antennas, which means less backward heating. Furthermore, the diameter of modified antennas is significantly reduced by using this embedding method.

      Design of antenna slots

      Increasing the number of slots in a slot antenna is another way to reduce backward heating. The multi-slot antennas can be obtained by cutting several annular slots on the outer conductor and welding the inner and outer conductors at the end of the coaxial cable, as shown in Fig. 6. The leaking current can be restrained successfully because of the mutual interference between phases of currents from different slots.
      Figure thumbnail gr6
      Fig. 6(A) Periodic multi-slot antenna (reproduced from
      [
      • Wang T.
      • Zhao G.
      • Qiu B.S.
      Theoretical evaluation of the treatment effectiveness of a novel coaxial multi-slot antenna for conformal microwave ablation of tumors.
      ]
      ), (B) Aperiodic tri-slot antenna (reproduced from
      [
      • Xu Y.S.
      • Moser M.A.J.
      • Zhang E.
      • et al.
      Large and round ablation zones with microwave ablation: A preliminary study of an optimal aperiodic tri-slot coaxial antenna with the pi-matching network section.
      ]
      ) (white: air).
      Based on simulation results, Keangin et al. [
      • Keangin P.
      • Rattanadecho P.
      • Wessapan T.
      An analysis of heat transfer in liver tissue during microwave ablation using single and double slot antenna.
      ,
      • Rattanadecho P.
      • Keangin P.
      Numerical study of heat transfer and blood flow in two-layered porous liver tissue during microwave ablation process using single and double slot antenna.
      ] found that an antenna with a single slot can generate a higher temperature level than that with double slots. Indeed, the double-slot antenna was found to be able to generate a larger and more conformal ablation zone pattern with less backward heating than the single slot antenna, supporting the potential benefit of multi-slot antennas. Two clinical trials of the two-slot antenna can be found in [
      • Saito K.
      • Yoshimura H.
      • Ito K.
      • et al.
      Clinical trials of interstitial microwave hyperthermia by use of coaxial-slot antenna with two slots.
      ]. The targeted tumor was completely covered by the ablation area, showing a satisfactory ablation performance with the two-slot antenna in the clinical setting.
      By adjusting the position and length of the slot in multi-slot antennas, different shape and size of ablation zones can be achieved for the treatment of specific tumors. For example, Liu et al. [
      • Liu W.M.
      • Lin X.Q.
      • Yang F.S.
      • et al.
      Microwave ablation antenna with the ablation pattern being a figure-of-eight.
      ] proposed specific model of double slot antenna
      which has the capability to form a figure-of-eight ablation pattern. Moreover, dual-slot antennas combined with a choke, a floating metallic sleeve, or a helical slot can also lead to the significant reduction in the backward heating and increase in the energy deposition into tissues [
      • Lee M.
      • Son T.
      Helical slot antenna for the microwave ablation.
      ,
      • Bertram J.M.
      • Yang D.S.
      • Converse M.C.
      • et al.
      Antenna design for microwave hepatic ablation using an axisymmetric electromagnetic model.
      ,
      • Ibitoye Z.A.
      • Nwoye E.O.
      • Aweda M.A.
      • et al.
      Optimization of dual slot antenna using floating metallic sleeve for microwave ablation.
      ].
      Wang et al. [
      • Wang T.
      • Zhao G.
      • Qiu B.S.
      Theoretical evaluation of the treatment effectiveness of a novel coaxial multi-slot antenna for conformal microwave ablation of tumors.
      ] developed the periodic multi-slot antenna by optimizing the length of slots using a genetic algorithm method achieving a near-spherical ablation zone, with minimal damage to surrounding normal tissues. They concluded that the antenna with ten periodic slots yielded more localized heating and a better conformal ablation zone pattern for spherical tumors when compared to other multi-slot antennas. However, they only selected minimum overtreatment ratio as sole optimization objective for antenna design and neglected the transmission efficiency of the antenna, leading to an ablation area of the optimized multi-slot antenna that was actually smaller than a single slot antenna under the same ablation duration and input power. To improve heating efficiency of multi-slot antennas, Gas [
      • Gas P.
      Optimization of multi-slot coaxial antennas for microwave thermotherapy based on the S11-parameter analysis.
      ] proposed to use the S11-parameter (representing the efficiency of antennas, discussed below) as the optimization objective to obtain the optimal location and size of slots of multi-slot antennas. But the ablation zone created by the optimized antennas was in a teardrop shape, which might not be suitable for conformal ablation of spherical tumors.
      To achieve both near-spherical ablation areas and good heating efficiency, Ge et al. [
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      ] proposed a periodic ten-slot antenna with an π impedance matching network, which was used to match the impedance between the antenna and the coaxial line for higher heating efficiency. For optimization objective they also chose the aspect ratio, in other words, aiming for spherical ablation zones. Both simulated and experimental ablation results showed this antenna had a better performance in generating larger and more conformal ablation zones.
      By simulation, Xu et al. [
      • Xu Y.S.
      • Moser M.A.J.
      • Zhang E.
      • et al.
      Large and round ablation zones with microwave ablation: A preliminary study of an optimal aperiodic tri-slot coaxial antenna with the pi-matching network section.
      ] further demonstrated that the aperiodic structure of slots could have a better performance than the periodic structure. They found that the optimized aperiodic tri-slot antenna had a competitive roundness (circularity: 0.92 vs. 0.99) with the optimized periodic ten-slot antenna, but much higher roundness (circularity: 0.92 vs. 0.66) than the periodic tri-slot antennas under the same conditions. To further improve heating efficiency, they also combined an π matching network with the aperiodic tri-slot coaxial antenna (Fig. 6B). The simulation results showed that the π-matched aperiodic tri-slot antenna generated a larger ablation zone with a similar roundness compared to the optimized periodic ten-slot antenna. Besides the competitive ablation performance, the tri-slot antenna is more easily fabricated than the ten-slot antenna.
      The comparison of the simulation results between the proposed multi-slot antennas and the single-slot antennas in liver tissues with a frequency of 2.45 GHz is tabulated in Table 2. It shows that the multi-slot antennas can achieve a more conformal ablation zone than is seen with the single-slot antenna. Multi-slot antennas with a π matching network can realize high heating efficiency and near-spherical ablation zone which is suitable for therapy of spherical tumors. Furthermore, the aperiodic structure of multi-slot antenna might have better performance in achieving large ablation zones than the periodic ones.
      Table 2Simulated results of the proposed multi-slot antennas in liver tissues with a frequency of 2.45 GHz.
      AntennaPower

      (W)
      Time

      (min)
      Overall Diameter

      (mm)
      Reflection coefficient

      (dB)
      Ablation Zone (cm2)Aspect RatioRef.
      Periodic ten-slot451033.81×3.770.99
      • Wang T.
      • Zhao G.
      • Qiu B.S.
      Theoretical evaluation of the treatment effectiveness of a novel coaxial multi-slot antenna for conformal microwave ablation of tumors.
      Single-slot30102.6−21.53.30×4.420.74
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      π-matched periodic ten-slot30102.6–23.53.69×3.750.98
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      π-matched aperiodic tri-slot45101.79–22.35.27×5.150.98
      • Xu Y.S.
      • Moser M.A.J.
      • Zhang E.
      • et al.
      Large and round ablation zones with microwave ablation: A preliminary study of an optimal aperiodic tri-slot coaxial antenna with the pi-matching network section.

      Balun-free operation

      To achieve the purpose of choking the electric currents excited on the outer conductors without increasing its invasiveness, Luyen et al. [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ] proposed a novel balun-free helical antenna, as shown in Fig. 7A. Unlike the normal monopole antenna, this antenna has a helical structure on its terminal end, which has the effect of reducing the length of the antenna (mentioned above). In general, a smaller S11 at operating frequency is preferable, because it means a higher power transmission efficiency. However, Luyen et al. found that the SAR pattern of the balun-free antenna under the first resonant frequency f1 exhibits long tails along the shaft of the antenna toward the input of the coaxial line, which would cause unwanted heating of the healthy tissue along the antenna shaft. But at the second resonant frequency f2, the SAR pattern was found highly localized [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]. Fig. 7B shows the simulated reflection coefficient of the antenna with the reference plane placed at the helix base (Fig. 7B). This is because the minimum current occurs at the feed point of the helix at this frequency, acting as a natural choke to restrain the leaking currents on the outer surface of the outer conductor of the coaxial cable. However, this area of minimal current results in a high feed-point impedance problem, causing inefficient energy transmission.
      Figure thumbnail gr7
      Fig. 7(A) Helical antenna without baluns (reproduced from
      [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]
      ), (B) Simulated |S11| of the antenna with the reference plane placed at the base of the helix (the first resonant frequency f1=0.9 GHz, and the second resonant frequency f2=1.9 GHz)
      [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]
      (with the permission of IEEE).
      This could be addressed by introducing an impedance matching section between the antenna and the coaxial line. The proposed helical antenna was designed to operate at 1.9 GHz due to the lack of microwave power generator at the ISM frequencies as claimed in [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]. Then, the prototype of this balun-free helical antenna implemented with the π-matching network was fabricated to test and compare its ablation effects with the floating sleeve dipole antennas in [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ].
      The experimental results of this study using ex vivo bovine liver samples are shown in Table 3. The ablation performance of the helical antenna is comparable to that of the floating sleeve dipole antennas regardless the ablation time but with a smaller antenna diameter (3.2 vs. 3.5 mm). Compared with the antennas that use coaxial baluns, the helical antenna showed promise as an alternative to existing MW antennas for reducing the overall diameter of the interstitial antenna. Furthermore, Luyen et al. [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ] claimed that the usage of the second resonant frequency can be also applied to monopole antennas with an arbitrary shape of inner conductors.
      Table 3Comparison of matched balun-free helical antenna and floating sleeve dipole antenna
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      .
      AntennaPower

      (W)
      Frequency

      (GHz)
      Overall diameter

      (mm)
      Time

      (min)
      Ablation zone (cm2)Aspect ratio
      Matched balun-free helical antenna421.93.255.5×30.55
      421.93.2107.5×4.30.57
      Floating sleeve dipole antenna421.93.555.7×3.40.60
      421.93.5107.3×3.80.52

      Structure design of outer conductor

      Besides methods mentioned above, the backward heating can also be addressed by changing the outer conductor structure of the coaxial cable because the reverse currents exist due to the unbalanced structure of the coaxial line. Fig. 8 shows a dipole antenna with a double-slot tapered balun [
      • Hung L.Y.
      • Hagness S.C.
      • Behdad N.
      A minimally invasive coax-fed microwave ablation antenna with a tapered balun.
      ]. One arm of the dipole was the two parallel strips located in the two slots, which were formed by gradually tapering the outer conductor of the coaxial cable, while the other one was the extension section of the inner conductor. These two arms of the antenna were able to balance the total currents at the operating frequency of the balun, so that the backward heating was effectively restrained. Experimental results on the use of this antenna prototype in ex vivo bovine livers showed that this antenna can generate a more conformal ablation zone compared to the conventional floating sleeve dipole antenna (0.76 vs. 0.65) with a much smaller antenna diameter (2.6 vs. 3.5 mm) [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      Reduced-diameter designs of coax-fed microwave ablation antennas equipped with baluns.
      ,
      • Hung L.Y.
      • Hagness S.C.
      • Behdad N.
      A minimally invasive coax-fed microwave ablation antenna with a tapered balun.
      ]. However, this particular design exhibits a better impedance matching from 5 to over 8 GHz than the clinical used 2.45 GHz.
      Figure thumbnail gr8
      Fig. 8Antenna with a double-slot tapered balun
      [
      • Hung L.Y.
      • Hagness S.C.
      • Behdad N.
      A minimally invasive coax-fed microwave ablation antenna with a tapered balun.
      ]
      (with the permission of IEEE).

      Addition of cooling system

      The addition of a cooling system to the antenna is another effective way to address the backward heating problem [
      • He N.
      • Wang W.
      • Ji Z.
      • et al.
      Microwave ablation: an experimental comparative study on internally cooled antenna versus non-internally cooled antenna in liver models.
      ,
      • Meloni M.F.
      • Chiang J.
      • Laeseke P.F.
      • et al.
      Microwave ablation in primary and secondary liver tumours: technical and clinical approaches.
      ,
      • Knavel E.M.
      • Hinshaw J.L.
      • Lubner M.G.
      • et al.
      High-powered gas-cooled microwave ablation: shaft cooling creates an effective stick function without altering the ablation zone.
      ]. A cooling system has been used in various commercially available MW antennas with favorable clinical outcomes [
      • Ziemlewicz T.J.
      • Hinshaw J.L.
      • Lubner M.G.
      • et al.
      Percutaneous microwave ablation of hepatocellular carcinoma with a gas-cooled system: initial clinical results with 107 tumors.
      ,
      • Horn J.C.
      • Patel R.S.
      • Kim E.
      • et al.
      Percutaneous microwave ablation of renal tumors using a gas-cooled 2.4-GHz probe: technique and initial results.
      ]. There are two types of cooling antennas available in clinic: the gas-cooled antennas [
      • Ziemlewicz T.J.
      • Hinshaw J.L.
      • Lubner M.G.
      • et al.
      Percutaneous microwave ablation of hepatocellular carcinoma with a gas-cooled system: initial clinical results with 107 tumors.
      ,
      • Horn J.C.
      • Patel R.S.
      • Kim E.
      • et al.
      Percutaneous microwave ablation of renal tumors using a gas-cooled 2.4-GHz probe: technique and initial results.
      ,

      Leapman M, Jayadevan R, Phillips C, et al. Percutaneous gas-cooled microwave ablation for small renal masses: the mount sinai experience. J Urol. 2014;191(4):E620.

      ] and water-cooled antennas [
      • Kuang M.
      • Lu M.D.
      • Xie X.Y.
      • et al.
      Liver cancer: Increased microwave delivery to ablation zone with cooled shaft antenna - Experimental and clinical studies.
      ,
      • Wang Y.
      • Sun Y.Y.
      • Feng L.
      • et al.
      Internally cooled antenna for microwave ablation: Results in ex vivo and in vivo porcine livers.
      ,
      • Zhou Q.
      • Jin X.
      • Jiao D.C.
      • et al.
      Microwave ablation: results in ex vivo and in vivo porcine livers with 2450-MHz cooled-shaft antenna.
      ]. A typical cooling antenna is shown in Fig. 9. In addition to addressing the backward heating problem, the technique of the cooling system in the antenna can suppress the unwanted heating in tissue from inside the antenna shaft (because of the above-mentioned impedance mismatching) and the over-hot ablation zone [
      • Brace C.L.
      Microwave tissue ablation: biophysics, technology, and applications.
      ], which should help prevent the collateral damages to the normal tissues. By eliminating excessive heat mentioned above, the cooling system also can enhance the energy deposition ability of the antenna [
      • Brace C.L.
      Microwave tissue ablation: biophysics, technology, and applications.
      ], so that the larger ablation zone can be expected due to more energy deposition. It is noteworthy that to minimize the electromagnetic loss caused by the cooling system during MWA, the material of coolant should be selected carefully. The addition of a cooling channel does inevitably increase the size of the original antenna (e.g. 2.46 vs. 0.86 mm [
      • Fallahi H.
      • Clausing D.
      • Shahzad A.
      • et al.
      Microwave antennas for thermal ablation of benign adrenal adenomas.
      ]). Though this issue can be alleviated by using less coolant flow or antennas with original smaller diameter, the cooling and energy handling capabilities can also be weakened, which should be taken into consideration in the process of conceptual design of antennas.
      Figure thumbnail gr9
      Fig. 9A commercially available water-cooled microwave antenna (MTC-3CA-II37, Nanjing Viking Jiuzhou Medical Equipment Co., Ltd).

      Designs to fulfill the directional ablation

      In most MWA procedures, a spherically symmetric ablation zone pattern is desirable and expected with a single antenna inserted into the center of the target tissue. Sometimes, multiple antennas are needed to achieve such an ablation zone in a larger dimension. Regardless, there is always the potential for damage to nearby critical structures such as blood vessels or bile ducts. One possible way to address this issue was proposed by Kitchin et al. [
      • Kitchin D.
      • Lubner M.
      • Ziemlewicz T.
      • et al.
      Microwave ablation of malignant hepatic tumours: Intraperitoneal fluid instillation prevents collateral damage and allows more aggressive case selection.
      ], at which fluid was injected by an image-guided needle between the target tissue and the surrounding normal tissues. However, this method requires experienced radiologists and causes additional burden to patients with the deep insertion of an extra needle.
      On the other hand, this collateral damage might be avoided by using a so-called directional antenna that can restrict the direction of the MW energy delivery. Directional antennas are characterized by a reflector that can make the power radiate in a certain direction; this reflection can be achieved by the addition of an additional metal catheter or outer conductor on the coaxial cable. To the best of authors’ knowledge, the directional antenna is still at the stage of laboratory investigation, and no commercial ones are yet available. As shown in Fig. 10, there are three types of directional antennas being investigated in literature. Fig. 10A shows a dipole antenna with directive radiation [
      • Alnassan H.
      • Kastler A.
      • Wang X.
      • et al.
      Modified dipole antenna for directional microwave ablation using 3D numerical simulation.
      ], which is realized by inserting the dipole antenna into a catheter with an opening that exposes 300 degrees of the circumference of the active portion of the antenna. The metal catheter is then used to guide the microwave energy through the ceramic active window. The air cavity between the outer conductor and the catheter was used for water cooling cycle. It was shown that this antenna can successfully achieve the directional ablation and its SAR patterns are uneven on the opposite sides. Nevertheless, this design has the drawback of being a relatively large antenna diameter. Besides, the heating on the non-active side was not well constrained, its depth could be nearly half of the heating depth on the active side.
      Figure thumbnail gr10
      Fig. 10Directional antennas: (A) Dipole directional antenna (reproduced from
      [
      • Alnassan H.
      • Kastler A.
      • Wang X.
      • et al.
      Modified dipole antenna for directional microwave ablation using 3D numerical simulation.
      ]
      ), (B) monopole directional antenna with water cooling (reproduced from
      [
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ]
      ) and (C) hybrid slot/monopole directional antenna (reproduced from
      [
      • Mohtashami Y.
      • Hagness S.C.
      • Behdad N.
      A hybrid slot/monopole antenna with directional heating patterns for microwave ablation.
      ]
      ) (white: air).
      As shown in Fig. 10B, McWilliams et al. [
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ] described a monopole antenna incorporating a hemi-cylindrical reflector. In the design, the monopole antenna was inserted into a stainless-steel tube and fixed by a bracket. One end of the stainless-steel tube near the antenna tip was cut into a semi-cylindrical shape, acting as a reflector to direct the microwave energy to the desired side. The other end of the tube was secured to a homeostatically-controlled valve for water cooling. A 3.5-mm (outer diameter) polyimide catheter was placed over the stainless-steel tube for protection. This antenna can realize almost minimal radiation of energy behind the reflector, but it also had the problem of larger overall diameter and relatively small power deposition within tissues. Sebek et al. [
      • Sebek J.
      • Curto S.
      • Bortel R.
      • et al.
      Analysis of minimally invasive directional antennas for microwave tissue ablation.
      ] investigated the monopole radiating tip and reflector shape of this directional antenna to reduce the antenna diameter and yield a larger ablation zone. Antennas were modified by moving the radiating tip closer to the catheter inner-wall and hence closer to the tissue. With such a configuration, it was possible to reduce the diameter of the antenna yet increase the ablation depth compared to the antenna proposed in [
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ]. Recently, Austin et al. [
      • Pfannenstiel A.
      • Sebek J.
      • Fallahi H.
      • et al.
      Directional microwave ablation: experimental evaluation of a 2.45-GHz applicator in ex vivo and in vivo liver.
      ] carried out preliminary experiments in ex vivo and in vivo porcine liver tissues using the prototype antenna originally reported by McWilliams et al. [
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ]. The study further demonstrates that the directional MW antenna has the potential to safely treat the tumors in sensitive areas, like the lungs or adrenal glands, while reducing the risk of collateral injury.
      Fig. 10C illustrates a monopole antenna with a portion of its outer conductor used as a reflector [
      • Mohtashami Y.
      • Hagness S.C.
      • Behdad N.
      A hybrid slot/monopole antenna with directional heating patterns for microwave ablation.
      ]. A semi-cylindrical slot is etched in the outer conductor to increase the field in desired direction and decrease the field behind the reflector. As an additional benefit, the reflector and slot can suppress the backward heating along the shaft of the antenna, thus avoiding the presence of the sleeve or choke. The problem of the impedance mismatch which is predominantly caused by the appearance of the semi-cylindrical reflector is resolved by using an internal matching network. This design has near four times overall ablation efficiency higher than that of the antenna proposed in [
      • McWilliams B.T.
      • Schnell E.E.
      • Curto S.
      • et al.
      A directional interstitial antenna for microwave tissue ablation: theoretical and experimental investigation.
      ], and a smaller overall diameter because an additional tube is not needed for cooling water.

      Designs to improve the efficiency

      One way to enhance MWA efficiency is nanoparticle-based therapeutic approach, such as magnetic nanoparticles and liposomes, which can be used as heat sources and reduce input power significantly to produce a similar ablation [
      • Soltani M.
      • Tehrani M.H.H.
      • Kashkooli F.M.
      • et al.
      Effects of magnetic nanoparticle diffusion on microwave ablation treatment: A numerical approach.
      ,
      • Minbashi M.
      • Kordbacheh A.A.
      • Ghobadi A.
      • et al.
      Optimization of power used in liver cancer microwave therapy by injection of Magnetic Nanoparticles (MNPs).
      ,
      • Wu S.S.
      • Zhang D.Y.
      • Yu J.
      • et al.
      Chemotherapeutic nanoparticle-based liposomes enhance the efficiency of mild microwave ablation in hepatocellular carcinoma therapy.
      ]. Another is to optimize the internal structure of the antenna to improve its power transmission efficiency, making more energy deposited into the tissue. In the MW antenna designs, a small value of reflection coefficient (S11) is preferred because a higher S11 indicates less electromagnetic power deposited into the tissue, which is caused by the impedance mismatch between the coaxial cable and the antenna [
      • Xu Y.S.
      • Moser M.A.J.
      • Zhang E.
      • et al.
      Large and round ablation zones with microwave ablation: A preliminary study of an optimal aperiodic tri-slot coaxial antenna with the pi-matching network section.
      ]. This mismatch phenomenon can cause additional heating damage to both surrounding tissues and the antenna. If the value of S11 is greater than −10 dB, more than 30% of the input power will be reflected back toward the microwave generator, which does not meet the general requirements of antenna design [

      Sawarbandhe MD, Naik SB, Satpute VR, et al. Coaxial Antenna for Microwave Ablation. International Conference on Distributed Computing, Vlsi, Electrical Circuits and Robotics. 2016:119-122.

      ].
      Unlike the antennas with the choke, sleeve and/or cap, which are found with a better impedance match [
      • Yassin M.M.
      • Tammam E.
      • Ibrahim A.A.
      • et al.
      A dual ring interstitial monopole antenna with spherical heating pattern for hepatic tumor microwave ablation.
      ,
      • Longo I.
      • Gentili G.B.
      • Cerretelli M.
      • et al.
      A coaxial antenna with miniaturized choke for minimally invasive interstitial heating.
      ,
      • Mohtashami Y.
      • Luyen H.
      • Sawicki J.F.
      • et al.
      Tools for attacking tumors.
      ], some antennas (e.g., monopole antennas) may not have a favorable impedance matching condition if they are not also specially designed for a low reflection coefficient. This is something that must be addressed to ensure a good ablation performance and to avoid failure of these antennas mid-procedure. Since the lossless transmission line can be used to synthesize an arbitrary impedance according to the transmission line theory, one effective method is to change a section of coaxial cable into the impedance matching network for impedance match between the coaxial line and the antenna. At present, there are mainly three types of matching networks that are commonly used in the design of antennas.
      The first design of the matching network is to use a quarter-wavelength transformer, as shown in Fig. 11A. The length of the transformer is a quarter of the wavelength, and the characteristic impedance (Z01) can be given as:
      Z01=Z0Zin
      (8)


      where Z0 and Zin are the characteristic impedance of the load impedance and the feeding coaxial line, respectively [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ].
      Figure thumbnail gr11
      Fig. 11(A) Quarter-wave transformer (reproduced from
      [
      • Luyen H.
      • Hagness S.C.
      • Behdad N.
      A balun-free helical antenna for minimally invasive microwave ablation.
      ]
      ), (B) π type impedance matching network (reproduced from
      [
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      ]
      ), (C) L type impedance matching network (reproduced from
      [
      • Mohtashami Y.
      • Luyen H.
      • Sawicki J.F.
      • et al.
      Tools for attacking tumors.
      ]
      ) (white: air).
      Furthermore, Z01 can also be depicted by the quarter-wavelength transformer as:
      Z01=LC
      (9)


      C=l2πεln(ba1),L=lμ2πln(ba1)
      (10)


      where C and L represent the capacitance and the inductance of the quarter-wavelength transformer, respectively, l is the length of the transformer part, ε and μ are the permittivity and permeability of the dielectric, respectively, a1 and b are the outer diameter of the inner conductor and the inner diameter of the outer conductor of the transformer, respectively.
      The second existing design is a π type impedance matching network, which is equivalent to two capacitors (C1 and C2) in parallel with a inductor (L1) in series [
      • Ge M.K.
      • Jiang H.J.
      • Huang X.Y.
      • et al.
      A multi-slot coaxial microwave antenna for liver tumor ablation.
      ], as shown in Fig. 11B. The capacitor can be obtained by thickening the outer conductor of the coaxial cable, which decreases the impedance of the impedance matching network. The inductor was designed by removing the dielectric material between the inner conductor and the outer conductor in a short section of the coaxial cable, which can increase the impedance of the impedance matching network. To get a better impedance match, the values of C1, C2 and L1 can be determined by
      C1=l12πεln(b1a),C2=l32πεln(b1a),L1=l2μ2πln(ba)
      (11)


      where l1, l2 and l3 are the component lengths of each part of π impedance matching network, as shown in Fig. 11(B), b1 is the inner diameter of the thickened outer conductor of the π matching network part and a is the inner diameter of the coaxial cable.
      The third existing matching network used is the L type one, which is equivalent to an inductor and a resistor in series with a parallel capacitor [
      • Mohtashami Y.
      • Luyen H.
      • Sawicki J.F.
      • et al.
      Tools for attacking tumors.
      ], as shown in Fig. 11C. Similarly, the values of C3, L2 and R can be determined by
      C3=l52πεln(b2a),L2=l4μ2πln(ba)
      (12)


      where l4 and l5 are the component lengths of each part of L matching network, as shown in Fig. 11C, and b2 is the inner diameter of the thickened outer conductor of the L matching network part.
      According to Eqns. (10)-(12), it is not difficult to find that the impedance matching networks of these three designs are all related to the component dimensions of the networks (i.e., the length and diameter), which provides the opportunity to optimize the structure of antennas for the optimal matching performance. However, these designs increase the difficulty in the manufacturing process, especially the π impedance matching network, which may be addressed with the advanced micro manufacture technique.
      To sum up, the panorama of MW antenna evolution reviewed in this study is illustrated in Fig. 12.
      Figure thumbnail gr12
      Fig. 12Development diagram of the existing MW antennas for MWA.

      Conclusion

      The MW antenna plays a vital role in the ablation of tumors, and many different types of MW antennae have been developed. A great deal of progress in the development of MW antennas has been made in response to various clinical requirements. This paper reviews the state-of-the-art of antenna designs and summarizes approaches for achieving three design criteria of the antenna to help invente novel antennas to advance this promising ablation technique to become an effective and safe and widely used modality in the treatment of patients with cancer.

      Funding

      This work was partially supported by National Natural Science Foundation of China under Grant 81801795 .

      Declaration of Competing Interest

      The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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