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Microwave ablation is a relatively new technology under development and testing to treat the same types of cancer that can be treated with radiofrequency (RF) ablation. Microwave energy has several possible benefits over RF energy for tumor ablation but, because clinical microwave ablation systems are not widespread, the underlying principles and technologies may not be as familiar. The basic microwave ablation system contains many of the same components as an RF ablation system: a generator, a power distribution system, and an interstitial applicator. This manuscript will attempt to provide an overview of each of these components, outline their functions and roles, and provide some insight into what every potential microwave ablation user should know about systems in development.
The rapid adoption of radiofrequency (RF) ablation has benefited many patients with primary and metastatic liver tumors (1, 2), and is being adapted for use in the lung, kidney and bone with some success (3–5). However, there are many drawbacks inherent to RF heating, including an inability to conduct current through charred tissue, a need for current return paths (ie, ground pads or additional interstitial electrodes), and a sizeable dependence on thermal conduction. For these reasons, RF energy creates relatively small zones of active heating, which can cause small ablations and poor results in areas near heat sinks such as large blood vessels (> 3 mm) and airways. These effects are amplified in high-impedance tissue like the lung. In addition, multiple electrodes have only been used effectively in a switched mode because of undue interactions between electrodes in close proximity (6).
Microwaves offer several advantages over RF energy for tumor ablation, including faster heating over a larger volume, less susceptibility to heat sinks or local perfusion, enhanced multiple-applicator support and no requirement for ground pads. Very few commercial devices are available but rapid commercial and academic development is aiming to make microwave ablation technology more accessible to physicians. However, the underlying technology of microwave ablation can often be unclear to the end user. The goal of this manuscript is to describe the main components of microwave ablation systems, including the power generation, distribution and delivery sub-systems, and explain how each might impact the efficacy, safety and cost of a microwave ablation system.
Microwaves generate heat through a process known as dielectric hysteresis: polar molecules (e.g., water) try to continuously realign with an applied electromagnetic (EM) field that alternates polarity billions of times per second (Figure 1). When the molecules fail to “keep up” with the alternating field, some of the microwave energy is absorbed by the material and converted to heat. The rate of heat generation (Qh) is proportional to the square of the applied electric field magnitude (E), or
where σ is effective conductivity (S/m), a measure of microwave absorption. Because of their high water content, most biological tissues have a relatively high conductivity and readily absorb microwave energy (7). For this reason, many insterstitial devices have been proposed to introduce microwave energy into the body (8–16). Limitations on field penetration and spatial resolution make interstitial antennas preferred for focal tumor ablation. The most general system design consists of an interstitial antenna (or array of antennas) receiving power from a microwave generator through a distribution system (Figure 2). The specifics of each of these subsystems, their technical specifications and most likely configurations for commercially available clinical systems will be discussed next.
Microwave ablation generators utilize one of two basic power sources: a magnetron or solid-state amplifier. A magnetron generates electromagnetic energy by accelerating electrons through a magnetic field inside of a resonant cavity (Figure 3). The geometry of the cavity determines the output frequency. Magnetrons are characterized by relatively high efficiency (> 70%), high output powers (>10 kW is common), high reliability and low cost. Their extremely large “Watts-per-dollar” ratio has made the magnetron the source-of-choice for microwave ovens, plasma generators, food treatment and industrial heating applications. However, magnetrons require a high-voltage power supply (which often entails a large, heavy transformer) that can be precisely controlled. Monitoring and output control systems within the generator may also be large and bulky due to their need to handle high output powers. However, the high output powers available from magnetrons can be used to power several antennas from a single source (17).
Solid-state generators create power in stages, with each stage consisting of a transistor-based amplifier that increases the power of the previous stage (Figure 4). Solid-state sources generally have a lower efficiency (< 30%), moderate output powers (< 150 W), high stability, good robustness and higher cost. However, they can be made smaller in size and are more controllable than magnetron devices. Due to their lower efficiency, solid-state sources generate large amounts of heat that need to be dissipated. Heating becomes more problematic as frequency increases or prolonged continuous-wave operation is required; 915 MHz solid-state sources are easier to come by and more reliable than their 2.45 GHz counterparts. One advantage to using solid state sources is that the pre-amplified signal can be monitored and controlled much easier than in magnetron sources. It remains to be seen how the competing interests of high output power and lightweight design will be resolved in clinical generator designs.
Regardless of power source, generators may also encompass some of the power distribution system. Current systems are capable of powering only one antenna per generator, but future generations may incorporate multiple-antenna operation into a single generator unit. Using separate generators makes advanced multiple-antenna capabilities all but impossible. Such capabilities have been investigated for microwave hyperthermia and are currently being researched for microwave ablation, but are likely several years from deployment (18–21). More information about power distribution options follows.
Electromagnetic energy is carried in transmission lines. One of the most popular is the coaxial transmission line (coaxial cable), which consists of an inner conductor, dielectric material and outer conductor (Figure 5). Coaxial cable is popular for many applications (e.g., cable television) because of its flexibility, compact size, excellent propagation characteristics and simplicity. For tumor ablation, coaxial cables can be used in various forms to carry power from the generator to the antenna. Antennas are typically built from a rigid form of coaxial cable as well.
Despite their many strengths, coaxial cables are limited in their ability to carry large amounts of power at microwave frequencies. As cable diameter decreases, power handling ability decreases precipitously (Figure 6). This limits how small and flexible the distribution cables can be, though proper material selection can improve the flexibility of most cables. A bigger issue lies in the antenna cable diameter, as will be discussed later.
Other components that may enter into the power distribution system include power splitters, phase shifters and amplitude modulation or switches. Power splitters use a specific transmission line geometry to divide an input power into a number of output channels, usually in equal proportions. While it is possible to split a single input into as many output channels as needed, a typical microwave ablation system would likely not have more than four output channels. Current current multiple-antenna RF systems support up to three electrodes and while cryoablation systems are capable of up to eight-probe operation, rarely are more than four used at a time.
Phase shifters and amplitude modulation would be used on systems offering phased-array operation of multiple antennas. Phased arrays have been used for hyperthermia, but have only begun to be researched for tumor ablation (18–21). The goal of a phased array is to overlap the electromagnetic waves in a way that increases (or decreases) the amount of heating. When using constructive interference, heating in the array can be increased by a factor of by N2, where N is the number of antennas used in the array. For example, three coherently phased antennas can deliver 9× more power than a single antenna (Figure 7). Finally, switches or continuous amplitude modulation may also be used in the power distribution system to selectively activate antennas.
Antennas come in a variety of formats, but all antennas are linked by their ability to transfer energy from a source to a load (i.e., from the generator to the tissue). Unlike RF ablation electrodes, antennas radiate energy by virtue of their geometry without ground pads or other electrodes. Thus, microwave fields may propagate through and heat any normal or malignant tissue, desiccated tissue, blood, cystic masses, vessels, etc.
Several different designs have been proposed for interstitial microwave heating(8–16). Most of these designs have an essentially needle-like geometry, but some loop and deployable designs have been described (22). Each antenna design attempts to satisfy some basic requirements: the antenna should be minimally invasive, highly efficient and radiate deep into the tissue to create large zones of active heating. Two metrics are often used to describe the performance of an antenna:
Generally, a balance exists between efficiency, heating pattern and invasiveness. For this reason, some antenna designs may be more suited to open or laparoscopic procedures, where invasiveness is not as much of a concern.
From a clinical perspective, antenna design is not the only factor that determines the final zone of ablation. Changes in tissue properties that occur during an ablation tend to alter the impedance matching of an antenna and the way in which EM energy propagates through the tissue. For this reason, antenna performance at the beginning of an ablation is often very different than at the end of the ablation. Other factors, such as nearby blood vessels, airways, bile ducts, bowel, ureters, etc. will also influence the actual antenna performance. Finally, thermal conduction from the active heating zone – which is independent of antenna geometry – has the most impact on the final ablation zone when ablation times longer than 6–7 min are used (Schramm and Haemmerich, presented at the World Congress of Interventional Oncology, 2006).
One major technical hurdle for microwave ablation is in antenna diameter. There is an inherent power handling-invasiveness trade-off with all microwave ablation antennas but it is known that increasing power is the easiest way to increase ablation zone size with a given antenna design (14, 23, 24). As power is increased, more heating of the feeding structure occurs, which can lead to unwanted ablation of the insertion track (Figure 10). One solution that is being researched to improve the power handling ability of small-diameter antennas is active cooling. Unlike cooled RF electrodes, which are designed to prevent char buildup at the electrode-tissue interface, microwave antennas are cooled to prevent overheating of the feeding structure and increase the power handling of the antenna. In a recent study, Kuang et al. found that cooling a 14-gauge antenna reduced unwanted heating along the feeding structure and allowed higher powers to be used, resulting in larger zones of ablation than a non-cooled antenna (25).
Three major components make up a microwave ablation system: the microwave generator, power distribution system and applicator antenna. Unlike RF ablation systems, microwave generators are capable of powering several antennas from the same source without the need for switching or bipolar techniques. The power distribution system may be a simple cable to transfer power directly to the antenna, or may contain components to control the phase, amplitude and duty cycle of multiple antennas. The antenna transfers energy into the tissue and could contain one of several designs, each having its own benefits and drawbacks for clinical applications. How a microwave ablation system will need to be configured for practical clinical use will depend on user feedback and clinical research with systems now coming into the marketplace.