1. Technical Field
The present invention relates to systems and methods for performing a medical procedure, wherein the medical procedure includes transferring energy from an energy source to a patient via a transmission line and, more particularly, dynamically matching energy source impedance to tissue impedance.
2. Background of Related Art
Historically, surgery was performed using only mechanical tools, such as mechanical cutting instruments, scalpels, bladed forceps, saws, rongeurs, and the like. However, in recent years, technology has improved such that surgeons now frequently use electromagnetic waves to render a wider variety of surgical effects, e.g., by selectively modifying tissue using electromagnetic energy to produce a specific effect. The characteristics of the electromagnetic energy applied to tissue strongly correlates to the effect that the energy has on the tissue. These characteristics are therefore changed in accordance with the desired tissue effect. Two types of electromagnetic energy that are commonly applied during surgery include radiofrequency (RF) electrosurgical energy and microwave electrosurgical energy. During most medical procedures in which an energy source is employed, the energy generated for the medical procedure is transferred to a patient via a transmission line. One example of a medical procedure employing an energy source is an RF or microwave ablation surgical procedure. In an ablation surgical procedure the energy generated may be an RF or microwave surgical signal having a frequency and a wavelength associated therewith.
During the ablation surgical procedure, the surgical signal may be transmitted to the patient via a transmission line. Generally, the transmission line employed may have losses associated therewith that may be attributable to many factors. Factors that can cause transmission line losses include at least the following: the type of material used for the transmission line, the length of the transmission line, the thickness of the transmission line, and impedance mismatch between the transmission line and tissue load.
Generally, electrosurgery utilizes an electrosurgical generator, an active electrode and a return electrode. The electrosurgical generator generates electrosurgical energy typically above 100 kilohertz to avoid muscle and/or nerve stimulation between the active and return electrodes when applied to tissue. During electrosurgery, current generated by the electrosurgical generator is conducted through the patient's tissue disposed between the two electrodes. The electrosurgical energy is returned to the electrosurgical source via a return electrode pad positioned under a patient (i.e., a monopolar system configuration) or a smaller return electrode positionable in bodily contact with or immediately adjacent to the surgical site (i.e., a bipolar system configuration). The current causes the tissue to heat up as the electromagnetic wave overcomes the tissue's impedance. Although many other variables affect the total heating of the tissue, usually more current density directly correlates to increased heating.
Microwave surgical procedures invoke the application of microwave energy to tissue. Unlike low frequency RF therapy that heats tissue with current, microwave therapy heats tissue within the electromagnetic field delivered by an energy delivery device (e.g., a microwave antenna). Microwave surgical procedures typically utilize a microwave generator and an energy delivery device that delivers the microwave energy to the target tissue. One type of energy delivery device is a coaxial microwave antenna that forms an approximate dipole antenna. Microwave surgical systems involve applying microwave radiation to heat, ablate and/or coagulate tissue. For example, treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors) or surrounding tissue. It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, by applying microwave energy to heat tumor cells to temperatures above 41° C. kills the tumor cells while adjacent healthy cells are maintained at lower temperatures avoiding irreversible cell damage. Another method used to treat diseased tissue is to resect a portion of the diseased organ, tissue or anatomical structure. For example, a liver may contain diseased tissue and healthy tissue. One treatment option is to pre-coagulate and ablate some of the liver tissue to facilitate resection of a portion of the liver including the diseased tissue. Microwave energy can be used during these types of procedures to pre-coagulate tissue prior to resection, to reduce bleeding during resection and to facilitate the actual resection of the tissue.
The microwave energy may be applied via an antenna that can penetrate tissue. There are several types of microwave antennas, such as monopole and dipole antennas. In monopole and dipole antennas, most of the microwave energy radiates perpendicularly away from the axis of the conductor. A monopole antenna includes a single, elongated conductor that transmits the microwave energy. A typical dipole antenna has two elongated conductors parallel to each other and positioned end-to-end relative to one another with an insulator placed therebetween. Each of the conductors is typically about ¼ of the length of the wavelength of the microwave energy making the aggregate length of both conductors about ½ of the wavelength of the microwave energy. Additionally, a microwave antenna may be adapted for use in a specific manner, for example, for endoscopic or laparoscopic (minimally invasive) procedures, for open procedures, and for percutaneous procedures.
It is known in the art that in order to maximize the amount of energy transferred from the source (microwave or RF generator) to the load (surgical implement or tissue), the line and load impedances should match. If the line and load impedances do not match (i.e. impedance mismatch) a reflected wave may be created. The ratio of the forward (primary) wave amplitude to the reflected wave amplitude is expressed as a reflection coefficient Γ. Standing waves within the transmission line may result from constructive and destructive interference between forward and reflected waves. The ratio between the wave maxima resulting from constructive interference and minima resulting from destructive interference is referred to as the voltage standing wave ratio, or VSWR. As an example, an unbalanced transmission line may exhibit an undesirably large VSWR up to around 4:1.
Standing waves created within the transmission line can contribute to the power loss associated with impedance mismatch, cause inaccurate energy dose (i.e., power) measurements, and impair monitoring of parameters associated with the surgical procedure. Moreover, standing waves may cause localized heating and failure of an interconnect (i.e., cable or coaxial cable), and cause premature wear and/or failure of the microwave or RF generator.
Further, during a typical ablation surgical procedure, the impedance at the surgical site changes over the course of the ablation procedure. This is because of tissue necrosis associated with the ablation surgical procedure. Generally, the energy source may include an impedance matching circuit and/or tuner, which may be configured to compensate for impedance changes at the surgical site.
Conventional impedance matching circuits may include devices such as motor-driven variable reactive elements, i.e., vacuum variable capacitors, and as a result may be large in size. In addition, because the energy source may have a much smaller wavelength than the length of the transmission line, it is often difficult to achieve accurate impedance matching in a rugged, reliable, and relatively compact design.