Microwave Ablation: Antenna Design and Application
Imagine a doctor diagnoses you with cancer. You would want the doctors to have the latest technology available for treating this cancer at their disposal.
You are not going to want them to use a previous-generation technology for treating this cancer, but the latest technology that has been proven to kill cancer. You would want the best chances of obliterating cancer and living as long as possible.
Microwave ablation technology is a relatively new way to treat various types of cancer in the human body. The antenna design is the most important aspect of this equipment as it pierces the tumor and emits the ablative energy into the tumor to quickly kill the diseased tissue.
Today we are going to talk about the pros and cons of various antenna designs and their applications. Let’s check them out.
The Microwave Ablation Antenna Defined
The entire microwave system for ablation consists of three parts. A microwave generator, a coaxial cable, and an antenna.
Microwave generators are commonly built using either solid-state devices or vacuum tubes, such as magnetrons found in home microwave ovens, to provide the power required to heat and kill cancerous tissue.
Solid-state amplifiers and oscillators operating at designated frequencies for medical equipment are readily available for this application and provide sufficient power for quickly heating and killing tumors. Magnetrons are also available for these systems but offer fewer features such as frequency and power control to the surgeon, so solid-state amplifiers and oscillators are preferred for these systems.
The coaxial cable carries the microwave energy from the microwave generator to the antenna that is embedded within a tumor. We use coaxial cable because of its relative flexibility to bend inside the human body to not damage healthy tissue and to optimally place the ablation antenna inside the tumor to be killed.
It is highly important in a hospital setting that coaxial cables be flexible and easy to connect to the microwave generator and ablation antenna. The coaxial cable is enclosed inside of a catheter inside the human body to maintain sterile conditions for the procedure, and the ablation antenna is located at the end of the catheter inside of a tumor.
The catheter is essential to maintain a sterile environment for the patient to prevent an infection from occurring from contamination by placing the antenna and coaxial cable inside the human body.
The antenna is the most key component of a microwave ablation system. Without the antenna, we could not deliver the energy to the cancerous tissue.
The ablation antenna is connected to the coaxial cable, both of which are enclosed by a sterile catheter. The ablation antenna is guided into the cancerous tumor by the surgeon by remotely bending the catheter to optimally guide the antenna into a specific location inside the tumor.
Some catheters have a white band, which indicates where the microwave radiation will emit from the antenna and be deposited into the tumor. This visual aid can assist the surgeon in locating the ablation antenna inside of a tumor.
Surgeons also use ultrasound imaging equipment to optimally locate the ablation antenna into a tumor inside the body using minimally invasive surgical techniques, which minimizes the possibility of an infection and speeds recovery after the ablation procedure.
Antenna Energy Output Design
Ablation antenna radiation patterns can be optimized to heat the tissue contained in specific shapes of tumors, so the shape of the heating zone can be optimized. Simple ablation antennas only radiate in the typical broadside or omnidirectional radiation pattern. This uniformly sends the radiation outward from the ablation antenna to destroy tissue in the shape of a cylinder.
However, many tumors treated with this therapy are also spherical, so the ablation antenna is placed down the center of this tumor. The broadside or omnidirectional radiation pattern is ideal for treating tumors of this shape.
Furthermore, omnidirectional radiation may not be ideal for cardiac ablation applications where the surgeon requires energy focused into a specific volume of tissue to be removed from a heart valve. In this case, the surgeon may want an antenna that radiates only out of the end of the catheter to create heat in a specific location within the heart. Ablation antennas can also be designed to radiate radially over a limited volume of a cylinder for a tumor adjacent to the antenna. Ablation antenna radiation patterns can be optimally selected for the location of the catheter with respect to the tumor or diseased tissue to be heated.
Finally, an ablation antenna system must be designed to maximize the transfer of power from the microwave generator to the tumor tissue, so power reflected by the antenna must be minimized. Why? This is necessary to maximize the amount of energy that is coupled into the tissue and to minimize wasted power.
Microwave power reflected from the ablation antenna causes heating of the coaxial cable. The heat dissipated by the coaxial cable can be sufficient to burn or damage tissue along the length of the catheter inside the human body, which is not part of the treatment procedure. Therefore, ablation antenna systems are designed to minimize reflected power for specific types of tumor tissue, so the right equipment must be used for each type of tumor being treated.
There are four designs that achieve this objective. These designs are based upon their radiation patterns.
There are three types of linear (wire) elements that have been used for microwave ablation antennas. The triaxial antenna is a coaxial monopole antenna that is inserted into a tumor using a catheter passing through a biopsy needle.
The triaxial antenna commonly reduces reflected power, commonly referred to as return loss, by more than 10 dB and limits heating along the coaxial cable.
Unfortunately, these antennas are mostly narrowband, which can cause hot-spots in the tissue or elongate the radiation patterns in the tumor tissue. This problem can usually be overcome with tip-loading.
This design uses one or more small slots cut into the outer conductor of a coaxial cable for radiation from an antenna contained inside a coaxial cable. Surgeons commonly use this ablation antenna for smaller volume treatments.
The biggest downside to this design is the backward heating along the antenna shaft.
This ablation antenna design has been improved by using an open sleeve. The open sleeve concept improved the absorption rate distribution and reduced the antenna’s sensitivity to insertion depth. It also eliminates shield radiation from the outer diameter of the coaxial cable.
However, the sleeves and chokes used for the slot antenna made can make the assembly bulky, which limits the ability of the surgeon to bend the assembly inside of the catheter to guide the antenna into a tumor.
Loop and Helix
These are the least common ablation antenna designs, but they useful for microwave ablation therapy. Looped antenna designs radiating in the axial mode are commonly used for cardiac ablation therapy. Others use the helix design for assisted angioplasty. Researchers have also tested the single and multi-loop designs for ablation therapy of liver tissue.
The Most Efficient Design
A surgeon needs the right tool for optimally ablating each type of tumor encountered. SiberSci, LLC can provide tumor tissue characterization, numerical modeling, and microwave ablation antenna design services, as well as experimental validation of these antennas. We take your performance specifications and surgical requirements and work with you in all areas from theoretical design, prototyping, laboratory testing, design optimization, and transitioning to manufacturing requirements.
Your clients will have the most efficient and adaptive microwave ablation machines on the market.
Contact us today and we’ll start solving your problems right away.