Technical Design Considerations for Selecting Selecting the Optimal Coaxial Cable for Use in Today’s RF/Microwave Ablation Applications

Originally published with Medical Design Briefs

Patient safety, cost, and efficiency will always be the top drivers shaping the evolution of healthcare. As technology advances, more medical and surgical procedures can be completed in noninvasive ways, reducing costs, risks, and recovery time.

These improvements include minimally invasive procedures that provide treatment options such as radiofrequency and microwave ablation. Radiofrequency ablation uses electrical energy, and microwave ablation uses microwave energy to ablate tissue using heat generated from alternating currents. These devices fall under the collective category of electrosurgical ablation. Ongoing advances in energy delivery have continually made this type of ablation system safer and more efficient.

Electrosurgical ablation systems operate in the ISM (industrial, scientific, medical) in the radiofrequency range of 915 MHz, 2.45 GHz, and 5.80 GHz at power levels of 50 W (+47 dBm) or more. This enables them to achieve thermal effects within biological tissue, heating precise areas with an optimal axial ratio to destroy abnormal cells, avoiding invasive surgical procedures and long recovery times. One of the most significant uses of electrosurgical ablation is on unresectable liver, lung, and kidney tumors. This technique is also used to shrink other growths in the body and treat various conditions, including chronic venous insufficiency in the legs and chronic back and neck pain.

THE ROLE OF COAXIAL CABLES IN ABLATION

Electrosurgical ablation devices consist of a generator and a handpiece with one or more electrodes. The generators produce a variety of electrical waveforms, and as these waveforms change, so do the corresponding tissue effects. The systems use tiny antennas or probes projected onto a patient’s body to focus electromagnetic (EM) energy on the treated tissues. The EM energy is coupled to the antenna or probe by a small-diameter, low-loss coaxial cable assembly.

There are several important coaxial cable assemblies for ablation systems, including the lead cable connecting the generator to the handpiece and the micro-coaxial cable within the handpiece. This application uniquely requires cables that balance optimal power delivery with thermal properties and limitations. As a result, coaxial cable assemblies for RF and microwave ablation are very specialized.

Since power handling becomes an issue when the cable size is limited, as it is in ablation, trade-offs are made to balance power efficiency, heat dissipation, flexibility, and ergonomics. Therefore, cable and connector designs must be customized to address specific application needs. Furthermore, choosing optimal coaxial cable assemblies to achieve ideal performance is crucial because life may depend on it.

Additionally, the ultimate conundrum with coaxial cable designs is that the smaller the device is, the higher the thermal density within power circuits. Often, designing the optimal assembly requires very specific cables for high power, with maneuverability to perform in environments with repeated movement; kink-resistant designs to help with repeated flexure and easy maneuvering in tight and compact places; and sufficient isolation to prevent interference. Key considerations in choosing a high-performance coaxial cable solution therefore include:

• Low loss.
• Power handling and thermal management.
• Reliability and safety.
• Shielding effectiveness.
• Ease of use and integration.

Low Loss. Cable loss or attenuation is a function of its dielectric and conductive materials, length, diameter, and operating frequency. What makes a cable more flexible or lower loss is how those building blocks are combined from a material standpoint, from the center connector and dielectric to the braid and the outer jacket.

For example, low loss is an essential requirement for lead cable assembly due to its long length. A lossy lead cable can significantly impact power transmission efficiency. The loss also increases with frequency, and excessive loss may cause the cable and ablation antenna’s temperatures to rise, resulting in a large amount of dissipated heat along the signal path to the antenna.

Power Handling and Thermal Management. An ablation device may need more than 100 W of output power from the generator to be effective. This presents a cabling challenge — because longer cables may be required to connect the generator and device, and as cable length increases, so does loss, reducing the power yield. For example, a lead cable assembly with 3 dB of insertion loss will deliver only half of the output power from the generator. The loss is dissipated through the cable as heat.

A cable with elevated surface temperature can be a risk to both the patient and the physician performing the procedure. Most medical procedures can only be safely executed within a controlled temperature range. Technical standards such as IEC 60601-1 provide acceptable temperature requirements.

Managing dissipated heat from cable loss may require active cooling to keep cable temperature at an acceptable level. Some manufacturers use liquid coolant to control the temperature throughout the cable assembly. Active cooling can be implemented as a closed-loop system, circulating fluid through one or several send and return lines separated from the electrical components.

Elements in a coaxial cable, such as metal braid and outer jacket, can aid thermal dissipation or act as insulators, preventing the operator and patient from direct contact with hot surfaces. The key is ensuring that the exterior device temperature doesn’t exceed a threshold deemed safe for the patient and the operator.

Reliability and Safety. Safe, reliable performance is closely related to power requirements and heat generation. The integrity of cable-connector junction and connector retention is also important because of the high level of RF or microwave energy transmitted. Intermittency or disruption in the high-power cable or wires connecting various control circuits would cause a severe safety hazard during an ablation procedure.

Electrosurgery applications like RF and microwave ablation do not tolerate equipment failure impacting safety. Therefore, the interconnect system must be reliable and error-proof, with secure and keyed connections to provide safe and efficient power transmission.

Shielding Effectiveness. Shielding effectiveness describes how well a coaxial cable assembly is isolated from nearby electrical devices and/or energy sources. This is key because high energy levels may interfere with other systems using the same frequencies, such as Wi-Fi in the 2.4 GHz band.

Ease of Use and Integration. Micro-coaxial cables used in ablation are typically integrated into higher-level assemblies such as a catheter, so ease of use and integration are essential considerations for optimizing clinical efficiency and ergonomics. The careful selection of cables and connectors can streamline integration and can ensure optimal performance.

Lightweight and seamless device handling are needed to facilitate precise movement and access to treatment spots, especially those in harder-to-reach locations. Cable flexibility can also impact the user’s comfort while performing a procedure. For example, a stiff cable with pronounced shape memory makes it difficult to handle and control the handheld device. Since an ablation procedure can take hours, fatigue can become a significant problem.

Cable flexibility is also essential to optimize device ergonomics. Minimizing interconnect size and weight helps when it can be accomplished without negatively affecting performance, safety, or ergonomics. The small size also enables procedures to be as minimally invasive as possible; smaller openings into the patient’s body help reduce infection risk and recovery time.

CABLING RECOMMENDATIONS

Hybrid Cable Assemblies. An optimal lead cable assembly for connecting the generator to the handpiece must balance many vital requirements in an ablation application, including:

• Low insertion loss.
• High flexibility.
• Low surface temperature.
• Lightweight.
• Integration.
• Aesthetics.

Hybrid cable assemblies provide an integrated solution for power, signal, and thermal management. This type of cable combines a high-power cable for microwave energy delivery, small signal wires that incorporate low voltage lines for LEDs, thermocouples, system control, and fluid lines that circulate a saline solution for cooling — in a single assembly. Hybrid cable assemblies are typically available as a cable bundle, single-ended assembly, or double-ended assembly.

However, this type of assembly is not a standard or off-the-shelf product. In designing a lead cable assembly like the one detailed above, medical device manufacturers should work with a highly qualified partner that is able to understand the needs of the complete device and customize a solution based on its unique needs. The partner should be able to integrate various components, including coaxial cables, insulated wires, optical fibers, fluid lines, connectors, and other mechanical features, into a single package.

Micro-Coaxial Cables. Micro-coaxial assemblies offer small-sized, low-loss coaxial cables that can be integrated into hybrid cable assemblies within the handpiece to deliver power or signals. These are usually flexible or semi-rigid cable assemblies packaged into the handheld ablation device, 047-sized or smaller. This low-loss cable assembly maximizes power transmission efficiency with the small diameter essential for minimally invasive procedures. Furthermore, they are ultra-flexible for easy maneuvering and waterproofed to withstand high fluid pressure in catheter assemblies with integrated cooling. Examples of micro-coaxial cables used in high-performance RF ablation applications include the PhaseTrack ^®-047, SF-047, and InstaBend^®-047 cables from Times Microwave Systems.

TAKEAWAYS

RF and microwave ablation systems present unique equipment challenges, demanding interconnect solutions that are very specialized. Therefore, one crucial guideline is to think of the entire interconnect system holistically in the earliest development and design stages. This can help avoid an unanticipated redesign, which can, in turn, impact regulatory approval and time to market.

As many next-generation electrosurgical systems enter development, the right interconnect components can differentiate between a marketing-leading solution and an average product.

The best results will be achieved by working with a solution partner with dedicated expertise in RF/coaxial cable technologies that can thoroughly understand the system. In addition, the supplier should have in-depth knowledge of RF technologies beyond coaxial cables and connectors; manufacturers should also take the time to learn about the host system and integration. For example, the requirements for thermal management, power delivery, and signal integrity may necessitate creating a hybrid cable assembly solution to address multiple needs.

Choose a supplier offering a broad range of products created from experience in performance-critical industries such as medical, military electronics, aerospace, space, and other rugged environments, and can design custom solutions for a specific application. The manufacturer’s expertise and design capabilities will allow your system to be developed for optimal performance.