A promising new method for the treatment of cancer tumors and metastases is microwave or RF ablation. This minimally invasive treatment makes it possible to operate in areas of the body that are inaccessible using conventional means and alleviates post-traumatic complaints, as well as offering significant cost savings. Such minimally invasive interventions are complemented by constantly improving imaging techniques that make it possible to localize even the tiniest tumors or metastases. All of these advantages mean that this new surgical technique offers excellent potential for growth.
Miniature probe disolates tumours
The way that ablation works is by releasing electromagnetic energy inside the tumor. This causes water molecules to vibrate, generating heat and destroying the diseased tissue, which is simple but effective. The great challenge, however, is to channel the microwave power required from the generator into the diseased tissue, adding as little loss as possible while keeping flexibility at an optimum, i.e. the cables, connectors, etc. The type of connectivity required is dictated by the specific application.
Although RF ablation is currently used to treat primary tumors and metastases in the lungs, liver, kidneys and bones, the technique is also increasingly being used for treating other organs. As a result, the system frequencies, the ablation time and the applied electromagnetic power vary considerably. These parameters, along with the ergonomic considerations of the operating environment need to be taken into account when deciding what cables/connectors to use.
Attenuation in coaxial structures
The transmission loss (attenuation), which indicates how much lower the outgoing power is in comparison with the incoming power in a cable, is a major consideration. In the equation:
Figure 1 Attenuation loss components.
this value is negative; however, to avoid confusion, attenuation is often stated as a positive number. Figure 1 shows the attenuation loss components of a cable, σa = conductivity of the inner conductor, σb = conductivity of the outer conductor, εr = dielectric constant and tangent δ = characteristic of insulator material. The total transmission loss is αtotal = αconductors + αdielectric.
The cable attenuation loss is the sum of the conductor losses (e.g. copper losses) and the dielectric losses. In Equations 2 and 3:
f = frequency (GHz) and the diameters d and D are in mm. Z is the characteristic impedance in Ohms [Ω], and ρrD and ρrD represent the material resistivities of the conductor in comparison to copper. That is: ρrD = 1 for a copper inner conductor and ρrD ≈ 10 for a steel outer conductor. δ is the loss angle of the insulating material.
Figure 2 Attenuation loss as a function of the three cable components.
Figure 2 shows the attenuation loss as a function of the three cable components. A lower attenuation loss can be achieved by the following:
- Large cable diameter
- High conductivity of the materials
- Low dielectric constant
- Small loss angle
Because the conductor losses increase proportionally to √f, whereas the dielectric losses increase directly proportionally to f, the losses from the polymer structures used in applications such as RF ablation are considerable (Equation 2 + Equation 3). The two parameters that need to be influenced are εr and tan δ.
Both values are directly linked physically to the presence of material and assume minimum values in a vacuum (εr = 1 and tan δ = 0). Furthermore, the polymers used must provide excellent stability in terms of mechanical and thermal loading, dielectric strength and, most importantly, process capability. The values for typical materials are shown in Table 1.
Figure 3 Supply cable and probe.
Ergonomic design
With regards to the surgical environment, many diverse requirements have to be met. For example, if a robust or even crush-resistant connection is required for a cable that is fed to the operating table, then it is desirable to have a cable with the smallest possible dimensions in the operating area. Also, the supply cable to a probe (see Figure 3) for liver tumor ablation should be as flexible as a cord and should remain positioned on the patient without losing its shape, whereas a cable used for intestinal operations should exhibit a ‘memory effect’ and should return to its original position in a controlled manner.
Surface properties regarding sterilization and ‘coolability’ as well as the choice of color in order to ensure visibility in the operating area and system matching are further examples of parameters that have to be considered when selecting the most suitable cable.
SELECTIVITY
A bent coaxial cable may, depending on the structure and material, develop forces and become deformed over time. Although in most cases this effect should be completely avoided, it can be desirable in certain special catheter applications. For such individual circumstances it is vital for the cable and connector manufacturer to work closely with the end user to select the right choice of connectivity products and develop test procedures to simulate the end-user applications.
Interdisciplinary approach
Just as today’s doctors are expected to be familiar with the use of state-of-the-art equipment, in order to supply and fit the right cables and connectors, engineers must have an understanding of how medical practitioners work and the environment in which they have to function. It is essential to select RF connectors that provide true and easily achievable connection.
This is a major consideration, for example, with the microwave-compatible connectors that an operating team would use, as partial or insecure mating could have serious consequences given the high power levels used in these applications. In such circumstances a push-pull connector that clicks audibly into place, reliably remains connected even when subjected to radial movements and is waterproof, would be the type of connector that should be considered.
Conclusion
RF ablation is a technique being increasingly used in the medical field. These applications are diverse, requiring a variety of cables and connectors to deliver the high power demanded. In order to provide effective, efficient and safe systems, engineers must calculate the attenuation loss of the cable and be aware of the specific characteristics of the cable/connector options on the market in order to be able to meet the specific demands of the medical environment.
Following a four-year apprenticeship as an electrician, Reto Germann gained an engineering degree in mechatronics from FHS St Gallen, Switzerland, and a management diploma from the Chinese-European International Business School (CEIBS), Shanghai, China. He joined HUBER+SUHNER as an Applications Engineer in 2000, became Application Engineering Manager, China, in 2005, and Distribution Manager, APAC a year later. Since 2008 he has been Market Manager Medical, Radio Frequency Division.
