Carbon Nanotubes as a Heating Agent in Microwave AblationIntroduction:Microwave Ablation (MWA) is a technique for ablating tumors using microwave frequency electromagnetic energy, to heat the tissue to temperature to 60 degrees Celsius, at which point cell death occurs. In this technique, microwave power is transmitted from a generator through a transmission line to an antenna, inserted into the tumor subcutaneously, where it is radiated as electromagnetic wave into tissue. Although MWA has been successfully completed in human clinical trials, there is room for improvement in the necessary duration and required power level used for the procedure.

Goal:We propose the use of Carbon Nanotubes (CNTs) to raise the effective conductivity of the targeted tissue, allowing it to be heated more quickly, or to be heated at the original speed but using less power

Methods and Materials:1. To simulate the results of the ablation experiments, I used the simulation program CST. I used the data for the complex permittivity of liver tissue from the (hung paper), which had been fit to a single pole cole-cole model (below), for the dielectric properties of the tissue in the simulation.

2. The effective conductivity of the tissue can be found using this equation, which I used to produce modified effective conductivities for the simulations:

σeff = ω * ɛr’’ * ɛ0

3. For the simulations, I recreated this floating sleeve antenna in CST:

4. I ran several microwave simulations using both the original effective conductivity and the effective conductivities that had been modified by a factor of 2x up to 8x. In order to produce the most highly localized SAR pattern- meaning the energy absorption of the tissue is limited to a single, localized area- I tweaked the length of the antenna’s floating sleeve to optimize it for the change in wavelength produced by the modified effective conductivity.

5. Not all of the simulations with modified effective conductivities were able to produce a localized SAR pattern even with a modified floating sleeve length, but for those simulations that did, I went on to run thermal simulations. The thermal simulations indicated the temperatures of the tissue throughout the ablation period.

6. For the simulations that were able to produce localized SAR patterns I also ran thermal simulations with a modified power level, to determine if a simulation with increased effective conductivity but with lower power can return a similar heating pattern as a simulation with the original effective conductivity. I ran thermal simulations with the modified effective conductivity at 50%, 65%, and 70% of the original power level.

Results and Discussion: The only simulation with a modified effective conductivity that returned a localized SAR pattern was the 2x effective conductivity.

The appropriate floating sleeve length for the antenna was roughly ¼ of the wavelength used in the simulation- 20mm for the original effective conductivity, and 20.5mm for the 2x effective conductivity. This longer sleeve length could be due to the slight change in the s11, or return loss, for the higher effective conductivities. The s11’s minimum shifted into longer wavelengths for the higher effective conductivities, so I lengthened the floating sleeve as well to accommodate the change.

These temperature graphs reflect the temperatures at various points in the tissue during the 5 minute ablation period. The points shown on this graph 5mm from the antenna tip, and at a 10mm, 15mm, or 20mm radius from the antenna’s center. The temperatures of both the original and modified effective conductivities are shown, the lines with the 2x effective conductivity indicated with a ‘2x’ in the key.

 

There are large differences in temperature between the different effective conductivities close to antenna, but as long as the temperature of the tissue is above 60 degrees Celsius, the temperature at which cell death occurs, it is irrelevant. More important is that radius of the ablation zone. Even at 20mm from the antenna, the 2x effective conductivity reaches cell death temperatures more quickly that the ablation with the original effective conductivity.

These pictures show the heating patterns of the original and 2x effective conductivity thermal simulations.  

The comparable heating patterns of the top two, despite the 2x effective conductivity pattern coming from just a 3 min. simulation, indicate that an increase in effective conductivity could be used to obtain the same ablation zone radius as the original in less time. The pictures underneath show the heating patterns of the 2x effective conductivity thermal simulation at 5 min, but using only 65% of the original power, and the original effective conductivity at 5 min. The comparable heating patterns between them suggests that increase in effective conductivity can allow for a decrease in power while maintaining the radius of the ablation zone. Note the difference in temperature scale between the two comparisons.

Conclusion:As evidenced by the increase in temperature due to a 2x increase in effective conductivity, a heating agent that modifies effective conductivity can also change the temperature of the ablation zone. More notably, the higher effective conductivity caused the radius of the ablation zone to grow more quickly in the thermal simulations. These changes could allow MWA procedure to be performed more quickly or using reduced power. Further research will be required to verify these results in actual liver tissue. CNTs have been shown to increase the effective conductivity of solutions, but their effects in animal tissue should be explored in order to determine their viability as a heating agent in Microwave Ablation.

References:1. D. Yang , J. M. Bertram , M. C. Converse , A. P. OâRourke , J. G. Webster , S. C. Hagness , J. A. Will and D. M. ��Mahvi  "A floating sleeve antenna yields localized hepatic microwave ablation", IEEE Trans. Biomed. Eng., vol. 53, no. 3, pp.533-537 20062. “Microwave ablation devices for interventional oncology” - Robert C Ward, Terrance T Healey, and Damian E Dupuy.

3. “Clinically relevant CNT dispersions with exceptionally high dielectric properties for microwave theranostic applications” - Shawn X. Xie, Fuqiang Gao, Sunny C. Patel, et al. 4. “The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues” - S Gabriel, R W Lau and C Gabriel, 2 April 1996

Original effective conductivity, after 5 min. of ablation

2x effective conductivity, after 3 min. of ablation

Original effective conductivity, after 5 min. of ablation

2x effective conductivity, after 5 min. of ablation, 65% power

Comparing Temperatures of Tissue at Points with Varying Effective Conductivities

Suzanne O’Meara, Hung Luyen, Prof. Susan Hagness