Time-multiplexed two-channel capacitive radiofrequency hyperthermia with nanoparticle mediation

Abstract

Background: Capacitive radiofrequency (RF) hyperthermia suffers from excessive temperature rise near the electrodes and poorly localized heat transfer to the deep-seated tumor region even though it is known to have potential to cure ill-conditioned tumors. To better localize heat transfer to the deep-seated target region in which electrical conductivity is elevated by nanoparticle mediation, two-channel capacitive RF heating has been tried on a phantom.

Methods: We made a tissue-mimicking phantom consisting of two compartments, a tumor-tissue-mimicking insert against uniform background agarose. The tumor-tissue-mimicking insert was made to have higher electrical conductivity than the normal-tissue-mimicking background by applying magnetic nanoparticle suspension to the insert. Two electrode pairs were attached on the phantom surface by equal-angle separation to apply RF electric field to the phantom. To better localize heat transfer to the tumor-tissue-mimicking insert, RF power with a frequency of 26 MHz was delivered to the two channels in a time-multiplexed way. To monitor the temperature rise inside the phantom, MR thermometry was performed at a 3T MRI intermittently during the RF heating. Finite-difference-time-domain (FDTD) electromagnetic and thermal simulations on the phantom model were also performed to verify the experimental results.

Results: As compared to the one-channel RF heating, the two-channel RF heating with time-multiplexed driving improved the spatial localization of heat transfer to the tumor-tissue-mimicking region in both the simulation and experiment. The two-channel RF heating also reduced the temperature rise near the electrodes significantly.

Conclusions: Time-multiplexed two-channel capacitive RF heating has the capability to better localize heat transfer to the nanoparticle-mediated tumor region which has higher electrical conductivity than the background normal tissues.

Fig. 1

A schematic diagram of the phantom. It consists of four copper electrodes a, a distilled water layer b, a normal-tissue-mimicking main body c and a tumor-tissue-mimicking insert d. I₁ and I₂ are the RF currents at channel 1 and channel 2, respectively

Fig. 2

A schematic diagram of the two-channel RF driving system

Fig. 3

A timing diagram of the alternating RF power delivery to the two channels. a RF amplifier output currents and the switching signals. b The RF current at each channel

Fig. 4

The RF switching circuit for two-channel driving. a A circuit diagram and b a photography of the RF switching circuit

Fig. 5

Impedance matching of the phantom circuit. a A circuit diagram of the impedance matching circuit and b a photography of the impedance matching circuit connected with the phantom. An external inductance L and two external capacitors, C_T and C_M, are connected with the phantom circuit modeled by a parallel circuit of C_phantom and R_phantom

Fig. 6

The FDTD model used for the electromagnetic and thermal simulations. It consists of four copper electrodes a, a distilled water layer b, a normal-tissue-mimicking main body c, a tumor-tissue-mimicking insert d, and an outer acrylic frame e

Fig. 7

RF heating experiments with intermittent MR thermometry. a The timing sequence of the RF heating and MR thermometry experiments. b A photography of the phantom placed in the birdcage coil near the 3T MRI magnet

Fig. 8

SAR and temperature maps in the simulations and experiments. a Simulated SAR maps for the one-channel and two-channel RF heating. b Simulated and experimental temperature maps at the time of 24 min RF heating

Fig. 9

An axial-view image of the phantom and oil tubes and the B_o-drift-induced phase map. a An axial-view MR image of the phantom and four oil tubes. b The B_o-drift-induced phase map computed by applying bilinear interpolation to the phases at the oil tube position, which demonstrates possible temperature errors ranging from −2.11 to 0.23 °C if not corrected

Fig. 10

The experimental temperature maps at the time of 8, 16, and 24 min RF heating. The top and bottom rows are the temperature maps for the one-channel and two-channel RF heating cases, respectively

Fig. 11

Temperature profiles along the central horizontal line of the phantom. a The profiles in the simulation and b in the experiment

Fig. 12

Temperature errors in MR thermometry. a The MR temperature maps taken without RF heating at the time of 8, 16, and 24 min. Phase correction reduces spatial temperature fluctuations on the temperature maps. b The temperature errors at the center of the phantom between the optic fiber sensor reading and MR thermometry