Heating patterns generated by phase modulation of a hexagonal array of interstitial antennas

Interstitial microwave transition from hyperthermia to ablation: historical perspectives and current trends in thermal therapy

This work reviews the transition from hyperthermia to ablation for cancer treatment with interstitial microwave (MW) antennas. Early work utilising MW energy for thermal treatment of cancer tissue began in the late 1970s using single antennas applied interstitially or the use of multiple interstitial antennas driven with the same phase and equal power at 915 or 2450 MHz. The original antenna designs utilised monopole or dipole configurations. Early work in thermal therapy in the hyperthermia field eventually led to utilisation of these antennas and methods for MW ablation of tumours. Efforts to boost the radiated MW power levels while decreasing antenna shaft temperatures led to incorporation of internally cooled antennas for ablation. To address larger tumours, MW treatment utilised arrays that were simultaneously activated by either non-synchronous or synchronous phase operation, benefiting both hyperthermia and ablation strategies. Numerical modelling was used to provide treatment planning guidance for hyperthermia treatments and is expected to provide a similar benefit for ablation therapy. Although this is primarily a review paper, some new data are included. These new data show that three antennas with 2.5 cm spacing at 45 W/channel and 10 min resulted in a volume of 89.8 cm(3) when operated synchronously, but only 53.4 cm(3) non-synchronously. Efficiency was 1.1 (synchronous) versus 0.7 (non-synchronous). MW systems, treatment planning, and image guidance continue to evolve to provide better tools and options for clinicians and patients in order to provide better approach and targeting optimisation with the goal of improved treatment for the patient.

Mix-mode technique of feeding arrays of dipole microwave antennae

A 4-element array of coherently driven dipole microwave antennae produces a hot spot in the central region of the implanted volume and cold spots in the peripheral regions of the implant. Conversely, an incoherently driven array of antennae predominantly heats peripheral (along the antennae) regions of the implant. These two modes of feeding the antennae are complementary in a sense that the cold zones obtained with the coherently driven antennae coincide with the hot zones obtained with the incoherently driven antennae and vice versa. The SAR distributions resulting from mixing these modes of feeding (mixed-mode technique) were studied theoretically and experimentally. A theoretical model that allowed calculation of SAR distributions of a 4-element array of microwave antennae fed coherently, incoherently or using a mixed-mode technique was developed in this work. The goal of the theoretical study was to determine the proper mix of the coherent and incoherent modes of feeding the antennae such that the adequately (enclosed within a 50% isoSAR surface) heated volume was maximized. In experimental studies, the antennae were driven in a cyclical manner with a duty cycle equal to the weight of the coherent mode in the mix. The duty cycle was defined as the ratio of the time the antennae were driven coherently to the total duration of the cycle. To facilitate the periodical change from the coherent to incoherent feeding, a special electromechanical switch was developed. This switch allowed a wide range of variation of the duty cycle and cycle period. Theoretical and experimental studies have demonstrated that, if the relative weight of the coherent feeding in the mix-mode technique was 30% (duty cycle = 0.3), the adequately heated volume was significantly larger and the SAR distribution was more uniform than those obtained with either the coherent or incoherent mode of feeding.

Three-dimensional finite element analysis of current density and temperature distributions during radio-frequency ablation

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. We used validated three-dimensional finite element models to perform these analyses. We found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density decreased more slowly with distance from the electrode surface. We analyzed the effects of three tissue-electrode angles: 0, 45, and 90 degrees. More power was needed to reach a maximal tissue temperature of 95 degrees C after 120 s when the electrode-tissue angle was 45 degrees. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45 degrees than for 0 and 90 degrees. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.

Effect of phase modulation on the temperature distribution of a microwave hyperthermia antenna array in vivo

Perfused, canine skeletal muscle and the brain tumour of a cancer patient were heated with an array of four parallel, interstitial antennas placed on the corners of a 2-cm square and driven at 915 MHz. The temperature distributions along the axial and diagonal catheters were measured with equal-phase driving of the antennas and with several time-varying schemes of driving phase differences among the antennas. When equal-phase driving was replaced by a rotating scheme of 90 degrees driving phase differences, the tissue area in the junction plane heated above a normalized index temperature of 0.6 increased by a factor of about 1.25. With a rotating phase of 135 degrees, the same area increased by a factor of about 1.6. The axial temperature distribution was not affected significantly by driving phase.