Electric-field distribution near rectangular microstrip radiators for hyperthermia heating: theory versus experiment in water

Evaluation of a Dielectric-Only Transmitarray for Generating Multi-Focusing Near-Field Spots Using a Cluster of Feeds in the Ka-Band

A transmitarray antenna is evaluated to generate a multi-focusing spot area in the Fresnel region of the antenna in the Ka-band. The antenna is designed to focus the radiated field at a certain point using a central feeding configuration. The number of feeds is increased to create as many focusing spots as feeds. The feeds are placed along an arc defined in the principal planes of the transmitarray, radiating independent near-field spots and providing a solution with a wide-angle spot scanning without an antenna displacement and a high isolation between feeds. To validate this concept, a transmitarray based on dielectric-only cells is designed and simulated under full-wave conditions. Then, this design is manufactured using a 3D printing technique, and the prototype is measured in a planar acquisition range. Measurements are performed for different feed positions in order to validate the multi-focusing capability of the antenna. Measurements and simulations show a high agreement and validate the proposed design technique.

Design and characterisation of miniaturised cavity-backed patch antenna for microwave hyperthermia

PURPOSE

The aim of this study was to describe the design and characterisation of a miniaturised 434 MHz patch antenna enclosed in a metal cavity for microwave hyperthermia treatment of cancer.

MATERIALS AND METHODS

Electromagnetic (EM) field distribution in the near field of a microstrip patch irradiating body tissue was studied using finite element method (FEM) simulations. Antenna miniaturisation was achieved through dielectric loading with very high permittivity, metal enclosure, patch folding and shorting post. Frequency dependent electrical properties of materials were incorporated wherever appropriate using dispersion model and measurements. Antenna return loss and specific absorption rate (SAR) at 434 MHz were measured on muscle phantoms for characterisation.

RESULTS

The design was progressively optimised to yield a compact 434 MHz patch (22 mm × 8.8 mm × 10 mm) inside a metal cavity (40 mm × 12 mm) with integrated coupling water bolus (35 mm). The fabricated antenna with integrated water bolus was self resonant at 434 MHz without load, and has better than -10 dB return loss (S11) with 13-20 MHz bandwidth on two different phantoms. SAR at 434 MHz measured using an infrared (IR) thermal camera on split phantoms indicated penetration depth for -3 dB SAR as 8.25 mm compared to 8.87 mm for simulation. The simulated and measured SAR coverage along phantom depth was 3.09 cm(2) and 3.21 cm(2) respectively at -3 dB, and 6.42 cm(2) and 9.07 cm(2) respectively at -6 dB. SAR full width at half maximum (FWHM) at 5 mm and 20 mm depths were 54.68 mm and 51.18 mm respectively in simulation, and 49.47 mm and 43.75 mm respectively in experiments. Performance comparison of the cavity-backed patch indicates more than 89% co-polarisation and higher directivity which resulted in deeper penetration compared to the patch applicators of similar or larger size proposed for hyperthermia treatment of cancer.

CONCLUSION

The fabricated cavity-backed applicator is self-resonant at 434 MHz with a negligible shift in resonance when coupled to different phantoms, Δf/f0 less than 1.16%. IR thermography-based SAR measurements indicated that the -3 dB SAR of the cavity-backed aperture antenna covered the radiating aperture surface at 5 mm and 20 mm depths. It can be concluded that the compact cavity-backed patch antenna has stable resonance, higher directivity and low cross polarisation, and is suitable for design of microwave hyperthermia array applicators with adjustable heating pattern for superficial and/or deep tissue heating.

Flow patterns and heat convection in a rectangular water bolus for use in superficial hyperthermia

This paper investigates both numerically and experimentally the spatio-temporal effects of water flow in a custom-made water bolus used for superficial hyperthermia generated by a 915-MHz, 4 x 3 microwave applicator array. Similar hyperthermia models referenced in the literature use a constant water temperature and uniform heat flux to describe conduction and convection energy exchange within the heating apparatus available to cool the tissue surface. The results presented in this paper show that the spatially varying flow pattern and rate are vital factors for the overall heat control applicability of the 5 mm thick bolus under study. Regions with low flow rates and low heat convection clearly put restrictions on the maximum microwave energy possible within the limits of skin temperature rise under the bolus. Our analysis is illustrated by experimental flow front studies using a contrast liquid set-up monitored by high definition video and complemented by numerical analysis of liquid flow and heat exchange within the rectangular water bolus loaded by malignant tissue. Important factors for the improvement of future bolus designs are also discussed in terms of diameter and configuration of the water input and output tubing network.

Performance evaluation of a conformal thermal monitoring sheet sensor array for measurement of surface temperature distributions during superficial hyperthermia treatments

PURPOSE

This paper presents a novel conformal thermal monitoring sheet (TMS) sensor array with differential thermal sensitivity for measuring temperature distributions over large surface areas. Performance of the sensor array is evaluated in terms of thermal accuracy, mechanical stability and conformity to contoured surfaces, probe self-heating under irradiation from microwave and ultrasound hyperthermia sources, and electromagnetic field perturbation.

MATERIALS AND METHODS

A prototype with 4 x 4 array of fiber-optic sensors embedded between two flexible and thermally conducting polyimide films was developed as an alternative to the standard 1-2 mm diameter plastic catheter-based probes used in clinical hyperthermia. Computed tomography images and bending tests were performed to evaluate the conformability and mechanical stability respectively. Irradiation and thermal barrier tests were conducted and thermal response of the prototype was compared with round cross-sectional clinical probes.

RESULTS

Bending and conformity tests demonstrated higher flexibility, dimensional stability and close conformity to human torso. Minimal perturbation of microwave fields and low probe self-heating was observed when irradiated with 915 MHz microwave and 3.4 MHz ultrasound sources. The transient and steady state thermal responses of the TMS array were superior compared to the clinical probes.

CONCLUSIONS

A conformal TMS sensor array with improved thermal sensitivity and dimensional stability was investigated for real-time skin temperature monitoring. This fixed-geometry, body-conforming array of thermal sensors allows fast and accurate characterization of two-dimensional temperature distributions over large surface areas. The prototype TMS demonstrates significant advantages over clinical probes for characterizing skin temperature distributions during hyperthermia treatments of superficial tissue disease.

Miniature dipole E-field probes for characterizing both phase and amplitude of microwave radiators for hyperthermia

This paper describes a system for characterizing th electric field patterns of microwave radiators in lossy media, in particular, of those used in hyperthermia treatments for cancer therapy. We discuss the design, fabrication and testing of small, minimally-perturbing electric field probes which are capable of measuring both amplitude and phase. An appropriate test configuration for mapping field patterns radiating from hyperthermia applicators (antennae) also is described. The system was developed specifically for the evaluation of applicators centered at 434 MHz and 915 MHz.

Large stationary microstrip arrays for superficial microwave hyperthermia at 433 MHz: SAR analysis and clinical data

Superficial hyperthermia with present day applicators provides a challenge when tumours exceed several cm in diameter. Unless microstrip applicators are scanned, the usable heat region often falls short of treating the entire region with 50% power or specific absorption rate (SAR). New microstrip applicator designs were evaluated through SAR analysis and compared to the traditional microstrip applicators used in the clinic at Dartmouth over the past six years. The new designs included fabricating thin archimedean spirals (1.0 mm strip width) incorporating dielectric substrate (epsilon = 5.3-10.8). The designs were optimized at 433 MHz for an arm length of 59 cm. Measurements in a plane 1.0 cm from the surface showed that thin spirals outperformed traditional designs by increasing the 50% SAR area by a factor of 2.5, while maintaining the same physical size. Arrays of four elements were fabricated from thin spirals, although SAR evaluation showed only 10-20% SAR between elements. Since this was deemed unacceptable and the design goal was to fabricate a stationary applicator that had at least 50% SAR between elements, dual element designs were created with gradually overlapped elements. It was found that overlapping three coils of the spiral created a large region that equalled or exceeded 50% SAR that could not be matched by single applicators. Coherent operation of the dual spiral array resulted in more central power deposition and incoherent operation resulted in more peripheral power deposition. SAR measurements at the fat/muscle interface showed an elongated heating pattern in hydroxyethylcellulose muscle equivalent phantom. Power deposition 1.0 cm deep in muscle retained the same basic size and shape with or without the fat layer. Patient treatments for chestwall tumours confirmed that the dual overlapping applicator heated a larger region without the sharp temperature peak associated with single applicators.

Pre-clinical evaluation of a microwave planar array applicator for superficial hyperthermia

Multi-element hyperthermia applicators have an advantage over single-aperture devices in that the power deposition pattern across the applicator surface may be adjusted to improve the resultant temperature distribution. This capability can be used to compensate for irregular tumour geometry as well as heterogeneity of thermal and power absorption parameters within the tissue. This paper evaluates the first commercially available microwave system of this type designed for superficial hyperthermia. The applicator (16-element planar array, 915 MHz, 15.2 x 15.2 cm footprint) was evaluated by the following: (1) measuring absolute SAR distributions in muscle-equivalent liquid phantom with an intervening 1 cm thick layer of fat phantom by scanning a calibrated E-field sensor, and (2) power output measurements using calorimetric methods. The SAR distributions measured for each individual aperture exhibited significant irregularities and differing power deposition patterns. A priori knowledge of these different power deposition characteristics was used to provide appropriate illumination schemes which could be used as initial starting points for producing clinically useful power deposition patterns. Measurements of these composite patterns demonstrate the adjustable nature and flexibility of the heating capabilities of this applicator, which includes 50% iso-SAR coverage that can be extended to the applicator perimeter. This clearly illustrates the clinical utility and potential advantages of this system over single-aperture devices for superficial hyperthermia.