Multilayer conformal applicator for microwave heating and brachytherapy treatment of superficial tissue disease.

PURPOSE: The purpose of this study was to construct and perform preliminary functionality evaluations of a multilayer conformal applicator with provisions for thermal monitoring, tight conformity and simultaneous microwave heating and brachytherapy treatment of large-area contoured surfaces. MATERIALS AND METHODS: The multilayer conformal applicator consists of thermal monitoring catheters for fibre-optic monitoring of skin temperatures, a waterbolus, a PCB microwave antenna array, a dielectric spacer for brachytherapy considerations, brachytherapy catheters for delivering HDR radiation and an inflatable air bladder for improving conformity to contoured surfaces. The applicator also includes an elastic attachment structure to hold the applicator securely in place on the patient. The conformity of the applicator to irregular surfaces was evaluated through CT imaging of the applicator fitted onto a life-sized human torso phantom. The fluid flow dynamics of the waterbolus, which impact the effectiveness of temperature control, were evaluated with thermometry during a 19 degrees C step change temperature of the circulating water. RESULTS: CT imaging showed improved conformity to the torso phantom surface following the application of gentle inward pressure from inflating the outer air bladder. Only a small number of 1-5 mm sized air gaps separated the conformal applicator and tissue surface. Thermometry testing of the bolus fluid flow dynamics demonstrated temperature uniformity within +/-0.82 degrees C across a 19 x 34 x 0.6 cm area bolus and +/-0.85 degrees C across a large 42 x 32 x 0.6 cm area bolus. CONCLUSION: CT scans of the applicator confirmed that the applicator conforms well to complex body contours and should maintain good conformity and positional stability even when worn on a mobile patient. Thermometry testing of two different waterbolus geometries demonstrated that uniform circulation and temperature control can be maintained throughout large, complex bolus shapes.

Evaluation of a dual-arm Archimedean spiral array for microwave hyperthermia.

PURPOSE: This effort describes a third-party performance evaluation of a novel, commercial, dual-armed Archimedean spiral array hyperthermia applicator. The applicator is analysed for its ability to couple efficiently into muscle equivalent phantom loads, operate over a broad bandwidth to help accommodate variable tissue properties and generate predictable and repeatable SAR contours that are adaptable to clinically probable disease shapes. MATERIALS AND METHODS: Characterization of the applicator includes E-field and return-loss measurements in liquid muscle tissue-equivalent phantom, as well as comparison of 'treatment-planning' simulations of several possible array SAR patterns with measured SAR from non-coherently driven spiral array antennae. RESULTS: The applicator demonstrates a reasonably low return loss over a large bandwidth and the ability to generate a very uniform heating pattern. Ability to adjust SAR contours spatially to fit specific shapes is also demonstrated. CONCLUSIONS: This device should prove a welcome addition to a currently limited set of superficial heating applicators to provide controllable heating of superficial tissue disease.

Phantom and animal tissues for modelling the electrical properties of human liver.

The dielectric properties of human liver were characterized over the frequency range of 0.3-3 GHz for freshly excised tissue samples of primary hepatocellular carcinoma, metastatic colorectal carcinoma, and normal liver tissues resected from the tumour margin. On average, the dielectric constant (epsilon(r)) of freshly excised human liver tumour was 12% higher than that of surrounding normal liver, and the electrical conductivity (sigma) of tumour was 24% higher. In order to establish suitable tissue models for human liver, the electrical properties were compared to measurements of homogenous phantom mixtures, in vitro bovine liver, and in vivo canine and porcine liver tissues. The data demonstrate that there are several animal tissues that can be used to model the average dielectric properties of human liver reasonably accurately, and use of the most readily available bovine liver appears well-justified, even when stored for up to 10 days in a refrigerator. Additionally, the dielectric properties of in vitro liver remained stable over a large temperature range, with sigma rising only 1.1%/ degrees C in porcine liver (15-37 degrees C) and 2.0%/ degrees C in bovine liver (10-90 degrees C), and epsilon(r) decreasing < or =0.2%/ degrees C in both tissues. This effort identifies homogeneous solid and liquid phantom models and several heterogeneous in vitro tissues that adequately model the dielectric properties of human liver tumours for use in quantitative studies of microwave power deposition in liver.

SAR pattern perturbations from resonance effects in water bolus layers used with superficial microwave hyperthermia applicators.

This study examines the effect of various thickness water bolus coupling layers on the SAR (Specific Absorption Rate) patterns from Dual Concentric Conductor (DCC) based Conformal Microwave Array (CMA) superficial hyperthermia applicators. Previous theory has suggested that water bolus coupling layers can be considered as a dielectric resonator; therefore, it is possible for the impinging electric field to stimulate volume oscillations and surface wave oscillations inside the water bolus. These spurious oscillations will destructively or constructively interact with the impinging electric field to cause a perturbation of the applicator SAR pattern. An experiment was designed which consisted of mapping the electric field produced by a four element DCC CMA applicator in liquid muscle phantom at depths of 5 and 10mm in front of four different thickness water boli; 0 (no bolus) 4, 9 and 13mm. Using the Finite Difference Time Domain (FDTD) method, SAR distributions were calculated for similar test cases. It was found that for water bolus thicknesses of 9mm or greater, there is a marked perturbation of both experimental and theoretical SAR distributions. It is believed that this perturbation is experimental confirmation of the volume and surface wave oscillation theory described by previous investigators.

Dual-mode antenna design for microwave heating and noninvasive thermometry of superficial tissue disease.

Hyperthermia therapy of superficial skin disease has proven clinically useful, but current heating equipment is somewhat clumsy and technically inadequate for many patients. The present effort describes a dual-purpose, conformal microwave applicator that is fabricated from thin, flexible, multilayer printed circuit board (PCB) material to facilitate heating of surface areas overlaying contoured anatomy. Preliminary studies document the feasibility of combining Archimedean spiral microstrip antennas, located concentrically within the central region of square dual concentric conductor (DCC) annular slot antennas. The motivation is to achieve homogeneous tissue heating simultaneously with noninvasive thermometry by radiometric sensing of blackbody radiation from the target tissue under the applicator. Results demonstrate that the two antennas have complimentary regions of influence. The DCC ring antenna structure produces a peripherally enhanced power deposition pattern with peaks in the outer corners of the aperture and a broad minimum around 50% of maximum centrally. In contrast, the Archimedean spiral radiates (or receives) energy predominantly along the boresight axis of the spiral, thus confining the region of influence to tissue located within the central broad minimum of the DCC pattern. Analysis of the temperature-dependent radiometer signal (brightness temperature) showed linear correlation of radiometer output with test load temperature using either the spiral or DCC structure as the receive antenna. The radiometric performance of the broadband Archimedean antenna was superior compared to the DCC, providing improved temperature resolution (0.1 degree C-0.2 degree C) and signal sensitivity (0.3 degree C-0.8 degree C/degree C) at all four 500 MHz integration bandwidths tested within the frequency range from 1.2 to 3.0 GHz.