Dual Concentric Conductor Research Articles

Vesicoureteral Reflux in Children: A Phantom Study of Microwave Heating and Radiometric Thermometry of Pediatric Bladder

We have investigated the use of microwave heating and radiometry to safely heat urine inside a pediatric bladder. The medical application for this research is to create a safe and reliable method to detect vesicoureteral reflux, a pediatric disorder, where urine flow is reversed and flows from the bladder back up into the kidney. Using fat and muscle tissue models, we have performed both experimental and numerical simulations of a pediatric bladder model using planar dual concentric conductor microstrip antennas at 915 MHz for microwave heating. A planar elliptical antenna connected to a 500 MHz bandwidth microwave radiometer centered at 3.5 GHz was used for noninvasive temperature measurement inside tissue. Temperatures were measured in the phantom models at points during the experiment with implanted fiberoptic sensors, and 2-D distributions in cut planes at depth in the phantom with an infrared camera at the end of the experiment. Cycling between 20 s with 20 Watts power for heating, and 10s without power to allow for undisturbed microwave radiometry measurements, the experimental results show that the target tissue temperature inside the phantom increases fast and that the radiometer provides useful measurements of spatially averaged temperature of the illuminated volume. The presented numerical and experimental results show excellent concordance, which confirms that the proposed system for microwave heating and radiometry is applicable for safe and reliable heating of pediatric bladder.

Non-invasive vesicoureteral reflux detection: Heating risk studies for a new device

Objective To investigate a novel non-invasive device developed to warm bladder urine and to measure kidney temperature to detect vesicoureteral reflux. Materials and methods Microwave antennas focused energy within the bladder. Phantom experiments measured the results. The heating protocol was optimized in an in-vivo porcine model, and then tested once, twice and three times consecutively in three pigs followed by pathologic examinations. Results Computer simulations showed a dual concentric conductor square slot antenna to be the best. Phantom studies revealed that this antenna easily heated a bladder phantom without over heating intervening layers. In-vivo a bladder heating protocol of 3 min with 30 W each to two adjacent antennas 45 s on 15 s off followed by 15 min of 15 s on and 45 s off was sufficient. When pigs were heated once, twice and three times with this heating protocol, pathologic examination of all tissues in the heated area showed no thermal changes. More intensive heating in the animal may have resulted in damage to muscle fibers in the anterior abdominal wall. Conclusions Selective warming of bladder urine was successfully demonstrated in phantom and animals. Localized heating for this novel vesicoureteral reflux device requires low-power levels and should be safe for humans.

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.

Non-invasive temperature profile estimation in a lossy medium based on multi-band radiometric signals sensed by a microwave dual-purpose body-contacting antenna

Microwave radiometry has during the past two decades been investigated as a non-invasive scheme for measurement of subcutaneous tissue temperatures, basically for monitoring and control in hyperthermic treatment of cancer. In this effort, we test a contact-type, dual-purpose antenna with integral water bolus. To overcome conflicting optimization criteria in the integration of this thermometry technique with heat applicators exhibiting a large effective field size during superficial hyperthermia, a stacked configuration design is proposed, where the radiometer receive antenna (Archimedean spiral) is located on the front (skin) surface of the water bolus and the heating antenna (Dual-Concentric Conductor aperture) is placed on the bolus back surface. The motivation is to achieve homogeneous tissue heating simultaneously with non-invasive thermography of the target tissue under the applicator. This paper addresses the feasibility of predicting one-dimensional depth temperature profiles from multi-band brightness temperatures. The performance is investigated statistically by a Monte Carlo technique on both simulated and real heated-phantom data using up to six radiometric bands. Analysis of measured data shows that during the transient heating period in a solid lossy phantom, the inversion technique exhibits a precision (2 † T ) and skewness (bias) of estimated compared to actual temperature profiles of better than - 0.38°C and - 0.55°C, respectively.

Theoretical characterization of dual concentric conductor microwave applicators for hyperthermia at 433 MHz

Radiation patterns from multi-aperture arrays of Dual Concentric Conductor (DCC) applicators for superficial microwave hyperthermia were calculated numerically using the Finite Difference Time Domain (FDTD) method at an operating frequency of 433MHz. Different aperture sizes were studied from 3.5-7cm square in six-element array configurations which may be considered as building block elements of much larger arrays. Once optimum parameters were established for gap width, inter-element spacing, and bolus thickness for each aperture size using repetitive parametric FDTD studies, a direct comparison of power deposition (SAR) patterns was performed for similar DCC arrays driven at 915MHz and 433MHz. Results show that clinically desirable uniformity and penetration of SAR can be obtained for square apertures in the range of 4-6cm per side at 433MHz, using 9.5-12.5mm thick water coupling boluses, as compared to the optimum 2-4.5cm square apertures and 5-10mm water bolus dimensions determined previously for 915MHz arrays. With an accompanying reduction in the lateral adjustability of power control, the 433MHz applicators can provide a significantly larger heating area than arrays with the same number of elements driven at a frequency of 915MHz. Penetration depth is similar for optimally sized apertures at both frequencies of operation, providing a maximum heating depth of 1-2cm for the DCC array applicators.

Theoretical characterization of dual concentric conductor microwave applicators for hyperthermia at 433 MHz

Radiation patterns from multi-aperture arrays of Dual Concentric Conductor (DCC) applicators for superficial microwave hyperthermia were calculated numerically using the Finite Difference Time Domain (FDTD) method at an operating frequency of 433MHz. Different aperture sizes were studied from 3.5-7cm square in six-element array configurations which may be considered as building block elements of much larger arrays. Once optimum parameters were established for gap width, inter-element spacing, and bolus thickness for each aperture size using repetitive parametric FDTD studies, a direct comparison of power deposition (SAR) patterns was performed for similar DCC arrays driven at 915MHz and 433MHz. Results show that clinically desirable uniformity and penetration of SAR can be obtained for square apertures in the range of 4-6cm per side at 433MHz, using 9.5-12.5mm thick water coupling boluses, as compared to the optimum 2-4.5cm square apertures and 5-10mm water bolus dimensions determined previously for 915MHz arrays. With an accompanying reduction in the lateral adjustability of power control, the 433MHz applicators can provide a significantly larger heating area than arrays with the same number of elements driven at a frequency of 915MHz. Penetration depth is similar for optimally sized apertures at both frequencies of operation, providing a maximum heating depth of 1-2cm for the DCC array applicators.

Performance Evaluation of Various Antenna Configurations for Microwave Thermography During Superficial Hyperthermia

Microwave thermography is currently under investigation for its potential in non-invasive monitoring and control of tissue temperature during superficial microwave hyperthermia treatments. In this effort, six antennas were tested for their suitability as radiometric probes in terms of penetration depth, signal-to-noise ratio, and the sensitivity and accuracy of measured temperature signals over the frequency range from 1.2-3.5 GHz. Due to the intended integration of this radiometry technique with large area multi-aperture heat applicators such as Dual Concentric Conductor (DCC) arrays, the size of each receive antenna was limited to 4 cm. The performance of logarithmic and Archimedean spiral microstrip structures was compared to dielectric loaded waveguide antennas. Results showed adequate signal-to-noise ratio for all antennas at frequencies above 2.5 GHz and for all antennas except the electrically small waveguides over the entire frequency range. Overall, the signal sensitivity from 1 to 3 GHz was approximately 2-3 times higher for the three broadband spirals than the waveguides or DCC apertures, and similar at the upper part of the band. Under clinically relevant conditions of probing the tissue load through a 5 mm water bolus maintained at constant 40° C, the spiral antennas all produced comparable accuracy of temperature measurements on the order of 0.15-0.2° C for a uniform temperature load.

Thermal and SAR characterization of multielement dual concentric conductor microwave applicators for hyperthermia, a theoretical investigation.

Six aperture array dual concentric conductor (DCO) microwave hyperthermia applicators were studied using theoretical models to characterize power deposition (SAR) and steady state temperature distributions in perfused tissue. SAR patterns were calculated using the finite difference time domain (FDTD) numerical method, and were used as input to a finite difference thermal modeling program based on the Pennes Bio-Heat Equation in order to calculate corresponding temperature distributions. Numerous array configurations were investigated including the use of different size DCC apertures (2, 3, and 4 cm), different spacing between apertures (1.0-2.0 cm), and different water bolus thicknesses (5-15 mm). Thermal simulations were repeated using blood perfusion values ranging from 0.5 to 5 kg/m3 s. Results demonstrate the ability of DCC array applicators to effectively and uniformly heat tissue down to a depth of 7.5-10 mm below the skin surface for a large number of different combinations of DCC element size, spacing, and water bolus thickness. Results also reveal the close correlation between SAR patterns and corresponding temperature distributions, verifying that design studies of the applicator can be performed confidently by analysis of SAR, from which the thermal behavior can be estimated. These simulations are useful in the design optimization of large microwave DCC array applicators for superficial tissue heating and for identifying appropriate aperture spacing and bolus thickness parameters for different size DCC aperture arrays and tissue blood perfusion conditions.

Transceiving antenna for homogeneous heating andradiometric thermometry during hyperthermia

The authors describe a conformal microwave applicator fabricated from thin, flexible, multi-layer printed circuit board material that facilitates the hyperthermic heating of surface areas covering contoured regions of the anatomy. An antenna structure which combines the axial beam characteristics of an Archimedian spiral antenna with the omni-directional radiation pattern of an annular slot dual concentric conductor antenna is studied with the aim of achieving homogeneous heating simultaneously with non-invasive thermometry of tissue under the applicator.

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.

Effect of complex bolus-tissue load configurations on SAR distributions from dual concentric conductor applicators. Specific absorption rate.

Power deposition [specific absorption rate (SAR)] distributions from a two-element array configuration of 4-cm-square 915-MHz dual concentric conductor (DCC) microwave antennas were characterized theoretically for several clinically realistic complex bolus-tissue load models using the finite difference time domain (FDTD) numerical method. The purpose of this effort was to determine the perturbing effects on SAR of three often unavoidable heterogeneities in the bolus-tissue load. The three cases studied in this work consist of bone (two ribs spaced 1 cm apart) embedded 5-mm or 1-cm deep in muscle or layered fat-muscle tissue, small air bubbles trapped between the coupling bolus and tissue surface, and variable thickness water bolus layer due to sharply contoured anatomy. Results of the FDTD simulations demonstrate rather small effects on SAR distribution for both rib-sized bones > or = 5-mm deep in muscle and small air pockets < or = 1-mm thick. Larger air bubbles > 1-cm diameter by 3-mm depth showed a distinct concentration of SAR near the lateral sides of the air bubbles, and a blocking effect under the bubbles when located directly under the center of a DCC aperture where there is a higher normal E-field component. Variation from 2.5- to 7.5-mm bolus thickness under the two aperture array produced only minor perturbation of the uniformity and penetration of SAR, along with minor reduction in SAR under the thicker bolus which should be accommodated sufficiently by changes in applied power to the array elements.

Radiation patterns of dual concentric conductor microstrip antennas for superficial hyperthermia.

The finite difference time domain (FDTD) method has been used to calculate electromagnetic radiation patterns from 915-MHz dual concentric conductor (DCC) microwave antennas that are constructed from thin and flexible printed circuit board (PCB) materials. Radiated field distributions are calculated in homogeneous lossy muscle tissue loads located under variable thickness coupling bolus layers. This effort extends the results of previous investigations to consider more realistic applicator configurations with smaller 2-cm-square apertures and different coupling bolus materials and thicknesses, as well as various spacings of multiple-element arrays. Results are given for practical applicator designs with microstrip feedlines etched on the backside of the PCB antenna array instead of previously tested bulky coaxial-cable feedline connections to each radiating aperture. The results demonstrate that for an optimum coupling bolus thickness of 2.5-5 mm, the thin, flexible, and lightweight DCC antennas produce effective heating to the periphery of each aperture to a depth of approximately 1 cm, and may be combined into arrays for uniform heating of large area superficial tissue regions with the 50% power deposition contour conforming closely to the outer perimeter of the array.

Effect of practical layered dielectric loads on SAR patterns from dual concentric conductor microstrip antennas

Radiation patterns of 2 and 4 cm square Dual Concentric Conductor (DCC) microstrip antennas were studied theoretically with Finite Difference Time Domain (FDTD) analysis and compared with experimental measurements of power deposition (SAR) in layered lossy dielectric loads. Single and array configurations were investigated with 915 MHz excitation applied across either one, two or four sides, or four corners of the square apertures. FDTD simulations were carried out for realistic models of a muscle tissue load coupled to the DCC antennas with a 5 mm thick bolus of either distilled water or low loss Silicone Oil. This study characterizes the effect on SAR of adding three additional thin dielectric layers which are necessary for clinical use of the applicator. These layers consist of a 0.1 mm thick dielectric coating on the array surface to provide electrical isolation of DCC apertures, and 0.15 mm thick plastic layers above and below the bolus to contain the liquid. Experimental measurements of SAR in a plane 1 cm deep in muscle phantom agree well with theoretical FDTD simulations in the multi-layered tissue models. These studies reveal significant changes in SAR for applicator configurations involving low dielectric constant (εr) layers on either side of a high εr water bolus layer. Prominent changes include a broadening and centring of the SAR under each aperture as well as increased SAR penetration in muscle. No significant differences are noted between the simple and complete load configurations for the low εr Silicone Oil bolus. Both theoretical and measured data demonstrate relatively uniform SAR distributions with 50% of maximum SAR extending to the perimeter of single and multi-aperture array configurations of DCC applicators when using a thin 5 mm water or Silicone Oil bolus.