Microwave Antenna Array Research Articles

Polyimide membrane for micro-heated gas sensor array

Presented here is the fabrication of a polyimide (PI) membrane for potential use as a low loss microheated metal oxide (MOS) gas sensor array. Such membranes are often made from dielectric layers of silicon nitride or silicon oxide on bulk etched silicon 〈100〉 to fabricate low power sensors. Due to the fragility of these types of membranes, the yield is not good and the dielectric thickness and active area is limited. The area of PI membrane is not limited by the fragility of the material due to its rugged nature. The thickness of polyimide membrane used is in the region of 1–10μm. The elastic modulus of the material enables it to be used in any number of applications ranging from heated sensor support to a microwave antenna array in the communication field. The application described here uses the thin rugged polyimide membrane to reduce the power consumption of a metal oxide gas sensor. Along with the fabrication of the polyimide membrane, an array of micro heaters is integrated on top of the membrane. A sputtered platinum (Pt) meander is resistively used to heat up the sensing layer to its elevated temperature of up to 350°C.

A conductive plastic for simulating biological tissue at microwave frequencies

A conductive plastic composite that exhibits complex dielectric properties similar to biological tissues over the electromagnetic spectrum of 300-900 MHz has been synthesized from compressed carbon black mixed with a castable thermoplastic (polyethyl methacrylate). This paper presents the techniques used to control the electrical properties of the conductive plastic and describes the challenges encountered in fabricating a material containing a high proportion of carbon black. While developed to serve as a housing material for a microwave antenna array for imaging biological bodies, the composite should be useful in any setting requiring a stable, solid, high loss material that simulates biological tissues over the microwave spectrum.

Preliminary report on treatment of bone tumors with microwave-induced hyperthermia.

Between July, 1992, and February, 1995, 62 patients with various bone tumors were treated with microwave-induced hyperthermia. The series had 47 cases of malignant tumors and 15 cases with benign tumors; most of the tumors occurred at or near knee joints (53/62 = 85.4%). The surgical procedure consisted of separating the tumorous segment from surrounding normal tissues with a safe margin, cooling the normal tissues (including the vital neurovascular bundle and the intrajoint structures) with a water circulation system while heating the tumor simultaneously with the microwave antenna array, and providing an adequate soft-tissue cover for the dead bone. The tumor core temperature and the surface temperature reached 108 and 65 degrees C, respectively. The duration of microwave irradiation was usually 40-50 minutes. Meanwhile, the temperature of the normal tissues was kept under 39 degrees C. The minimal and maximal periods of clinical observation were 3 months and 36 months, respectively, and the mean follow-up period was 17 months. The 62 cases were evaluated from both oncological and orthopedic points of view. Five cases had local recurrence and required amputation. The 57 other cases had excellent local control. Six malignancy cases died of lung metastasis during a period of 1-2 years. Pathological fracture occurred at devitalized bone in five cases. In most of the cases, the knee joints functioned well, were stable and painless, and had almost full range of motion. Single-photon emission-computed tomography study in 16 cases revealed that revascularization of the devitalized tumorous bone segment could be accomplished in 1 year or more. These results show that the use of microwave hyperthermia for the treatment of bone tumors can be considered to be a definitive operation procedure that is safe and is well tolerated by patients. The oncological and orthopedic results are very encouraging.

On-axis DC sputtered YBa/sub 2/Cu/sub 3/O/sub 7/(-x) films up to 2" in diameter for microwave antenna arrays

We have extended the planar on-axis DC sputtering process under high oxygen pressure (1-4 mbar) to the epitaxial growth of YBa/sub 2/Cu/sub 3/O/sub 7-x/ films on LaAlO/sub 3/ substrates up to 2 in diameter without scanning. The stability of the accordingly large plasma turned out to depend critically on the homogeneity of the stoichiometric target and its bonding to the water-cooled sputtering cathode. The radiation-shielded heater plate provided temperature uniformity of /spl plusmn/4K at 890/spl deg/C to the 2 substrates. The quality of the large unpatterned films was controlled by inductive as well as by new microwave test systems based on niobium choke-flange or sapphire-loaded cavities. These have been designed for highly sensitive scanning measurements of the surface resistance R/sub s/ at 87 GHz and high power microwave tests at 20 GHz, respectively. Homogeneous low R/sub s/ values have been achieved on the large films up to surface fields of 5 mT at 77 K. A frequency diplexer at 5 GHz for antenna arrays was designed with lumped elements and patterned from a 1 film by means of photolithography, ion milling and wet etching. Its measured performance agrees well with numerical simulations. >

Theoretical limits of SAR distributions of a four‐element square array of dipole‐type antennas

Four-element dipole microwave antenna arrays with square insertion patterns are commonly used clinically for interstitial hyperthermia. One major disadvantage with this type of antenna array is the presence of a large dead length at the tips because the current gradually decreases from maximum at the junctions to zero at the tips. This dead length is usually 1.5-2 cm along the central axis of a 2 x 2 cm array of regular dipole antennas. Many attempts to improve the performance of dipole antenna arrays have been made by designing antennas with increased current at the tips. While some dipole antennas of new design show negligible dead lengths at close proximity in the single-antenna configuration, phantom experiments have demonstrated that these antennas exhibit at least 1.1 cm dead space along the central axis of the four-antenna array. Therefore, there seems to be a limit to which the array dead length can be reduced by the improvements in the dipole-type antenna design. The goal of this work is to find the theoretical minimum of the array dead length. This was done by assuming a uniform current distribution along the entire antenna. The specific absorption rate (SAR) patterns were calculated for an array with an insertion depth of 7 cm (resonant length for 915 MHz) and a variable spacing between antennas (1-3 cm). It was found that there is a dead length of 6 mm along the central axis of the 2 x 2-cm array with the uniform current distribution, which can be considered as the theoretical limit of the dead length for this array.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficient computation of SAR distributions from interstitial microwave antenna arrays

Electromagnetic and thermal modelling aids quantitative treatment by hyperthermia by enabling estimations of the temperature within a treatment volume to be given at locations other than those at which probes exist. The computation of SAR distributions within interstitial microwave dipole arrays is required both in the and near regions of each dipole. Expressions for the approximate solution of the electric field distribution from an asymmetric imbedded dipole are derived and are shown to be accurate for the calculation of SAR distributions in the field. However, inaccuracies in the near result in the need to evaluate an exact solution in this region, requiring a far more CPU intensive solution. To overcome this in the near a combination solution has been derived from the approximate and exact solutions and shown to be in good agreement with the exact solution. The computation of the combination solution requires significantly (70-80%) less CPU time than the exact solution. >

Theoretical and experimental analysis of air cooling for intracavitary microwave hyperthermia applicators.

An intracavitary microwave antenna array system has been developed and tested for the hyperthermia treatment of prostate cancer at Thayer School of Engineering and Dartmouth-Hitchcock Medical Center. The antenna array consists of a choked dipole antenna inserted into the urethra and a choked dipole antenna eccentrically embedded in a Teflon obturator inserted into the rectum. To prevent unnecessary heating of the healthy tissue that surrounds each applicator, an air cooling system has been incorporated into the rectal applicator. The air cooling system was designed and modeled theoretically using a numerical solution of heat and momentum equations within the applicator, and an analytical solution of the Pennes bioheat equation in tissue surrounding the applicator. The 3-D temperature distribution produced by the air-cooled rectal applicator was measured in a perfused canine prostate.

SAR distributions in interstitial microwave antenna arrays with a single dipole displacement.

The use of interstitial microwave antenna array hyperthermia (IMAAH) as a treatment for cancer, in conjunction with radiation therapy and chemotherapy, has been investigated widely. The heating pattern produced by a coherently phased 915 MHz asymmetric antenna array displays the maximum power deposition in the array center. This paper investigates the effect of variable insertion depth between antennas of an array on the heating patterns produced. The "study" of this heating behavior demonstrates a similar effect to that of the variably phased arrays, showing a shift of the heating peak towards the periphery of the tumor, offering a more simple method for the clinical treatment of such tumors.

Three-dimensional theoretical SAR and temperature distributions created in brain tissue by 915 and 2450 MHz dipole antenna arrays with varying insertion depths

Theoretical three-dimensional power deposition and temperature distributions were calculated for interstitial hyperthermia microwave antenna arrays driven at 915 and 2450 MHz in brain tissue. Four dipole antennas were assumed to be placed in a 2 x 2 cm array with varying insertion depths in cylindrical tumour models. The bioheat transfer equation was solved for the three-dimensional steady-state temperature distributions using a finite element method. Homogeneous and non-homogeneous blood flow models were considered. As a basis of comparison of the various temperature distributions, the volume of the tumour heated to greater than or equal to 43 degrees C was calculated. SAR distributions calculated for the 915 MHz antenna arrays in brain tissue were very similar to those calculated for muscle. The 2450 MHz arrays showed similar behaviour to the 915 MHz arrays; however, as the insertion depth increased from slightly less than a full-wavelength there was a single hotspot centred at the antenna junction. For the 2450 MHz arrays, the predicted therapeutic tumour volumes were relatively constant over the entire range of insertion depths considered, and in fact, for most insertion depths considered, the model predicted the 2450 MHz arrays would heat larger therapeutic volumes than the 915 MHz arrays. For the 915 MHz array, at insertion depths between 7.8 and 14.6 cm there was a sharp decrease in the predicted therapeutic volume due to a proximal secondary hotspot in the normal tissue causing overheating. However, when the same size tumour at the same insertion depth was heated with the 2450 MHz array, the hotspot was in the tumour, adding to the volume of tumour that was heated to therapeutic temperatures.

Three-dimensional theoretical temperature distributions produced by 915 MHz dipole antenna arrays with varying insertion depths in muscle tissue

Interstitial microwave antenna array hyperthermia (IMAAH) systems are currently being used in the treatment of cancer. The insertion depth of an interstitial microwave antenna, defined as the length of the antenna from the tip to the point of insertion in tissue, affects its ability to produce uniform power deposition patterns in tumor volumes. The effect of varying insertion depths on the ability of an IMAAH system to heat two theoretical tumor models was examined. Four dipole microwave antennas were implanted in a 2 × 2 cm array and driven at 915 MHz in muscle tissue. The explicit power deposition patterns were calculated for each insertion depth using known theory. The bioheat transfer equation was solved for the 3-dimensional steady-state temperature distributions in cylindrical and ellipsoidal tumor models using a finite element method. Homogeneous and nonhomogeneous blood flow models were considered. As a basis of comparison of the various temperature distributions, the volume of tumor heated to ≥ 43°C was calculated. Under the conditions of this study, the insertion depth was shown to have a significant effect on the ability of an IMAAH system to heat the tumor volumes. A sharp decrease in the percentage of tumor volume heated to ≥ 43°C was seen for insertion depths between 7.8 and 14.6 cm. At an insertion depth of 11.7 cm ( 3 4 λ) there was virtually no heating of the tumor. Regions of elevated power occurred outside of the desired treatment volume, stressing the importance of adequate thermometry techniques and demonstrating the need for hyperthermia treatment planning prior to implantation of an antenna array. Plots of the power deposition patterns and the corresponding temperatures produced in the diagonal plane of the antenna arrays are present.

Phantom studies and preliminary clinical experience with the BSD 2000

The BSD 2000 system is an array of microwave antennas operating in the 60-120 MHz range. It is a four-quadrant regional hyperthermia device with phase control permitting the operator to choose the point of constructive interference. A computer preplanning system is provided. We have compared the computer predictions with measured relative specific absorption rate (SAR) distributions in muscle equivalent phantoms and pig cadaver. The predicted SAR distribution is in qualitative agreement with observation, although differences of approximately 3-5 cm in the location of iso- SAR contours were noted. The longitudinal dependence of relative of distance cephalad to the central plane of the power distribution was measured, and it was found that a length of 15-20 cm was covered by the 50% iso-SAR. SAR measurements in the spinal canal of a pig cadaver showed that, even at resonance frequency, the measured SAR in the cervical spine was small (0.07 of central axis SAR). However, the spinal canal SAR as measured in the central plane of an evenly balanced configuration was 0.6 of the peak SAR. Marked reduction of the spinal canal SAR could be obtained with steering techniques and was in good agreement with the computer preplan. This paper reviews preliminary clinical experience with 17 patients. All but 2 patients were treated with steering techniques to permit partial sparing of normal tissues. The goal of maintaining central tumour temperature at or above 42 degrees C for at least 30 min was maintained in 41 of 67 sessions (61%) and in at least one session for all but 2 patients. Seventy-seven per cent of monitored tumor points attained at least 42 degrees C in patients for whom thermal mapping was performed. Significant cardiovascular stress developed during hyperthermia in two patients. There have been five complete responses and seven partial responses in the 17 patients.

A theoretical evaluation of the performance of the Dartmouth IMAAH system to heat cylindrical and ellipsoidal tumour models

The Dartmouth interstitial microwave antenna array hyperthermia (IMAAH) system was evaluated based on its ability to heat idealized three-dimensional cylindrical and ellipsoidal tumour models with lengths between 2 and 16 cm. The evaluation was based on computer simulations of the three-dimensional temperature distributions produced by the explicit theoretical power deposition pattern of resonant dipole antenna arrays driven at 433, 915 and 2450 MHz. The theoretical calculations were performed using three-dimensional tumour models with constant thermal and dielectric tissue properties. Each 2 X 2 cm antenna array was evaluated on the basis of its ability to heat a specified tumour volume with various blood flow patterns. The bioheat transfer equation was solved in three dimensions using a Galerkin finite-element method. In general, under the conditions of this study, in all cases examined larger percentages of the ellipsoidal tumour volumes were heated to therapeutic temperatures (greater than or equal to 43 degrees C) than the cylindrical tumours. The 2450 MHz antenna array was the most effective at heating the shorter tumours, while the 433 MHz antenna array heated the longer tumours most effectively. The antennas driven at 915 MHz were most effective at heating the medium-length tumours, especially tumours with high blood flow. The results of this computerized comparative thermal dosimetry study provide information on the feasibility of modelling tumours as simple geometries, and serve to advance three-dimensional interstitial hyperthermia treatment planning.

The modification of specific absorption rates in interstitial microwave hyperthermia via tissue-equivalent material bolus

Patterns of specific absorption rates generated by interstitial, microwave antenna arrays must be experimentally ascertained and quantified to facilitate their clinical incorporation. Phantom studies involved the use of four single-gap, coaxial antennas oriented in a 2 cm square array. These dipoles were driven in phase by a microwave generator at a frequency of 915 MHz. The inherent limitations in modifying the specific absorption rate patterns were addressed with the addition of bolus to the phantom. These additions of Guy's muscle tissue-equivalent material were made either proximal or distal to the phantom proper. Experiments conducted in the presence and absence of tissue-equivalent material bolus showed the ability to achieve broader bands of 50% power deposition in certain bolus conditions. These heating patterns were sufficiently reproducible and predictable to warrant clinical application of the bolus addition. A through-and-through method of catheter implantation allowed for bolus addition when deemed necessary. Treatments with veterinary and human patients using the bolus method to modify heating patterns yielded augmented patterns of power deposition. The effective length of the antennas that would radiate efficiently was essentially broadened via introduction of a microwave-interacting medium. As a result of the tissue equivalent material's ability to absorb microwave power, it was necessary to interpose minimally-interactive styrofoam spacers to limit heat transfer effects at the tissue-bolus interfaces.

Microwave hyperthermia induced by a phased interstitial antenna array

An interstitial microwave antenna array for hyperthermia cancer treatment is investigated. The purpose is to generate both uniform and controlled nonuniform temperature distributions in biological tissue by modulating the phases of the signals applied to each antenna. The array has four antennas positioned on the corners of a 2 cm square. The distributions of absorbed power within the arrays are computed and then converted into temperature distributions through a heat conduction simulation. The temperature patterns over phantom muscle are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel to the antennas). It is found that by proper phase modulation of radiofrequency signals applied to each antenna, a uniform heating can be produced in the entire array volume.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of insertion depth on the theoretical SAR patterns of 915 MHz dipole antenna arrays for hyperthermia.

Interstitial microwave antenna array hyperthermia (IMAAH) is presently used clinically in the treatment of cancer. This paper presents the theoretical specific absorption rate (SAR) patterns of 915 MHz microwave antenna arrays for varying insertion depths. The antennas were oriented in a 2 x 2 cm square array. Insertion depth, defined as distance from skin to antenna tip, ranged from 5.9 to 17.6 cm. Two different antenna configurations were considered. In the first the antenna had a distal section a quarter-wavelength long (resonant case), while the second had a distal section approximately 13% longer than a quarter-wavelength (non-resonant case). SAR patterns were calculated from theoretical expressions, and displayed as lines of constant SAR normalized to the maximum SAR value in the array. The results show that regions of concentrated power deposition or 'hotspots' occurred in the centre of the array and moved in a complex but predictable fashion as insertion depth was varied. For insertion depths shorter than a resonant half-wavelength, there occurred one hotspot distal to the antenna junctions. As insertion depth was increased beyond a resonant half-wavelength, the hotspot moved proximal to the antenna junctions and eventually split in two. For depths very much longer than a resonant half-wavelength a hotspot centred about the antenna junction dominated the SAR pattern. For the resonant case the maximum SAR was often along the central axis of the array, while for the non-resonant case the maximum SAR was at the antennas with a local maximum on the central axis.

Changes in heating patterns of interstitial microwave antenna arrays at different insertion depths.

The changes in heating patterns of interstitial microwave antennas at different insertion depths were investigated in a static phantom at 915 MHz. Antennas for the Clini-Therm Mark VI system were inserted 5-15 cm into muscle-equivalent material, through nylon catheters. The phantom was heated with arrays of antennas at 2 cm spacings for 60 s at 15 W per antenna. Midplane and transverse heating patterns were determined thermographically with the antennas inserted parallel or perpendicular to the split of the phantom. Hot spots, independent of heating near the junction plane, were observed in the midplane of the 2 x 2 and 2 x 4 arrays at 2.8 cm from the insertion end. The magnitudes of these hot spots were reduced by 40-45 per cent as insertion depth was increased from 7 to 10.5 cm. With insertion depths of more than 11.5 cm the hot spots gradually diminished and heating occurred mostly near the junction plane. The observed heating patterns were caused by changes in impedance of the antenna arrays at different insertion depths. The impedance mismatch had resulted in different wave propagation within the tissue material which produced different radiation patterns. During treatments, because heating varies with insertion depth, great care must be exercised in monitoring temperatures.

Applications of lasers and fiber-optic systems in control of the formation of microwave signals and their distribution in antenna arrays

This review is based on the non-Soviet literature and deals with the applications of lasers and fiber-optic systems in phased microwave antenna arrays. An analysis is made of the promising variants of fiber-optic systems for the distribution of microwave signals and the requirements which must be satisfied by components of such systems are formulated. An account is given of the advantages of using fiber-optic beam-forming systems for antenna arrays, of fiber-optic devices for processing microwave signals, and of laser control of the generation of microwave oscillations. A review is given of ways for improving optical and optoelectronic components so as to achieve fast modulation of the intensity of laser radiation as well as reduction of noise and nonlinear distortions in a communication channel.

Comparison of power deposition by in-phase 433 MHz and phase-modulated 915 MHz interstitial antenna array hyperthermia systems

The interstitial microwave antenna array hyperthermia (IMAAH) system produces a pattern of specific absorption rate (SAR) that is nonuniform within a 2-cm square array when driven in phase at 915 MHz. It was found that phase modulation makes the time-averaged SAR pattern significantly more uniform in planes perpendicular to the antennas. To drive antennas in phase at 433 MHz similarly improves SAR uniformity when the antennas are of resonance length. >

The distribution of power and heat produced by interstitial microwave antenna arrays: II. The role of antenna spacing and insertion depth

The distribution of power and temperature generated by 915 MHz interstitial microwave antenna arrays was studied in static muscle-equivalent phantoms and both perfused and non-perfused canine thigh muscle. These arrays, which would form the geometric basis of larger volume implants, consisted of four parallel antennas oriented such that transverse to their long axes they formed the corners of a square. Arrays with 2 and 3 cm sides were compared at various depths of insertion where the nodes for all four antennas were coincident at the same depth. The position relative to the antenna nodes of the maximum power and highest temperature within the array volume varied with the depth of insertion of the antennas. Though power dropped rapidly distal to the nodes at all depths, a shift in the location of the maximum power proximal to the nodes resulted in an increase in the effective heating volume at certain insertion depths. For 2 cm array spacing the highest power and temperature were measured along the central axis of the array at all insertion depths. However, arrays using 3 cm spacing generated their maximum power adjacent to the antennas with only 50% of this level occurring along the central axis. When the temperature produced by 3 cm arrays was measured in phantoms midway through simulated 30minute hyperthermia treatments, the effect of thermal conduction on the temperature distribution was evident. Though power was only 50% centrally, the highest temperatures occurred there. This same pattern of central heating occurred in perfused canine muscle demonstrating the importance of conductive and conductive heat redistribution in reducing thermal gradients within the array volume.

Optimum data quantization in microwave imagery and its effect upon image quality

An optimum quantization scheme for compressing the aperture data in a microwave antenna array is derived. The optimization criterion is maximization of the quality of the output image from the system. The procedure can be applied to a quantizer designed for any number of bits per sample. It is shown that hard limiting the input to a phased array while leaving the phase unaltered preserves much of the image integrity. Amplitude-only information destroys it. Hard limiting plus one bit of phase information sometimes suffices. Hard limiting plus two bits of signal phase per sample often results in acceptable imagery. A minimum of 4 bits of information per complex signal sample guarantees a relatively good image, provided that 3 bits are used to represent the phase and 1 bit is used to represent the amplitude. Eight bits are required to assure a high-fidelity image. When the number of bits per data sample is no more than 8, the number of bits assigned to phase should be two larger than the number of bits assigned to amplitude. For finer quantization, equal numbers of bits should be assigned to amplitude and phase. >