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P R Stauffer

Publications and source records attributed to P R Stauffer.

At least 37 records · Page 2Linked to original sources
PubMed1988

Thermal distribution studies of helical coil microwave antennas for interstitial hyperthermia.

An implantable 915 MHz helical coil antenna was developed for improved localization and control of interstitial microwave hyperthermia. The radiating element consisted of a fine wire coil wound back over the inner conductor of a miniature semi-rigid coaxial cable in place of the terminal portion of outer conductor. The power deposition profiles from single helical coil antennas were studied both in homogeneous phantom and in muscle tissue in vivo and compared to those of single half-wavelength linear dipole antennas. The effects of variable coil length, turn density, and antenna insertion depth in tissue were characterized. The helical coil antennas produced a well-localized heating pattern with a sharp falloff of temperature in both directions axially from the coil element. One of the best heating patterns was obtained with a 35 turn, 35 mm long helical coil element which was separated from the antenna feedline outer conductor by a 1 mm gap (HCS-35(1)/36). This antenna showed a marked shift of the effectively heated volume toward the antenna tip and essentially no dependence of the heating pattern on insertion depth. In contrast, the axial power deposition profiles of dipole antennas were strongly affected by insertion depth and exhibited an inadequately heated area at the antenna tip even with 1/2-3/4 wavelength insertion. Thermal distribution studies showed that the single helical coil microwave antenna provided more predictable, well-localized heating of deep-seated tissues, with minimal requirement for over-implanting of the treatment volume.

Animals↗
PubMed1988

Interstitial helical coil microwave antenna for experimental brain hyperthermia.

A helical coil 2450-MHz microwave antenna was used to induce interstitial hyperthermia in normal dog brain. The HCS-10(1)/11 antenna consisted of a miniature semirigid coaxial cable around which a fine wire coil with 10 turns per 1-cm length was wound. A single antenna and two or three temperature probes were implanted stereotactically, and the temperature distributions surrounding the antenna were measured and compared to those induced using a dipole antenna. The helical coil antenna produced well-localized temperature distributions at depths that were symmetrical around the coil and that extended to the antenna tip. There was minimal variation of the heating patterns with insertion depth using the HCS-10(1)/11 antenna and no excessive heating of extracerebral tissues. In contrast, 2450-MHz dipole antennas induced temperatures of 43 to 46 degrees C at the brain surface and extracerebral tissues (skull, muscle, and scalp), with a relatively uniform but lower temperature in the targeted brain volume. One week after hyperthermia treatment, the thermal lesions induced by the helical coil antenna were visualized using computed tomography. The heating patterns correlated well with the location of the heat lesions and were reproducible among animals. The results indicated that the helical coil antenna could be used to induce localized hyperthermia at specific depths in normal brain without inducing unacceptable heating of the brain surface or extracerebral tissues. Consequently, this antenna seems to be suitable for studying the response of normal brain after a heat insult and may be effective in the application of interstitial microwave brain hyperthermia for malignant brain tumors.

Animals↗
PubMed1988

[Characterization of helical coil microwave antenna for interstitial hyperthermia].

Implantable microwave antennas for interstitial hyperthermia have been constructed by using a fine wire helical coil extension of the coax feedline inner conductor. The effects of the variable coil length the winding density, and the antenna insertion depth in the tissue on the heating patterns from a single antenna were characterized in phantom models. Results indicated that the helical coil microwave antenna provided localized heat at depth near the antenna tip, and the heating patterns were easily modified by minor changes in the coil length, winding density, and altering the separation gap. The characteristics of the helical coil microwave antenna may increase its potential application to cases of interstitial hyperthermia.

Hyperthermia, Induced↗
PubMed1986

Interstitial microwave hyperthermia in a canine brain model.

A dual frequency microwave system was constructed for interstitial heating of brain tissue. Single-junction dipole antennas were tested in a phantom model and in normal dog brain to determine how variations in physical factors affected temperature distributions. Non-survival studies were performed at both 915 and 2450 MHz to determine heating patterns that could be achieved within normal brain using this system. Chronic survival studies were performed using a single dipole antenna inserted laterally into one hemisphere of brain and driven at 2450 MHz. Temperatures of 43 or 44 degrees C for 30 min at a reference point 0.5 cm from the antenna junction were used to induce a thermal lesion of approximately 1 cm diameter in the right cerebral hemisphere of dogs. Neurologic and physical changes in dogs were monitored daily for up to 16 weeks after induction of cerebral lesions. The extent and development of thermal lesions was monitored with weekly computed tomographic (CT) examinations and, after death, at histopathologic examination. Results of the phantom studies showed that the longitudinal heating pattern was bell-shaped at both frequencies used and that there was some variation in heating length that depended on insertion depth. Acute studies in dog brain showed that 915 MHz antennas implanted less than 6.5 cm deep produced erratic heating patterns that usually included excessive heating of the surface of the brain. Conversely, 2 cm-long antennas driven at 2450 MHz gave reproducible temperature distributions both longitudinally along and radially away from the antenna. The steepest gradients--about 1 degree C/mm--occurred in the radial direction away from the antenna junction. A single 30 min heat treatment produced a large focal lesion that consisted of central coagulation necrosis surrounded by a sharply demarcated hypervascular zone. Edematous changes were minimal and were observed only during the first week after treatment. As assessed by serial CT scans, thermal lesions reached a maximum size by the first week after treatment and were essentially resolved by 16 weeks after treatment.

Animals↗
PubMed1986

[Hyperthermia of malignant brain tumors--development of an interstitial microwave hyperthermia system and study of thermodosimetry and heat toxicity in a canine brain model].

Hyperthermia is a promising important adjunct to conventional treatment of malignant brain tumors and available data suggest a strong biological rationale for its use. Hyperthermia has been shown to sensitize tumors to radiation, particularly radiation delivered at low dose rates typical of brachytherapy. To complement our clinical study of interstitial brachytherapy of malignant brain tumors, a microwave interstitial heating system with data acquisition has been constructed at UCSF. This system was evaluated in a phantom and in a series of non-survival and survival experiments for heating patterns and heat toxicity in 24 adult canine brains. A single 2450 MHz microwave heating antenna and 3 fiberoptic thermometry probes were placed stereotaxically into the frontal white matter of 24 dogs. Temperature measurements were made at 1 mm increments along the antenna length 5 mm from the antenna axis and radially away from the antenna junction. For the survival studies, minimum temperatures 43 degrees C, 44 degrees C, and 45 degrees C were maintained at points 5 mm from the antenna junction for 30 minutes. Hyperthermia-induced tissue damage was measured weekly following the heat treatment using CT. In general, it was easy to maintain a steady temperature state for a long time period. Heating patterns tended to be ellipsoidal, corresponding relatively well with those predicted by the phantom trials. Heat toxicity was examined by sequential quantitative CT scans in chronic trials and correlated with histopathology following sacrifice at 1-16 weeks. Neurological deficits were minimal and were restricted to the first day after treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

Directional power deposition from direct-coupled and catheter-cooled interstitial ultrasound applicators.

This research represents an experimental investigation of the directional power deposition capabilities of interstitial ultrasound applicators intended for applications in hyperthermia and thermal surgery for cancerous or benign disease. Direct-coupled and catheter-cooled ultrasound applicators were fabricated using cylindrical piezoceramic transducers sectored to produce 90 degrees, 180 degrees or 270 degrees active acoustic zones. The applicators were characterized through measurements of acoustic power output and intensity beam distributions in degassed water, in vitro temperature measurements in a perfused kidney model, and in vivo temperature distributions in pig thigh muscle. The angular power deposition patterns obtained in water were closely correlated to the resultant temperature distributions measured in the perfused kidney and in vivo pig thigh muscle. These sectored catheter-cooled and direct-coupled devices both demonstrated the ability to generate high temperatures (>50 degrees C) at sustained high power output levels (6-12 W) without degradation of the ultrasound transducers. Directional control of the energy deposition from the sectored ultrasound applicators was verified with corresponding temperature profiles in both the in vitro and in vivo experiments, as well as with angularly shaped thermal lesions. This is significant in that it demonstrates that heating in the angular expanse can be controlled with interstitial ultrasound applicators, thus providing more conformal thermal therapy by directing the thermal energy in the targeted tissue while protecting non-targeted tissue from thermal damage.

Animals↗
PubMedDate not supplied

Combination of transurethral and interstitial ultrasound applicators for high-temperature prostate thermal therapy.

The purpose of this study was to determine the feasibility of using a transurethral ultrasound applicator in combination with implantable ultrasound applicators for inducing thermal coagulation and necrosis of localized cancer lesions or benign disease within the prostate gland. The potential to treat target zones in the anterior and lateral portions of the prostate with the angularly directive transurethral applicator, while simultaneously treating regions of extracapsular extension and zones in the posterior prostate with the directive implantable applicators in combination with a rectal cooling bolus, is evaluated. Biothermal computer simulations, acoustic characterizations, and in vivo thermal dosimetry experiments with canine prostates were used to evaluate the performance of each applicator type and combinations thereof. Simulations have demonstrated that transurethral applicators with 180-270 degrees acoustic active zones can direct therapeutic heating patterns to the anterior and lateral prostate, implantable needles can isolate heating to the posterior gland while avoiding rectal tissue, and that the combination of applicators can be used to produce conformal heating to the whole gland. Single implantable applicators (1.8 mm OD x 10 mm long, approximately 180 degrees active sector, approximately 7 MHz, direct-coupled type) produced directional thermal lesions within in vivo prostate, with temperatures >50 degrees C extending more than 10 mm radially after 10-15 min. Combination of interstitial applicators (1-2) and a transurethral applicator (3-2.5 mm OD x 6 mm long, approximately 180 degrees active sector, 6.8 MHz, 6 mm OD delivery catheter) produced conforming temperature distributions (48-85 degrees C) and zones of acute thermal damage within 15 min. The preliminary results of this investigation demonstrate that implantable directional ultrasound applicators, in combination with a transurethral ultrasound applicator, have the potential to provide thermal coagulation and necrosis of small or large regions within the prostate gland, while sparing thermally sensitive rectal tissue.

Acoustics↗
PubMedDate not supplied

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.

Biophysical Phenomena↗
PubMedDate not supplied

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.

Animals↗
PubMedDate not supplied

Implantable helical coil microwave antenna for interstitial hyperthermia.

An implantable helical coil microwave antenna has been developed for improved localization and control of interstitial hyperthermia for deep-seated tumours. A helical coil structure was employed as an extension of the inner conductor at the terminal portion of a miniature semi-rigid coaxial cable. The antennas were constructed with three different connection configurations of the helical coil to the feedline, and with several coil turn densities during the optimization of heating characteristics. In order to compare relative antenna heating performance, a set of quantitative parameters was introduced. Power deposition profiles of 2450 MHz helical coil antennas were studied in both phantom models and muscle tissue in vivo, and compared to those of commonly used dipole antennas. Optimal antenna performance was obtained with a 10-turn per 1 cm helical coil connected to the inner conductor at the tip and separated from the outer conductor by a 0.1 cm gap (HCS-10). These antennas produced a well-localized heating pattern with a sharp falloff of temperature in both directions axially from the coil element. For half-wavelength insertion depths, the effective heating length (50 per cent of maximum SAR) of HCS-10 antennas matched that of standard dipole antennas, but was shifted down towards the tip. For shorter and deeper antenna insertion depths the HCS-10 heating pattern remained similarly localized to the region surrounding the helical coil with minimal cold zone at the tip. In contrast, the dipole antenna heating pattern changed significantly depending on insertion depth, with an unavoidable 0.2-0.7 cm cold region at the antenna tip and elevated surface temperatures for short insertion depths.

Animals↗
PubMedDate not supplied

Comparative thermal dosimetry of interstitial microwave and radiofrequency-LCF hyperthermia.

Steady-state temperature distributions induced by commercial radiofrequency localized current field (RF-LCF) and microwave (MW) interstitial heating systems were compared in dog thigh muscle in vivo using repeated 15-min heating experiments in the same implant site. Control experiments consisting of up to nine successive, identical heat trials with either modality verified that induced temperature distributions could be duplicated reliably. For all comparative dosimetry experiments a square array of parallel heat sources and thermometry probes was inserted percutaneously through a 5 mm grid Plexiglas template to a depth of 7.0-8.0 cm. Metal trocar electrodes were left at the corners of square arrays for two or three successive RF-LCF heat trials. After the metal trocars were removed, two or three more heat trials were performed using dipole microwave antennas in Teflon catheters at the same four positions. The three-dimensional temperature distributions within the array boundaries were characterized by mapping up to 11 fibre optic temperature probes in 1 cm increments during the steady-state plateau of each trial. The distributions were analysed quantitatively in terms of the percentage of measured points which achieved at least 50 per cent of the maximum array temperature increase above baseline (delta Tmax). Results showed that the RF-LCF technique heated more uniformly with depth along the bare metal electrodes and more consistently within the array boundaries than the microwave dipole antennas. For all array spacings studied (1.0-3.5 cm), the RF electrodes heated approximately 10-20 per cent more of the array volume to greater than 50 per cent of delta Tmax.

Animals↗
PubMedDate not supplied

Experimental assessment of phased-array heating of neck tumours.

An investigation of phased-array microwave systems (PAMS) for non-invasively inducing hyperthermia, primarily in neck lesions, has been done with implications for applications at other sites such as lung and pelvis. Our general approach was to combine numerical and analytical approaches with parallel experimental studies. In this paper we will concentrate only on the experimental aspects. The object, such as a homogeneous cylindrical phantom or a neck phantom, was encircled with several standard applicators driven by a single source, but with relative phase and amplitude control over each applicator. The relative phases of the applicators were adjusted by using an implanted monopole antenna connected to an HP network analyser. Power was applied and the specific absorption rate (SAR) was determined by using split phantoms and thermography or by measuring temperature transients dT/dt, recorded by implanted thermometer probes. We found that at 915 MHz for our applicators (SMA Co.) the centre of an 11 cm diameter muscle-like phantom heated to about 33% of the value at the surface in front of the applicator. Similarly, we were able to show significant SAR at the centre of realistically sized neck phantoms using four phased apertures of 915 MHz. Furthermore, substantial improvement was observed if the frequency was lowered to about 400 MHz.

Animals↗
PubMedDate not supplied

Normal brain response after interstitial microwave hyperthermia.

The effects of interstitial hyperthermia were assessed in the normal brain after a single 30 min treatment using 2450 MHz microwaves. A single helical coil microwave antenna was inserted into the frontal white matter and reference temperatures of 40, 41, 42, 43 and 44 degrees C maintained for 30 min along a temperature sensing probe 5 mm away and parallel to the antenna. The extent of hyperthermia damage was quantified weekly for 6 weeks using computed tomography (CT), and changes in regional cerebral blood flow (rCBF), tissue vascularity and mean transit time were determined using ultrafast CT. Qualitative histopathological analysis was carried out on tissues at various times after heating. Heat lesions were radiographically characterized by rapid development and resolution and consisted of an area of focal low density surrounded by a ring of contrast enhancement. Histologically, the focus of low density corresponded to regions of tissue necrosis, whereas the ring enhancement showed local reactive changes, including endothelial cell proliferation and infiltration with macrophages. Tissue necrosis occurred at temperatures greater than 43.9 +/- 1.5 degrees C (mean +/- standard deviation), and volumes of necrosis and ring enhancement were at a maximum 1 week following treatment. Relative to the contralateral, unheated hemisphere, rCBF in the heated brain appeared to be reduced for the first 3 weeks after treatment but approached normal by week 4. The mean transit time of blood was increased for weeks 1-3 compared to the untreated hemisphere, and tissue vascularity reached a maximum, 3 weeks after treatment. The rapid CT changes together with ultrafast CT and histopathological findings suggest that focal heat lesions in the brain stimulate a significant and rapid vascular response.

Animals↗