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J W Strohbehn

Publications and source records attributed to J W Strohbehn.

At least 19 recordsLinked to original sources
PubMed1992

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 x 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 greater than or equal to 43 degrees 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 greater than or equal to 43 degrees C was seen for insertion depths between 7.8 and 14.6 cm. At an insertion depth of 11.7 cm (3/4 lambda) 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.

Equipment Design↗
PubMed1991

Experimental brain hyperthermia: techniques for heat delivery and thermometry.

An experimental canine brain model was developed to assess the effects of hyperthermia for a range of time and temperature endpoints, delivered within a specified distance of an interstitial microwave antenna in normal brain. The target temperature location was defined radially at 5.0 or 7.5 mm from the microwave source at the longitudinal location of maximum heating along the antenna in the left cerebral cortex. Temperatures were measured with fiberoptic probes in a coronal plane at this location in an orthogonal catheter at 1.0 mm intervals. Six antennas were evaluated, including dipole, modified dipole, and four shorted helical antennas with coil lengths from 0.5 to 3.9 cm. Antenna performance evaluated in tissue equivalent phantom by adjusting frequency at a fixed insertion depth of 7.8 cm or adjusting insertion depth at 915 MHz showed dipoles to be much more sensitive to insertion depth and frequency change than helical antennas. Specific absorption rate (SAR) was measured in a brain/skull phantom and isoSAR contours were plotted. In vivo temperature studies were also used to evaluate antenna performance in large and small canine brain tissues. A helical antenna with a 2.0 cm coil length driven at 915 MHz was chosen for the beagle experiments because of tip heating characteristics, well-localized heating along the coil length, and heating pattern appropriate to the smaller beagle cranial vault. Verification of lesion dimensions in 3-D was obtained by orthogonal MRI scans and histology to document the desired heat effect, which was to obtain an imagable lesion with well-defined blood-brain-barrier breakdown and necrotic zones. The desired lesion size was between 1.5 to 2.5 cm diameter radially, in the coronal plane with the greatest diameter.

Animals↗
PubMed1990

Absorbed power deposition for various insertion depths for 915 MHz interstitial dipole antenna arrays: experiment versus theory.

Dipole antennas are commonly used in interstitial clinical hyperthermia treatments because of their compatibility with brachytherapy techniques and their good power deposition patterns when used in arrays. For accurate treatment planning, however, there must be a comprehensive knowledge base to predict the power deposition patterns when insertion depth is a non-resonant length. This is especially true for insertion depths that result in significant power deposition outside of the antenna junction plane and presumably outside of the tumor volume. A computer controlled measurement system was used with a muscle equivalent phantom to make measurements of specific absorption rate (SAR) or absorbed power per unit mass of tissue at 598 points in a plane. The diagonal plane was the measurement plane of choice because it characterized the SAR profiles at the array center as well as areas in the proximity of the antennas. Dartmouth dipole antennas were used (0.9 mm O.D.) in brachytherapy catheters with inner catheters (2.2 mm O.D./1.2 mm I.D.). The resonant half-wavelength of this dipole antenna/catheter combination is 7.8 cm. A choke modification of the dipole was also investigated. Four antennas were used in a boxlike configuration with 2.0 cm separation. Insertion depths of 5.9, 7.8, 9.8, 12.7, 15.6 and 17.6 cm were used. The hA subsection (junction to tip) was held constant at 3.9 cm. Plots were made of the experimental SAR data normalized to the maximum SAR measured in the plane. Theoretical plots were calculated in the same plane for each of the insertion depths. SAR comparisons were also made longitudinally along the central axis of the array and through the antenna junctions in the diagonal plane for resonant half-wavelength insertion depth. Experimental results verified theoretical predictions of the existence of a secondary hot-spot in the center of the array, but outside of the antenna junction plane and approximately a quarter-wavelength from the insertion point. This secondary hot-spot appears for all insertion depths greater than 10 cm. At longer insertion depths approaching a full wavelength, however, this secondary peak is not dominant. Choke antennas demonstrated a solution to the problem of shifting SAR patterns with varying insertion depths by restricting the active length of the antenna.

Absorption↗
PubMed1989

The stereotactic operating microscope: accuracy refinement and clinical experience.

Accuracy of a stereotactic operating microscope, by which imaging data may be superimposed on the operative field without a stereotactic frame, has been most limited by the resolution of imaging information. Using newer algorithms and pilot pole calibration of the digitizer, an error in registration of 2 mm and in contour display of 3 mm has been demonstrated. Greatest utility of the system clinically has been in providing navigational guidance to small lesions undergoing resection.

Brain Neoplasms↗
PubMed1989

Optimization of the absorbed power distribution for an annular phased array hyperthermia system.

One of the systems under investigation for producing hyperthermia noninvasively for treating deep-seated tumors is the annular phased array. This device consists of two rings of eight electromagnetic apertures that are placed concentrically about the long axis of the patient and radiate energy toward the center. Previous theoretical and clinical studies have concentrated primarily on systems where the amplitude and phase of the signal applied to each aperture were the same, and these studies have shown that the system is capable of depositing power deep within the patient. Nevertheless, in many situations the system was not capable of producing desirable temperature distributions in the tumor and normal tissue. In this paper we report on a 2-dimensional theoretical investigation where an optimization routine was used to select the amplitude and phases of each of eight apertures. The optimization procedure and resulting calculations were based on CT scans of patients with tumors. The electrical and thermal properties of the different organs and tissues were taken into account. The optimization routine tried to achieve uniform absorbed power in the tumor region with zero absorbed power outside. Using the optimized amplitudes and phases, the SAR (specific absorption rate, W/kg) was calculated for the array. The results show that in general the optimization procedure was successful in that the power deposited within the tumor volume was increased with less power deposited into normal tissue when compared to the equal amplitude and phase case. This SAR data was then used as the input to a program based on the bioheat transfer equation, which calculated the temperature distribution in the patient model for an assumed set of blood perfusion rates. Depending on the location, size of the tumor, and blood perfusion rates, the improvement in the percentage of the tumor brought to therapeutic temperature varied from 0% to as much as 80%.

Humans↗
PubMed1989

Thermal conduction effects associated with temperature measurements in proximity to radiofrequency electrodes and microwave antennas.

The smearing effects due to thermal conduction along various, nonenergized, interstitial devices were quantified in a flow cell-thermal step gradient. An insulated cylindrical flow cell with a high (ca 45 degrees C, 1.12 cm i.d., 1.6 cm o.d.) temperature region surrounded by a low (ca 37 degrees C) temperature region was used to compare temperature profiles measured with a thermocouple sensor inside a Stanford radiofrequency (RF) hyperthermia/brachytherapy catheter, a BSD instrumented microwave (MW) antenna (i.e., thermistor integrated into a dipole antenna) and a Dartmouth MW antenna with a juxtaposed optical sensor. Two parameters were used to quantify the thermal smearing of each interstitial device in the flow cell: (a) the maximum temperature difference (MTD) and (b) the full- width- half-maximum (FWHM) of the high temperature region. The "true" temperature maximum (45.4 degrees C) and distribution (FWHM = 1.65 +/- 0.06 cm) were measured with an optical sensor. These data indicate that the BSD instrumented MW antenna significantly smeared the true temperature profile (MTD = 2.7 degrees C, FWHM = 2.1 cm), as did the Dartmouth MW antenna (MTD = 1.5 degrees C, FWHM = 1.7 cm). The Stanford RF catheter, when insulated, resulted in minimal smearing (MTD = 0.3 degrees C, FWHM = 1.9 cm). Moreover, when the insulation was removed so the RF electrode was exposed to the thermal step gradient, smearing was again minimal (MTD = 0.3 degrees C, FWHM = 1.9 cm).

Brachytherapy↗
PubMed1989

A frameless stereotaxic operating microscope for neurosurgery.

A new system, which we call the frameless stereotaxic operating microscope, is discussed. Its purpose is to display CT or other image data in the operating microscope in the correct scale, orientation, and position without the use of a stereotaxic frame. A nonimaging ultrasonic rangefinder allows the position of the operating microscope and the position of the patient to be determined. Discrete fiducial points on the patient's external anatomy are located in both image space and operating room space, linking the image data and the operating room. Physician-selected image information, e.g., tumor contours or guidance to predetermined targets, is projected through the optics of the operating microscope using a miniature cathode ray tube and a beam splitter. Projected images superpose the surgical field, reconstructed from image data to match the focal plane of the operating microscope. The algorithms on which the system is based are described, and the sources and effects of errors are discussed. The system's performance is simulated, providing an estimate of accuracy. Two phantoms are used to measure accuracy experimentally. Clinical results and observations are given.

Algorithms↗
PubMed1989

Interstitial thermoradiotherapy.

The more recent engineering and clinical aspects of interstitial hyperthermia are reviewed. The advantages and difficulties of microwave, radiofrequency, and ferromagnetic seeds are evaluated and some future directions for improvements are outlined.

Brachytherapy↗
PubMed1987

Iridium-192 brachytherapy in combination with interstitial microwave-induced hyperthermia for malignant glioma.

A phase I clinical trial assessing the feasibility and safety of hyperthermia in combination with 192Ir brachytherapy (60 Gy) for the treatment of malignant glioma now includes 14 patients. Hyperthermia (42-43 degrees C at tumor margin for 60 min) has been induced using stereotactically implanted afterloading catheters and a microwave (915 or 2,450 MHz) antenna array. Thermometry recorded along each catheter confirms the general ability of the technique to heat such volumes, but thermal heterogeneities are documented. Transient or permanent worsening of previous neurologic deficit, seen in 7 patients, has been the most common morbidity.

Adult↗
PubMed1987

Two-dimensional ultrasonic tissue characterization: backscatter power, endocardial wall motion, and their phase relationship for normal, ischemic, and infarcted myocardium.

To understand the possible differences in reflected ultrasonic energy from normal, ischemic, and infarcted myocardium, we studied 20 open-chest dogs with a commercially available two-dimensional ultrasonic scanner. Echocardiographic radiofrequency images of anterior myocardium were obtained serially during complete coronary occlusion for 2 hr (n = 15) or 5 hr (n = 10), or after temporary coronary clamping for 15 min with release for 1 hr (n = 5). We investigated two variables: the cyclic backscatter power and the phase difference among endocardial wall motion (EWM), cyclic backscatter power (BSP), and left ventricular pressure (LVP). The cyclic BSP decreased from a control (nonischemic) level of 5.1 +/- 0.8 to 2.3 +/- 0.7 dB during ischemia (up to 30 min after coronary ligation). The phase difference between the EWM and BSP progressed from a control (nonischemic) value of 38 +/- 20 to 115 +/- 23 degrees during ischemia. For the infarction period (2 to 5 hr after coronary ligation), the cyclic BSP progressively returned toward baseline control levels to 4.0 +/- 1.2 dB, but the phase had increased further to 170 +/- 28 degrees. The reperfusion study showed a similar decrease in cyclic BSP and an increase in phase after arterial clamping and both returned to near-normal nonischemic values upon arterial release. Simultaneous LVP recordings were performed to assess the phase contribution of endocardial dyskinesis to the total phase difference measurement. At 5 hr the dyskinesis had contributed 46% to the total phase difference, while the backscatter power contributed 54%. However, the EWM contribution occurred immediately while BSP contribution changed progressively during the 5 hr study period.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

A theoretical comparison of the temperature distributions produced by three interstitial hyperthermia systems.

Three interstitial hyperthermia systems were compared on the basis of their abilities to heat tumors: ferromagnetic seeds, microwave antennas, and rf electrode needles. The theoretical calculations were performed using two-dimensional tumor models containing static tissue and electrical properties and various blood flow patterns. Basic assumptions were kept the same to permit a meaningful comparison of the different systems. Power deposition patterns were calculated based on the appropriate theoretical expressions. The bioheat transfer equation was solved using a finite element method. Temperature distributions were calculated for four 1 or 2 mm diameter implants on the corners of 1, 2, and 3 cm squares. The tumor was assumed to be circular with a diameter 1 cm longer than the diagonal of each square. Three blood flow models were considered: homogeneous, nonhomogeneous, and concentric annulus perfusion models. The blood flow ranged from 0 to 100 ml/100 g/min with the ratio of tumor/normal tissue blood flow ranging from 0 to 40. A Hyperthermia Equipment Performance (HEP) rating was used as a criterion for comparing the temperature distributions from the various cases examined. As expected, the higher HEP ratings were generally produced by decreasing the spacing between the implants and/or lower blood perfusion rates. Under the conditions of this study, the microwave antennas were able to adequately heat a larger number of cases to therapeutic temperatures than either of the other two modalities.

Brachytherapy↗
PubMed1986

Two-dimensional ultrasonic variation in myocardium throughout the cardiac cycle.

This study was performed to investigate the two-dimensional unprocessed radiofrequency (rf) ultrasonic backscatter from myocardium throughout the cardiac cycle. Eleven mongrel dogs underwent left lateral thoracotomy and had two-dimensional echocardiographic studies performed using a 5 MHz transducer. Regions of interest, selected to encompass a portion of the myocardium, were transported at 33 ms intervals to a host computer and disc. Results show a reproducible cyclic variation in backscatter power throughout the cardiac cycle for the anterior wall. The backscatter power was highest during diastolic relaxation and lowest during systolic contraction. The average change was 4.5 dB.

Animals↗
PubMed1986

Interstitial hyperthermia and iridium brachytherapy in treatment of malignant glioma. A phase I clinical trial.

An oncolytic effect of hyperthermia in the 42 degrees to 43 degrees C range has been previously demonstrated in cell culture and animal models. To apply this modality clinically, an interstitial microwave antenna array system has been developed for the delivery of controlled hyperthermia to an intracranial tumor volume, and a Phase I clinical trial involving six patients with malignant gliomas was undertaken. The protocol to study technical feasibility and patient tolerance combined interstitial iridium-192 irradiation and interstitial hyperthermia with 60-minute hyperthermia sessions immediately before and after brachytherapy. After-loading catheters suitable for both treatment modalities were implanted using a computerized tomography-assisted technique. Thermometry data confirmed the ability of a microwave antenna system to achieve reliable temperature distributions, and reasonable patient tolerance was documented.

Adult↗