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J W Strohbehn

Publications and source records attributed to J W Strohbehn.

54 records · Page 3Linked to original sources
PubMed1982

A simulation study of motion artefacts in computed tomography.

In order to increase understanding of artefacts caused by patient motion in computed tomography, rigid translational and rotational motions in the plane of reconstruction have been considered. Motion artefacts were simulated on a digital computer and demonstrated in actual CT scans of a phantom. The types of motion artefacts observed included streaking, boundary splitting and subtle distortions of shape. A method for removing artefacts from a CT image without rescanning the patient is discussed, and some examples of possible improvement are shown using the simulation model. It is assumed that perfect knowledge of patient motion is known since this represents the theoretical limit on possible improvement to the degraded image. A qualitative discussion of streaking, the most common type of motion artefact, is also included.

Humans↗
PubMed1979

Mechanism of obstruction of closed-wound suction tubing.

The mechanism of obstruction of closed-wound drainage tubing was investigated by means of coagulation tests performed on wound drainage fluid and by examination of the contents of the tubes after their removal. Although clotting is commonly thought to be responsible for the obstruction, the wound drainage fluid was found to be essentially incoagulable and little fibrin was evident within the tubes. By contrast, bits of tissue were frequently found within the tubes, and these frequently virtually occluded the lumen. This observation, that tissue fragments are responsible for tube obstruction, permits a rational approach to the solution of this problem. For example, meticulous wound flushing and irrigation, or perhaps tubing of different design, might lead to a reduced incidence of tube failure.

Blood Vessels↗
PubMed1979

An invasive microwave antenna for locally-induced hyperthermia for cancer therapy.

A microwave system has been developed and characterized for delivering heat directly into tumors. This system employs a microwave power source (3-10W) operating in the 500 MHz to 1.3 GHz frequency range, coaxial transmission line, and a monopole antenna. Absorbed power was measured in saline, in tissue equivalent phantoms, and in tumors in live and dead mice. Antennas were designed to operate at 500 MHz and 1 GHz, and the critical design parameters have been identified for this system. Analytical and experimental results obtained in our laboratory suggest that this system is capable of providing controlled temperature distributions appropriate for hyperthermia in animal tumors. Theoretical results predict that 3 GHz may be an optimum choice for this system in animal tumors of approximately 1 cm diameter; the microwave antenna system provides a heat distribution superior to that obtained using a resistance heater of similar dimensions. We propose that further development of this approach may overcome some of the problems associated with other systems which use external radiation sources, and implications for clinical application of this system are discussed.

Animals↗
PubMedDate not supplied

Mathematical model of conventional tomography.

A Fourier decomposition approach is used to study the imaging properties of conventional tomography. Spatial frequency response curves (MTFs) are calculated for both linear and axial transverse tomography. These curves depend on the product of the spatial frequency of the sinusoidal density variations in thin layers parallel to the tomographic plane and the distance between such layers and the tomographic plane. Based on the spatial frequency response curves, a quantitative definition of tomographic layer thickness is given. Furthermore, the spatial frequency response curves suggest that unattenuated low-frequency information from outside the tomographic layer limits the resolution in conventional tomograms and that high-pass spatial filtering of the image may substantially improve the diagnostic quality of tomographic images, particularly in the identification of boundaries.

Mathematics↗
PubMedDate not supplied

Linearizing mechanisms in conventional tomographic imaging.

Implicit in the concept of conventional tomography and in any attempt to characterize the tomographic process by a modulation transfer function is the assumption that the tomographic process is linear. A Fourier decomposition approach and an analysis of nonlinear contributions to the integrated tomographic image intensity are used in this paper to establish the validity of this assumption and to determine the mechanisms by which the tomographic process is effectively linearized.

Energy Transfer↗
PubMedDate not supplied

Some aspects of optimization of an invasive microwave antenna for local hyperthermia treatment of cancer.

Hyperthermia has emerged as a promising alternative or adjunct to other forms of cancer therapy. In order to utilize hyperthermia in very localized volumes immersed in regions of vital normal tissue, an invasive microwave coaxial monopole antenna has been developed. An experimental approach has been taken to characterize and optimize the electromagnetic properties and heating capabilities of bare and insulated antennas imbedded in tissue equivalent phantoms and dog brain. Four methods have been used to visualize the thermal profiles of the microwave probes: the liquid crystal technique, the gelatin technique, and the direct measurement of temperature with thermistor probes in phantom and dog brain. Among the parameters studied are: antenna impedance, insertion depth, antenna insulation (dielectric constant and thickness), shaft insulation, and frequency.

Electromagnetic Phenomena↗
PubMedDate not supplied

Theoretical temperature distributions for solenoidal-type hyperthermia systems.

One type of system proposed for producing hyperthermia for cancer therapy of deep-seated tumors consists of a cylindrical conducting coil (solenoid) in which the patient (or a limb of the patient) is placed. The current in the conducting coil creates an alternating magnetic field inside the patient, which induces eddy currents in the tissue. The eddy currents deposit power and hence produce heating. In this paper, steady-state temperature profiles are calculated analytically, assuming a homogeneous one-dimensional model which includes blood perfusion. Numerical methods are used to calculate the temperature distributions as a function of time for this homogeneous model and to calculate the temperature profiles when there are radial variations in tissue parameters including blood flow. Based on these computer simulations, it would appear that in most cases a solenoid-type hyperthermia system will not be able to provide deep-seated heating in the abdomen or thorax, but should be able to raise temperatures to therapeutic levels to about 6 cm depth in the abdomen and in the muscle layer in the thorax.

Hot Temperature↗
PubMedDate not supplied

Transient finite element analysis of thermal methods used to estimate SAR and blood flow in homogeneously and nonhomogeneously perfused tumour models.

A two-dimensional time-dependent finite element model was developed to evaluate thermal techniques for estimating blood flow and specific absorption rate (SAR). In these computer simulations, homogeneously and nonhomogeneously perfused tumour models were heated by a 915 MHz interstitial microwave antenna array. Representative blood flow values were assigned within the tumour, and the applied SAR distribution was based on a previously developed antenna theory. SAR values were estimated from the power-on transient temperatures, and blood flow values were estimated from thermal clearance data after power was discontinued. These estimated parameters were then compared to the known 'true' blood flow and SAR values throughout the treatment region. SAR values could be predicted with reasonable accuracy throughout most of the heated region independent of local blood flow. For a homogeneous model, thermal clearance was found to yield reasonably accurate blood flow estimates at high perfusion rates and less accurate estimates at lower perfusion rates. However, for the inhomogeneous model, the blood perfusion estimates were generally poor, and an average blood flow value for the tumour was obtained with little ability to resolve the differences in perfusion between regions. Using temperatures observed early in the cool-down curve resulted in improved spatial resolution, but increased the contribution of thermal conduction to the blood flow estimates. A single time-constant exponential thermal decay curve was found to be a necessary but not sufficient condition for reliable blood flow estimates using this technique.

Absorption↗
PubMedDate not supplied

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.

Absorption↗
PubMedDate not supplied

Theoretical investigation of a phased-array hyperthermia system with movable apertures.

A four-applicator phased-array hyperthermia system with movable apertures (MA) is compared with an eight-applicator annular phased-array hyperthermia system with fixed apertures (AA) in terms of the HEP (hyperthermia equipment performance) values, based on two-dimensional models and the bioheat transfer equation. A hybrid element method is used to calculate the zeta-directed two-dimensional electric field with the inhomogeneities in tissue properties taken into account. The amplitudes and phases of each applicator are then optimized with the objective of uniform power deposition in the tumour and no power deposited in normal tissues. The temperature distributions under different blood flow conditions are obtained by solving the bioheat transfer equation using the finite element method. It is found that among the seven patient models studied, the MA and AA in general perform equally well when the tumour has zero blood flow, or equally poorly when the tumour has a blood flow larger than 5 ml/100 g per min. The performance of AA is often significantly better than that of MA when the tumour blood flow is 2.7 ml/100 g per min. The effects of different weighting functions are evaluated. We show that even if uniform absorbed power density (ARD = absorption rate density) could be achieved in the tumour volume with zero ARD in normal tissue the entire tumour would still not be brought to 43 degrees C or greater. However, it is found that the performance of uniform ARD in the tumour is on average far better than either the AA or MA, and choosing the uniform ARD as the objective function improved 35% of the cases for AA and 16% for MA. The optimization formula includes a weighting function that can be varied for different tissues. By decreasing the weights in regions of high blood flow the HEP values can sometimes be improved quite noticeably. Finally, the importance of the locations of applicators is studied. The results obtained indicate that the applicators should be placed about 5 cm or more away from the patient body (assuming water is the coupling medium) to ensure good HEP ratings.

Blood Flow Velocity↗
PubMedDate not supplied

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.

Evaluation Studies as Topic↗
PubMedDate not supplied

Techniques for intraoperative hyperthermia with ultrasound: the Dartmouth experience with 19 patients.

Over the course of 3 years, tumours of 19 patients were heated with ultrasound in the operating room during surgical resection. Immediately following intraoperative radiation therapy, thermocouples were inserted into tumour and adjacent normal structures. Patients were then given a 60-min heat treatment with ultrasound after a 10-15-min heatup period. Temperatures were measured at a total of 133 fixed locations for the 19 patient series. Temperature mapping was done in the tumour volume when logistically feasible. Treatment sites included colorectal (n = 3), portahepatus (n = 1), pancreas (n = 7), liver (n = 1), pelvis (n = 3), sacrum (n = 2), and abdomen (n = 2). A sterile, constant-volume water circulating system was utilized to control surface temperatures. Three generations of completely immersible transducers were designed over the course of this study with a 4-cm height specification. Since the ultrasound transducer was assembled on the sterile field during surgery, a 1, 2 or 3 MHz ceramic element was placed in either a 6, 8 or 10 cm diameter aluminium housing to conform the acoustic field to the tumour size. Average of the maximum temperatures attained was 46.6 degrees C. Temperature with which 90% of all measured points equalled or exceeded (T90) was 39.2 degrees C. The T50 was 42.9 degrees C. This compared favourably with T90 and T50 of 38.8 and 41.9 degrees C, respectively, in our outpatient clinic series, in which superficial tumours were treated with a similar external applicator, and patient tolerance was often a treatment limitation.

Combined Modality Therapy↗
PubMedDate not supplied

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.

Body Temperature↗
PubMedDate not supplied

Pattern of response to interstitial hyperthermia and brachytherapy for malignant intracranial tumour: a CT analysis.

Interstitial microwave hyperthermia in combination with iridium-192 brachytherapy has been administered to 23 cases of malignant brain tumours in a phase one clinical trial to assess the feasibility and safety of this treatment. In order to quantify the acute and long-term response of tumour and surrounding brain to this treatment, a morphometric computed tomography scan analysis was performed in 18 evaluable patients. Volumes defined by the outer margin of the contrast-enhancing rim, by the hypodense necrotic region within the enhancing rim and by the surrounding hypodensity region were calculated from computer measurements. Hyperthermia equipment performance (HEP) was calculated for the evaluation of heating. After the treatments, the volume of the inner hypodensity region decreased in seven patients and the volume increased in 11 patients. In five patients, the outer margin of the contrast-enhancing lesion showed an initial increase in volume followed by a decrease and in these patients higher HEP and longer survival were observed significantly. The volume of the surrounding hypodensity region varied following treatments, but in most instances, the region subsequently increased in the interval immediately prior to death. Contribution of heat effect to these changes are discussed and the significance of aggressive heating, which provides transient opening of blood brain barrier, is shown.

Adult↗
PubMedDate not supplied

Performance of an adaptive MIMO controller for a multiple-element ultrasound hyperthermia system.

A prototype adaptive automatic control algorithm was implemented to regulate temperatures measured at several points in a tumour by adjusting the power applied to several ultrasound transducers. The goal was to control the temperatures under the elements of a mosaic applicator individually without any priori knowledge of which probes are under which elements. The control algorithm was devised for clinical applications where the position of each probe with respect to the heat sources is difficult to determine precisely. Instead, the program 'learns' the relationship between the inputs (power levels) and the outputs (temperatures) automatically. Based on the observed transfer function relating the power at m sources to the temperatures n probes, where n and m are not necessarily the same, a new method was used to implement a feedback controller. This method simplifies the design of the controller for a multiple-input/multiple-output (MIMO) system, while taking into account the coupling that may exist between the various elements of the system. As a result of using an adaptive scheme, the regulator continuously tracks changes in the system, such as blood flow variations or patient motion, by modifying its control parameters. The algorithm performance has been tested in simulations as well as experiments in dog thigh and a perfused kidney model.

Algorithms↗
PubMedDate not supplied

Hyperthermia equipment evaluation.

This article looks briefly at the advances over the past 2 decades in the development of equipment for administering clinical hyperthermia, and observes that while there has been much progress, the equipment available today still does not meet the design criteria articulated 20 years ago. The assertation is made that the testing of hyperthermia equipment in animals has not addressed the questions most of interest in the clinic, and a suggestion is made for a more rigorous way to evaluate clinical equipment. Finally some areas where significant progress in equipment development seems possible are mentioned.

Equipment Design↗