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Biomedical subjects

R B Roemer

Publications and source records attributed to R B Roemer.

At least 19 recordsLinked to original sources
PubMed1992

Optimal power deposition with finite-sized, planar hyperthermia applicator arrays.

Improved hyperthermia applicator technology is allowing finer spatial power resolution within the heated tissue volume. Effective utilization of these planar applicator arrays requires an understanding of the interrelationships between the lateral dimensions of the tumor and the applicators, the power field produced by the applicators, the amount of surface cooling, the tumor tissue blood perfusion, and the normal tissue blood perfusion. These interrelationships are investigated using three-dimensional power patterns and temperature fields produced by optimizing the power amplitudes of the individual applicators located within an array of small, but finite, planar applicators. Five major conclusions are obtained. First, optimization works and is effective in determining optimal power fields. Second, for optimal treatments the lateral dimensions of a single superficial applicator need to extend beyond the tumor boundary. Third, surface cooling is needed to reduce the high normal tissue temperatures at shallow depths. Fourth, finer power resolution becomes more important as the tumor size decreases, but, little improvement in the temperature field is achieved beyond a 3 x 3 array configuration. Fifth, increasing the normal blood perfusion rate can decrease the temperature on the tumor boundary if direct power deposition on that boundary is unavailable.

Equipment Design↗
PubMed1992

The effects of large blood vessels on temperature distributions during simulated hyperthermia.

Several three-dimensional vascular models have been developed to study the effects of adding equations for large blood vessels to the traditional bioheat transfer equation of Pennes when simulating tissue temperature distributions. These vascular models include "transiting" vessels, "supplying" arteries, and "draining" veins, for all of which the mean temperature of the blood in the vessels is calculated along their lengths. For the supplying arteries this spatially variable temperature is then used as the arterial temperature in the bioheat transfer equation. The different vascular models produce significantly different locations for both the maximum tumor and the maximum normal tissue temperatures for a given power deposition pattern. However, all of the vascular models predict essentially the same cold regions in the same locations in tumors: one set at the tumors' corners and another around the inlets of the large blood vessels to the tumor. Several different power deposition patterns have been simulated in an attempt to eliminate these cold regions; uniform power in the tumor, annular power in the tumor, preheating of the blood in the vessels while they are traversing the normal tissue, and an "optimal" power pattern which combines the best features of the above approaches. Although the calculations indicate that optimal power deposition patterns (which improve the temperature distributions) exist for all of the vascular models, none of the heating patterns studied eliminated all of the cold regions. Vasodilation in the normal tissue is also simulated to see its effects on the temperature fields.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMed1991

Treatment of malignant brain tumors with focused ultrasound hyperthermia and radiation: results of a phase I trial.

Hyperthermia delivered by scanned focused ultrasound was combined with external beam radiation to treat 15 patients with primary malignant tumors of the brain. A preliminary craniectomy was performed to avoid attenuation of the ultrasound beam by the skull, and multiple thermal sensors were employed to ascertain intratumoral temperatures. The target temperature was 42.5 degrees C at the tumor boundary. This was attained at more than one point during every complete treatment, while a mean temperature in excess of 42 degrees C was achieved within the scanned tumor volume during at least 1 treatment in 11 patients. Technical problems and toxicities are described.

Adolescent↗
PubMed1991

Development of scanned focussed ultrasound hyperthermia: clinical response evaluation.

Selective heating of irregularly shaped tumors at depth can now be accomplished through focussing and controlled scanning of energy deposition patterns by ultrasound. A scanned focussed ultrasound (SFUS) hyperthermia system developed at the University of Arizona has been used to deliver 220 treatments to 87 tumors in 71 patients with extracranial malignancies between October 1986 and May 1990. Patients received an average of three SFUS hyperthermia treatments, spaced weekly, during ongoing fractionated radiotherapy. The most common anatomic sites treated were the pelvis (22 patients), chest wall or breast (14), neck (8), and axilla (7), while the most common histologies were adenocarcinoma (36), squamous cell carcinoma (11), and melanoma (10). Concurrent radiotherapy was delivered (range 1000-7640 cGy, mean 4320 cGy) to 67 SFUS hyperthermia patients; 4 received concomitant chemotherapy. Tumor volumes ranged from 1-2100 cubic centimeters (mean 325 cc), and 75% were located at depths greater than 3 cm from skin. A 62% overall response rate was observed, with 22% of treated tumors demonstrating a complete response (defined as complete disappearance of treated tumor), and 40% exhibiting a partial response (defined as greater than or equal to 50% reduction in tumor volume). Dramatic local pain reduction was achieved in 42% of the tumors treated. The acute tolerance of SFUS hyperthermia was quite good, and chronic toxicities (persistent skin blisters/burns) were identified in two patients. The versatility of the SFUS system is discussed, as well as its future potential for improving control of advanced loco-regional malignancies treated with curative intent.

Combined Modality Therapy↗
PubMed1991

Estimating three-dimensional temperature fields during hyperthermia: studies of the optimal regularization parameter and time sampling period.

During hyperthermia therapy it is desirable to know the entire temperature field in the treatment region. However, accurately inferring this field from the limited number of temperature measurements available is very difficult, and thus state and parameter estimation methods have been used to attempt to solve this inherently ill-posed problem. To compensate for this ill-posedness and to improve the accuracy of this method, Tikhonov regularization of order zero has been used to significantly improve the results of the estimation procedure. It is also shown that the accuracies of the temperature estimates depend upon the value of the regularization parameter, which has an optimal value that is dependent on the perfusion pattern and magnitude. In addition, the transient power-off time sampling period (i.e., the length of time over which transient data is collected and used) influences the accuracy of the estimates, and an optimal sampling period is shown to exist. The effects of additive measurement noise are also investigated, as are the effects of the initial guess of the perfusion values, and the effects of both symmetric and asymmetric blood perfusion patterns. Random perfusion patterns with noisy data are the most difficult cases to evaluate. The cases studied are not a comprehensive set, but continue to show the feasibility of using state and parameter estimation methods to reconstruct the entire temperature field.

Algorithms↗
PubMed1990

Effects of physical parameters on high temperature ultrasound hyperthermia.

The purpose of this research was to investigate the feasibility of inducing perfusion independent, predictable therapeutic thermal dose using high power ultrasonic pulses. Computer simulations were used to study the effects of blood perfusion, tissue properties, transducer characteristics, and treatment geometry on the temperature elevation and thermal dose delivered by short ultrasonic pulses. Experiments were conducted in vitro and in vivo to investigate the effects of blood perfusion changes. Results show that short pulse lengths (less than or equal to 2 s) and small focal diameters (approximately 3 mm) give temperature elevations and thermal doses which are nearly perfusion independent. Normal fluctuations in tissue properties should not have a significant effect on the treatment provided that proper choice of transducer is made for each individual application.

Animals↗
PubMed1990

Computer-controlled dynamic phantom for ultrasound hyperthermia studies.

A dynamic phantom system for use in evaluating hyperthermia heating equipment has been designed, constructed, and tested. It consists of four in vitro preserved canine kidneys perfused with an 80% ethanol preparation by a single pump, plus four computer-controlled valves and four flow meters. The flow rate to each kidney is computer-controlled, giving flexibility in the types of spatial and temporal flow patterns possible. Examples are given for both step and ramp changes in flow, and for a temperature dependent flow which has been used to simulate physiological responses to elevated temperatures.

Animals↗
PubMed1989

A spherical source model for the thermal pulse decay method of measuring blood perfusion: a sensitivity analysis.

The thermal pulse-decay method, as developed and analyzed by Chen et al. [1-6], is a thermal clearance technique that uses a small thermistor probe for determining the blood perfusion and thermal conductivity of the tissue immediately surrounding the probe. They described the energy transfer of the probe/tissue system mathematically with a simple analytical model, the point source model, which assumes that the heating source is infinitely small. This paper introduces a new, more accurate analytical description that assumes the heating source is spherically symmetric with a finite radius. A numerical study of these two alternative mathematical models is presented in which the solutions of each model are compared to transient temperature decay data generated from a detailed finite difference simulation of the probe/tissue system. The accuracy and sensitivity of the predictions of each of these models to variations in tissue thermal conductivity and perfusion, probe characteristics, and heating time are presented. In all cases, the accuracy of the spherical source model was better than the point source model. It is also shown that the spherical source model can accurately predict low rates of perfusion (on the order of 1 kg/m3 s) unlike the point source model. The spherical source model allows for the possibility of the measurement probes to be calibrated for an "effective bead radius" which accounts for the nonideal characteristics of the probe, thereby giving even more accurate determinations of perfusion.

Blood Circulation↗
PubMed1988

Scanned focussed ultrasound hyperthermia: initial clinical results.

Between November 1986 and July, 1987, a preliminary study to determine the feasibility of scanned focussed ultrasound for clinical hyperthermia at various sites was conducted. Fourteen patient (17 tumors) have been treated using a microprocessor-controlled apparatus developed at the University of Arizona by modifying a commercially available diagnostic ultrasound unit. We have treated nine pelvic tumors, four extremity tumors, two brain tumors, and two extracranial head and neck tumors for a total of 42 treatments. Multipoint thermometry was achieved for all patients, with 2-25 (mean = 10) points monitored during each treatments within the scanned tumor volume. Average maximum temperature within the scanned tumor volume was 44.2, 44.7, 44.8, and 42.0 degrees C for pelvic, extremity brain, and extracranial head and neck tumors, respectively; similarly, 55%, 45%, 71%, and 0 of monitored points exceeded 42.5 degrees C. Pain limited applied power in 15 of 42 treatments, and bone pain with a periodicity similar to the scanning periodicity was seen in 11 treatments. A non-randomized comparison of temperatures achieved using scanned focussed ultrasound to those achieved using the microwave annular array and the CDRH Helix suggests that scanned focussed ultrasound may have promise and potential advantages in heating selected pelvic tumors.

Brain Neoplasms↗
PubMed1985

Obtaining local SAR and blood perfusion data from temperature measurements: steady state and transient techniques compared.

A series of analyses and experiments was performed to determine the extent that SAR and blood perfusion information can be extracted from steady state temperature values and from transient temperature measurements following a step change in applied power. Multiple local temperature measurements were made in canine thighs heated by 2450 MHZ microwaves to evaluate two parameters: the local absorbed power in the tissue, and the local "effective blood perfusion." The theoretical bases for these calculations are presented in order to identify their underlying assumptions and to obtain a unified basis for comparison of the various calculation methods used by previous investigators. From energy balance considerations it can be shown that the local absorbed power can be obtained from either the rate of increase of temperature following a step increase in power, or from the rate of decrease in temperature immediately following a step decrease in power. These theoretical observations are verified experimentally by comparing the SAR results at fixed positions in canine thighs as calculated from both increasing and decreasing power steps. For decreasing power steps, the resulting decreasing temperature curves can also be used to calculate an effective blood perfusion rate if thermal conduction is included. Alternatively, this same effective blood perfusion rate can be calculated from steady state data. (These two approaches have been used by previous investigators to determine "blood perfusion" values. We have added the modifier "effective" to specifically denote the presence of thermal conduction effects in such perfusion calculations.) From our experimental results and theoretical calculations it appears that differences between the predictions of the two calculation methods arise from changing thermal conduction values during the cooling period of the thermal clearance method. The steady state calculation approach is easier to apply than the washout method, but it requires the additional knowledge of the local SAR value. It is important to realize that effective blood perfusion values calculated using thermal techniques are subject to large errors under conditions where thermal conduction is important, unless this conduction is explicitLy included in the calculation. Such effective blood perfusion values should not be quantitatively compared to values calculated from non-thermal techniques that are not affected by thermal conduction. Unless such conduction effects are known to be negligible, effective perfusion values are only qualitative indicators of the presence of changes in blood perfusion.

Absorption↗
PubMed1985

A comparative evaluation of unconstrained optimization methods applied to the thermal tomography problem.

In cancer hyperthermia treatments, it is important to be able to predict complete tissue temperature fields from sampled temperatures taken at the limited number of locations allowed by clinical constraints. An initial attempt to do this automatically using unconstrained optimization techniques to minimize the differences between experimental temperatures and temperatures predicted from treatment simulations has been previously reported [1]. This paper reports on a comparative study which applies a range of different optimization techniques (relaxation, steepest descent, conjugate gradient, Gauss, Box-Kanemasu, and Modified Box-Kanemasu) to this problem. The results show that the Gauss method converges more rapidly than the others, and that it converges to the correct solution regardless of the initial guess for the unknown blood perfusion vector. A sensitivity study of the error space is also performed, and the relationships between the error space characteristics and the comparative speeds of the optimization techniques are discussed.

Hot Temperature↗
PubMed1985

Oscillatory temperature response to constant power applied to canine muscle.

A previously unreported oscillatory temperature response has been observed in canine thigh muscle subjected to an abrupt application of a constant specific energy absorption rate (SAR). The power was applied to anesthetized dogs (stage 3 plane of anesthesia) using 2,450-MHz microwaves. Five types of responses have been identified that have occurrences depending on the maximal temperature produced by the applied power. In particular, for SAR values resulting in sufficiently high initial temperatures, self-sustaining temperature oscillations lasting over 4 h have been observed with amplitudes up to 7 degrees C. The temperature oscillations are believed to be caused by oscillations in the local blood perfusion rate.

Animals↗
PubMed1979

Thermal response of skin to application of localized pressure.

Pressure was applied to a small circular area of the skin over the trochanter of human subjects for variable time periods. The local skin temperature response was monitored during and after pressure application. Pressures of 100, 200, and 300mmHg were applied for time periods of between 11 and 67 minutes. Upon the release of pressure the local skin temperature rose quickly and then gradually decreased. Peak temperature responses occurred at between 3 and 5 minutes after pressure release. Larger pressure and longer application times produced larger peak temperature increases, with the largest individual increase being 1.9C. The data derived from the study tend to quantify what clinicians have long appreciated in a qualitative sense, especially in areas where decubitus ulcers most often occur.

Adult↗
PubMed1978

Automated upper extremity progressive resistive exercise system.

This paper describes a semiautomated, motorized skateboard and skate system for use in progressive resistive arm exercises. This experimental system has been evaluated in an occupational therapy clinic for the past two years and found to be valuable in the treatment of impaired upper extremity function. It is superior in function and design to the previously used apparatus and provides the following features: adjustable load (force), adjustable range of motion limits, an automatic method of counting the correct repetitions of the prescribed exercise, the visual feedback to patients regarding their performance. Based on therapists' evaluations, the system's three main advantages are: a decrease in the amount of therapist/attendant time needed for the exercise program, improved performances by the patients, and a better, quantitative measure of the patients' progress.

Adult↗
PubMed1977

Voice controlled wheelchair.

A voice recognition and control system for use by quadriplegics and other severely physically disabled individuals in controlling a wheelchair has been developed and tested. Several wheelchair compatible features are included in the design--low power consumption (approximately 20 watts), small size (10"x11"x14"), low weight (approximately 22 lb), and no need for wheelchair modification (existing joystick control cable plugs directly into this system). The current system uses an eight word vocabulary (e.g., "forward," "slower," "right,") and a feedback control system to maintain the chair speed and direction. A recognition rate of more than 90% has been attained in laboratory tests with experienced speakers. Future system improvements are planned to increase the recognition rate and practical utility of the chair. The results of laboratory tests with normal and quadriplegic subjects indicate that voice commands are potentially a useful and practical means of wheelchair control. The effects of environmental noise are investigated and found to be minimally deleterious.

Biomedical Engineering↗