H. Frederick Bowman

A new technique utilizing thermistor probes for the measurement of thermal properties of biomaterials.

Abstract The thermal limitations inherent with the use of invasive thermistor probes in the measurement of thermal properties of biomaterials have been investigated. An electronic temperature controller has been developed which provides a nearly instantaneous step rise in average probe resistance (temperature). The method of experimentally determining the heat rate required to maintain the average probe temperature constant and incorporation of that heat rate into the general heat diffusion equation provides a solution which allows the determination of both thermal conductivity and diffusivity values with improved accuracy. The method is general to all media which wet the surface of the probe; the need for calibrating media is avoided. The solution also predicts the minimum required sample size.

THE BIO‐HEAT TRANSFER EQUATION and DISCRIMINATION OF THERMALLY SIGNIFICANT VESSELS

The recent renewal of interest in the use of localized hyperthermia for the treatment of malignant tissue is motivating a concurrent effort to achieve a greater understanding of the transport of thermal energy in tissue volumes of the same scale. The clinical requirement for well-managed application of hyperthermia is the ability to produce specific, well-characterized temperature elevations in carefully delineated volumes of tissue that comprehend the malignancy. The engineering requirement is to be able to predict the resultant temperature distribution in a given tissue or tumor volume for a given absorbed thermal dose and to be able to modulate the temporal and spatial characteristics of the absorbed thermal dose so as to produce the desired or optimum temperature distribution for the specific malignancy be:ng treated. On the engineering and physics side of this multidisciplinary problem, the principal emphasis to date has been focused on investigating the potential and limitations of the various alternative heating and temperature-sensing techniques. No clear technique of choice has yet emerged, and indeed, given the great variation in type, size, location, and distribution of malignant tissue, it may not be possible to define a single technique that will prove to be applicable generally. Ideally, one would like to be able to image, heat, and sense the temperature field of the tumor mass or malignancy noninvasively and in three dimensions. Though it is theoretically possible to obtain three-dimensional temperature distributions noninvasively, this capability has not been demonstrated clinically. Thus, it is necessary to use invasive temperature probes to quantify the temperature elevation at specific sites, from which an estimate of the temperature profile in the remaining malignant and normal host tissue is derived. When the temporal and spatial distribution of the absorbed thermal dose and the thermal transport properties of the neoplastic and normal host tissue are known, the temperature field can be determined by evaluation of the appropriate biothermal diffusion equation; and the predicted temperature can be independently verified by one or more temperature sensors. Careful attention should be focused on what constitutes an “appropriate” biothermal diffusion equation for a given tissue region and desired hyperthermic state. The bio-heat equation due to Pennes,’ which includes the effects of metabolic heat generation and tissue perfusion (but neglects the effect of major vessels), is one such equation that has been rather widely used without rigorous evaluation of its limits of

The temperature distribution in a semi-infinite body due to surface absorption of laser radiation

Abstract An exact steady state and transient solution for the temperature distribution in a semi-infinite body with a Gaussian distribution heat source at the body surface is developed and presented. This solution has direct application to steady state laser heating for target materials which have a high absorption coefficient at the laser wavelength. The solution also provides an upper bound for the case of pulsed laser heating. Approximate solutions for the steady and transient cases are used for comparison and it is shown that the exact solutions are the asymptotic limit of the approximate solutions.

Ligament and tendon oxygenation measurements using polarographic oxygen sensors

This pilot study was designed to investigate a new flexible polarographic oxygen sensor for intraarticular oxygen tissue tension monitoring, as a first step in designing a combined simultaneous oxygen and perfusion monitor. The tendoachilles in the sheep provided the initial testing model, which was followed by in vivo testing in the human knee. In the animal model, the tendoachilles was perforated by a single needle through which the new flexible multichannel polarographic oxygen sensor probe was passed. Variations in blood and oxygen delivery to the hindlimb were then produced by tourniquet and fraction of inspired oxygen (FiO2) changes. The oxygen sensors appropriately recorded current variations proportional to the hypothesized oxygen delivery. In the human model, the anterior cruciate ligament (ACL) in five human knees was exposed during routine total knee replacement surgery and stripped of the fat pad and synovial attachments. The probe was inserted into the substance of the ligament from distal to proximal. Oxygen measurements were taken with the tourniquet elevated and then inflated, in an effort to evaluate the response of the oxygen sensors and to establish the degree of oxygenation that the bone blood supply provides to the ACL. In the sheep, 100% of the time (13 of 13 events), the current changed appropriately to a change in the FiO2. Ninety-four percent of the time (17 of 18 events), the current changed appropriately to a tourniquet change. In the human ACL, the probe was 83% sensitive (5 of 6 events) to a tourniquet change. The FiO2 changes were inconclusive owing to an insufficient amount of time allowed for tissue perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Model and solution for the thermal response of blood-perfused tissue during laser hyperthermia

The monitoring and control of the thennal dose in non-contact laser hyperthermia treatment of tumors requires a model and solution of the lasertissue interaction. Presented are two models for the thermal response of tissue during continuous wave, noncontact, laser hyperthermia treatment of tumors. The first model is where all the laser energy is absorbed at the surface of the tissue and the second model is where the laser energy penetrates into the tissue before absorption. Both models include the effect of blood perfusion in the solution for the temperature rise. It is shown that perfusion has a negligible affect on the temperature distribution for the surface absorption of laser energy. However, in the case of laser energy penetration, perfusion has a significant effect which increases with increasing optical penetration.