William J. Fry

Determination of Absolute Sound Levels and Acoustic Absorption Coefficients by Thermocouple Probes—Experiment

A detailed theoretical analysis of the operation of thermocouple probes used to determine absolute sound levels or acoustic absorption coefficients is presented.The probe consists of a thermocouple imbedded in a sound absorbing medium which closely matches in density and sound velocity the medium in which the sound level is to be determined. In use the transducer which generates the acoustic field is excited to generate sound pulses with a rectangular envelope. The initial time rate of change of the temperature at the thermocouple junction is determined. In addition to the measurement of the temperature change, the calculation of the absolute sound intensity requires only a knowledge of the absorption coefficient of the imbedding material and its heat capacity per unit volume at the temperature at which the measurements are made.The theoretical discussion includes an analysis of the relation between the temperature rise at the junction resulting from absorption in the body of the imbedding medium and the ...

Physical Factors Involved in Ultrasonically Induced Changes in Living Systems: I. Identification of Non‐Temperature Effects

The results of the first step in a systematic investigation of the mechanism of the action of ultrasound on tissue are reported. The temperature changes resulting from absorption of acoustic energy were determined while irradiation was in progress.Experimental evidence is presented which demonstrates the existence of non‐temperature effects in various nerve tissue preparations.

Ultrasonic Absorption and Reflection by Lung Tissue

The acoustic reflection and absorption coefficients of both normal and diseased (pneumonitis) excised lung tissue (dog) were experimentally determined at a frequency of 098 Mc/sec. It is found that the physiological saline-lung interface reflects 50% of the sound energy falling on it at normal incidence. The acoustic amplitude absorption coefficient per unit path length of lung tissue is 47 cm−1. The very high absorption exhibited can be explained as caused by radiation of acoustic energy by the pulsating gaseous structures in the lung tissue. The theory indicates that the absorption coefficient of lung tissue should approach a minimum as the frequency is increased above 1 Mc/sec and should then increase at still higher frequencies. The diseased lung exhibited an acoustic absorption coefficient approximately 25% less than that of normal lung specimens.

Physical Factors Involved in Ultrasonically Induced Changes in Living Systems: II. Amplitude Duration Relations and the Effect of Hydrostatic Pressure for Nerve Tissue

The results of experiments with frogs under a hydrostatic pressure demonstrate that cavitation is not an important factor in the mechanism of production of paralysis of the hind legs of frog by ultrasonic (frequency one megacycle) irradiation over the lumbar enlargement region of the spinal cord. Experimental results indicate that a linear relation exists between the reciprocal of the minimum exposure time for paralysis and the acoustic amplitude. This result is readily described in terms of a one factor rate process. On the basis of this experimentally determined relation, it is shown that time rate of change of temperature cannot be correlated with the observations. It is concluded on the basis of a theoretical calculation that absorption of ultrasound at interfaces in the spinal cord does not result in minute hot regions.Further work on summation of subparalytic doses, spaced apart at various time intervals, indicates that the recovery process following exposure to a subparalytic dose of ultrasonic rad...

Electrical Stimulation of Brain Localized without Probes—Theoretical Analysis of a Proposed Method

A method of electrically stimulating neural elements confined within small volumes of brain at predetermined deep locations without employing probes is considered theoretically. The basic principle of the method is the partial rectification in the focal region of an ultrasonic field of the alternating current that flows in the entire brain, or a major portion thereof, in response to an externally applied electric field of the same frequency. Since the magnitude of the electrical conductivity of the tissue varies with the temperature the adiabatic temperature, changes produced by the acoustic disturbance cause a periodic variation in the conductivity that results in a net unidirectional transfer of charge when the frequenices of the two fields are equal. The condition for stimulation is expressed quantitatively by a relation involving the amplitudes of the electric and acoustic field parameters, the thermal coefficient of electric conductivity of the tissue, the threshold quantity of charge that must be tr...

Ultrasonic Visualization System Employing New Scanning and Presentation Methods

New methods for the ultrasonic visualization of tissue, achieved by employing a newly developed versatile instrumentation system, are outlined and their application to the examination of brain is illustrated. The complete system, whose operating characteristics are described in detail, incorporates an on‐line medium‐sized digital computer, implementing omnidirectional scanning, utilizing three translational and two rotational degrees of freedom; relief display, which combines intensity modulation and deflection of the oscillograph beam by the echo signals; and segmental construction of echograms, which incorporates wide dynamic range combined with high resolution via program control of receiver gain as a function of range and echo strength. Omnidirectional scanning permits the more complete viewing of tissue interfaces than is possible with the usual compound‐scanning methods. Relief display provides an additional parameter as compared to the common flat format for the presentation of echo information in ...

Ultrasonic Absorption Microscope

This paper constitutes a progress report on the development of an ultrasonic instrument for detecting microstructure. The principle of operation of the device (designated by the term “ultrasonic microscope”) is as follows: The specimen is imbedded in a liquid, which closely matches it in acoustic impedance, and is irradiated with short pulses of high‐frequency sound. Some of the ultrasonic energy is absorbed in the specimen and the remainder leaves it and excites a small thermoelectric probe placed immediately adjacent to the region being investigated. As the specimen, which is between the sound source and the probe, is moved in a direction normal to the direction of sound propagation, a varying acoustic signal will be detected by the probe. This is a consequence of the fact that the amount of acoustic energy absorbed by the specimen in the portion immediately adjacent to the probe is determined by the structure of that part of the specimen.Preliminary experimental results obtained with such a “microscope...

Temperature Changes Produced in Tissue during Ultrasonic Irradiation

This paper is concerned with the technique of temperature measurement in living tissue during irradiation by high intensity ultrasound. The interpretation of data obtained by the use of thermocouples is presented. The specific biological object used in this study is the spinal cord of rat exposed by laminectomy. This particular preparation serves to illustrate the relative importance of the heat conduction process in contributing to the temperature change as a function of the proximity of the imbedded thermocouple to bone and the time elapsed after initiation of the exposure.The ultrasonic frequency used in these studies was 980 kc. The sound intensities incident on the cord were between 60 and 80 watts/cm2.The experimental results presented in the paper are used to obtain values for the acoustic absorption coefficient of the tissue of the spinal cord. The range of values obtained for the intensity absorption coefficient per centimeter from measurements made on six adult rats at various positions in the s...

Action of Ultrasound on Nerve Tissue—a Review

This review is concerned with (1) certain physiological and structural changes produced in tissues of the central nervous system caused by high level ultrasound and (2) investigations into the physical mechanisms underlying these changes.The cell bodies of neurons in the central nervous system are particularly susceptible to change by ultrasound. The effect of irradiation is immediately evident as a loss of function which may be reversible or irreversible depending on the dosage. Irreversible changes in function are accompanied by changes in the structure of the cell. The susceptibility of neurons studied so far is graded according to size, the larger neurons exhibiting a greater susceptibility. The dose of ultrasound can be adjusted to cause irreversible changes in neurons without causing any observable damage to the vascular and supporting components of the tissue. This selective and specific effect of ultrasound is being used as a tool in neuroanatomical studies now in progress and has considerable pot...

Ultrasonic Irradiation of the Central Nervous System at High Sound Levels

High level ultrasound produces, under properly controlled dosage conditions, selective changes in the central nervous system. The physical mechanism of the action of the sound requires elucidation. Some of the problems associated with determining the physical mechanism are discussed and a preparation and procedure are described which are appropriate for accurately determining dosage relations for such a study. The quantitative results obtained with this preparation are presented.