Biomedical subjects

R E Apfel

Publications and source records attributed to R E Apfel.

25 records · Page 2Linked to original sources

PubMed1990

Thresholds for transient cavitation produced by pulsed ultrasound in a controlled nuclei environment.

Transient cavitation is a discrete phenomenon that relies on the existence of stabilized nuclei, or pockets of gas within a host fluid, for its genesis. A convenient descriptor for assessing the likelihood of transient cavitation is the threshold pressure, or the minimum acoustic pressure necessary to initiate bubble growth and subsequent collapse. An automated experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz. A fluid jet was used to convect potential cavitation nuclei through the focal region of the insonifying transducer. Potential nuclei tested include 1-microns polystyrene spheres, microbubbles in the 1- to 10-microns range that are stabilized with human serum albumin, and whole blood constituents. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Measurements of the transient cavitation threshold in water, in a fluid of higher viscosity, and in diluted whole blood are presented. These experimental measurements of cavitation thresholds elucidate the importance of ultrasound, host fluid, and nuclei parameters in determining these thresholds. These results are interpreted in the context of an approximate analytical theory for the prediction of the onset of cavitation.

Acoustics↗

PubMed1989

Practical neutron dosimetry with superheated drops.

The Superheated Drop Detector (SDD) is a new kind of neutron detector based on the same principle as that of the bubble chamber, except the superheated material is in drop form suspended by a gel or polymer as first described by Apfel (U.S. Patent 4,143,274). Previously, we have developed a theoretical model for the prediction of the threshold neutron energy to nucleate bubbles in our superheated materials and a model for the calculation of the energy dependent response function of SDD. The results of the theoretical calculations agree with the experimental measurements reasonably well. We found the measured response curve of one of our SDDs follows the same trend as the ideal ICRP dose equivalent response curve within 40% for neutrons of energy above 100 keV and within a factor of 10 below 100 keV. Therefore, it is possible to make a rem-response Superheated Drop Detector. We have further calibrated our SDD with 252Cf neutron sources (both bare and D2O moderated). The results are in reasonable agreement with our theoretical predictions, which proves that our model is valid not only for monoenergetic neutrons but also for neutrons from a radioactive source. The principles of operation of our detector, which can be used in neutron dosimetry, area monitoring, and neutron spectrometry, are discussed.

Neutrons↗

PubMed1988

Thresholds for cavitation produced in water by pulsed ultrasound.

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.

Acoustics↗

PubMed1988

Application of 30-MHz acoustic scattering to the study of human red blood cells.

A technique for simultaneously measuring the scattering amplitude of individual particles at two angles is applied to human red blood cells. Using a Rayleigh scattering model, the density and compressibility of the cells may be determined given a priori knowledge of their volume. A calibration method relying on measurements of the bulk properties of particle suspensions is described. Red cell properties in hypotonic and hypertonic hosts are compared with a homogeneous mixture model, and a linear relation between hemoglobin content and scattering amplitude at a 90 deg scattering angle is established.

Calibration↗

PubMed1986

Prediction of tissue composition from ultrasonic measurements and mixture rules.

A methodology is presented for predicting the composition of tissues from measurements of the density, sound velocity, and acoustic nonlinear parameter, using mixture laws for the density, compressibility, and nonlinear parameter. It is shown that the mixture law for the nonlinear parameter plays an essential part in this methodology, which leads to the prediction of the volume fractions of water, protein, and fat in a given tissue. Data from the literature for solutions, blood, normal tissue, and cancerous tissue are investigated, and predicted fractions are consistent with tissue compositional information available in handbooks. More experimental work is needed with tissues of known composition in order to more fully test the proposed methodology.

Adipose Tissue↗

PubMed1985

Shape oscillations of microparticles on an optical microscope stage.

A modulated acoustic radiation pressure technique to produce quadrupole shape oscillations of drops ranging in diameter from 50-220 micron has been used by us. These drops have been suspended by acoustic levitation in a small chamber mounted on a stage of an optical microscope, which allowed easy viewing. The fission of drops and the deformation of sea urchin eggs were also observed.

Acoustics↗

PubMed1982

Acoustic cavitation: a possible consequence of biomedical uses of ultrasound.

Those concerned with acoustic cavitation often use different measures and nomenclature to those who employ ultrasound for medical purposes. After illustrating the connections between the two, acoustic cavitation phenomena are divided into two classes: (1) relatively moderate amplitude changes in the bubble size that occur during each acoustic cycle, as with rectified diffusion and resonant bubble motion, and (2) rather dramatic changes in the bubble radius that occur in one cycle. It is seen that pulse-echo diagnostic equipment can excite the dramatic changes whereas continuous wave therapeutic equipment will excite the slower, but no less important, changes. The ranges of the acoustic variables and material states for which these phenomena are possible are quantified. It is shown that whereas the concept of an ultrasonic (energy) dose may be appropriate for the effects of acoustically induced heating or resonant bubble motion. It is inappropriate when discussing the effects of the transient type of cavitation that can occur from short, high amplitude acoustic pulses.

Acoustics↗