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Biomedical subjects

W L Nyborg

Publications and source records attributed to W L Nyborg.

29 records · Page 2Linked to original sources
PubMed1982

Ultrasonic microstreaming and related phenomena.

It has been shown in recent work that biological cells and tissues are affected by low intensity ultrasound in the lower megahertz frequency range if small gas-filled channels or pores are present, with dimensions in the range of a few microns. When these gas bodies pulsate in response to the ultrasound they act on surrounding media by unique forms of radiation pressure, forces and torque; in liquids small-scale eddying is generated, known as microstreaming. It is probably by combinations of these phenomena that biological effects are produced.

Biophysical Phenomena↗
PubMed1979

Platelet aggregation induced by ultrasound under specialized conditions in vitro.

Human platelets were induced by 2.1-megahertz ultrasound to form aggregates around gas-filled pores in membranes immersed in platelet-rich plasma. The spatial peak intensities required were only about 16 to 32 milliwatts per square centimeter. Ultrasound generated by a medical Doppler device, whose intensity exceeded this, induced aggregate formation under the same conditions.

Blood Platelets↗
PubMedDate not supplied

Temperature elevation in a beam of ultrasound.

Computations are made of the temperature elevation expected at points along the axis of an unfocussed beam of ultrasound in a homogeneous absorbing medium. A simplified model is used in which the intensity is assumed uniform over a cross section. Heat conduction is taken into account, but not convection or perfusion. Results are presented for frequencies from 1 to 10 MHz and beam diameters from 0.2 to 2.4 cm. The results include limiting temperatures, reached at infinite time, as well as temperature vs time characteristics. As an example for certain conditions (transducer diameter 1.2 cm, frequency 4 MHz, intensity at the transducer 0.1 W/cm2, tissue acoustically similar to liver, transducer thermal conductivity similar to that of tissue) the temperature elevation produced at a distance 2 cm from the transducer is calculated to be 0.61 degrees C after 253 s; it then increases more and more slowly, approaching, but never exceeding 1.46 degrees C.

Hot Temperature↗
PubMedDate not supplied

Ultrasonic gas-body activation in Drosophila.

The role of gas bodies in the interaction of Drosophila eggs and larvae with ultrasonic fields has been studied (1) by direct microscopic examination, (2) by observation of gas body activation during sonation with a special microscope, (3) by direct measurement of the volume fraction of gas in eggs and (4) through the absorption of ultrasound. All approaches provide support for the postulate that these organisms contain a rich distribution of gas bodies which play a dominant role in the acoustic properties of eggs and are the most probable site of action for ultrasonic biological effects on these organisms.

Absorption↗
PubMedDate not supplied

Optimization of exposure conditions for medical ultrasound.

Optimum exposure conditions are values of acoustic exposure parameters (e.g., intensity, pressure amplitude, time) which yield maximum net medical benefit in a therapeutic or diagnostic procedure. It is generally agreed that if the exposure values are too low, the procedures are ineffective, while if they are too high, harm will result. However, little quantitative information exists on what specific values the parameters should have to give greatest net benefit in various applications. The determination of optimum exposure conditions is an important goal, but the process is likely to be a long one, requiring efforts by many individuals and groups. In this review several approaches to the problem are discussed, and difficulties are pointed out.

Animals↗
PubMedDate not supplied

Hyperthermic teratogenicity, thermal dose and diagnostic ultrasound during pregnancy: implications of new standards on tissue heating.

Hyperthermia is a recognized teratogen in mammalian laboratory animals and is a suspected teratogen for humans. The purpose of this synopsis is to reanalyse existing data on hyperthermia-induced teratogenic effects in experimental mammalian systems in terms of a thermal dose (temperature:time) concept, and then to illustrate the utility of this concept to human situations involving potential thermal increments to post-implantation embryos and foetuses. For example, the threshold temperature elevation for hyperthermia-induced teratogenic effects in experimental mammals is estimated (but not rigorously tested) to be approximately 1.5 degrees C above core values for exposures of long duration, possibly with a thermal dose of approximately 5 min duration or more at 4 degrees C. This level of tissue temperature increment is within the capability of some modern diagnostic ultrasound (DUS) devices sold within the USA and abroad. Epidemiological studies have not indicated any hazard from the use of DUS, but such studies are limited in sensitivity and were conducted with DUS devices whose acoustic outputs were relatively low compared to those presently available. After a regulatory change that allowed for substantially increased acoustic outputs, modern DUS devices were mandated to provide the user with on-screen information (the Thermal Index, or 'TI') about ultrasound-induced temperature increments in the target tissue. The TI is generally accurate to within a factor of 2, but the factor may be as high as 6 in certain obstetric settings. Thus, informed use of and attention to the TI is strongly advised, with this admonition gaining increased emphasis if the present regulations regarding allowable acoustic outputs of DUS devices were to be further relaxed or eliminated.

Animals↗