A comparison of two calibration methods for ultrasonic hydrophones.

Although miniature ultrasonic hydrophones are frequently used to measure the acoustic pressure distributions from diagnostic ultrasound sources, relatively little attention has been devoted to the methods for absolute calibration of these hydrophones. In this study a polyvinylidene (PVDF) hydrophone was used to compare two calibration methods currently in use. One is based on a reciprocity technique and the second involves the planar scanning of a source transducer having a known radiated ultrasonic power. The reciprocity method revealed that the hydrophone response did not vary by more than +/- 1.6 dB from -262.8 dB re IV/microPa over the frequency range of 1-10 MHz. For the planar scanning technique seven ultrasound sources between 1-10 MHz were used, and all calibration points were within +/- 0.5 dB of the corresponding points found by the method of reciprocity.

Considerations in ultrasound therapy and equipment performance.

A regulatory performance standard for ultrasound therapy equipment has been published by the Bureau of Radiological Health under the authority of The Radiation Control for Health and Safety Act of 1968. To clarify this standard and its intents, a number of topics relevant to the use of ultrasound in physical therapy are discussed, including the physics of ultrasound, equipment output measurements, and the harmful biological effects of ultrasound.

A plane layered model to estimate in situ ultrasound exposures.

Determination of in situ ultrasound exposures in experimental animals and patients is necessary to assess any potential hazard of ultrasound implied by reported biological effects in animals. A layered model was used to calculate total attenuation of ultrasound pulses as they passed through tissue layers to an organ site, thus enabling calculation of in situ intensities from free-field transducer emissions. The model assumes plane homogeneous layers of tissue of known thickness, attenuation and acoustic impedance. Calculations based on the model, compared with in situ measurements in a human cadaver specimen, were accurate to within approximately +/- 3 dB. We then calculated in situ clinical exposures using published obstetric B-scans. Total attenuation to the fetus averaged approximately 11 dB at 3.5 MHz.

Development of health risk evaluation data for diagnostic ultrasound: a historical perspective.

The growth of ultrasound applications in diagnostic medicine has helped stimulate related biological effects investigations. Current data related to effects associated with diagnostic ultrasound indicate the need for additional research on cell surface structures, motility and developmental effects. Research on biological effects, especially for evaluating in vivo end points analogous to those employed by investigators using in vitro systems and simple organisms, are needed. Particular emphasis relative to potential effects on fetal and embroyonic development is indicated. Limited data also suggest the need to investigate possible effects on the immune response. There is a growing realization of the potential importance of nonthermal effects and increasing evidence that the temporal peak intensity is potentially related to the production of some effects. A number of recent comprehensive reviews have helped identify, analyze and evaluate some of the relevant data.

In vitro ultrasonic imaging of flow through prosthetic heart valves.

A rapid, inexpensive, portable technique has been developed for the qualitative and semiquantitative evaluation of in vitro flow characteristics through prosthetic heart valves. The technique combines a cardiac pulse duplicator and any diagnostic, linear sequential array, ultrasonic imaging system. The pulse duplicator has been modified to include an acoustically transparent aortic section which contains the prosthetic valve to be evaluated. The acoustically transparent section is fabricated from Rho-c rubber and enables direct flow-imaging for several centimeters on both sides of the prosthesis. Aerated tap water is used as a contrast medium. Forward and reverse flow, laminar flow complex eddy patterns, regurgitation, and jets are easily observed in real time over a wide field of view. Time-exposure photography of sequential images and subsequent off-line calculations enable point-by-point determinations of flow velocities. This work allowed preliminary evaluations of four cardiac valves: Bjork-Shiley, St. Jude, caged disk, and Starr-Edwards.

Compilation of reported biological effects data and ultrasound exposure levels.

A compilation of reports of biological effects and related ultrasound exposure levels can be useful in the identification of trends in effects data that may be developing in the literature. The exposures in the experiments reviewed were at frequencies used in medicine. For short (10 minutes or less) continuous wave exposure, with the exception of behavioral effects where variable results have been observed, this tabulation did not identify any in vivo or in vitro effects at spatial peak, temporal average (SPTA) intensities less than 100 mW/cm2. However, a number of effects using pulsed sources with SPTA intensities under this level have been reported. There are several frequently recurring topics in recent reports which include developmental effects and microstructure changes at the cellular level. In addition to animal investigations, there are some limited human epidemiological studies reporting an association between medical ultrasound exposure in utero and developmental effects. Although sufficient research has not been completed to draw conclusions, the findings deserve further investigation. A systematic pattern of directed research on a large scale is not apparent. The available data consist of many scattered studies conducted by individual investigators and groups throughout the world.

Relationship between image quality and ultrasound exposure level in diagnostic US devices.

Ultrasonographic B-mode images were obtained at various exposure levels with three real-time diagnostic scanners. Adult human and tissue-equivalent phantom images were compared in terms of diagnostic content and depth of penetration. For the exposure level settings used, spatial-peak pulse-average intensities ranged from approximately 10 to 500 W/cm2. At the 3.50-3.75-MHz nominal frequencies used in the study, images of the human abdomen showed little discernible change in quality with varying exposure levels. However, phantom tests confirmed that depth of penetration is a function of exposure level. The results suggest that a judicious use of exposure level and receiver gain controls can be a practical means for minimizing patient exposure to ultrasound without sacrifice of diagnostic effectiveness.