Tech Briefs

Ultrasonic Bubble-Sizing Instrument

Bubbles associated with decompression sickness can be detected noninvasively.

An ultrasonic instrument has been developed for measuring the sizes of bubbles in the human body. A primary example is that of bubbles associated with decompression sickness (the "bends"); these bubbles consist mostly of nitrogen and can occur in both blood vessels and extravascular tissues. The bubbles can lodge (embolize) in vessels in the lungs and elsewhere, causing any of a variety of pathological conditions. Gaseous emboli that cause decompression sickness pose a serious risk of injury to aviators, astronauts, divers, and other individuals who are exposed to varying environmental pressures. Gaseous emboli can also be serious complications of cardiopulmonary bypass operations, introduction of air during cardiotomy, and cavitation bubbles generated by replacement heart valves.

The present instrument can be expected to aid research on decompression sickness and other gaseous embolic events by providing a capability to: (1) distinguish between gaseous and thrombotic emboli, and (2) measure the sizes of gaseous emboli noninvasively. It could be useful in research on heart valves, development of coronary-bypass machines, and cardiotomy.

The physical basis of the instrument is the use of ultrasound to excite the resonant behavior (oscillatory expansion and contraction) of gaseous emboli. The resonant behavior is a function of the bubble diameter and has been well characterized analytically, so that little or no calibration of the instrument is needed. Nongaseous (e.g., thrombotic) emboli do not display the same resonant characteristics as do gaseous emboli; this difference can be a means for distinguishing between the two types of emboli.

The instrument includes several ultrasonic transducers. One of them, denoted the pump transducer, is driven at a relatively low ultrasonic frequency (fp) and is used to excite the fundamental vibrational mode of any bubbles that may be present. The other transducers are operated at a higher carrier frequency (fi) in a pulse/echo mode and are part of an ultrasonic imaging subinstrument used to observe bubbles as they resonate. The instrument also includes signal-processing and -analysis equipment.

To interrogate the measurement volume for a particular bubble size, one simultaneously insonifies the measurement volume with (1) a pump signal with fp set at the resonance frequency for that bubble size and (2) the higher-frequency imaging signal. Properly excited, the bubbles respond nonlinearly to the ultrasonic signals. In this case, they act as signal mixers; consequently, measurable signals at fi + fp and fifp are present in the high-frequency return signal if and only if there is a gaseous bubble with a fundamental resonance frequency equal to fp. The range of bubble sizes of interest is scanned by varying fp frequency over the appropriate range.

This work was done by Patrick J. Magari, Robert Kline-Schoder, and Brant Stoedefalke of Creare, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-22980.