Creare Engineering Services – In Vivo Bubble Detection

In Vivo Bubble Detection Creare engineers have developed a way to better characterize the cause and onset of decompression sickness (DCS). Caused by the formation of small bubbles in the bloodstream, DCS has long been a major concern in any activity requiring significant change in the ambient environmental pressure. Divers returning from significant depths require regimented decompression stops on the way up. Likewise, astronauts preparing for extra-vehicular activity (EVA) in space suits pressurized to only a third of an atmosphere, must undergo up to two hours of oxygenation prior to suiting up. Both activities significantly decrease work efficiency in their respective environments. Despite over a century of dealing with DCS, tabular decompression tables remain statistical and experiential—and in fact, the appropriate decompression time can vary from person to person, and from time to time within a given individual. With funding from NASA, Creare engineers have developed an approach to monitor and measure non-invasively the formation of nitrogen bubbles both in the bloodstream and in tissue, for the purpose of generating data to better characterize and qualify the physical etiology of DCS. The instrument uses diagnostic levels of ultrasound to detect and size intravascular gas bubbles in the range of 30 to 200 microns, and extravascular bubble micronuclei of 1 to 30 microns that are believed to be present in tissue under normal conditions, and theorized to be the precursors of the intravascular bubbles observed in DCS. A body of anecdotal evidence suggests that reducing the number of tissue micronuclei by various pre-breathe strategies might greatly reduce the risk of DCS. Alternatively, detecting the quantity and/or size of micronuclei may predict the risk of DCS.


The theoretical basis for operation of the instrument developed by Creare is the exploitation of the nonlinear resonant properties of bubbles, which are strongly correlated to bubble size. Ultrasonic energy at two frequencies is used to resonate the bubbles, and the reflected signal is processed to detect and size the bubbles. Tissue is insonified with a combination of carefully selected frequencies—a lower “pump” frequency and a higher “image” frequency—and a frequency analysis of the return signal is performed. Bubbles act as non-linear mixers such that if a bubble that resonates at the lower pump frequency is contained in the measurement volume there will be energy in the return signal at both the sum and difference of the two frequencies. Without a resonant bubble present, there will be no significant energy at these sidebands. The efficacy of this method has been demonstrated by Creare with in-vivo tests. In addition to providing valuable data to aid in the understanding of DCS, such an instrument may possess value as a tool to guide the pre-breathe schedules of astronauts, high-altitude aviators, and special operations personnel performing high-altitude, low-opening (HALO) parachute missions. The instrument could improve the efficiency and cost of these missions by minimizing pre-breathe time and potentially improve safety by providing a quantitative means of assessing the risk of DCS for a given individual on a given day. In the case of the spacesuit environment, the instrument also has the potential to replace the in-suit Doppler ultrasound systems currently being designed as a means for providing an early warning of the onset of DCS, since it can potentially detect the symptoms of DCS sooner and is insensitive to motion.