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Wen-Shiang Chen is on the leading edge of research in the nascent medical discipline of therapeutic ultrasound. His work focuses on the physical characteristics of ultrasound contrast agents (UCAs), and the mechanisms governing their therapeutic application. Wen received an M.D. from the Department of Medicine, National Taiwan University, Taipei in 1991 and continued resident training at the University Hospital's Department of Physical Medicine and Rehabilitation, where he first encountered therapeutic ultrasound. When the hospital asked for doctors to do more research in the area, Wen knew that he would have to study abroad to develop the expertise needed. Wen came to the university's Department of Bioengineering in 1997, and joined Lawrence Crum's laboratory at the Center for Industrial and Medical Ultrasound soon thereafter.
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UCAs are bubbles engineered to a size of about 5 microns. Because of their small size they can inhabit any tissue in the body, including brain capillaries, and they have no deleterious effects. Wen has demonstrated that by gradually increasing the acoustic pressure level on contrast bubbles they lose their acoustic scattering effect—this fragmentation threshold describes the bubbles' shattering and loss of effectiveness as an imaging agent.
The first trace of inertial cavitation (IC) activity—the growth and violent collapse of bubbles in response to an applied sound field—was found near this acoustic pressure level. Upon follow-up experiments he was surprised to find that further increases in acoustic pressure did not induce scattering until another threshold, the sustained IC threshold, was realized. The sustained IC threshold likely describes the self-replenishing ability of cavitation nuclei to form microbubbles anew. These thresholds depend on frequency, pulse length, and pulse repetition.
These UCA properties have important clinical applications, especially for stroke and myocardial infarction patients. Coupled with tiny transducers affixed to the tips of catheters, UCAs can be activated at the treatment site with ultrasound and speed thrombolysis, perhaps by the oscillation of the bubbles at their sustained cavitation threshold.
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