Sandra L. Poliachik, poliachi@u.washington.edu
Appl. Phys. Lab.
Univ. of Washington
1013 NE 40th St.
Seattle
WA 98105,
Popular Version of Paper 2aBB4
Wednesday Morning, May 31
139th ASA Meeting, Atlanta, GA
Just as a magnifying glass can focus light and deposit energy into a concentrated area, ultrasound transducers can be constructed to focus sound and deposit acoustic energy into a concentrated area. This property of ultrasound is being harnessed in research attempting to develop an ultrasound device that will halt internal bleeding non-invasively. This process has been termed "acoustic hemostasis".
Normally, when you cut yourself your body reacts to the wound by forming a clot that stops the bleeding. The clot is initiated because one of the components of blood, the platelet, is activated by the collagen that is exposed when a wound is created. The platelet sticks to the collagen, spreads out, releases its contents and recruits other platelets to form a platelet plug. Coagulation then occurs to stabilize the plug by forming a fibrin mesh.
One of the challenges in developing an acoustic hemostasis device is to identify the mechanisms that allow acoustic energy to enhance the clotting process. One likely mechanism is heating, which occurs with focused acoustic energy as it does with light energy in the magnifying glass example. The capability to heat tissue with focused ultrasound can be used to cauterize a wound. Other more subtle effects of ultrasound application may also be active in stimulating clot formation.
Current research at the University of Washington has shown that high intensity focused ultrasound (HIFU) can enhance platelet activity in samples that have insufficient heating to stimulate platelets. Platelets can be activated with HIFU, and the activated platelets have been shown to aggregate under constant HIFU exposure. In addition, platelets exposed to HIFU tend to attach themselves to collagen strands.
What effects of the ultrasound causes the platelet activity? Platelets can be activated by shear, and the HIFU sound field causes streaming in samples which may be sufficient to shear and activate platelets. Alternatively, a process called cavitation -- the oscillation of gas bubbles due to application of ultrasound, can create local shearing conditions that activate platelets or may tear a few platelets apart, leading to activation of other platelets.
These processes are under investigation in an attempt to identify the mechanisms responsible for enhanced platelet activity as a result of HIFU exposure. The conditions of HIFU exposure may then be controlled to optimize acoustic hemostasis.