Therapeutic Ultrasound, Part II

Acoustic hemostasis is a new field of ultrasound research in which high intensity focused ultrasound (HIFU) is used to induce hemostasis in actively bleeding, injured solid organs and blood vessels. In animal studies, moderate to profuse bleeding from parenchymal and vascular injuries has been arrested within approximately 1 minute of HIFU application, even when a large dose of heparin was administered. Moreover, acoustic hemostasis has shown promise in cauterizing large regions of liver, providing a method for bloodless resectioning of abnormal tissue. Two distinct physical mechanisms of HIFU appear to contribute to hemostasis: (1) a thermal mechanism in which absorption of sound leads to temperature elevations, and (2) mechanical mechanisms (acoustic cavitation) in which gas and vapor-filled voids oscillate with large displacement amplitudes. While the thermal mechanism results in a temperature increase in excess of 70°C in about 1 second, the mechanical mechanism appears to result in structural disruption of tissue and possible release of coagulation-inducing tissue factors. Of utmost importance in further development of HIFU as a clinical tool is targeting and monitoring of the HIFU treatment. We have obtained initial success in integrating HIFU with ultrasound imaging so as to develop an image-guided therapy system. Image-guided acoustic hemostasis may provide a valuable method of hemostasis in surgical and prehospital settings with applications in trauma and elective surgery.