Page 28 - Volume 8, Issue 4 - Winter 2012

DISINTEGRATION OF TISSUE USING
HIGH INTENSITY FOCUSED ULTRASOUND: TWO APPROACHES THAT UTILIZE SHOCK WAVES
Adam Maxwell
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory
University of Washington
Seattle, Washington 98105
and
Department of Urology
University of Washington
Seattle, Washington 98105
Oleg Sapozhnikov
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory
University of Washington
Seattle, Washington 98105
and
Department of Acoustics, Physics Faculty
Moscow State University
Moscow 119991, Russia
Michael Bailey
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory
University of Washington
Seattle, Washington 98105
Lawrence Crum
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory
University of Washington
Seattle, Washington 98105
Zhen Xu
Department of Biomedical Engineering University of Michigan
Ann Arbor, Michigan 48109
Brian Fowlkes
Department of Biomedical Engineering University of Michigan
Ann Arbor, Michigan 48109
and
Department of Radiology University of Michigan
Ann Arbor, Michigan 48109
Charles Cain
Department of Biomedical Engineering University of Michigan
Ann Arbor, Michigan 48109
Vera Khokhlova
Center for Industrial and Medical Ultrasound, Applied Physics Laboratory
University of Washington
Seattle, Washington 98105
and
Department of Acoustics, Physics Faculty
Moscow State University
Moscow 119991, Russia
Introduction
Surgery is moving more and more
toward minimally-invasive proce-
dures – using laparoscopic approach-
es with instruments inserted through
tiny incisions or catheters placed in
blood vessels through puncture sites.
These techniques minimize the risks to
the patient such as bleeding complications or infection dur- ing surgery. Taken a step further, high-intensity focused ultrasound (HIFU) can provide a tool to accomplish many
1-5
For many years, HIFU surgery was centered on utilizing a thermal effect— tissue heating and denaturation caused
6
by absorption of ultrasound. As the
heating rate is dependent on local acoustic intensity, the temperature rises significantly enough to ablate tissue only in the focal region. While thermal
ablation is the dominant interaction at lower HIFU focal intensities, higher intensities can introduce other bioeffects (Fig. 2). If the temperature rises to 100oC during sonication, boiling bubbles appear in the tissue, inducing additional mechanical as well as thermal damage. At higher focal inten- sities, mechanical effects of the ultrasound wave itself
7,8
With HIFU, an ultrasound transducer can be positioned outside the body and focused through the skin and overlying tissue to deliver high-amplitude ultrasound to a target structure such as a tumor (Fig. 1). Absorption of acoustic energy within the focal volume is high enough to rapidly heat the tissue, effectively ‘cooking’ it within seconds or even a frac- tion of a second. This procedure also removes the need for a sterile operating room: without the risk of infection, HIFU noninvasive therapy could be done in the doctor’s
office or outpatient clinic.
of the same procedures without any incision at all.
24 Acoustics Today, October 2012
“Ultrasound-induced tissue disintegration opens a new direction in development of HIFU medical technology”
The large tension phase of the wave can cause sporadic inertial cavitation or even a cloud of cavita- tion bubbles in the focal region in tissue—a process where the small gas bubbles grow and violently collapse, creating destructive effects on the tissue. Nonlinear propagation effects result in formation of high-amplitude shock waves around the focus which themselves create mechanical stress in the tissue. In addition, significantly enhanced heat deposi-
become significant.