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The Risk of Exposure to Diagnostic Ultrasound in Postnatal Subjects

Nonthermal Mechanisms

National Center for Physical Acoustics, University of Mississippi, University, Mississippi USA (C.C.C.); Department of Electrical Engineering, University of Rochester, Rochester, New York USA (E.L.C.); Department of Physics, University of Vermont, Burlington, Vermont USA (W.L.N.); Department of Radiology, University of Michigan, Ann Arbor, Michigan USA (P.L.C.); Department of Electrical and Computer Engineering, University of Illinois, Urbana, Illinois USA (L.A.F.); and Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, Washington (M.R.B.).

Address correspondence to Charles C. Church, PhD, National Center for Physical Acoustics, University of Mississippi, 1 Coliseum Dr, University, MS 38677 USA. E-mail: cchurch{at}olemiss.edu

Abstract

This review examines the nonthermal physical mechanisms by which ultrasound can harm tissue in postnatal patients. First the physical nature of the more significant interactions between ultrasound and tissue is described, followed by an examination of the existing literature with particular emphasis on the pressure thresholds for potential adverse effects. The interaction of ultrasonic fields with tissue depends in a fundamental way on whether the tissue naturally contains undissolved gas under normal physiologic conditions. Examples of gas-containing tissues are lung and intestine. Considerable effort has been devoted to investigating the acoustic parameters relevant to the threshold and extent of lung hemorrhage. Thresholds as low as 0.4 MPa at 1 MHz have been reported. The situation for intestinal damage is similar, although the threshold appears to be somewhat higher. For other tissues, auditory stimulation or tactile perception may occur, if rarely, during exposure to diagnostic ultrasound; ultrasound at similar or lower intensities is used therapeutically to accelerate the healing of bone fractures. At the exposure levels used in diagnostic ultrasound, there is no consistent evidence for adverse effects in tissues that are not known to contain stabilized gas bodies. Although modest tissue damage may occur in certain identifiable applications, the risk for induction of an adverse biological effect by a nonthermal mechanism due to exposure to diagnostic ultrasound is extremely small.

Key Words: cavitation • intestinal hemorrhage • lung hemorrhage • mechanical effects • nonthermal mechanism

Abbreviations: ALARA, as low as reasonably achievable • CW, continuous wave • MI, mechanical index • PRF, pulse repetition frequency

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