Biological effects of megahertz-frequency diagnostic ultrasound are thoroughly monitored by professional societies throughout the world. A corresponding, thorough, quantitative evaluation of the archival literature on the biological effects of low-frequency vibration is needed. Biological effects, of course, are related directly to what those exposures do physically to the tissue-specifically, to the shear strains that those sources produce in the tissues. Instead of the simple compressional strains produced by diagnostic ultrasound, realistic sources of low-frequency vibration produce both fast (similar to 1,500 m/s) and slow (1-10 m/s) waves, each of which may have longitudinal and transverse shear components. Part 1 of this series illustrates the resulting strains, starting with those produced by longitudinally and transversely oscillating planes, through monopole and dipole sources of fast waves and, finally, to the case of a sphere moving in translation-the simplest model of the fields produced by realistic sources. (C) 2016 World Federation for Ultrasound in Medicine & Biology.

A sphere moving back and forth in tissue generates the kinds of complex displacement fields that are used in elastography. The analytical solution of Hans Oestreicher for this phenomenon [(1951). J. Acoust. Soc. Am. 23, 704-714] gives an understanding of the transverse and longitudinal, fast and slow waves that are generated. The results suggest several ways to determine the absorption coefficients of tissues, which together with phase velocity permit the computation of both the real shear modulus and the shear viscosity as functions of frequency.

This review considers three general classes of physical as opposed to phenomenological models of the shear elasticity of tissues. The first is simple viscoelasticity. This model has a special role in elastography because it is the language in which experimental and clinical data are communicated. The second class of models involves acoustic relaxation, in which the medium contains inner time-dependent systems that are driven through the external bulk medium. Hysteresis, the phenomenon characterizing the third class of models, involves losses that are related to strain rather than time rate of change of strain. In contrast to the vast efforts given to tissue characterization through their bulk moduli over the last half-century, similar research using low-frequency shear data is in its infancy. Rather than a neat summary of existing facts, this essay is a framework for hypothesis generation-guessing what physical mechanisms give tissues their shear properties.

Overview and Findings This in vitro study investigated cellular responses to ultrasound exposure related to multidrug resistance (MDR) to chemotherapy in a drug-resistant tumor cell line (an MDR phenotype of the human hepatocarcinoma cell line HepG2). Previous studies by this group reported the selection of optimum parameters for ultrasound exposure of the cell line and the increased effectiveness of chemotherapeutic agents against the cell line with ultrasound exposure. The current study indicated an increased retention of adriamycin, a chemotherapy drug, induced by ultrasound exposure. The authors hypothesized that ultrasound can increase uptake and retention of chemotherapeutic agents via inhibiting the functions of ABC transporter proteins and alter the expression of key molecular components of the apoptotic processes in the MDR cancer cells. The study reviewed herein examined cellular effects that might explain ultrasound-enhanced drug retention and thus the reported reduction in drug resistance. Tests were conducted to compare the control or ultrasound-treated drug-resistant cell line. Exposure was done with an 800-kHz ultrasound transducer (2-cm diameter and unfocused) aimed upward in a water tank at a tube containing 1 mL of cell suspension. The single exposure condition was of 9 seconds’ duration at a spatial-peak pulse-average intensity of 0.43 W/cm2 with a 60% duty cycle (without addition of preformed microbubbles). Assessments of ultrasound effects were conducted using: (1) immunostaining of cell membrane transport proteins, (2) measurement of gene expression for the transport proteins, (3) apoptosis, and (4) expression of genes related to apoptosis. The numbers of cells positive for the transport proteins and their gene expression were reduced by ultrasound exposure. Ultrasound exposure also increased apoptosis and expression of apoptosis-related genes. The authors assert that these results indicate a potential role for ultrasound treatment of tumors to improve chemotherapeutic efficacy in the face of MDR.

(AIUM) Bioeffects Committee provides information to the AIUM membership on issues pertaining to the biological effects of ultrasound, especially when these issues relate to the safety of clinical ultrasound. A primary responsibility of the committee is to evaluate research reports on biological effects. This report was evaluated by committee members and selected experts in the clinical and experimental areas pertinent to the study. Bioeffects Committee members: Melvin Stratmeyer, PhD, chair; Timothy Bigelow, PhD, vice chair; Jacques Abramowicz, MD; Michael Bailey, PhD; Charles Church, PhD; Gregory Czarnota, PhD, MD; Cheri Deng, PhD; Kullervo Hynynen, PhD; Michael Kolios, PhD; Inder Raj Makin, MD, PhD; Douglas Miller, PhD; Michael Oelze, PhD; Jean Lea Spitz, MPH, RDMS; Marvin Ziskin, MD. Resource Members: John Abbott, PhD; Lori Barr, MD; Stephen Bly, PhD; Andrew Brayman, PhD; Paul Carson, PhD; Edwin Carstensen, MS, PhD; Francis Duck, PhD; Floyd Dunn, PhD; Peter Edmonds, PhD; Leon Frizzell, PhD; James Greenleaf, PhD; Gerald Harris, PhD; Christy Holland, PhD; Christopher Merritt, MD; Morton Miller, PhD, MS; Wesley Nyborg, PhD; Narendra Sanghvi, MSEE; Paul Smolenski; Thomas Szabo, PhD; Jinxing Tan, PhD; Kai Thomenius, PhD; James Zachary, DVM, PhD. Liaison members: Jennifer Bagley, MPH, RDMS, RVT; Stanley Barnett, MSc, PhD; Paul Smolenski; Tony Whittingham, PhD. Executive Committee liaison: J. Brian Fowlkes, PhD. Board of Governors liaison: Dev Maulik, MD, PhD. Staff liaison: Kathi Borok, BS, RDMS, RDCS.

Particle displacements can be much greater near bubbles than they would be in a homogeneous liquid or tissue when exposed to an acoustic wave. In a plane wave, shear and bulk strains are of the same order of magnitude. In contrast, for a bubble oscillating close to its resonance frequency, the shear strain in the medium near the bubble is roughly four orders of magnitude greater than the bulk strain. This can lead to shear strains of a few percent even with acoustic excitation pressures far below the pressure thresholds required to cause inertial cavitation. High shear strains near oscillating bubbles could potentially be the cause of bioeffects. After acoustic exposures at audio frequencies, hemorrhages in tissues as diverse as lung, liver, and kidney have been observed at shear strains on the order of 1%.

Normal liver tissue is soft and pliable. With inflammation, however, many of the cells die and are replaced by collagenous fibrils and the tissue gets stiffer. The progress is often slow—extending over decades in many cases. When liver stiffness increases by a factor of about five, the condition is called cirrhosis, a disease with serious medical implications. After the onset of cirrhosis, the probability of developing hepatic cancer increases at the rate of about 5% per year. Precise, noninvasive measurement of liver stiffness, a simple application of elastography, promises to be a safe, inexpensive method to monitor the progress of liver patients, improve outcome, save many lives and much suffering and reduce the cost of medical care. (E-mail: ecarsten@rochester.rr.com)

This report responds to a request from a deputy editor of the Journal of Ultrasound in Medicine (JUM) for guidelines for measurement and reporting of acoustic output and exposure. The request was addressed to the American Institute of Ultrasound in Medicine' s Technical Standards and Bioeffects Committees, which appointed a task group to draft a response. A basic premise of scientific reporting is the expectation that another investigator will wish to replicate a reported study. Therefore, it is essential that all pertinent information available or accessible to an investigator be reported.

Ultrasound (US) accelerates enzymatic fibrinolysis in vitro and in animal models, and may be a useful adjunctive therapy for clinical thrombolysis. Successful clinical application will depend on the selection of appropriate US parameters to optimize fibrinolytic enhancement while limiting adverse effects, including heating. Most studies have been done at megahertz frequencies, but tissue penetration is better and heating less at lower frequencies. We have, therefore, now investigated the effects of continuous-wave and pulsed US on fibrinolysis at midkilohertz frequencies. Fibrinolysis with tissue plasminogen activator (t-PA) was measured by solubilization of radiolabeled fibrin exposed to a calibrated US field in a temperature-controlled water bath. There was significant enhancement of fibrinolysis at frequencies of 27, 40 and 100 kHz, with the greatest effect observed at 27 kHz. The largest effect was observed with continuous-wave US, but significant acceleration was also observed with peak intensities of 1 W/cm(2) duty cycles of 10% and 1%. At a 10% duty cycle, there was approximately 60% of the fibrinolytic enhancement observed with continuous-wave exposure, indicating a clear advantage of pulsing to optimize fibrinolytic effect and limit exposure. We conclude that US in the range of 27 to 100 kHz is effective in accelerating fibrinolysis at intensities and pulsing conditions that minimize the probability of heating and cavitation in clinical applications.

This note compares theoretical predictions of pressure waves scattered by free gas bubbles with recent acoustical determinations of cavitation thresholds for individual microbubbles of the surfactant-stabilized contrast agent Sonazoid(R). The results indicate that surfactant-coated microbubbles undergo "stable" (i.e., repetitive) inertial cavitation above a threshold of 0.3 to 0.4 MPa at 2.5 MHz, and that irreversible postcollapse bubble fragmentation usually requires much higher pressures (approximately 1.5 MPa). Adverse bioeffects can be expected in vivo far below these fragmentation pressures when contrast agents are present. With diagnostically relevant exposures, the threshold for the generation of petechiae in skeletal muscle is approximately 0.6 MPa at 2.5 MHz.

The response of the lung to exposure to low-frequency underwater sound was investigated for frequencies of 100–500 Hz. Several different experimental and theoretical approaches demonstrated that the lung responds to low-frequency underwater sound as a resonant structure. Measurements of acoustic scattering near murine and rat lung indicated that the response of the lung to low-frequency acoustic fields can be described well by the theory of linear oscillations of a bubble in water. The measured mean resonance frequency was 325 Hz for adult mice (30 g), 420 Hz for young mice (10 g), and 175 Hz for rats (320 g). Noninvasive measurements of the displacement amplitude of the lung using an ultrasonic pulse-echo ranging technique confirmed that the lung oscillates in response to exposure near the resonance frequency. At the resonance frequency the response of the lung is maximized and the thresholds for damage to the lung and surrounding tissues (such as liver) were lowest. For exposure of adult mice at the resonance frequency, the threshold for lung hemorrhage and for liver hemorrhage was ∼184 dB re: 1 μPa (i.e., 1.6 kPa).

At least three physical processes have the potential to produce therapeutic benefits through ultrasound exposure. These include heating, cavitation, and a related group of second-order mechanical effects that involve radiation forces and streaming on a macroscopic or microscopic scale. In most applications of ultrasound in physical therapy, deep heating is the postulated rationale for its use. Hyperthermia as an adjunct to cancer therapy requires sophisticated control of ultrasonic heating. At higher levels, ultrasound can be used to destroy precisely selected tissues obviating the need for surgical intervention. In this application, nonlinear propagation can be used to selectively increase the absorption parameters of the target tissues. At low amplitudes the response of bubbles is dominated by the acoustic pressure. At a critical acoustic pressure, however, the inertia of the surrounding medium becomes controlling. At this threshold acoustic pressure a 10% or 20% increase in acoustic pressure leads to an increase in the collapse pressure in the bubble by orders of magnitude. Radiation forces can be detected by auditory and tactile sensors. Macroscopic acoustic streaming may alter transport across biological membranes and microscopic streaming exerts stresses on the membranes and the cytoplasmic and nuclear contents of cells.

Until the mid 1970s, it was generally assumed that, with the short pulses of ultrasound (US) used in medical diagnosis, there was little need for concern about the possibility of inertial cavitation in vivo. This assumption came into question when experimental evidence indicated that killing of fruit fly larvae by diagnostically relevant US was associated with the presence of gas in the respiratory apparatus of the organisms. Independent theoretical contributions by Flynn and Apfel in the early 1980s made it clear that complacency in regard to cavitation was not warranted. Later, the mammalian lung, as with larva, was shown to be particularly vulnerable when it contained air. Yet, overall evidence suggests that lung hemorrhage is not consistent with the classical picture of inertial cavitation. Most recently, however, hemolysis and hemorrhage associated with the use of contrast agents have provided nearly incontrovertible evidence of the occurrence of cavitation in vivo.