Andrew A. Brayman

A review of in vitro bioeffects of inertial ultrasonic cavitation from a mechanistic perspective

This selective review of the biological effects of ultrasound presents a synopsis of our current understanding of how cells insonated in vitro are affected by inertial cavitation from the standpoint of physical and chemical mechanisms. The focus of this review is on the physical and chemical mechanisms of action of inertial cavitation which appear to be effective in causing biological effects. There are several fundamental conditions which must be satisfied before cavitation-related bioeffects may arise. First, bubbles must be created and then brought into proximity to cells. Exposure methods are critical in this regard, and simple procedures such as rotation of a vessel containing the cells during exposure can drastically alter the results. Second, once association is achieved between bubbles and cells, the former must interact with the latter to produce a bioeffect. It is not certain that the inertial event is the prime mechanism by which cells are lysed; there is evidence that the turbulence associated with bubble translation may cause lysis. Additionally, there appear to be chemical and other physical mechanisms by which inertial cavitation may affect cells; these include the generation of biologically effective sonochemicals and the apparent emission of ultraviolet (UV) and soft X-rays. The evidence for inertial cavitation occurring within cells is critically reviewed.

Lysis of erythrocytes by exposure to CW ultrasound

The threshold for lysis of erythrocytes suspended at concentrations of 0.5-1% in saline or plasma in rotating cylindrical exposure vessels is approximately spatial peak intensities of 2 W/cm2 at 1 MHz continuous wave (CW). Results of a series of experiments in which cell concentration, viscosity and gas composition of the suspending medium and rotation speed of the exposure vessel were varied combined with observations of sonoluminescence are all consistent with a hypothesis that cells are lysed by inertial (transient) acoustic cavitation. For the proposed mechanism to operate in cell suspensions, it is necessary that bubbles be brought into contact with the cells. Rotation of the chamber recycles bubbles that are driven by radiation forces to the far wall of the chamber in a matter of milliseconds. The physical and chemical properties of the wall of the chamber appear to be important as stabilizing sites for nuclei that serve as seeds for cavitation events.

Erosion of artificial endothelia in vitro by pulsed ultrasound: acoustic pressure, frequency, membrane orientation and microbubble contrast agent dependence.

The erosion of cells from fibroblast monolayers simulating the vascular endothelium by 20 micros pulses of ultrasound at 500 Hz PRF was studied in relation to the peak negative acoustic pressure (P-; 0.0-2.5 MPa), ultrasound (US) frequency (1.0, 2.1 or 3.5 MHz), orientation of the monolayer (i.e., simulating the sites of ultrasound entry/exit from a blood vessel) and the presence or absence of a microbubble contrast agent (3 Vol% Albunex). The a priori hypotheses were that erosion of the monolayers would: 1. arise due to insonation treatment, 2. arise as a consequence of cavitation activity and, thus, increase with increasing P- at constant frequency, and decrease with increasing frequency at constant P-, 3. be significantly increased by the presence of a microbubble contrast agent, and 4. have a weak dependence on monolayer orientation. The data support these hypotheses. Under the most severe exposure conditions used, most of the affected cells appeared to have been lysed; however, a substantial number of viable cells were dislodged from the monolayer surface.

Transient poration and cell surface receptor removal from human lymphocytes in vitro by 1 MHz ultrasound

The study objective was to gain insight into ultrasound-induced, sub-lytic cell surface modifications. Two primary hypotheses were tested by flow cytometric methods; viz., sonication will: 1. remove all or part of a specific cell surface marker in lymphocytes surviving insonation, and 2. induce transient pores in the cell membranes of some surviving cells. RPMI 1788 human lymphocytes were exposed in vitro to 1-MHz, continuous-wave ultrasound (approximately 8 W/cm2 ISP) for 30 s, which lysed approximately 50% of the cells. Insonation: 1. altered cell morphology, increasing the population of cells of reduced size but high structure (designated as population R2), many of which were nonviable, and diminishing the population of cells of large size and high structure (designated as population R1), most of which were viable, 2. diminished the fluorescence signal from the pan B lymphocyte marker CD19 in populations R1 and R2 to equivalent extents, and 3. increased by approximately 7-fold the number of transiently permeabilized cells in R1, as evidenced by simultaneous uptake of propidium iodide and fluorescein diacetate. The results indicate that ultrasound-induced CD19 removal from R1 cells can occur without accompanying gross membrane loss. The cell morphology/mortality shifts indicate that the ultrasound-induced morphological change is associated with lethal membrane poration, suggesting that the diminished CD19 fluorescence signal from insonated R2 cells arises partly by simultaneous loss of membrane fragments, CD19 and cytoplasm.

Effect of a Stabilized Microbubble Contrast Agent on CW Ultrasound Induced Red Blood Cell Lysis In Vitro

Human red blood cells (RBCs) in vitro at various fractional hematocrits (HCTs) were exposed for 60–120 seconds in a dialysis tubing vessel to 1 MHz continuous wave ultrasound (0–5 W/cm2 SPTA intensity); exposure vessels were either rotated at 200 rpm or stationary. Some RBC suspensions also contained Albunex® (ALX; a commercially‐produced microbubble clinical ultrasound contrast agent) at final concentrations ranging from 0–41 μL/mL RBC suspension. Isonation was either by one transducer or by two opposing, continuously‐gated, balanced transducers. For the vessel rotation / no rotation experiments, ultrasound‐induced cell lysis was always increased with the ALX regimen relative to that of the no‐ALX regimen at HCTs up to about 10%; at higher HCTs (including whole blood), ultrasound‐induced hemol‐ysis essectially ceased to occur. The data are consistent with reports of the ineffectiveness of continuous wave ultrasound at 3–5 W/cm2 to lyse cells in vitro at physiological cell densities with or without echocontrast medium supplemantation.

Hemolysis of albunex-supplemented, 40% hematocrit human erythrocytes in vitro by 1-MHz pulsed ultrasound: Acoustic pressure and pulse length dependence

The tested hypothesis was that ultrasound-induced hemolysis in blood supplemented with a microbubble contrast agent varies with ultrasound intensity and pulse duration. Human erythrocytes in autologous plasma containing 3.6% v:v Albunex microspheres were exposed to 1.07-MHz ultrasound pulses of 5 to 1000 mus at SPTP intensities of 0 to 1100 W/cm2. The dependence of hemolysis on the mechanical index (MI) value of the exposures was also examined. Ultrasound-induced hemolysis: (1) was evident at all pulse/intensity combinations; (2) increased generally with increasing pulse duration at constant intensity; and (3) increased with increasing MI at constant pulse duration. For pulses of 10 to 30 mus, ultrasound-induced hemolysis remained low (< or = 2%) at MI values < approximately 2 and increased sharply with further increase in MI; for 5-mus pulses, this abrupt increase in hemolysis was associated with a larger MI (approximately 3).

Effect of a Stabilized Microbubble Echo Contrast Agent on Hemolysis of Human Erythrocytes Exposed to High Intensity Pulsed Ultrasound

Microbubble contrast agents have been shown to enhance ultrasonic cell lysis in vitro when exposed to continuous‐wave ultrasound having spatial peak temporal average (SPTA) intensities of a few W/cm2. The response is strongly dependent upon the hematocrit (HCT) of the cell sample; detectable cell lysis essentially disappears as the HCT approaches 5%‐10%. This study was conducted to determine whether high intensity pulsedsound is an effective lytic agent in the presence of preexisting potential cavitation nuclei (Albunex® contrast agent). Human erythrocytes weresuspended in autologous plasma to HCTs ranging from 1%–40%. Suspensions were exposed or sham exposed for 60 seconds to focused, pulsed ultrasound. The pulse duration was 1 msec, and the pulse repetition frequency was 20 Hz. The pressure amplitudes, spatial peak pulse average (SPPA) intensity, and SPTA intensity were 4.7 MPa peak positive pressure, ‐2.7 MPa peak negative pressure, 420 W/cm2, and 8.5 /cm2, respectively. Samples were exposed to ultrasound in a dialysis membrane exposure vessel rotating at 200 rpm. When included in the erythrocyte samples, the Albunex concentration was 35 μL/mL suspension. Significant ultrasound‐induced hemolysis in the absence of Albunex was observed only at the lowest HCT value tested (1%). In the presence of Albunex significant cell lysis was observed at all tested HCT values. The relative fraction of cells lysed by the combination of ultrasound exposure and Albunex diminished with increasing HCT, but the number of cells lysed per sample was nearly constant over the range of 5%–40% HCT. The ultrasound exposure parameters used in this study differ substantially from those associated with diagnostic imaging equipment; it is not valid to infer from the present results that the use of Albunex in diagnostic applications will induce or enhance hemolysis in vivo.

Obstetric ultrasonography: A biophysical consideration of patient safety—The “rules” have changed☆☆☆★★★♢

We address the issue of health and safety in relation to exposure to diagnostic ultrasound, with particular emphasis given to obstetrics. In terms of fetal and maternal outcomes, the epidemiologic record of diagnostic ultrasound is exemplary but is primarily made on the basis of data derived from clinical devices whose outputs were relatively low compared with what is now allowable and available. The power outputs of clinical devices have been increasing over the past decade such that the potential for thermal and nonthermal insults is increased. For obstetric devices that use these higher outputs, the Food and Drug Administration now requires the presentation of 2 on-screen indexes, the thermal index and the mechanical index, in recognition of the 2 major mechanisms by which ultrasonography is known to affect cells and tissues. Greater responsibility for patient safety is now placed on the diagnostician; for the new indexes to be meaningful the diagnostician must be cognizant of the health and safety implications. The purpose of this article is to provide some guidance to the obstetrician in interpreting the indexes and to review the current status of ultrasonography biophysics in relation to the use of diagnostic ultrasound in obstetrics.

Hemolysis of 40% hematocrit, Albunex®-supplemented human erythrocytes by pulsed ultrasound: Frequency, acoustic pressure and pulse length dependence

The dependence of hemolysis produced by pulsed ultrasound on ultrasound frequency, acoustic pressure and pulse length was explored. Human erythrocytes (40% hematocrit; in Albunex-supplemented autologous plasma) were exposed (60 s) to 20 or 200 microns pulses of ultrasound at frequencies of 1.02, 2.24 or 3.46 MHz and at peak negative pressures [P-] ranging from 0.0 to approximately 3.0 MPa in 0.5 MPa increments. The duty factor was 0.01. At each frequency, hemolysis increased with increasing acoustic pressure and depended weakly on pulse duration. At relatively high acoustic pressures, hemolysis depended strongly on ultrasound frequency; at lower pressures, the frequency dependence was weaker. The potential clinical significance of ultrasonic hemolysis is discussed.

Sonolysis of Albunex®-supplemented, 40% hematocrit human erythrocytes by pulsed 1-MHz ultrasound: pulse number, pulse duration and exposure vessel rotation dependence

The hypotheses tested were that sonolysis of erythrocytes in the presence of a gas-based ultrasound contrast agent in vitro will be related quantitatively to the duration and number of ultrasound pulses applied using a constant pulse repetition period and, at least qualitatively, to the total exposure duration (i.e., the product of pulse number x pulse duration). An objective was to determine the influence of sample rotation during insonation on the amount of hemolysis produced under these conditions. Human erythrocytes, suspended to 40% hematocrit in autologous plasma containing 3.6% (V:V) Albunex, were exposed/sham-exposed to 1-100 pulses of 1-MHz ultrasound (6.2 MPa peak positive, 3.6 MPa peak negative acoustic pressures; I(SPTP) approximately 800 W/cm2) using a 1-s pulse repetition period. Pulse durations ranged from 20-20,000 micros; samples were either stationary or rotated (200 rpm) during insonation. Hemolysis was independent of vessel rotation treatment at all tested pulse durations and pulse numbers. Levels of hemolysis statistically greater than in sham-exposed samples were obtained with > or = 50 pulses of 20 micros duration, and > or = 1 pulse of 200, 2000 or 20,000 micros duration. Hemolysis increased with increasing pulse number and pulse duration. Approximately equivalent levels of hemolysis were produced by different pulse number x pulse duration combinations, yielding the same total exposure duration.