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

Ultrasound-Mediated Drug Delivery: Sonoporation Mechanisms, Biophysics, and Critical Factors

Sonoporation, or the use of ultrasound in the presence of cavitation nuclei to induce plasma membrane perforation, is well considered as an emerging physical approach to facilitate the delivery of drugs and genes to living cells. Nevertheless, this emerging drug delivery paradigm has not yet reached widespread clinical use, because the efficiency of sonoporation is often deemed to be mediocre due to the lack of detailed understanding of the pertinent scientific mechanisms. Here, we summarize the current observational evidence available on the notion of sonoporation, and we discuss the prevailing understanding of the physical and biological processes related to sonoporation. To facilitate systematic understanding, we also present how the extent of sonoporation is dependent on a multitude of factors related to acoustic excitation parameters (ultrasound frequency, pressure, cavitation dose, exposure time), microbubble parameters (size, concentration, bubble-to-cell distance, shell composition), and cellular properties (cell type, cell cycle, biochemical contents). By adopting a science-backed approach to the realization of sonoporation, ultrasound-mediated drug delivery can be more controllably achieved to viably enhance drug uptake into living cells with high sonoporation efficiency. This drug delivery approach, when coupled with concurrent advances in ultrasound imaging, has potential to become an effective therapeutic paradigm.

Mechanisms underlying sonoporation: Interaction between microbubbles and cells

The past several decades have witnessed great progress in "smart drug delivery", an advance technology that can deliver genes or drugs into specific locations of patients' body with enhanced delivery efficiency. Ultrasound-activated mechanical force induced by the interactions between microbubbles and cells, which can stimulate so-called "sonoporation" process, has been regarded as one of the most promising candidates to realize spatiotemporal-controllable drug delivery to selected regions. Both experimental and numerical studies were performed to get in-depth understanding on how the microbubbles interact with cells during sonoporation processes, under different impact parameters. The current work gives an overview of the general mechanism underlying microbubble-mediated sonoporation, and the possible impact factors (e.g., the properties of cavitation agents and cells, acoustical driving parameters and bubble/cell micro-environment) that could affect sonoporation outcomes. Finally, current progress and considerations of sonoporation in clinical applications are reviewed also.

The effect of 220 kHz insonation scheme on rt-PA thrombolytic efficacy in vitro

Ultrasound-enhanced recombinant tissue plasminogen activator (rt-PA) thrombolysis is under development as an adjuvant to ischemic stroke therapy. The goal of this study was to design a pulsed ultrasound (US) exposure scheme that reduced intracranial constructive interference and tissue heating, and maintained thrombolytic efficacy relative to continuous wave (CW) insonation. Three 220 kHz US schemes were evaluated, two pulsed insonation schemes (15 cycles, 68 µs pulse duration, 33% or 62.5% duty cycle) and an intermittent CW insonation scheme (50 s active, 30 s quiescent) over a 30-min treatment period. An in silico study using a finite-difference model of transcranial US propagation was performed to estimate the intracranial acoustic field and temperature rise in the skull for each insonation scheme. In vitro measurements with flow were performed to assess thrombolysis using time-lapse microscopy. Intracranial constructive interference was not reduced with pulsed US using a pulse length of 15 cycles compared to intermittent CW US. The 33.3% duty cycle pulsed US scheme reduced heating in the temporal bone as much as 60% relative to the intermittent CW scheme. All insonation schemes promoted sustained stable cavitation in vitro and augmented thrombolysis compared to rt-PA alone (p   <  0.05). Ultraharmonic (UH) and harmonic cumulative energy over a 30 min treatment period was significantly higher (p   <  0.05) for the intermittent CW US scheme compared to either pulsed US scheme. Despite the difference in cavitation emissions, no difference was observed in the clot lysis between the three US schemes. These findings demonstrate that a 33.3% duty cycle pulsed US scheme with a 15-cycle burst can reduce bone heating and achieve equivalent thrombolytic efficacy as an intermittent CW scheme.

The effect of ultrasound-related stimuli on cell viability in microfluidic channels

Background

In ultrasonic micro-devices, contrast agent micro-bubbles are known to initiate cavitation and streaming local to cells, potentially compromising cell viability. Here we investigate the effects of US alone by omitting contrast agent and monitoring cell viability under moderate-to-extreme ultrasound-related stimuli.

Results

Suspended H9c2 cardiac myoblasts were exposed to ultrasonic fields within a glass micro-capillary and their viability monitored under different US-related stimuli. An optimal injection flow rate of 2.6 mL/h was identified in which, high viability was maintained (~95%) and no mechanical stress towards cells was evident. This flow rate also allowed sufficient exposure of cells to US in order to induce bioeffects (~5 sec), whilst providing economical sample collection and processing times. Although the transducer temperature increased from ambient 23°C to 54°C at the maximum experimental voltage (29 V_pp), computational fluid dynamic simulations and controls (absence of US) revealed that the cell medium temperature did not exceed 34°C in the pressure nodal plane. Cells exposed to US amplitudes ranging from 0–29 V_pp, at a fixed frequency sweep period (t_sw = 0.05 sec), revealed that viability was minimally affected up to ~15 V_pp. There was a ~17% reduction in viability at 21 V_pp, corresponding to the onset of Rayleigh-like streaming and a ~60% reduction at 29 V_pp, corresponding to increased streaming velocity or the potential onset of cavitation. At a fixed amplitude (29 V_pp) but with varying frequency sweep period (t_sw = 0.02-0.50 sec), cell viability remained relatively constant at t_sw ≥ 0.08 sec, whilst viability reduced at t_sw < 0.08 sec and minimum viability recorded at t_sw = 0.05 sec.

Conclusion

The absence of CA has enabled us to investigate the effect of US alone on cell viability. Moderate-to-extreme US-related stimuli of cells have allowed us to discriminate between stimuli that maintain high viability and stimuli that significantly reduce cell viability. Results from this study may be of potential interest to researchers in the field of US-induced intracellular drug delivery and ultrasonic manipulation of biological cells.

Ultrasound destruction of air microemboli as a novel approach to brain protection in cardiac surgery

OBJECTIVE

Evaluation of a novel approach to eliminate air microemboli from extracorporeal circulation via ultrasonic destruction.

DESIGN

In vitro proof-of-concept study.

SETTING

Research laboratory.

PARTICIPANTS

None.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

An extracorporeal circulation device was filled with human blood circulating at 3 L/min. Air bubbles were injected into the system. For bubble destruction, the blood in the tubing system was repeatedly insonated for 3 minutes using a therapeutic 60-kHz device, with variation of intensity and duty cycle settings, ranging from 0.2 W/cm² to 1.0 W/cm² and from duty cycle 60% to continuous wave (CW). Number and diameter of air microemboli were counted upstream and downstream of the ultrasound device by a 2-channel microemboli Doppler detector. For safety assessment, circulating blood was insonated continuously for 2 hours at 0.8 W/cm² CW and compared with circulation without insonation; and standard blood parameters were analyzed. Without treatment, 1,313 to 1,580 emboli were detected upstream, diameter ranging between 10 and 130 μm. Ultrasound treatment eliminated up to 87% of all detected bubbles in cw application (p<0.01) and showed comparable effects at intensities from 0.4 W/cm² to 1.0 W/cm² cw. Bubbles sized>15 μm almost were eliminated completely (p<0.001). Pulsed wave application rendered inferior results (p>0.05). No relevant changes of blood parameters were observed compared with control circulation.

CONCLUSIONS

Ultrasound destruction of air emboli is a very efficient method to reduce number and size of emboli. Within the limits of safety assessment, the authors could not detect relevant side effects on standard blood parameters.

Contrast agent-free sonoporation: The use of an ultrasonic standing wave microfluidic system for the delivery of pharmaceutical agents

Sonoporation is a useful biophysical mechanism for facilitating the transmembrane delivery of therapeutic agents from the extracellular to the intracellular milieu. Conventionally, sonoporation is carried out in the presence of ultrasound contrast agents, which are known to greatly enhance transient poration of biological cell membranes. However, in vivo contrast agents have been observed to induce capillary rupture and haemorrhage due to endothelial cell damage and to greatly increase the potential for cell lysis in vitro. Here, we demonstrate sonoporation of cardiac myoblasts in the absence of contrast agent (CA-free sonoporation) using a low-cost ultrasound-microfluidic device. Within this device an ultrasonic standing wave was generated, allowing control over the position of the cells and the strength of the acoustic radiation forces. Real-time single-cell analysis and retrospective post-sonication analysis of insonated cardiac myoblasts showed that CA-free sonoporation induced transmembrane transfer of fluorescent probes (CMFDA and FITC-dextran) and that different mechanisms potentially contribute to membrane poration in the presence of an ultrasonic wave. Additionally, to the best of our knowledge, we have shown for the first time that sonoporation induces increased cell cytotoxicity as a consequence of CA-free ultrasound-facilitated uptake of pharmaceutical agents (doxorubicin, luteolin, and apigenin). The US-microfluidic device designed here provides an in vitro alternative to expensive and controversial in vivo models used for early stage drug discovery, and drug delivery programs and toxicity measurements.

Intravascular inertial cavitation activity detection and quantification in vivo with Optison

Inertial cavitation (IC) is an important mechanism by which ultrasound (US)-induced bioeffects can be produced. It has been reported that US-induced in vitro mechanical bioeffects with the presence of ultrasound contrast agents (UCAs) are highly correlated with quantified IC "dose" (ICD: cumulated root-mean-squared broadband noise amplitude in the frequency domain). The ICD has also been used to quantify IC activity in ex vivo perfused rabbit ear vessels. The in vivo experiments reported here using a rabbit ear vessel model were designed to: (1) detect and quantify IC activity in vivo within the constrained environment of rabbit auricular veins with the presence of Optison and (2) measure the temporal evolution of microbubble IC activity and the ICD generated during insonation treatment, as a function of acoustic parameters. Preselected regions-of-interest (ROI) in the rabbit ear vein were exposed to pulsed focused US (1.17 MHz, 1 Hz PRF). Experimental acoustic variables included peak rarefaction pressure amplitude ([PRPA]: 1.1, 3.0, 6.5 or 9.0 MPa) and pulse length (20, 100, 500 or 1000 cycles). ICD was quantified based on passive cavitation detection (PCD) measurements. The results show that: (1) after Optison injection, the time to onset of measurable microbubble IC activity was relatively consistent, approximately 20 s; (2) after reaching its peak value, the IC activity decayed exponentially and the half-life decay coefficient (t(1/2)) increased with increasing PRPA and pulse length; and (3) the normalized ICD generated by pulsed US exposure increased significantly with increasing PRPA and pulse length.

Correlation of cavitation with ultrasound enhancement of thrombolysis

Pulsed ultrasound, when used as an adjuvant to recombinant tissue plasminogen activator (rt-PA), has been shown to enhance thrombolysis in the laboratory as well as in clinical trials for the treatment of ischemic stroke. The exact mechanism of this enhancement has not yet been elucidated. In this work, stable and inertial cavitation (SC and IC) are investigated as possible mechanisms for this enhancement. A passive cavitation detection scheme was utilized to measure cavitation thresholds at 120 kHz (80% duty cycle, 1667 Hz pulse repetition frequency) for four host fluid and sample combinations: plasma, plasma with rt-PA, plasma with clot and plasma with clot and rt-PA. Following cavitation threshold determination, clots were exposed to pulsed ultrasound for 30 min in vitro using three separate ultrasound treatment regimes: (1) no cavitation (0.15 MPa), (2) SC alone (0.24 MPa) or (3) SC + IC combined (0.36 MPa) in the presence of rt-PA. Percent clot mass loss after each treatment was used to determine thrombolysis efficacy. The highest percent mass loss was observed in the stable cavitation regime (26%), followed by the combined stable and inertial cavitation regime (20.7%). Interestingly, the percent mass loss in clots exposed to ultrasound without cavitation (13.7%) was not statistically significantly different from rt-PA alone (13%) [p > 0.05]. Significant enhancement of thrombolysis correlates with presence of cavitation and stable cavitation appears to play a more important role in the enhancement of thrombolysis. (E-mail: ).

Electro-sensitisation of mammalian cells and tissues to ultrasound: a novel tumour treatment modality

This study demonstrates that mammalian cell targets (erythrocytes and tumour cells) may be sensitised to ultrasound using electric pulses and this combination treatment results in destruction of those cells in vitro. It further demonstrates that when a tumour mass is treated in vivo using combined electric field and ultrasound therapy, significant retardation of tumour growth has been observed using a mouse tumour model. We suggest that combined electric field and ultrasound (CEFUS) therapy may provide a novel, drug-free treatment modality for cancer.

Vascular effects induced by combined 1-MHz ultrasound and microbubble contrast agent treatments in vivo

Previous in vivo studies have demonstrated that microvessel hemorrhages and alterations of endothelial permeability can be produced in tissues containing microbubble-based ultrasound contrast agents when those tissues are exposed to MHz-frequency pulsed ultrasound of sufficient pressure amplitudes. The general hypothesis guiding this research was that acoustic (viz., inertial) cavitation, rather than thermal insult, is the dominant mechanism by which such effects arise. We report the results of testing five specific hypotheses in an in vivo rabbit auricular blood vessel model: (1) acoustic cavitation nucleated by microbubble contrast agent can damage the endothelia of veins at relatively low spatial-peak temporal-average intensities, (2) such damage will be proportional to the peak negative pressure amplitude of the insonifying pulses, (3) damage will be confined largely to the intimal surface, with sparing of perivascular tissues, (4) greater damage will occur to the endothelial cells on the side of the vessel distal to the source transducer than on the proximal side and (5) ultrasound/contrast agent-induced endothelial damage can be inherently thrombogenic, or can aid sclerotherapeutic thrombogenesis through the application of otherwise subtherapeutic doses of thrombogenic drugs. Auricular vessels were exposed to 1-MHz focused ultrasound of variable peak pressure amplitude using low duty factor, fixed pulse parameters, with or without infusion of a shelled microbubble contrast agent. Extravasation of Evans blue dye and erythrocytes was assessed at the macroscopic level. Endothelial damage was assessed via scanning electron microscopy (SEM) image analysis. The hypotheses were supported by the data. We discuss potential therapeutic applications of vessel occlusion, e.g., occlusion of at-risk gastric varices.

Bubble dynamics and size distributions during focused ultrasound insonation

The deposition of ultrasonic energy in tissue can cause tissue damage due to local heating. For pressures above a critical threshold, cavitation will occur, inducing a much larger thermal energy deposition in a local region. The present work develops a nonlinear bubble dynamics model to numerically investigate bubble oscillations and bubble-enhanced heating during focused ultrasound (HIFU) insonation. The model is applied to calculate two threshold-dependent phenomena occurring for nonlinearly oscillating bubbles: Shape instability and growth by rectified diffusion. These instabilities in turn are shown to place physical boundaries on the time-dependent bubble size distribution, and thus the thermal energy deposition.

The relation between cavitation and platelet aggregation during exposure to high-intensity focused ultrasound

Our previous study showed that high-intensity focused ultrasound (HIFU) is capable of producing "primary acoustic hemostasis" in the form of ultrasound (US)-induced platelet activation, aggregation and adhesion to a collagen-coated surface. In the current study, 1.1 MHz continuous-wave HIFU was used to investigate the role of cavitation as a mechanism for platelet aggregation in samples of platelet-rich plasma. A 5 MHz passive cavitation detector was used to monitor cavitation activity and laser aggregometry was used to measure platelet aggregation. Using spatial average intensities from 0 to 3350 W/cm2, the effects of HIFU-induced cavitation on platelet aggregation were investigated by enhancing cavitation activity through use of US contrast agents and by limiting cavitation activity through use of an overpressure system. Our results show that increased cavitation activity lowers the intensity threshold to produce platelet aggregation and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation.

The relevance of cell size on ultrasound-induced hemolysis in mouse and human blood in vitro

This paper describes a further test of the hypothesis that cell size is an important physical parameter in ultrasound (US)-induced hemolysis, that is, the larger the cell the greater the potential for sonolysis by a cavitational mechanism. Mouse (M) and human (Hu) erythrocytes in vitro were used; their mean corpuscular volumes were 49.0 and 89.5 fL, respectively. At a US exposure in vitro in the presence of Albunex that yielded an average of 36.8% hemolysis for M blood, the Hu blood yielded an average of 54.0% hemolysis. The data supported the hypothesis. This paper also briefly discusses the difficulty of extrapolating sonolytic in vitro results to those derived in vivo.

Bioeffects caused by changes in acoustic cavitation bubble density and cell concentration: a unified explanation based on cell-to-bubble ratio and blast radius

Acoustic cavitation has been shown to load drugs, proteins and DNA into viable cells as a complex function of acoustic and nonacoustic parameters. To better understand and quantify this functionality, DU145 prostate cancer cell suspensions at different cell concentrations (2.5 x 10(5) to 4.0 x 10(7) cells/mL) were exposed to 500 kHz ultrasound (US) over a range of acoustic energy exposures (2 to 817 J/cm(2); peak negative pressures of 0.64 to 2.96 MPa; exposure times of 120 to 2000 ms) in the presence of different initial concentrations of Optison contrast agent bubbles (3.6 x 10(4) to 9.3 x 10(7) bubbles/mL). As determined by flow cytometry, molecular uptake of calcein and cell viability both increased with increasing cell density; viability decreased and uptake was unaffected by increasing initial contrast agent concentration. When normalized relative to the initial contrast agent concentration (e.g., cells killed per bubble), bioeffects increased with increasing cell density and decreased with increasing bubble concentration. These varying effects of contrast agent concentration and cell density were unified through an overall correlation with cell-to-bubble ratio. Additional analysis led to estimation of "blast radii" over which bubbles killed or permeabilized cells; these radii were as much as 3 to 90 times the bubble radius. Combined, these results suggest that extensive molecular uptake into cells at high viability occurs for low-energy exposure US applied at a high cell-to-bubble ratio.

Comparative hemolytic effectiveness of 1 MHz ultrasound on human and rabbit blood in vitro

This project continued testing of the general working hypothesis that cell size is a physical determinant in extent of ultrasound (US)-induced hemolysis, the larger the cell the greater the lysis. For this project, the specific hypothesis tested was that human erythrocytes, being larger than rabbit erythrocytes, would be the more sensitive to sonolysis induced by inertial cavitation in the presence of Albunex, a US contrast agent. The rationale behind this hypothesis was 1. an earlier-published analytic construct indicating an inverse relation between particle size and the shear force required for deformation, and 2. a number of independent demonstrations that, among sized populations of erythrocytes, an inverse relation exists between erythrocyte volume and mechanically-induced shear forces in the cell-bathing medium; namely, the larger the cell, the less shear force required to rupture the cell's membrane. The present data support the hypothesis; over six independent trials, the mean corpuscular volumes of human (H) and rabbit (R) erythrocytes were 89.5 and 64.1 microm(3), respectively, H > R (p << 0.001), and the ratio of US-induced hemolysis in H to R blood in vitro was 1.12:1.0 (p < 0.004).

Inertial cavitation dose and hemolysis produced in vitro with or without Optison

Gas-based contrast agents (CAs) increase ultrasound (US)-induced bioeffects, presumably via an inertial cavitation (IC) mechanism. The relationship between IC dose (ICD) (cumulated root mean squared [RMS] broadband noise amplitude; frequency domain) and 1.1-MHz US-induced hemolysis in whole human blood was explored with Optison; the hypothesis was that hemolysis would correlate with ICD. Four experimental series were conducted, with variable: 1. peak negative acoustic pressure (P-), 2. Optison concentration, 3. pulse duration and 4. total exposure duration and Optison concentration. P- thresholds for hemolysis and ICD were approximately 0.5 MPa. ICD and hemolysis were detected at Optison concentrations >/= 0.01 V%, and with pulse durations as low as four or two cycles, respectively. Hemolysis and ICD evolved as functions of time and Optison concentration; final hemolysis and ICD values depended on initial Optison concentration, but initial rates of change did not. Within series, hemolysis was significantly correlated with ICD; across series, the correlation was significant at p < 0.001.

The pulse length-dependence of inertial cavitation dose and hemolysis

Gas-based ultrasound (US) contrast agents increase erythrocyte sonolysis, presumably via enhancing inertial cavitation (IC) activity. The amount of IC activity (IC "dose") and hemolysis generated by exposure to 1.15 MHz US were examined with different US pulse lengths, but with the same delivered acoustic energy, for Optison and Albunex. The hypotheses were that 1. at longer pulse lengths, IC would generate more bubbles that could nucleate additional IC activity; 2. if the interval between pulse pairs were short enough for the next pulse to hit derivative bubbles before their dissolution, more IC could be induced; and 3. hemolysis would be proportional to IC activity. Two types of studies were performed. In the first, bubble generation after each burst of IC activity was quantified using an active cavitation detector (ACD), for different pulse lengths (5, 10, 20, 30, 50, 100 or 200 cycles), but the same pressure level (3 MPa) and total "on" time (173.16 ms). Low concentrations of either Optison or Albunex were added into the tank with high-intensity and interrogating transducers orthogonal to each other. For pulse lengths > 100 cycles, and pulse repetition intervals < 5 ms, a "cascade" effect (explosive bubble generation) was observed. In the second, IC was measured by passive detection methods. IC dose and hemolysis were determined in whole blood samples at a pressure level (3 MPa) and interpulse interval (5 ms) that induced the "cascade" effect. Each blood sample was mixed with the same number of contrast microbubbles (Optison approximately 0.3 v/v % and Albunex approximately 0.5 v/v %), but exposed to different pulse lengths (5, 10, 20, 30, 50, 100 or 200 cycles). With Optison, up to 60% hemolysis was produced with long pulses (100 and 200 cycles), compared with < 10% with short pulses (5 and 10 cycles). Albunex generated considerably less IC activity and hemolysis. The r(2) value was 0.99 for the correlation between hemolysis and IC dose. High pulse-repetition frequency (PRF) (500 Hz) generated more hemolysis than the low PRF (200 Hz) at 3 MPa. All experimental results could be explained by the dissolution times of IC-generated bubbles.

Biological and environmental factors affecting ultrasound-induced hemolysis in vitro: 1. HIV macrocytosis (cell size)

This paper reports the results of a further test of the hypothesis that the extent of ultrasound (US)-induced cell lysis in the presence of a US contrast agent to enhance cavitational effects is a function of cell size. The present data support the hypothesis. Human adult erythrocytes in vitro derived from patients with HIV (n = 15) and apparently healthy individuals (n = 15) were compared for US-induced hemolysis in vitro. The anticoagulated whole blood from patients with HIV and macrocytic erythrocytes had significantly greater (p <0.0001) mean corpuscular volume (MCV) and a significantly greater (p <0.03) extent of US-induced hemolysis in vitro relative to blood from apparently normal, healthy individuals. As a control to determine if disease state (i.e., HIV infection per se) might be a contributing factor in US-induced hemolysis in vitro, the blood from patients with HIV and apparently normal MCVs (n = 15) was also tested against an additional population of apparently normal, healthy individuals (n = 15); there were no statistically significant differences in MCVs or US-induced hemolysis between the two groups (p >> 0.05). There were also no statistically significant differences in viscosities or hematocrits of the whole blood or plasma in vitro from HIV-macrocytic or apparently healthy individuals but, for all blood types, a pooled correlation existed between hematocrit and whole blood viscosity.

Microbubbles induce renal hemorrhage when exposed to diagnostic ultrasound in anesthetized rats

The generation of ultrasound (US) bioeffects using a clinical imaging system is controversial. We tested the hypothesis that the presence of microbubbles in the US field of a medical imager induces biologic effects. Both kidneys of anesthetized rats were insonified for 5 min using a medical imaging system after the administration of microbubbles. One kidney was insonified using a continuous mode (30 Hz) and the opposite kidney was insonified using an intermittent (1 Hz) technique. The microbubbles were exposed to three different transducer frequencies and four transducer output powers. After insonification, the animals were euthanized, the kidneys were removed and their gross appearance scored under "blinded" conditions using a defined scale. After the administration of microbubbles, US imaging of the kidney caused hemorrhage in the renal tissue. The severity and area of hemorrhage increased with an increase in the transducer power and a decrease in the transducer frequency. Intermittent insonification in the presence of microbubbles produced a greater degree of renal hemorrhage than continuous imaging techniques.

Ultrasonic contrast agents: safety considerations reviewed

Ultrasonic contrast agents are usually comprised of a stabilised shell encapsulating a gas bubble. When these are introduced in the body they increase the acoustic scattering from the tissues through which they pass, and especially from the vasculature. Their primary uses lie in cardiological and oncological imaging. However, these microbubbles have the potential to act as centres for acoustic cavitation activity, and so it is important to consider the safety of their use from an acoustic standpoint. The addition of ultrasonic contrast agents to in vitro suspensions of red blood cells has been shown to lead to haemolysis when the sample is exposed to ultrasound at levels which leave the cells unharmed in their absence. In vivo the infusion of gas bubble contrast agents into experimental animals has been shown to enhance the incidence of petechiae and haemorrhage in the intestine. The Mechanical Index (MI) thresholds for the effects seen in vitro lie within the range of MIs available with diagnostic clinical scanners, but in vivo the thresholds lie at the top end of the exposure levels available clinically. No adverse effects in humans arising from the ultrasonic exposure of these contrast agents have been reported to date.

Photodisruptive laser nucleation of ultrasonic cavitation for biomedical applications

Pulses of high intensity laser light, when focused into transparent materials, may produce localized electron-ion plasmas through optical breakdown. By simultaneously incorporating the resulting volume of vaporized material within the focal volume of a high intensity ultrasound source, the photodisruption (1.05 microm wavelength) void served as a nucleation site for ultrasonic cavitation. Dilute suspensions of canine erythrocytes in phosphate buffered saline were exposed in a flow-through exposure chamber and the percentage of lysed cells was used as a measure of the biologically effective cavitation activity produced in the chamber. Brief (about 30 micros) acoustic emissions were detected from the photodisruption alone (indicating laser nucleation of bubbles), but the cell lysis produced was undetectable against the background. However, combined exposure greatly increased both the duration of the acoustic emissions (up to 1.5 ms) and the amount of cell lysis above an ultrasonic pressure amplitude threshold of about 4.3 MPa at 2.5 MHz. The amount of cell lysis (sometimes approaching 100%) increased with increasing ultrasonic intensity, laser pulse energy and laser PRF. Addition of 5% serum albumin enhanced the effect, apparently by stabilizing bubbles and nuclei. Photodisruptive laser nucleation of ultrasonic cavitation can provide controlled and synergistic enhancement of bioeffects.

A comparison of the hemolytic potential of Optison and Albunex in whole human blood in vitro: acoustic pressure, ultrasound frequency, donor and passive cavitation detection considerations

This project tested the hypothesis that a "second-generation" ultrasound (US) contrast agent (Optison), offering extended echogenicity over that of its "first-generation" predecessor (Albunex), would have the greater potential for sonolysis of human erythrocytes in vitro. Whole human blood, obtained from apparently healthy donors, was anticoagulated and subsequently exposed in vitro to US in the presence of one of each or neither of the two US contrast agents. The US exposures were for 30 s and involved frequency (1.0, 2.2 and 3.4 MHz) and amplitude (approximately 2.8 to 0.38 MPa P(-)) regimens; pulse duration (200 micros) and interpulse interval (20 ms) were held constant. The data supported the hypothesis, with an overall ratio of approximately 2.5 for relative extent of background-corrected US-induced hemolysis of the Optison/Albunex regimens. Passive cavitation detection analyses corroborated the results obtained with hemolysis.

Comparative sensitivity of human fetal and adult erythrocytes to hemolysis by pulsed 1 MHz ultrasound

Human fetal and adult erythrocytes differ significantly in mean corpuscular volume (MCV), the fetal cells being larger than adult cells and diminishing in MCV as gestational age (GA) increases. Previous studies have shown that the sensitivity of erythrocytes from different species to lysis by mechanically applied shear stress increases as MCV increases. The tested hypotheses in the present project were: 1. fetal erythrocytes would be more sensitive to sonolysis than adult erythrocytes because of the former's larger size, and 2. erythrocyte sonolytic sensitivity would scale with MCV. Fetal and adult erythrocytes were resuspended to 40% hematocrit in oxygenated isotonic saline solution and 500 microL aliquots were exposed for 60 s to 200 micros bursts of 1-MHz ultrasound (US) (peak pressures: approximately 4.8 MPa positive, approximately 2.7 MPa negative; duty factor = 0.01), either with or without 3.6 volume % Albunex (ALX) present. Background-corrected hemolysis was indistinguishable from zero in sham-exposed fetal or adult erythrocyte suspensions. Without ALX, mean background-corrected US-induced hemolysis was significantly greater than zero for fetal and adult cells (0.42 +/- 0.15% vs. 0.62 +/- 0.15), but fetal cell lysis was not significantly greater than adult cell lysis. With ALX, US-induced hemolytic yields increased approximately 80-fold (fetal: 50.53 +/- 2.14; adult: 46.40 +/- 1.85%), and were significantly higher for fetal than for adult cells. There was also a statistically significant correlation between MCV and US-induced background-corrected hemolysis. Thus, the two hypotheses were supported.

Thresholds for inertial cavitation in albunex suspensions under pulsed ultrasound conditions

Stabilized microbubbles used as echo-contrast agents can be destroyed by ultrasonic irradiation. We have identified two pressure thresholds at which these microbubbles undergo inertial cavitation (here, defined as the collapse of gas bubbles followed by emission of an acoustic broadband noise). The first threshold (P1) corresponds to the pressure at which all the microbubbles in a cavitation field lose their property as an effective scatterer because of fragmentation or deflation. The second threshold (P2) is associated with the acoustic reactivation of the remnants of the contrast agents and is related to the onset of more violent inertial cavitation. P1 and P2 were measured as a function of the concentration of Albunex (Molecular Biosystems Inc., San Diego, CA) contrast agent, the number of transmitting acoustic cycles, and the pulse repetition frequency (PRF). The ultrasound frequency used was 1.1 MHz, and the peak negative acoustic pressures ranged from 0 to 8 MPa. Our results, measured in Isoton II (Coulter Diagnostics, Miami, FL) and whole blood solutions, showed that P1 increased with increasing Albunex concentration and decreased with increasing PRF, whereas P2 decreased with increasing Albunex concentration and was independent of the PRF. Both P1 and P2 decreased with increasing number of acoustic cycles N for N < 10 and were independent of the number of cycles for N > 10. Ultrasound images of Albunex acquired by a commercial scanner showed echo enhancement not only at pressure levels below P1 but also at levels above P2. The threshold P2 was achieved at ultrasound energies above the diagnostic level. Inertial cavitation produced at P2 was associated with a higher level of hemolysis compared with P1. The results of this investigation have potential significance for both diagnostic and therapeutic ultrasound applications.

The search for cavitation in vivo

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.

Comparative sensitivity of human and bovine erythrocytes to sonolysis by 1-MHz ultrasound

This project tested the hypothesis that human erythrocytes, being larger than bovine erythrocytes, would be the more sensitive to sonolysis induced by inertial cavitation. The rationale behind this hypothesis was an earlier demonstration that, among sized populations of erythrocytes, an inverse relation existed between erythrocyte volume and mechanically-induced shear forces in the surrounding medium; viz, the larger the cell, the less shear force required to rupture the cell's membrane. At low erythrocyte densities (i.e., approximately 5% hematocrit) the hypothesis was supported; at high cell densities (i.e., approximately 35% hematocrit) it was not supported. The data are consistent with an ultrasound (US)-induced symmetric implosion of affected gas nuclei as causing the effect at low cell densities; under such conditions there is ample spacing among cells for US-induced symmetric growth and collapse of gas nuclei and the concomitant production of radially-expanding shock waves (which lyse the cells); at high cell densities there is not sufficient spacing among cells for US-induced symmetric growth and collapse of bubbles and an alternative mechanism, possibly asymmetric bubble collapse, becomes operational.

Diagnostic ultrasound activation of contrast agent gas bodies induces capillary rupture in mice

Interaction of diagnostic ultrasound with gas bodies produces a useful contrast effect in medical images, but the same interaction also represents a mechanism for bioeffects. Anesthetized hairless mice were scanned by using a 2.5-MHz transducer (610-ns pulses with 3.6-kHz repetition frequency and 61-Hz frame rate) after injection of Optison and Evans blue dye. Petechial hemorrhages (PHs) in intestine and abdominal muscle were counted 15 min after exposure to characterize capillary rupture, and Evans blue extravasation was evaluated in samples of muscle tissue. For 5 ml small middle dotkg(-1) contrast agent and exposure to 10 alternating 10-s on and off periods, PH counts in muscle were approximately proportional to the square of peak negative pressure amplitude and were statistically significant above 0.64 MPa. PH counts in intestine and Evans blue extravasation into muscle tissue were significant above 1. 0 MPa. The PH effect in muscle was proportional to contrast dose and was statistically significant for the lowest dose of 0.05 ml small middle dotkg(-1). The effects decreased nearly to sham levels if the exposure was delayed 5 min. The PH effect in abdominal muscle was significant and statistically indistinguishable for uninterrupted 100-s exposure, 10-s exposure, 100 scans repeated at 1 Hz, and even for a single scan. The results confirms a previous report of PH induction by diagnostic ultrasound with contrast agent in mammalian skeletal muscle [Skyba, D. M., Price, R. J., Linka, A. Z., Skalak, T. C. & Kaul, S. (1998) Circulation 98, 290-293].

Cavitation nucleation agents for nonthermal ultrasound therapy

The use of a nucleation-promoting agent can greatly enhance therapeutically useful nonthermal bioeffects. A blank agent (saline), Optison ultrasound contrast agent, a stabilized perfluoropentane droplet suspension (SDS), and retained air space were compared as nucleation agents in whole blood. Fresh canine whole blood with added agent was exposed in 1.3-ml disposable pipette bulbs to lithotripter shock waves (2-Hz rate; +24.4, -5.2 MPa peak pressure amplitudes). Cavitation activity was assessed by measuring hemolysis. The droplet suspension performed nearly as well as retained air when added at a concentration sufficient to provide a roughly equal volume of gas after vaporization. Optison also yielded nucleation, but a concentration of 10%-20% was needed for large enhancement of hemolysis comparable to 5% SDS. Exposure at room temperature, which was less than the 29 degrees C boiling point of perfluoropentane, eliminated the enhancement of the hemolysis effect relative to the blank. Application of 100-kPa excess pressure during exposure reduced but did not eliminate the nucleation ability of Optison, SDS, or retained air. However, this small pressure (relative to the peak positive pressure of the shock waves) eliminated the hemolysis induced with the blank agent. The stabilized perfluoropentane droplet suspension appears to be a good nucleation agent for nonthermal ultrasound therapy applications.

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.

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.

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.

Non-invasive assessment and control of ultrasound-mediated membrane permeabilization

PURPOSE

Ultrasound has been shown to transiently permeabilize biological membranes, thereby facilitating delivery of large compounds such as proteins and DNA into cells and across tissues such as skin. In this study, we sought to quantitatively determine the dependence of cell membrane permeabilization on ultrasound parameters and to identify acoustic signals which correlate with observed membrane permeabilization.

METHODS

Bovine red blood cells were exposed to ultrasound at 24 kHz over a range of controlled conditions. The degree of membrane permeabilization was measured by release of hemoglobin and was determined as a function of ultrasound parameters and measured acoustic signals.

RESULTS

These studies showed that permeabilization increased with incident ultrasound pressure, increased with total exposure time above a threshold of approximately 100 msec, showed a weak dependence on pulse length with a small maximum at 3 msec, and did not depend on duty cycle under the conditions examined. Using measured acoustic spectra we found that red blood cell membrane permeabilization correlated best with the pressure measured at half the driving frequency (f/ 2 = 12 kHz) and its ultraharmonics, less strongly with the broadband noise pressure measured between peaks, and least strongly with pressure measured at the driving frequency and its higher harmonics. Permeabilization caused by ultrasound applied at any set of conditions tested in this study could be well predicted by the parameter tau x Pf/2, which characterizes the total cavitational exposure.

CONCLUSIONS

This study provides a quantitative guide to designing ultrasound protocols useful for drug delivery. The acoustic measurements support the hypothesis that ultrasonic cavitation is the mechanism by which membranes are permeabilized. They also suggest that measurable acoustic signals can provide noninvasive, real-time feedback about membrane permeabilization and drug delivery.

Enhancement of ultrasonically-induced hemolysis by perfluorocarbon-based compared to air-based echo-contrast agents

Hemolysis induced by ultrasonic activation of various contrast-agent gas bodies was investigated. Canine whole blood, with high concentrations of the agents held in 1 mm thick chambers, was exposed in the nearfield of a 2.4-MHz ultrasound beam in a 37 degrees C water bath. Sterile phosphate buffered saline (PBS) served as a control agent without gas bodies. Albunex (Mallinckrodt Medical, St. Louis, MO) and Levovist (Schering AG, Berlin, Germany) represented the air-based contrast agents. The experimental agents FS069 (Optison, Molecular Biosystems Inc., San Diego, CA) and modified MRX-130 (ImaRx Pharmaceutical Corp., Tucson, AZ) represented perfluorocarbon-based contrast agents. No significant ultrasonically-induced hemolysis was detected for the PBS or Levovist suspensions. After 1 s continuous exposure, ultrasonically-induced hemolysis was significant for Albunex at 0.4 MPa or higher pressure amplitudes, for FS069 at 0.2 MPa and for modified MRX-130 at 0.4 MPa. Hemolysis found after pulsed exposure with 10 micros pulses and 1 ms pulse repetition period was significant for Albunex, FS069 and modified MRX-130 above thresholds of 1.1 MPa, 0.57 MPa and 1.6 MPa, respectively. FS069 led to more hemolysis after pulsed mode exposures of 1 s duration or longer than did Albunex. Reduced concentrations of gas bodies gave increased thresholds and reduced hemolysis. These results indicate that improvements in persistence of contrast agents, which increase their clinical utility, may also enhance the potential for cavitational bioeffects.

Ultrasonically induced hemolysis at high cell and gas body concentrations in a thin-disc exposure chamber

Ultrasound image contrast may be enhanced by injecting gas bodies into the blood. This in vitro study was undertaken to assess the potential for induction of hemolysis due to ultrasonic activation of the contrast agent gas bodies. Canine whole blood with Albunex (Mallinckrodt Medical, St. Louis, MO, USA) was exposed to near-field ultrasound beams in 1-mm-thick chambers held stationary (i.e., not rotated) in a 37 degrees C water bath. At 2.25 MHz, statistically significant hemolysis occurred in 0.5 hematocrit, 50% Albunex suspensions for 0.28-MPa, 1-s continuous exposure and for 0.58-MPa, 100-s exposures with 10-microsecond pulses and 1.0-ms pulse repetition period. Continuous exposure durations as short as 10 ms produced about 4.5% hemolysis, which only increased slightly to about 5.5% after 100 s. At a constant 1.6 MPa, hemolysis increased with increasing gas body concentration and with decreasing cell concentration. Hemolysis decreased with increasing frequency in a 50/50 mixture of whole blood and Albunex, with thresholds rising from 0.12 MPa continuous (1 s) and 0.47 MPa pulsed (10 microseconds:1.0 ms for 100 s) at 1.06 MHz to 0.47 MPa continuous and 1.9 MPa pulsed at 5.3 MHz.

Ultrasound contrast media and their use in obstetrics and gynecology

Contrast media have gained acceptance to enhance ultrasonography in many fields of medicine; in particular, cardiology. Several agents have been described and many more are being manufactured and tested. By increasing the number of strong sound scatterers, these agents improve images by increasing the amount of echoes. This is true both for grey-scale and color or Doppler imaging. Their use in obstetrics is very limited at the moment because of safety issues. In a laboratory setup, they have been shown to markedly enhance placental imaging. In gynecology, imaging of the uterine cavity and Fallopian tubes is greatly improved. A potential area where ultrasound contrast may find a role is gynecological oncology. Vascularity is increased in many tumors, but usually vessel diameter is small and velocity low. One can therefore expect future use of the ultrasound contrast agents in ovarian or other gynecological neoplasms.

The influence of contrast agents on hemorrhage produced by lithotripter fields

Ultrasonic contrast agents greatly increase the side effects of low-amplitude lithotripter fields in mice. Using a piezoelectric lithotripter, adult mice were exposed to 200 lithotripter pulses with a peak positive pressure amplitude of 2 MPa. During the exposure period, mice were injected with approximately 0.1 mL of the ultrasonic contrast agent Albunex. For comparison, another group of mice experienced the same lithotripter exposures, but were not injected with contrast agent. Following exposures, animals were sacrificed and observed for hemorrhage in various organs and tissues. Mice exposed to the lithotripter field alone had minimal hemorrhage only in the intestine and lung. In comparison, mice injected with Albunex during exposure exhibited extensive hemorrhage in the intestine, kidney, muscle, mesentery, stomach, bladder, seminal vesicle and fat.

Hemolysis in vivo from exposure to pulsed ultrasound

Ultrasonically induced hemolysis in vivo when a commercial ultrasound contrast agent, Albunex, was present in the blood. Murine hearts were exposed for 5 min at either 1.15 or 2.35 MHz with a pulse length of 10 microseconds and pulse repetition frequency of 100 Hz. During the exposure period, four boluses of Albunex were injected into a tail vein for a total of approximately 0.1 mL of Albunex. Following exposure, blood was collected by heart puncture and centrifuged, and the plasma was analyzed for hemoglobin concentration. With Albunex present in the blood, the threshold for hemolysis at 1.15 MHz was 3.0 +/- 0.8 MPa (mean +/- SD) peak positive pressure (approximately 1.9 MPa negative pressure, approximately 180 W cm-2 pulse average intensity). For the highest exposure levels (10 MPa peak positive pressure at the surface of the animal), the mean value for hemolysis was approximately 4% at 1.15 MHz and 0.46% at 2.35 MHz, i.e., the threshold at 2.35 MHz is > 10 MPa peak positive pressure. In contrast, hemolysis in control mice receiving saline injections at 10 MPa or sham-exposed (0 MPa) mice receiving Albunex was approximately 0.4%.

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.

Acoustic cavitation nuclei survive the apparent ultrasonic destruction of Albunex microspheres

The hypothesis tested was that gas bodies capable of nucleating violent cavitation activity in vitro would survive the rapid disruption of Albunex microspheres by 1-MHz ultrasound. Human erythrocyte hemolysis was used as a proxy measure of cavitation. Fluid (5% human serum albumin [HSA]) with or without Albunex (ALX) was exposed or sham-exposed to 1-MHz ultrasound (P+ = 1.25 +/- 0.01 MPa, P- = 0.81 +/- 0.01 MPa; ISPTP approximately 35 W/cm2) for 60 s using 10-microseconds pulses and a duty factor of 0.5. An equal volume of whole human blood was then added to the fluid, followed by a second 60-s treatment. Insonation of cell suspensions prepared in previously sham-exposed HSA + ALX fluid produced about 4% hemolysis, a level significantly greater than in the controls. Insonation of cell suspensions prepared in previously insonated HSA + ALX fluid produced about 0.4% hemolysis; this also differed significantly from the controls. The data thus support the hypothesis.