E. Carr Everbach

Cavitational mechanisms in ultrasound-accelerated thrombolysis at 1 MHz

Inertial cavitation is hypothesized to be a mechanism by which ultrasound (US) accelerates the dissolution of human blood clots when the clot is exposed to a thrombolytic agent such as tissue plasminogen activator (t-PA). To test this hypothesis, radiolabeled fibrin clots were exposed or sham-exposed in vitro to 1 MHz c.w. US in a rotating sample holder immersed in a water-filled tank at 37 degrees C. Percent clot dissolution after 60 min of US exposure was assessed by removing the samples, centrifuging, and measuring the radioactivity of the supernatant fluid relative to the pelletized material. To suppress acoustic cavitation, the exposure tank was contained within a hyperbaric chamber capable of pneumatic pressurization to 10 atmospheres (gauge). Various combinations of static pressure (0, 2, 5, and 7.5 atm gauge), US (0 or 4 W/cm(2) SATA), and t-PA (0 or 10 microg/mL) were employed, showing statistically significant reductions in thrombolytic activity as static pressure increased. To gain further insight, an active cavitation detection scheme was employed in which 1-micros duration tonebursts of 20-MHz US (< 1 kPa peak negative pressure, 1 Hz PRF) were used to interrogate clots subjected to US and static pressure. Results of this cavitation detection scheme showed that scattering from within the clot and broadband acoustic emissions that were both present during insonification were significantly reduced with application of static pressure. However, only about half of the acceleration of thrombolysis due to US could be removed by static pressure, suggesting the possibility of other mechanisms in addition to inertial cavitation.

Bioeffects Considerations for Diagnostic Ultrasound Contrast Agents

Diagnostic ultrasound contrast agents have been developed for enhancing the echogenicity of blood and for delineating other structures of the body. Approved agents are suspensions of gas bodies (stabilized microbubbles), which have been designed for persistence in the circulation and strong echo return for imaging. The interaction of ultrasound pulses with these gas bodies is a form of acoustic cavitation, and they also may act as inertial cavitation nuclei. This interaction produces mechanical perturbation and a potential for bioeffects on nearby cells or tissues. In vitro, sonoporation and cell death occur at mechanical index (MI) values less than the inertial cavitation threshold. In vivo, bioeffects reported for MI values greater than 0.4 include microvascular leakage, petechiae, cardiomyocyte death, inflammatory cell infiltration, and premature ventricular contractions and are accompanied by gas body destruction within the capillary bed. Bioeffects for MIs of 1.9 or less have been reported in skeletal muscle, fat, myocardium, kidney, liver, and intestine. Therapeutic applications that rely on these bioeffects include targeted drug delivery to the interstitium and DNA transfer into cells for gene therapy. Bioeffects of contrast‐aided diagnostic ultrasound happen on a microscopic scale, and their importance in the clinical setting remains uncertain.

Correlation of ultrasound-induced hemolysis with cavitation detector output in vitro

A 20-MHz passive acoustic detector was used to quantify the amount of transient acoustic cavitation occurring in a sample exposed to intense pulsed ultrasound. A dilute suspension of human erythrocytes with and without a microbubble echo-contrast agent was exposed in vitro to 500 W/cm2 (SPPA) ultrasound of center frequency 1 MHz and tone burst duration 20, 100, 200, 500 and 1000 microseconds at a pulse repetition frequency of 20 Hz. Inertial cavitation occurring within the sample, as measured by the temporal average of the detector output, correlated well with hemolysis, suggesting that violent bubble collapse is responsible for cell damage. The result also raises the prospect of cavitation monitoring as a possible predictor of adverse bioeffects when echo-contrast agents are used clinically.

American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound: Executive summary

The continued examination of potential biological effects of ultrasound and their relationship to clinical practice is a key element in evaluating the safety of diagnostic ultrasound. Periodically, the American Institute of Ultrasound in Medicine (AIUM) sponsors conferences bringing experts together to examine the literature on ultrasound bioeffects and to develop conclusions and recommendations related to diagnostic ultrasound. The most recent effort included the examination of effects whose origins were thermal or nonthermal, with separate evaluations for potential effects related to fetal ultrasound. In addition, potential effects due to the introduction of ultrasound contrast agents were summarized. This information can be used to assess risks in comparison to the benefits of diagnostic ultrasound. The conclusions and recommendations are organized into 5 broad categories, with a comprehensive background and evaluation of each topic provided in the corresponding articles in this issue. The following summary is not meant as a substitute for the detailed examination of issues presented in each of the articles but rather as a means to facilitate further study of this consensus report and implementation of its recommendations. The conclusions and recommendations are the result of several rounds of deliberations at the consensus conference, subsequent review by the Bioeffects Committee of the AIUM, and approval by the AIUM Board of Governors.

Endoscopic measurement of lesion size: Improved accuracy with image processing

Endoscopic measurement of lesions is of great importance in the design and performance of clinical trials, as, for example, in studies of ulcer disease. Endoscopes are constructed with wide-angle lenses that significantly distort the image by creating a relative compression of points in its periphery. We have recently developed a computer program to correct the distortion of the wide-angle lens. We sought to determine the accuracy of the currently used open-biopsy forceps measurement technique and compare it to that of an image-processing technique designed to correct image distortion. The overall error of the open-biopsy forceps technique using an in vitro ulcer model was under-estimation of lesion size by 41.8% +/- 23.3%. When image processing was used to correct distortion, error was significantly decreased to 1.8% +/- 2.2% (p < 0.05). In vivo measurements were made using an inserted object of known size (coated chewing gum). The mean error of the forceps technique in vivo was 26.5% +/- 5.7% (under-estimation of size), which improved significantly to an error of 2.8% +/- 3.2% (p < 0.05) with the image-processing technique. We conclude that image processing significantly enhances the accuracy of measurement at endoscopy.

Internal stress wave measurements in solids subjected to lithotripter pulses

Semiconductor strain gauges were used to measure the internal strain along the axes of spherical and disk plaster specimens when subjected to lithotripter shock pulses. The pulses were produced by one of two lithotripters. The first source generates spherically diverging shock waves of peak pressure approximately 1 MPa at the surface of the specimen. For this source, the incident and first reflected pressure (P) waves in both sphere and disk specimens were identified. In addition, waves reflected by the disk circumference were found to contribute significantly to the strain fields along the disk axis. Experimental results compared favorably to a ray theory analysis of a spherically diverging shock wave striking either concretion. For the sphere, pressure contours for the incident P wave and caustic lines were determined theoretically for an incident spherical shock wave. These caustic lines indicate the location of the highest stresses within the sphere and therefore the areas where damage may occur. Results were also presented for a second source that uses an ellipsoidal reflector to generate a 30-MPa focused shock wave, more closely approximating the wave fields of a clinical extracorporeal lithotripter.

Effect of acoustic cavitation on platelets in the presence of an echo-contrast agent

A suspension of human platelets in autologous plasma or buffer solution with and without a microbubble echo-contrast agent was exposed in vitro to 730 W/cm2 (ISPPA) ultrasound pulses of duration 40-160 microseconds at 1 MHz and 20-Hz pulse repetition frequency. Inertial cavitation occurring within the samples was monitored during the exposures and a measure of average cavitational activity was calculated for each 5-min exposure. This quantity, with the other acoustic parameters, accounted for up to 75% of the variation in the destruction of platelets as measured by Coulter counter and 83.5% of the release of bound radiolabel using a multiple-interaction statistical model. When the echo-contrast agent was absent, negligible cavitation occurred and the amount of platelet destruction was statistically indistinguishable from sham (no-ultrasound) exposures. Therefore, microbubble echo-contrast agents may interact with ultrasound to cause platelet lysis through the mechanism of inertial cavitation.

Effects of Attenuation and Thrombus Age on the Success of Ultrasound and Microbubble-Mediated Thrombus Dissolution

The purpose of this study was to examine the effects of applied mechanical index, incident angle, attenuation and thrombus age on the ability of 2-D vs. 3-D diagnostic ultrasound and microbubbles to dissolve thrombi. A total of 180 occlusive porcine arterial thrombi of varying age (3 or 6 h) were examined in a flow system. A tissue-mimicking phantom of varying thickness (5 to 10 cm) was placed over the thrombosed vessel and the 2-D or 3-D diagnostic transducer aligned with the thrombosed vessel using a positioning system. Diluted lipid-encapsulated microbubbles were infused during ultrasound application. Percent thrombus dissolution (%TD) was calculated by comparison of clot mass before and after treatment. Both 2-D and 3-D-guided ultrasound increased %TD compared with microbubbles alone, but %TD achieved with 6-h-old thrombi was significantly less than 3-h-old thrombi. Thrombus dissolution was achieved at 10 cm tissue-mimicking depths, even without inertial cavitation. In conclusion, diagnostic 2-D or 3-D ultrasound can dissolve thrombi with intravenous nontargeted microbubbles, even at tissue attenuation distances of up to 10 cm. This treatment modality is less effective, however, for older aged thrombi.

An interferometric technique for B/A measurement

An isentropic phase method is described for measuring in vitro the acoustic nonlinearity parameter B/A of several aqueous buffers, protein solutions, lipid oils, and emulsions. The technique relies upon the use of an acoustic interferometer to measure the small changes in sound speed that accompany a rapid hydrostatic pressure change of between one and two atmospheres. Average accuracies of 0.85% are attainable with this method.

Biological and environmental factors affecting ultrasound-induced hemolysis in vitro: 2. Medium dissolved gas (pO2) content

The data collected in this project supported the a priori hypothesis that the concentration of dissolved oxygen in whole human blood in vitro affected the extent of ultrasound (US)-induced hemolysis under conditions conducive to the occurrence of inertial cavitation. Aliquots of whole human blood in vitro with a relatively high O(2) level had statistically significantly more 1-MHz US-induced hemolysis than aliquots with a relatively low O(2) level in the presence of controlled gas nucleation (Albunex or ALX, supplementation), with US-induced hemolytic yields being substantially less at 2.2- and 3.5-MHz exposures or in the absence of ALX-supplementation at otherwise comparable acoustic pressures, pulse lengths and duty factors. Passive cavitation detection (pcd) measures indicated a linear relationship for hemolysis up to about 70% and pcd values (R(2) = 0.99).