Lack of bioeffects of ultrasound energy after intravenous administration of FS069 (Optison) in the anesthetized rabbit

Therapeutic Ultrasound in Cardiovascular Medicine

An advantage of therapeutic ultrasound (US) is the ability to cause controlled biological effects noninvasively. Depending on the magnitude and frequency of exposure parameters, US can interact in different ways with a variety of biological tissues. The development and clinical utility of therapeutic US techniques are now rapidly growing, especially with regard to the application of US pulses for cardiac pacing and the potential treatment of cardiovascular diseases. This review outlines the basic principles of US-based therapy in cardiology, including the acoustic properties of the cardiovascular tissue, and the use of US in therapeutic cardiovascular medicine.

Ultrasound-biophysics mechanisms

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

Mechanical Bioeffects of Ultrasound

Ultrasound is used widely in medicine as both a diagnostic and therapeutic tool. Through both thermal and nonthermal mechanisms, ultrasound can produce a variety of biological effects in tissues in vitro and in vivo. This chapter provides an overview of the fundamentals of key nonthermal mechanisms for the interaction of ultrasound with biological tissues. Several categories of mechanical bioeffects of ultrasound are then reviewed to provide insight on the range of ultrasound bioeffects in vivo, the relevance of these effects to diagnostic imaging, and the potential application of mechanical bioeffects to the design of new therapeutic applications of ultrasound in medicine.

Ultrasound agents may open the blood-brain barrier in rats and aggravate pathologic consequences of experimental head trauma

Unilateral intracarotid injection of contrast agents may considerably destabilize the blood-brain barrier in rats. This leads to vasogenic edema in the ipsilateral hemisphere. Mortality and extravasation increased significantly when administration of these ultrasound contrast agents was followed by mild traumatic brain injury. Direct administration to the cerebral circulation is, therefore, indicative for edema-related pathology and may amplify the consequences of experimental neurotrauma.

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).

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.

Optison (FS069) disrupts the blood-brain barrier in rats

Optison is a new echocardiographic contrast agent, designed for IV injection, that is very useful in delineating cardiac structures during ultrasound examination. Because Optison could be a valuable adjunct in the diagnosis and evaluation of congenital heart disease, this study was undertaken to assess its effects on the blood-brain barrier when introduced directly in the cerebral circulation, as might occur with some congenital lesions. In this study, Sprague-Dawley rats were anesthetized, and Optison, at various dosages, was injected into the carotid artery. After this, Evans blue dye, a marker for blood-brain barrier disruption, was injected at different time intervals. Gross and histologic examination of the animals' brains revealed disruption of the blood-brain barrier that appeared to be Optison-dosage-dependent. Although the mechanism for this disruption is unclear, it may be related to the use of octofluoropropane gas used in the Optison as a contrast medium. Further studies are necessary to determine the pathologic consequences of Optison's effects on the blood-brain barrier.

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].

Destruction of contrast microbubbles and the association with inertial cavitation

The destruction of insonified Sonazoid microbubbles and its association with inertial cavitation in vitro utilizing an active acoustic detector was investigated. The experimental observation indicated that contrast microbubbles could be damaged at moderate acoustic pressures of 0.6-1.6 MPa (0.4-1.0 in mechanical index, MI). A damaged bubble could be dissolved into the medium on the order of 1 ms, implying that the destruction at moderate pressures is a relatively slow (relative to inertial bubble collapse), nonviolent dissolution process following the disruption of encapsulating surface materials. Inertial cavitation events in the presence of contrast microbubbles were observed using multiple highly intense ultrasound (US) pulses (>1.6 MPa). This observation suggested that intense US might disintegrate contrast microbubbles, and fragments of disintegrated microbubbles could be activated by an upcoming highly intense imaging pulse. The above results imply that inertial cavitation is unlikely to take place in the presence of Sonazoid contrast microbubbles when exposed to diagnostic US with an MI <1.

The use of a thrombus-specific ultrasound contrast agent to detect thrombus in arteriovenous fistulae

RATIONALE AND OBJECTIVES

To evaluate the use of a new thrombus-specific ultrasound contrast agent, MRX-408, in the ultrasonic detection of thrombus in arteriovenous (AV) fistulae.

METHODS

Six purpose-bred mongrels with two AV fistulae each were imaged with gray-scale ultrasound 7 weeks after graft implantation before and after the intravenous bolus injection of MRX-408 (a GPIIb receptor-targeted ultrasound contrast agent). Pre- and postcontrast videotaped segments were randomized and reviewed by four radiologists blinded to the presence of thrombus in the grafts.

RESULTS

After the use of MRX-408, there was improved visualization of thrombus within the grafts (P < 0.0001). This was due to the enhancement of the thrombus (P < 0.0001). The improved visualization and contrast enhancement were more marked in the grafts that contained thrombus nonhyperechoic to surrounding soft tissues.

CONCLUSIONS

MRX-408 demonstrated better visualization of thrombus within AV fistulae. This was shown in both patent and occluded grafts. These results are encouraging and suggest that this contrast agent merits further development.

Ultrasound-induced cell lysis and sonoporation enhanced by contrast agents

The enhancement of ultrasound-induced cell destruction, lysis, and sonoporation in low cell concentration suspensions (2 x 10(5)/mL) by the presence of contrast agents (gas bubble to cell ratio = 230) was demonstrated using cervical cancer cells (HeLa S3) suspensions containing micron-size denatured albumin microspheres filled with air (Albunex) or octafluoropropane (Optison). The suspensions were insonificated by 2-MHz continuous or tone burst ultrasound in near field. The spatial peak-pressure amplitude was 0.2 MPa. The enhancement of cell destruction due to Optison was shown to be much higher than that due to Albunex for similar bubble concentration and ultrasound conditions. For tone burst exposures, significant lysis and sonoporation only occurred in the presence of a contrast agent. The majority of the bioeffects observed occurred in the first 5 min of exposure. The relationship between the enhancement of bioeffects and duty cycle of tone burst ultrasound appears to indicate that both stable gas spheres of contrast agents and cavitation nuclei created by the disruption of the gas spheres play a significant role in causing the bioeffects.