Edwin L. Carstensen

Prediction of nonlinear acoustic effects at biomedical frequencies and intensities

Abstract Some fundamentals of nonlinear acoustics are reviewed to facilitate their consideration in biomedical ultrasound. The phenomena described include acoustic nonlinearity, finite amplitude distortion, shock formation, harmonic components, nonlinearly induced absorption, saturation, and the influence of these effects on ultrasonic beams. The simplified results of several theoretical derivations are presented and employed in illustrative calculations and plots. These maybe used to ascertain the importance of nonlinear effects in applications involving plane waves, spherically diverging waves, and spherically converging (focused) waves. A discussion of relevant experiments is given, along with some comments on possible consequences in diagnostic, surgical, and theraputic applications.

Enhancement of fibrinolysis in vitro by ultrasound.

The effect of ultrasound on the rate of fibrinolysis has been investigated using an in vitro system. Plasma or blood clots containing a trace label of 125I fibrin were suspended in plasma containing plasminogen activator and intermittently exposed to continuous wave 1-MHz ultrasound at intensities up to 8 W/cm2. Plasma clot lysis at 1 h with 1 microgram/ml recombinant tissue plasminogen activator (rt-PA) was 12.8 +/- 1.2% without ultrasound and was significantly (P = 0.0001) increased by exposure to ultrasound with greater lysis at 1 W/cm2 (18.0 +/- 1.4%), 2 W/cm2 (19.3 +/- 0.7%), 4 W/cm2 (22.8 +/- 1.8%), and 8 W/cm2 (58.7 +/- 7.1%). Significant increases in lysis were also seen with urokinase at ultrasound intensities of 2 W/cm2 and above. Exposure of clots to ultrasound in the absence of plasminogen activator did not increase lysis. Ultrasound exposure resulted in a marked reduction in the rt-PA concentration required to achieve an equivalent degree of lysis to that seen without ultrasound. For example, 15% lysis occurred in 1 h at 1 microgram/ml rt-PA without ultrasound or with 0.2 microgram/ml with ultrasound, a five-fold reduction in concentration. Ultrasound at 1 W/cm2 and above also potentiated lysis of retracted whole blood clots mediated by rt-PA or urokinase. The maximum temperature increase of plasma clots exposed to 4 W/cm2 ultrasound was only 1.7 degrees C, which could not explain the enhancement of fibrinolysis. Ultrasound exposure did not cause mechanical fragmentation of the clot into sedimentable fragments, nor did it alter the sizes of plasmic derivatives as demonstrated by SDS polyacrylamide gel electrophoresis. We conclude that ultrasound at 1 MHz potentiates enzymatic fibrinolysis by a nonthermal mechanism at energies that can potentially be applied and tolerated in vivo to accelerate therapeutic fibrinolysis.

Lung damage from exposure to pulsed ultrasound

Motivated by a recent finding that threshold pressures for hemorrhage in mouse lung exposed to the fields of an electrohydraulic lithotripter were less than 2 MPa, we extended the exposures to pulsed ultrasound. Sharply defined thresholds of the order of 1 MPa were found with 10 microseconds length pulses and roughly twice that value for 1 microsecond pulses. The thresholds at 4 MHz are greater than at 1 MHz. The thresholds are comparable for focused and unfocused fields. As would be expected for a cavitation-like phenomenon, temporal average intensity is a very poor predictor of this effect. In the extreme case, lesions were found at temporal average intensities on the order of 1 mW/cm2.

Enhancement of Fibrinolysis With 40-kHz Ultrasound

BACKGROUND Ultrasound at frequencies of 0.5 to 1 MHz and intensities of > or =0.5 W/cm2 accelerates enzymatic fibrinolysis in vitro and in some animal models, but unacceptable tissue heating can occur, and limited penetration would restrict application to superficial vessels. Tissue heating is less and penetration better at lower frequencies, but little information is available regarding the effect of lower-frequency ultrasound on enzymatic fibrinolysis. We therefore examined the effect of 40-kHz ultrasound on fibrinolysis, tissue penetration, and heating. METHODS AND RESULTS 125I-fibrin-radiolabeled plasma clots in thin-walled tubes were overlaid with plasma containing tissue plasminogen activator (tPA) and exposed to ultrasound. Enzymatic fibrinolysis was measured as solubilization of radiolabel. Tissue attenuation and heating were examined in samples of porcine rib cage. Fibrinolysis was increased significantly in the presence of 40-kHz ultrasound at 0.25 W/cm2, reaching 39+/-7% and 93+/-11% at 60 minutes and 120 minutes, compared with 13+/-8% and 37+/-4% in the absence of ultrasound (P<0.0001). The acceleration of fibrinolysis increased at higher intensities. Attenuation of the ultrasound field was only 1.7+/-0.5 dB/cm through the intercostal space and 3.4+/-0.9 dB/cm through rib. Temperature increments in rib were <1 C/(W/cm2). CONCLUSIONS These findings indicate that 40-kHz ultrasound significantly accelerates enzymatic fibrinolysis at intensities of > or =0.25 W/cm2 with excellent tissue penetration and minimal heating. Externally applied 40-kHz ultrasound at low intensities is a potentially useful therapeutic adjunct to enzymatic fibrinolysis with sufficient tissue penetration for both peripheral vascular and coronary applications.

Demonstration of nonlinear acoustical effects at biomedical frequencies and intensities

Abstract Examples of nonlinear acoustic phenomena in water in the range of frequencies and intensities of interest for biomedical ultrasound are provided. Total intensities, including all harmonics generated in the medium have been measured with a spherical radiometer. The fundamental component of the waves have been measured with a miniature probe hydrophone and low pass filter. Simple adaptation of the theory summarized in a companion paper leads to predictions which are in excellent agreement with the observations. The illustrated phenomena must be considered in studies of the biological effects of ultrasound as well as in the applications of ultrasound in medicine.

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.

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%.

Determination of the Acoustic Properties of Blood and its Components

Measurements of absorption and velocity of sound in blood, plasma, and solutions of albumin and hemoglobin have been carried out in the frequency range 800–3000 kc and temperature range 5–45°C. The absorption departs only slightly from a linear dependence upon frequency. Absorption for the various solutions is in direct proportion to protein content. It is concluded that the acoustic properties of blood are largely determined by the proteins which it contains.

Acoustic Properties of Hemoglobin Solutions

Difference methods have been used to measure both the absorption and the velocity of sound in solutions of hemoglobin in the range from 0.3 to 10 Mc, and for various temperatures and concentrations. The absorption varies as a linear function of frequency over a large part of the range covered. Measurements show marked dispersion in the velocity of sound over the entire frequency range.

Shear properties of mammalian tissues at low megahertz frequencies

Rough values for the transverse‐wave specific acoustic impedance Zs=Rs+jXs, transverse velocity of sound cs, and transverse absorption coefficient αs have been measured for canine liver, kidney, muscle tissues, and for packed red blood cells in the frequency range from 2 to 14 MHz at 25°C. The ranges of the results are Rs=700–3000 Ωmech/cm2, Xs=400–4000 Ωmech/cm2, cs=900–10 000 cm/sec, and αs=2000–30 000 np/cm. The corresponding results for shear stiffness μ1 and viscosity μ2 are μ1<107 dyn/cm2 and μ2=4–30 centipoise. At these frequencies the viscosities are orders of magnitude less than those reported at 0.5–5 kHz by Oestreicher (1951). The low impedances (viscosities) correspond to low velocities and extremely high absorption coefficients for shear waves in tissue.Subject Classification: [43]80.20, [43]80.30.