Cardiac sonographers' communication
Ultrasound contrast physics: a series on contrast echocardiography, article 3*,**,,★★,

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Abstract

J Am Soc Echocardiogr 2000;13:959-67.

Introduction

This third article in a series on contrast echocardiography addresses the physics of microbubbles. The properties of ultrasonographic contrast agents and the manner in which bubbles react within an ultrasound beam will be discussed in general. It is important to realize that the field of contrast echocardiography is changing as ultrasonographic equipment manufacturers modify signal processing for contrast-enhanced images; at the same time, manufacturers of contrast agents are developing new microbubbles. Therefore the sonographer will need a greater understanding of microbubble composition, infusion techniques, physics, and optimal machine settings. This article serves as a primer to elucidate the interactions between ultrasound and ultrasonographic contrast agents at the current level of technology.

Section snippets

Current contrast agents

The current generation of contrast agents comprises small, stabilized, gas-filled microbubbles that can pass through the smallest capillaries. Table 1 compares the size of new contrast agents to red blood cells, agitated saline solution, and Albunex (the first ultrasonographic contrast agent approved by the US Food and Drug Administration).

. Bubble size

TypeSize (microns)
Microbubble<100
Red blood cell6-8
Saline solution16*
Albunex3-5†
New contrast agents2.5‡
*Approximate size. †1995 Food and Drug

Harmonics

All objects have an inherent or natural frequency. As an object oscillates at this natural frequency, a series of oscillations may also be produced at multiples of that natural frequency. Harmonics are also generated by microbubbles when they are insonified within an acoustic field. Two unique aspects of microbubbles are that they give off a nonlinear response and they expand to a greater degree than they can be compressed. Figure 2 shows the difference between a bubble with a linear response

Response of bubbles to ultrasound

Figure 3 shows the response of these microbubbles within an ultrasonic (acoustic) field.

. As acoustic power is increased, microbubbles move from a linear response to a nonlinear, harmonics-producing response. At maximum power, destruction of the microbubbles occurs. (Illustration courtesy of DuPont Pharmaceuticals Company.)2

At low power output (PO) settings, there is mostly a linear response (fundamental enhancement) with some generation of harmonic frequencies. As the PO is increased, the

Mechanical index

An understanding of the mechanical index (MI) (acoustic PO of the ultrasonographic system) and how it affects microbubbles is critical in optimizing echocardiographic contrast images. The MI is a unit-less number that serves as an indicator of the nonthermal bioeffects. It is a measurement of negative acoustic pressure within the ultrasound field and is calculated at the depth where energy concentration is highest. An important point is that energy concentration is not even throughout the

Contrast artifacts

Attenuation, a commonly seen artifact, is caused by the high echogenicity of microbubbles. When a high concentration of microbubbles exists, a larger portion of the ultrasound energy is backscattered and unable to transmit through to the far field structures. Attenuation is seen as shadowing or darker areas in the mid to far field. Figure 5 is an example of an apical 4-chamber view with too high a concentration of microbubbles in the left ventricle.

. Apical 4-chamber view of the left ventricle

Tissue harmonics

Although this article concentrates on the physics of contrast, it is important to note that myocardial tissue itself generates some harmonic frequencies by a different mechanism. With contrast, harmonics are caused by the bubbles giving off a nonlinear response to compression and expansion. In tissue harmonics, the mechanism is the result of propagation of ultrasound through the myocardium. The nonlinear response in tissue occurs because the speed of sound is higher during compression than in

Conclusion

Encapsulated gas microbubbles capable of traversing the pulmonary microcirculation are now commercially available. These ultrasonographic contrast agents provide an increased backscattered signal from the blood pool, greatly enhancing endocardial border definition in technically inadequate studies. Harmonic imaging enhances the differences between contrast-generated signals and those originating from tissue. An understanding of the basic physical principles of microbubbles and their behavior in

Acknowledgements

The authors gratefully acknowledge technical support for this article by DuPont Pharmaceuticals Company.

References (3)

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The American Society of Echocardiography (ASE) is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to sponsor continuing medical education for physicians. The ASE designated this educational activity for 1 hour of Category 1 credit of the AMA Physicians' Recognition Award. The ASE adheres to ACCME Standards regarding industry support of continuing medical education. Disclosures of faculty and commercial sponsor relationships, if any, have been indicated.

**

Target Audience: Participation should include individuals from the fields of cardiac sonography, cardiovascular anesthesiology, cardiovascular medicine, cardiovascular surgery, pediatric cardiology, neurology, and nursing, as well as medical residents/fellows, and students.

Educational Objective(s): Upon completing the reading of this article the participant should be able to:

  • 1.

    Discuss what microbubbles are and their ideal properties.

  • 2.

    Understand the importance of mechanical index (MI) and how to change it.

  • 3.

    Discuss the difference between fundamental and second harmonic imaging.

  • 4.

    Identify potential artifacts, such as attenuation and swirling, and how to correct them.

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The estimated time for this CME activity is approximately 1 hour.

Reprint requests: David Adams, RDCS, Duke University Medical Center, Box 3818, Durham, NC 27710 (E-mail: [email protected]).

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