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The last decade has seen a revolution in the technique of contrast echocardiography, with detailed understanding of the interaction between microscopic air bubbles and ultrasound leading to rapid development in both contrast and imaging technology. Several transpulmonary contrast agents have been developed, capable of crossing the pulmonary vascular bed and hence achieving left heart opacification following intravenous injection. Combined with major developments in cardiac ultrasound equipment, left heart contrast echocardiography has moved from research into mainstream clinical echocardiography and we stand on the threshold of myocardial contrast echocardiography as a tool for routine clinical use in the assessment of the patient with ischaemic heart disease. Optimal use of these techniques requires detailed understanding of the agents used, mode of administration, and optimum machine settings. Basic knowledge of the physics of ultrasound is now crucial if the potential for contrast echocardiography is to be realised in clinical practice.
▸ CONTRAST ECHOCARDIOGRAPHY: PHYSICAL PRINCIPLES
Blood appears black on conventional two dimensional echocardiography, not because blood produces no echo, but because the ultrasound scattered by red blood cells at conventional imaging frequencies is very weak—several thousand times weaker than myocardium—and so lies below the displayed dynamic range. Contrast ultrasound results principally from the scattering of incident ultrasound at a gas/liquid interface, increasing the strength of returning signal. However, the bubble/ultrasound interaction is complex and its nature has only recently been fully elucidated. Understanding this interaction is key to performing, understanding, and interpreting a contrast echo study.
When insonated, gas bubbles pulsate, with compression at the peak of the ultrasound wave and expansion at the nadir. Electron microscopy studies have eloquently demonstrated the extent of this volume pulsation with bubble radius changing by a factor of 20 or more. In an ultrasound beam with a frequency of 3 MHz, this will result in bubble oscillation three million times per …
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