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Contrast echocardiography encompasses a broad variety of different applications whereby cardiovascular ultrasound is used to detect microbubble contrast agents that produce a strong acoustic signal. Ultrasound contrast agents are currently approved for use in cardiology for the purposes of opacifying the left ventricular chamber in order to aid interpretation of echocardiography, yet their off-label use has been applied for myocardial perfusion imaging in ischaemic heart disease for over a decade. This article provides a broad overview of some of the future uses of contrast ultrasound in patients with cardiovascular disease that are on the horizon, including: (1) evaluating vascular anatomy and plaque neovascularisation; (2) detecting abnormal microvascular function; (3) assessing peripheral vascular disease with stress/rest limb perfusion imaging; (4) molecular imaging of cardiovascular disease; (5) site targeted delivery of therapeutic genes or drugs; and (6) ultrasound facilitated thrombolysis.
The acoustic bubble
Signal enhancement during contrast enhanced ultrasound (CEU) relies on the detection of the acoustic energy produced by encapsulated microbubbles or other acoustically active particles. The basis for signal generation from these contrast agents is their compressibility.w1 w2 Ultrasound contrast agents contain gases or, less commonly, emulsions with a low vapour pressure. Hence, they are several orders of magnitude more compressible than water or tissue and, because they are also smaller than the wavelength of diagnostic ultrasound, undergo volumetric oscillation in the pressure fluctuations of an ultrasound field.w1 The ‘vibration’ of microbubbles produces, among other forms of energy (light, heat), strong backscattered acoustic pressure signals that produce contrast enhancement on ultrasound imaging.
As predicted by the Rayleigh–Plesset principles, the magnitude of bubble oscillation and, hence, signal generation is influenced by the compressibility and density of the gas, the frequency and power of …