Echocardiography, the use of diagnostic medical ultrasound to evaluate the heart and proximal great vessels, complements other diagnostic procedures by quantifying chamber dimensions, wall thicknesses, and the dynamic events of the cardiac cycle; it also allows visualization of the anatomy and motion of valves and visualization of congenital abnormalities ranging from a defect in the interventricular septum to a stenotic pulmonary valve. Blood flow velocity is also commonly measured, and turbulent blood flow is identified using Doppler echocardiography. Pressure gradients, blood flow volumes, and several indices of cardiac function can be calculated. Echocardiography can also identify changes in myocardial tissue texture indicative of ischemia and fibrosis and delineate masses, valvular vegetations, pericardial effusion, and many other features previously verifiable only with cardiac catheterization or at necropsy.
There are four main types of echocardiography: three-dimensional, two-dimensional, M-mode (one-dimensional), and Doppler. Two-dimensional echocardiography provides a wedge-shaped, two-dimensional image of the heart in real-time. Several standard long-axis and short-axis views obtained from standard imaging windows on the thorax have been developed for dogs, cats, horses, and cows. M-mode echocardiography is produced by a one-dimensional beam of ultrasound that penetrates the heart, providing an “ice-pick view” over time. The tissue interfaces that are encountered by the beam are then plotted on a screen. This mode of evaluation has been typically used to measure chamber dimensions, wall thickness, valve motion, and great vessel dimensions but, as frame rate of two-dimensional echocardiography has improved, this mode has lost some of its usefulness. Three-dimensional echocardiography is the newest modality and is still in its infancy. Doppler echocardiography uses the principle of changing frequency of the ultrasonic beam after it contacts a moving structure (eg, RBCs, cardiac wall) to measure velocity. Doppler echocardiography is further divided into spectral (pulsed and continuous wave), color flow, and tissue Doppler echocardiography. Color flow Doppler echocardiography is a form of pulsed Doppler echocardiography prone to aliasing when high velocity flow is encountered, allowing high velocity (and therefore turbulent) flow in the heart and great vessels to be detected. Continuous wave Doppler is used to quantitate high velocity flow and thus used to calculate pressure gradients, most commonly across the regions of valves, using the modified Bernoulli equation (4 × velocity2). Tissue Doppler imaging is used to measure the lower velocity motion of cardiac structures, most commonly ventricular walls, in an attempt to quantitate regional myocardial function. Variations include measuring strain and strain rate.
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