During the clinical application of echocardiography, several imaging modalities are employed. M-mode, or motion mode, is used clinically for the assessment of valve motion, chamber sizes, aortic root size, wall thickness, and ventricular function (Figure 1). M-mode pulses a narrow ultrasound beam in a single plane through the heart, producing images of the tissue in that plane with a very high temporal and spatial resolution. 2D imaging produces an arc of ultrasound beams from a single transducer head to create echocardiography images of a cross-sectional view of the heart. The majority of cardiac images are observed in this modality. Two of the more important capabilities of 2D echocardiography are the ability to calculate the ejection fraction of the left ventricle and the stroke volume, which can be used to calculate the cardiac output when the heart rate is known.

Doppler ultrasound uses the Doppler principle to determine the velocity of blood within the heart. A frequency shift occurs when the ultrasound waves interact with objects in motion, such as red blood cells within blood (Figure 2). This detectable frequency shift, which is also dependent upon angle of incidence of the ultrasound beams with the blood flow, is then used to determine the velocity of the blood flow. Using a modified Bernoulli equation, the velocity is used to estimate chamber pressures and pressure gradients. Continuous wave Doppler is capable of detecting the Doppler shift of very high velocities, but does not localize where along the transmission path the shift occurs. Pulsed wave Doppler uses bursts of ultrasound alternating with pauses to detect Doppler shifts. This technique allows for the localization of the Doppler shift, and hence velocity or pressure measurements (Figure 3). Finally, color Doppler flow mapping plots the Doppler shifts on top of the 2D echocardiography images to show the direction of flow with respect to the transducer head (Figure 4). Flow toward the transducer head is always shown in red, while flow away from the transducer head is shown in blue. A third color, usually yellow or green, shows areas of accelerated or turbulent flow, which is useful for determining valvular regurgitation, visualization of intracardiac shunting, or assessment of arterial connections.