Two Dimensional Color Doppler Ultrasound

Two-dimensional echocardiography provides real-time dynamic ultrasound images that are used to evaluate cardiac anatomy and function. This noninvasive technique is widely used to evaluate cardiac motion and function, but it was only with the introduction of pulsed and continuous wave Doppler acquisition systems that the direction, character, and velocity of blood flow could be appreciated. The most dramatic addition is real-time two-dimensional Doppler color flow mapping, which allows for real-time display of multiple sampling sites where flow velocity is expressed as a color-coded map overlaid on the anatomical cardiac images. Imaging intracardiac blood flow in two dimensions simultaneously with morphologic display in real time is perhaps one of the most important advances in medical Doppler technology.

The Doppler principle states that the frequency of transmitted sound is altered when the source is moving in relation to the observer. This alteration in frequency, known as Doppler shift, is used to define the direction and velocity of blood flow through the heart and great vessels. The relationship between blood flow Vand the Doppler shift is expressed mathematically by the formula c Fr — Ft

where the Doppler shift (Fr — Ft) is the difference between the frequency of the transmitted ultrasound wave (Ft) and the frequency of the ultrasound wave that returns to the transducer (Fr), c is the speed of sound in tissue, and 0 is the angle between the direction of blood flow and the propagation direction of the ultrasound beam.

Measurement of flow velocity from Doppler ultrasound has been used to evaluate the presence and severity of various valvular diseases. In addition to the velocity itself, it can be used to estimate the pressure gradient across a valve. This is done by applying a simplified form of the Bernoulli principle. AP = 4 V2, where AP is the pressure gradient and V the peak velocity. All current echocardiography equipment has software that allows the estimation of peak velocity and mean gradient across a valve.

The dynamic display of Doppler shift information is usually obtained using color-coded overlays directly on the standard echocardiographic images (Fig. 9). This information is displayed on two-dimensional or M-mode images with direction relative to the transducer coded by color (conventionally, red = toward, blue = away), and velocity information coded by shade within each color (lighter = faster, darker = slower).

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