Doppler

Doppler ultrasonography is one of the 3 pillars of clinical ultrasonography, in addition to M-mode and B-mode imaging. The underlying basis of Doppler imaging is the Doppler effect—the change in frequency of transmitted ultrasound due to relative motion between the ultrasound transducer and the location/object of imaging interest. This highlights one point of differentiation for Doppler imaging. Whereas for M-mode and B-mode, optimal image generation requires a 90° angle of insonation, Doppler imaging is optimal when angle of insonation is parallel to the direction of movement (so either 0° or 180°). The change in frequency is proportional to the velocity of object motion. In clinical imaging, there are four commonly used types of Doppler ultrasonography—Pulsed Wave Doppler, Color Flow Doppler, Continuous Wave Doppler, and Tissue Doppler Imaging. These modes facilitate the use of Doppler ultrasonography for spatial mapping of blood flow in cardiac and vascular chambers; estimation of flow velocities; measurement of pressure gradients across valves; and measurement of the velocity of myocardial contraction (useful in the evaluation of ventricular systolic and diastolic function). These hemodynamic and flow variables are derived using the modified Bernoulli equation. Doppler derived images and data can be displayed, superimposed on a B-mode image of the heart or as a Doppler spectral graph of flow velocities over time. For all these applications, one important consideration is the Nyquist limit (the maximum velocity that can be accurately measured) and is equal to half the pulse repetition frequency. If the frequency change exceeds the Nyquist limit, then signal aliasing occurs and interferes with accurate estimation of peak velocities. Continuous Wave Doppler is not subject to the Nyquist limit and is therefore, one of several options for mitigating aliasing.