Sound waves, generated by the ultrasound transducer, are mostly longitudinal waves with alternating pressure deviations causing local compression and rarefaction which propagate through a medium ( blood, tissues etc ). For simplification these are described as sinusoidal waves.
The Doppler effect in ultrasound is used to detect and evaluate blood flow and velocities. Echoes returning from a moving structure (Fr) like a red blood cell will have a different frequency as compared to the original frequency ( Ft ) of the echo signal. This difference is called the Doppler shift which can be used to calculate direction and velocity of blood flow. This Velocity is calculated and displayed on the monitor. The equation used to calculate is given below.
This is a color map of Doppler shifts. Red color indicates blood flow towards the ultrasound probe while blue means blood away from the probe. Commonly used for diagnosis of DVT. Image1 below shows a color Doppler application for a venous doppler study.
Continuous wave Doppler
Ultrasound probe has 2 piezoelectric elements. One for continuous transmission of signal and the other to continuously receive the signal. Commonly used for high velocities like aorta and aortic valves. Note that this will measure a wide range of velocities along the transmission line but continuous wave Doppler is position insensitive. Image 2 shows a 5 chamber apical view in which continuous wave Doppler is used across the aortic valve to assess for aortic stenosis.
Pulse wave Doppler
The same piezoelectric element is used to send and receive the signal. This Doppler application is used to assess for velocities at a particular location. Commonly used to assess for Left ventricular outflow tract( LVOT) velocity and velocity time integral ( VTI ). Image 3 shows another 5 chamber apical view in which pulse wave Doppler is used at the LVOT. Please note the difference in the Doppler envelope for continuous wave and pulse wave Doppler.
This Doppler application measures flow velocities of tissues. Tissues have higher amplitudes but lower velocities. This is very commonly used to assess diastolic function of the heart. Image 4 is a 4 chamber apical view where tissue Doppler is being used to assess diastolic function.
For color doppler, the flow colors are based on the velocity mode map and can vary. In the image 1 the color box is pointed to the left. This represents where the probe is. If the blue color was on top of the velocity mode map then this picture would actually show flow towards the probe.