1.8 Doppler Echocardiography
Doppler Echocardiography is a method by which one is able to identify the direction and velocity of blood flow. Doppler can be used for various purposes in echocardiography. It should be an integral part of every examination. To correctly interpret Doppler information, one must understand how the technique works and its limitations.
The principles of Doppler as we use it in echocardiography were discovered by Christian Doppler, an astronomer who lived in the mid 19th century in Salzburg, (Austria). Christian Doppler was able to show that the observed frequency of a wave depends on the relative speed of the source and the observer. This principle enabled him to explain the color of binary stars.
The principles of Doppler have many uses in daily life. Just think of the sound of an ambulance or a motorcycle as it speeds towards you and then away from you. You will notice that the sound of the vehicle has a higher frequency (pitch) as it comes towards you and a lower frequency as it moves away.
This principle can be applied to ultrasound waves to determine the velocity and direction of moving blood. Just the way stars change their color depending on how quickly they travel, we can determine the velocity of blood cells by measuring the magnitude of the frequency shift between the send and the receive signal. Furthermore, we can determine whether blood is flowing towards the transducer or away from it, depending on whether the phase shift is positive or negative.
One can also apply the Doppler principle to tissue (such as the myocardium). It is a technique that provides crucial functional information. Tissue Doppler will be discussed in greater detail in Chapter 22 (Other Techniques).
Information obtained by Tissue Doppler Imaging
- Tissue Velocity
- Strain rate
Tissue Doppler is also angle dependent.
As the Doppler formula shows, the calculated velocity also depends on the angle of insonation (cos α). The angle of insonation greatly influences measurements. The more perpendicular the ultrasound beam is aligned to the direction of blood flow, the greater is the measurement error.
This is a crucial point to be considered when using Doppler. Care must be taken to align the ultrasound beam as parallel to blood flow as possible.
Doppler information can be obtained either with "spectral Doppler" or with color Doppler. Both provide information about blood flow velocity and its direction. However, they employ different display formats, yield different functional information, and can be applied in different situations. Nevertheless, both are an integral part of every echo exam.
The Doppler formula permits us to calculate velocities (i.e. blood) - based on the Doppler shift between the send and receive signal.
The measured velocity greatly depends on the angle between blood flow and the ultrasound beam. Always try to be as parallel as possible.
|PW Doppler||Low velocity (< 1.5 m/s)||site specific|
|CW Doppler||High velocity (> 1.5m/s)||site unspecific|
|Tissue Doppler||Lower velocity, higher amplitude|