Textbook of Color Doppler Imaging Satish K Bhargava
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The Story of Doppler1

Doppler color flow mapping, a new and exciting advancement in cardiac ultrasound is a method whereby blood flow is imaged and displayed on 2D image.
The first description of the physical principle used in color flow devices is attributed to Johann Christian Doppler, an Austrian Mathematician and Scientist who lived in the first half of the nineteenth century. Doppler's first descriptions concerned changes in wavelength of light as applied to astronomical events. In 1842, he presented a paper entitled “on the colored light of double stars and some other heavenly bodies” in which he postulated that certain properties of light emitted from stars depend upon the relative motion of the observer and the wave source. He suggested that the colored appearance of certain stars was caused by their motion relative to the earth, the blue ones moving toward earth and the red ones moving away. He drew an analogy of a ship moving to meet or retreat from incoming ocean waves. The ship moving out to sea would meet the waves with more frequency than a ship moving towards the shoreline. Interestingly Doppler never extrapolated his postulates to sound waves.
There was immediate criticism of Doppler. Just like today, critics abounded. Among them was Buys Ballot who in 1844 stated he simply did not believe Doppler. There is rather amusing account of the difficulties Buys Ballot encountered in attempting to disclaim the Doppler Effect. In 1845, he borrowed a steam locomotive from Dutch Government and arranged for a trumpet player to ride a fiat car as it approached and then left a station. Two other trumpet players were positioned on the ground one to either side, where an observer with the ability to appreciate perfect pitch listed to all the trumpets playing the same note. Following a hail- storm and other delays, the experiment finally took place. The note was higher in pitch as it departed when compared with trumpets on the ground. Aside from verifying Doppler's observations, this experiment proved that “getting started in Doppler” was difficult to understand even then.
Even with this scientific verification, Buys Ballot and others continued to level strong criticism. Those struggling to understand the Doppler principle will be interested to know that while Doppler's postulate concerning frequency shift from moving objects was ultimately shown to be correct his extrapolation about color shift of light from stars was later proven to be wrong. He incorrectly assumed that all the stars emitted white light. In reality the colors and lines of the various stars are a function of thin surface temperature rather than their direction or velocity of movement.
We are familiar with the Doppler effect in everyday life. For example an observer stationed on a highways overpass easily notices that the pitch of the sound made from the engine of a passing automobile changes from high to low as the car approaches and then passes into the distance.2
The engine is emitting the same sound as it passes beneath, but the observer notices a change in pitch depending upon the speed of automobile and its direction.
Doppler effect is now employed in modern astronomy. It has practical application in radar detection of storm and is used in modern weather forecasting. It can help to form the “radar trap” used by police on modern highways to detect speeding automobiles in developed countries.
The medical applications of Doppler are dependent upon the use of ultrasound and have been in practice for sometime. Doppler systems emit a burst of very high frequency sound termed as ultrasound that is reflected off the moving red blood cells and then returned at a different frequency dependent upon the speed and direction of the moving blood. The result information is displayed as various waveforms on the velocity spectral analysis. The clinical uses of blood flow imaging systems have expanded immensely since the first measurement of flow in the heart that was performed by Satomura in 1956.
Despite its widespread use, Doppler methods and principle are difficult to understand and implement without considerable training and experience.