Doppler ultrasound

a group of ultrasound techniques exploiting the Doppler effect to measure or image blood flow velocity. The major techniques are pulsed Doppler ultrasound, continuous wave (CW) Doppler, colour Doppler sonography, and power Doppler sonography.

Short historical review
The Doppler effect was first described by the Austrian mathematician and physicist, Johann Christian Doppler (1803-1853). In his famous article of 1842, he describes how the phenomenon affects the observed light waves from stars having a movement relative to the observer. If the star is moving towards the observer, the frequencies of the observed light waves are slightly higher than the emitted frequencies, and vice versa. The change in frequency can be used to estimate the speed of the star relative to the observer. This Doppler effect is, however, applicable to any kind of wave, whether electromagnetic and mechanical, and thus also to ultrasound.

The first use of ultrasound for medical diagnosis, came in the 1940s with attempts at ultrasonographic cross-sectional imaging (see ultrasonography). In 1954, H.P. Kalmus described how flow velocity in fluids could be determined by measuring the phase difference between an upstream and downstream ultrasonic wave. His "upstream - downstream" method was further developed by D.L. Franklin et al. who in 1959 produced a flowmeter that could be mounted directly on blood vessels. Short ultrasound pulses were transmitted through the vessel lumen between two piezoelectric crystals, and the difference in transit time between upstream and downstream ultrasound pulses was used for measurement of instantaneous flow velocity.

The fact that the Doppler frequency shift could be used for the detection of blood velocity patterns, was shown by S. Satomura in 1959. By means of transcutaneously applied ultrasound he could visualize the patterns of flow velocity in superficial peripheral arteries.

In 1964, D.W. Baker and H.F. Stegall presented the first Doppler instrument intended for the transcutaneous measurement of blood flow velocity in man. They used the continuous wave Doppler principle with two piezoelectric crystals, one continuously transmitting ultrasound, and the other continuously receiving the echoes. The change in frequency from emission to reception of the echoes was used for the estimation of blood flow velocity. Approximately five years later, pulsed Doppler instruments were introduced, allowing blood flow velocity measurements at predetermined depths.

In 1974, F.E. Barber et al. described the first combined use of B-mode ultrasonography and pulsed Doppler velocity detection, introducing the term duplex scanning. By means of a multi-gated system, the Doppler signals were used for production of a two-dimensional image, bright spots on the monitor indicating presence of blood flow velocity above a certain threshold. The Doppler image was superimposed on the B-mode image, thus producing a "duplex image". Flow velocity was not measured.

In the late 1970s, combined real-time B-mode imaging and pulsed Doppler blood flow velocity measurement became available, and this is the method which today is referred to as duplex scanning. Colour Doppler sonography with its colour coding of blood flow velocities, was introduced in the 1980s, followed by power Doppler sonography in the early 1990s. The combination of duplex scanning and colour Doppler sonography is sometimes called "triplex scanning". Recently, Doppler techniques have also been used to measure tissue velocity. Tissue Doppler imaging of the myocardium can estimate myocardial strain and strain rate.

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