Physical and Technical Principles of Doppler Sonography

Abstract

The frequency shift postulated in 1842 by Christian Doppler as a consequence of a relative movement between wave transmitters and receivers was only utilized for the first time to capture intra-corporeal movements in the middle of the 20th century. During these initial experiments, Satomura first used an ultrasound beam focused on the heart to measure the contractile movements of the myocardium and later demonstrated that this method can also be used to detect blood flow.

When exposed to ultrasound, the frequency reflected by the corpuscular blood components changes as the reflectors approach the probe or move away from it. Since this frequency shift is proportionate to the flow velocity, by comparing the transmission and reception frequency, the velocity can be determined and displayed in the form of a spectral curve.

In CW (Continuous Wade) mode, all flows within the beam are captured, while PW (Pulsed Wave) mode allows the selective detection of flows along a narrowly circumscribed space known as the sample volume. To be able to visualize this sample volume, PW Dopplers are generally combined with imaging ultrasound equipment to create “duplex systems” and the measuring window displayed in the form of a cursor in the B image.

The color-coded Doppler creates a number of tiny measuring locations and selectively displays the documented flows in the form of corresponding pixels on a screen. The color (usually red and blue) allows conclusions to be drawn about the direction of flow, while the flow velocity is expressed by the brightness of the corresponding pixels.

Echo signals that do not have their frequency shifted are evaluated as reflections of static structures and are displayed in the form of gray values. As a result, the color-coded (Color Flow) mode corresponds to a B-image with the location-correct display of flow information.

The Doppler ultrasound capture of flows is to a large degree dependent on the angle between the ultrasound beam and the flow axis, can sometimes fail when capturing very high velocities and demonstrates only limited spatial resolution. Additional alternative methods have also recently become available that are based on subtraction methods and which go some way towards overcoming the described shortcomings.