The principle of ultrasound: Difference between revisions

Color Flow Doppler uses pulsed Doppler technique.  The velocity data is encoded in color, and it reports mean velocities.  Since it is a pulsed Doppler technique, it is subject to range resolution and aliasing.  Color data is extremely complex and consumes significant computational resources, thus several assumptions are made to speed up this process.  Returned echo frequencies are compared to a predetermined threshold to decide whether this is a 2D image vs Doppler shift.  Once the computer decides that the frequency is low enough to be a Doppler shift data, repetitive sampling determines the mean velocity and variance.  Then a color is assigned using a color look-up table rather than doing a discrete Fourier transform for each data point.  Velocities that move toward the transducer are encoded in red, velocities that move away are encoded in blue.  One must remember that the color jets on echo are not equal to the regurgitant flow for a number of reasons.  The regurgitant flow is a three dimensional structure with jet momentum being the primary determinant of jet size.  This parameter is effected by the jet velocity as well as flow rate.  Blood pressure will affect the velocity and thus the regurgitant flow.  Chamber constraints will have an effect on the appearance of the color jet, especially eccentric jets.  Lastly, the settings of the echo machine will have an effect on how the color flow jet appears on the screen.  

Feigenbaum's Echocardiography, 7th Edition

Sidney K. Edelman, PhD.  Lecture notes from 2005 ASCeXAM Review course