14 Chapter 3: System Overview
Velocity Color Doppler (VCD)
In Velocity Color Doppler, a real-time, two-dimensional cross-section of blood flow is displayed. The 2D
cross-section may be presented as a rectangle, parallelogram, trapezoid, sector, or a full circle, depending on the
particular transducer used.
The 2D cross-section is presented as a full color display, with various colors being used to represent the velocity,
both positive and negative, of the blood flow echoes. Often, to provide spatial orientation, the full color blood flow
cross-section is overlaid on top of the gray scale cross-section of soft tissue structure (2D echo). For each pixel in
the overlay, the decision of whether to display VCD, gray scale (echo) information or a blended combination is
based on the relative strength of echoes from the soft-tissue structures and from the red blood cells.
A high pass filter (wall filter) is used to remove the signals from stationary or slowly moving structures. Tissue
motion is discriminated from blood flow by assuming that blood is moving faster than the surrounding tissue,
although additional parameters may also be used to enhance the discrimination. The remaining signal after wall
filtering may be averaged over time (persistence) to present a steady state image of blood flow distribution.
Variance information may also be displayed to provide information when large variance is observed in the velocity
information.
Magnitude
Estimation
This module combines real and imaginary components to estimate magnitude spectrum.
The magnitude of Doppler spectrum is calculated using “cordic approximation”, which is
done by estimating the complex magnitude by successive approximation. The vectors are
reflected into the first quadrant by taking the absolute value, then rotating them
sequentially by halved degrees.
Temporal
Averaging
Spectral lines are produced by the FFT module at a constant rate. This module averages the
appropriate number of spectral lines to produce output lines at the desired display scroll
rate.
Compression The compression module maps the input spectral magnitude values to display output
values.
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