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Factors Affecting Color Flow Images: Clutter (Wall) Filters

Along each color scan line the color algorithm receives frequency shifts from moving tissue as well as from blood. Assigning color to moving anatomic structures helps ultrasound distinguish moving structures from moving blood. This coloring on anatomic structures is called “ghosting” or “flashing". To minimize flashing ultrasound systems employ clutter filters (also called wall filters). The color PRF provides the basis for filters, which attenuate blood, flow signals below a certain velocity-related flow detection threshold. Attenuating these low velocity signals helps to eliminate low velocity wall motion. Elimination of low velocity flow also occurs with these filters. Most color flow mapping systems allow the choice of a clutter filter that balances flashing and low velocity blood flow. Factors Affecting Color Flow Images: Pulse Repetition Frequency (PRF) The depth of the PW Doppler sample volume determines the PRF (and consequently the Nyquist limit). Data points along the scan line display different frequency shifts resulting in the depth determining the PRF. In PW Doppler, the sample volume depth is typically shallower than the bottom of the sector image. Therefore, in PW Doppler the PRF is generally higher than in color flow imaging. On many color flow-imaging systems, you can change the PRF by adjusting the color scale. Changing the scale affects the system color velocity sensitivity. Increasing the PRF by increasing the scale setting allows display of higher velocities without aliasing. However, increasing the PRF also changes the clutter filter threshold, filtering out lower velocity signals previously displayed. Decreasing the PRF via the color scale filters the low velocity signals displayed. é PRF é Maximum measurable mean velocity ê Sensitivity to low velocity blood flow             ê PRF ê Maximum measurable mean velocity é Sensitivity to low velocity blood flow