To create a black and white image, ultrasound systems interrogate each scan line once for tissue information. However, to achieve a reliable picture of complex blood flow movement over time, color flow imaging systems must interrogate each color scan line several times for flow information.

Packet size is the term for the number of times the ultrasound system consecutively interrogates each color scan line for blood flow data. A packet size of eight, for example, means that each color scan line is interrogated eight times.

Increasing the packet size (sensitivity), as demonstrated in image A, increases the color velocity accuracy but decreases the frame rate. A decrease in packet size (B) results in decreased color velocity quality and an increased frame rate. To some degree, you can compensate for the trade-off between packet size and frame rate by decreasing the color.