In color Doppler imaging, the packet size refers to the number of what?

• A. Pulses per second
• B. Sample volumes per scan line
• C. Scan lines per unit area
• D. Pulse-listen cycles per acoustic scan line

Answer: (D)

Packet size in color Doppler imaging is the number of pulse-listen cycles used to form one Doppler ensemble along a single scan line. Each cycle is a transmit followed by a listening interval to gather echoes. Using more cycles in the ensemble improves the Doppler velocity estimate by increasing signal averaging and reducing noise, but it also slows down the frame rate because more transmissions are required per line. This is distinct from pulses per second (PRF), which is about how often you transmit in time, from how many depth samples you collect along a line, or from how many scan lines you acquire per area. So, the packet size equals the number of pulse-listen cycles per acoustic scan line.