The packet size in color Doppler refers to the number of:

• A. Pulses per frame
• B. Pulses per acoustic scan lines
• C. Pulses per second
• D. Sample volumes per scan line

Answer: (B)

Packet size in color Doppler is the number of transmitted pulses used to form the Doppler ensemble along each scan line. This ensemble length determines how many samples are averaged to estimate velocity for every color sample along a line. Using more pulses per line improves the reliability and noise performance of the velocity estimate, but it lowers the frame rate because more transmissions are required per line. Using fewer pulses per line increases the frame rate but can reduce accuracy and sensitivity of the Doppler information. It isn’t about pulses per second (that’s the PRF), nor about how many sample volumes are along the line, nor about pulses per frame across the whole image.