The dwell time (also called ensemble length or packet size) is the transmitting duration of ultrasonic waves focused at one place or the Doppler samples for each line of sight. The reduction of the dwell time is of use if threshold pressures are exceeded.
The sensitivity to slow flow and accuracy of Doppler measurement increase with longer dwell times. With increased dwell times, the frame rates decrease and the capability of Color Doppler images to depict fast changes in hemodynamics is limited.

The duty factor is the product of the pulse duration and the pulse repetition frequency (the lapse of time the transducer is actively transmitting sound).
Most transducers are acting as the receiver 99 % of the time with a duty factor of 1.

See also Dwell Time.

Same as Dwell Time or Packet Size.

The frame rate is the number of images displayed per second. One frame represents one complete sweep of the ultrasound beam and contains many pulse-listen cycles. The frame rates decrease with an increased dwell time.

The packet size is the number of sound bursts transmitted per color sector line. The accuracy of the estimated Doppler velocity is influenced by the packet size. A large packet size increases the accuracy of the mean flow velocity by decreasing the frame rate, the line density, or the sector angle.
Dwell time, ensemble length or packet size are synonymous.