For the flowprobe a vessel is positioned between transducers which generate wide beams of ultrasound to fully illuminate the vessel. The ultrasound beams alternately intersect the flowing blood in upstream and downstream directions. The flowmeter derives an accurate measure of the changes in transit time influenced by the motion of the blood.

See also Bi-directional Flow.

Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.

The Doppler angle (theta) is the angle of incidence between the ultrasound beam and the estimated flow direction (parallel to the long axis of the vessel). If the beam is parallel to the flowing blood, the Doppler angle is zero, and the determination of flow is most accurate.

See also Beam Vessel Angle, Doppler Effect and Doppler Ultrasound.

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 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.