Doc #: 60-00118 Rev. A
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spectrogram. If the envelope is not following the edge, on the patient report manually locate the
maximum and minimum velocities of the spectrogram edges.
The velocity measurements are derived from computational analysis of the spectrogram.
Because the spectrogram waveform may depict blood flow that is complex, the numerical results
remain subject to clinical interpretation.
The velocity measurements are affected by the following conditions:
•
The Doppler Angle
•
Envelope Edge Uncertainty
•
Beat to Beat Variation
•
Velocity Scale Variables
•
Filter Setting
•
Acoustic Velocity
Doppler Angle
– the angle between the direction of movement of reflectors, such as red blood
cells, and the effective direction of the ultrasonic beam, which is normal to the wavefront.
Envelope Edge Uncertainty
– the velocity envelope is designed to follow the instantaneous
maximum velocity of the flow signal waveform in the spectrogram. For complex blood flows, the
spectral edge of the waveform is subject to interpretation.
Beat to Beat Variation
– the maximum, minimum and mean velocities are calculated from a
cardiac cycle and are averaged over time.
Velocity Scale Variables
– the maximum measurable envelope velocity is affected by the Pulse
Repetition Frequency (PRF) and baseline settings. The upper limit is indicated by the vertical
scale of the spectrogram. The range is reduced at lower PRF settings and when the baseline is
set at a higher position on the spectrogram. The depth set by the user determines the PRF.
Filter Setting
– the minimum measurable envelope velocity is affected by the Filter (clutter filter
cutoff frequency). The higher the filter setting, the higher the minimum measurable velocity.
Calculations
– the calculations are based on a nominal acoustic velocity of 1540 m/s through
tissues.
