Doppler Imaging

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

Print this page

This article have been viewed 1130 times

8 Doppler Imaging

The Doppler shift is the change in frequency of sound when the sound wave strikes a moving object. This means the frequency of the transmitted and reflected sound waves is not the same. Doppler shifts in clinical imaging are in the audible range (±10 KHz). Red blood cells are the primary reflectors that produce Doppler shifts. Ultrasound machines can color-encode the mean velocity (color Doppler), variance within the sample volume (variance Doppler), and power spectrum of the frequency shift (power Doppler).¹

The optimal spectral Doppler angle is 30 to 60 degrees. Doppler angles greater than 60 degrees result in small Doppler shifts. Doppler angles less than 30 degrees result in loss of signal due to refraction.

Aliasing (incorrect or ambiguous estimation of the velocity) occurs when the velocity scale is set too small relative to the actual velocities. Wrap-around transition between positive and negative velocity on spectral Doppler tracings indicates aliasing; therefore, the peak velocities are off-scale and not accurately estimated. This occurs because the pulse repetition frequency is insufficiently low relative to the frequency of the Doppler signal (a consequence of the sampling or Nyquist theorem).

Color Doppler is traditionally shown with the Nyquist velocity limits. Color aliasing is displayed as reversed flow within laminar flow areas, with no intervening black stripe between them. With true flow reversal, the transition has an intervening black stripe, indicating no flow estimation. This narrow colorless area occurs because of the absence of a Doppler shift where flow is perpendicular to the angle of insonation.

Clinical Pearls

• Blood has a low ultrasound attenuation coefficient. Red blood cells are the primary reflectors within blood.

• In power Doppler, the gain threshold can be adjusted to the level at which there is no observed signal in bone.²

• In low-flow states (e.g., heart failure or atrial arrhythmias), aggregates of red blood cells can cause spontaneous contrast within blood vessels.

FIGURE 8-1 An example of color Doppler imaging during axillary block. A short-axis view of the neurovascular bundle is displayed.

FIGURE 8-2 Long-axis view of the axillary artery and its profunda branch in conventional B-mode imaging (A) and with power Doppler (B).

1 Bude RO, Rubin JM. Power Doppler sonography. Radiology. 1996;200:21–23.

2 Rubin JM. Musculoskeletal power Doppler. Eur Radiol. 1999;9(Suppl 3):S403–S406.

Related

Atlas of Ultrasound-Guided Regional Anesthesia 2e