Colour Flow Doppler
Colour flow Doppler is an exciting mode of ultrasound that permits detection of fast moving elements. This tutorial will focus on cardiac and vascular ultrasound–but these principles remain the same independent of the structure being examined.
The Doppler shift
When blood cells move towards the transducer—sound is reflected with higher frequency than transmitted frequency; the probe registers this as a red shift or towards the transducer.
When blood cells are moving away from the transducer, the Doppler shift is negative, the reflected frequency is lower than the transmitted frequency; the machine registers this as a blue shift or away from the transducer.
A simple way to remember this is BART or blue away, red towards. Be aware that these colours can be reversed manually on many ultrasound machines.
In this apical 4-chamber the colour box is over the tricuspid valve. You can see clear diastolic flow from RA into RV. You can also see a blue regurgitant jet away from the transducer (tricuspid regurgitation). This jet is clearly more than just blue–keep reading!
Probe angle and BART
Keep in mind that the direction the probe is oriented can influence the colour–this is especially important in vascular ultrasound! You can make the colour change with a simple redirection of the probe.
Probe angle and blood velocity
Also be aware that the alignment of the Doppler beam and blood flow matters. The best case scenario is that the blood flow is parallel to the Doppler beam (measured velocity is 100% of true velocity). This means that the probe may pickup less than 100% of true velocity with poor alignment.
A cosine of 90
Be aware that the Doppler shift equation includes a cosine angle to account for angle of flow with reference to the Doppler beam. At a cosine of 90, no Doppler shift is read and this is displayed as black. In this clip–you can see parts of the abdominal aorta appear to have no flow. In actuality there is flow here, but the cosine of the Doppler beam and flow is 90.
If the velocity on the color scale is exceeded, then a phenomenon called “aliasing” occurs, in which you sett a mosaic pattern of colors. In reality, this is the highest velocities of both towards and away from the probe–and directionality is lost. Nonetheless, we make use of this feature to detect both regurgitation and stenosis as blood flow speeds up in both conditions!
Why colour Doppler in the PLAX view?
A common question is why is the color box applied to the aortic and mitral valve in the parasternal long axis view if the cosine appears to be 90? Well the truth is –is that this angle is seldom truly 90 and these views much qualitative information about the nature and size of the jet. Also note that the bigger the color box, the more information that requires computing and this will degrade the 2D image.
Colour box placement
Placement of the color box is especially important–whether the lesion is regurgitant or stenotic. The key is to be able to capture the neck of the jet and the distal jet expansion. For regurgitant lesions (see adjacent clip), this means placing the box in front of the valve and over the receiving chamber (where regurg jet spews into). In stenosis, you will want to capture the valve and downstream.
Setting the Nyquist limit
The color scale has a velocity displayed in both towards and away-from directions. For cardiac, typically it is between 60 and 70 cm/s. This velocity is the “Nyquist limit.” If this velocity is too low, then color Doppler is too sensitive and increases the risk of noise; if too high, it will become less sensitive and this can dampen the signal. This also means that an inappropriately set Nyquist limit can really mislead novices in both directions–to both minimize and maximize the appearance of a valve lesion.
Setting the color gain
While many think that gain only exists on 2D ultrasound, it also exists on colour Doppler. If the gain is too high then you risk the phenomenon of “colour confetti” where the colour spills beyond the normal anatomic boundaries.
Setting the color gain
These same principles of Nyquist limit and colour gain apply to vascular ultrasound.