Doppler Flashcards
Describe the Doppler shift
Frequency of a wave appears to change if the source and the receiver are moving relative to each other
Doppler equation
fD = (2 x f x v x cos theta)/ c
What is the maximum Doppler angle allowable?
60 degrees
As the velocity error becomes larger with larger Doppler angles
What does pulsed Doppler do?
Assess flow in a small volume, typically a few mm in size
Uses short pulses of ultrasound. Echo signals from the appropriate depth are selected for Doppler processing.
Doppler signal is displayed as a spectral trace and blood velocity can be measured
What is the Doppler PRF?
The frequency at which the Doppler pulses are emitted. This is limited by the depth of the range gate.
This also limits the maximum Doppler shift that can be detected - PRF/2 = Nyquist limit
What is the function of the wall filter?
Movement of vessel walls produces much stronger signals than that of the scatterers we are interested in. The wall filter is set at a level to remove the signals from vessel walls by detecting the lower velocity at which they move, while including the relevant flow within the vessel.
Describe patient exposure in Doppler modes
Power transmission is higher in Doppler modes than in B mode. Additionally, the beam is stationary, so the power is concentrated in the tissues along the beam.
Why does the spectral display look like it does?
The Doppler sample volume contains a large number of red cells, each a scatterer. These RBCs each travel at slightly different velocities. This therefore is displayed as a spectrum of Doppler shift values.
Describe the function of colour Doppler
The ultrasound beam remains in one position for a small number of transmit pulses, then steps along the probe and repeats the process. This allows the beam to move along the colour box, building up a colour image.
The machine calculates and displays:
- Mean Doppler shift - colour
- Doppler variance - measure of the range of Doppler values in the sample volume
- Doppler power - strength of the signal
Then the machine produces a grey-scale image which it overlays with the colour
What assumption does colour Doppler make?
Doppler angle = 0 degrees
Therefore cannot be used to make velocity calculations
What are the drawbacks for colour, compared to PW Doppler?
Less accurate in determining Doppler shift
Less able to detect slow moving blood
Less effective in removing unwanted Doppler signals
How does power mode colour Doppler work?
Rather than displaying mean Doppler shift, it displays Doppler signal strength
What are the benefits of power Doppler over colour Doppler?
It is not influenced by velocity or direction, only where there is moving blood
Frequency aliasing does not affect the image, meaning PRF can be optimised to detect flow without concerns about aliasing
Also displays flow, even when flow direction is perpendicular to the beam (due to intrinsic spectral broadening)
