Topic 1 Doppler principles Flashcards
Define spectral Doppler
The spectral shift frequencies are analysed into a graph of frequency shift against time, with the brightness of the pixels on the screen representing the amplitude of the returning frequency shifts.
What is the doppler equation?
f= (2 x transmitting frequency x velocity of target)/speed of sound x cosine of the angle of the beam to target
What happens to the returning frequency if blood is moving towards the transducer?
The returning frequency will be higher
What happens to the returning frequency if blood is moving away from the transducer?
The returning frequency will be lower
How can you enhance colour flow?
By heel toeing the probe, you can enhance colour flow by reducing the insonation angle and minimising aliasing.
When is continuous wave Doppler used?
Blood pressure
Ankle brachial index
List some advantages of continuous wave Doppler
- Does not suffer from aliasing as the receiving and transmitting crystals are in constant use
- greatly advantageous in echo where high velocity jets need to be measured
- measuring blood pressure in a convenient way
- can assess VVs bedside to indicate presence of competence at the SPJ or SFJ
- bedside assessment of patency of arteries or grafts
List some limitations of continuous wave Doppler
- Cannot discriminate depth of returning signal
- cannot estimate angle of insonation of the vessel
- only frequency information can be obtained, not velocity
- multiple vessels in one beam path will be interrogated simultaneously
How does pulsed Doppler allow for depth and size of sampling to be controlled?
By controlling the timing that the signal is received, as speed of sound is constant.
What is spectral aliasing?
The most common spectral artefact
Why does spectral aliasing occur?
If one or more frequencies received by the transducer is greater than half the pulse repetition frequency, then the system will represent the frequency at a lower rate. This is because the frequency will not be sampled twice within each cycle, so the estimated frequency will be lower than the true frequency.
How do you avoid aliasing?
Sample the frequency at less than PRF/2
How does colour mapping occur?
A colour map is created by sending multiple beams along each beam path within the colour box. Each returning beam is compared to the previous to extract the average changes in phase and frequency at each point along the beam line. Autocorrelation processes this information and estimate an average speed, direction and variance.
What is blooming artefact?
Caused by over-gaining and exaggerated by a low priority write setting and high colour sensitivity setting.
Describe directional ambiguity?
When there is simultaneous forward and reverse flow in a vessel. May be artefactual or may be normal (eg. carotid bulb) or in an aneurysm/dissection
