Topic 1 - Doppler principles Flashcards
What does the Doppler equation measure?
a change in the reflected sound frequency generated by motion of the source or the detector
What is the Doppler principle?
the principal which explains how frequencies change as they reflect from a moving object.
What is B mode ultrasound?
‘Brightness mode. This is the grey-scale image where the brightness of the pixel represents the amplitude of the reflected beam from the tissue.
What is Spectral Doppler ultrasound?
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 colour Doppler?
Using autocorrelation, the averaged Doppler shift frequencies are displayed in real time on the B-mode image
What are the advantages of continuous wave Doppler?
does not suffer from aliasing because the receiving and transmitting crystals are in constant use
• great advantage in echocardiography where high velocity jets need to be measured.
What are the disadvantages of continuous wave Doppler?
- unable to discriminate the depth of the returning signal or estimate the angle of insonation to the vessel being examined.
- Thus only frequency information can be obtained (not velocity) and multiple vessels in the one beam path will all be interrogated simultaneously.
How does PW Doppler work?
- uses short bursts of emitted ultrasound to sample the blood moving past the beam
- pulsing of the beam leaves regular intervals when the elements of the transducer are not transmitting ultrasound.
- While the crystals are not transmitting, they are able to receive the returning frequencies from the blood vessel
- allows the depth of the sampling (gate) and the size of the sample (gate size) to be controlled
When will PW Doppler systems aliase?
• Despite its advantages, pulsed Doppler systems will produce aliasing if the frequencies being sampled are too high for the pulse rate (pulse repetition frequency) to sample properly.
How does pulsed wave spectral Doppler work?
- The spectral information derived from the returning pulsed Doppler system is recorded in real time (time domain)
- and then analysed in small segments to determine the frequencies that are present and what amplitude or strength each frequency has (frequency domain).
- Each sample analysed is then represented as a single vertical line on the spectral display which represents the range of frequencies present and is brightness modulated to show the amplitude of the analysed frequencies.
- This sampling and analysis process is repeated on consecutive samples during the recording to create a continuous display of the returning frequencies for as long as the spectral Doppler is activated.
The direction of colour flow is determined by which 3 variables?
o Direction of the vessel orientation on the screen
o Choice of colour map
o Orientation of the transducer and colour box
Describe spectral aliasing
- if one or more frequencies received by the transducer is greater than half the pulse repetition frequency (PRF) then the system represents the frequency (velocity) at a much lower value.
- This is because the frequency will not be sampled twice within each cycle and the estimated frequency will be less than the true frequency.
- Therefore the frequency being sampled must be less than PRF/2
Describe colour aliasing
- Even though colour represents an average velocity change, it is assigned a progressively changing hue as frequency (velocity) increases.
- Like pulsed Doppler, when the frequency being sampled is more than twice the pulse repetition frequency then the system represents the frequency (velocity) at a much lower value.
- This aliasing causes the hue to be represented by the colour flow oriented in the opposite direction and should not be confused with reversal of blood flow
What is blooming artefact?
This is generally caused by over gaining and exaggerated by a low priority write setting and high colour sensitivity settings.
When the colour bleeds into the vessel walls and soft tissues
When can directional ambiguity occur?
- when the sample volume covers a region of flow separation such as in a dilated carotid bulb or in an aneurysm.
- It may also occur if the sample volume is sampling from the true and false lumen of a dissection.
Where can mirror artefact occur in vascular work?
the subclavian artery which is reflected against the lung and forms a third image
Define a bio effect and a bio hazard
Bio-effect = a known effect of the interaction of ultrasound radiation and human tissue. Bio-hazard = a known damaging consequence of a bio-effect, associated with some risk.
What is cavitation?
Describes the formation of gas bubbles from the fluctuations of the ultrasound pressure wave.
What is stable cavitation?
the ultrasound pressure wave interacts with gas bubbles already present and causes them to expand and contract.
As they do they grow in size and they can resonate with the pressure wave of a certain frequency once they reach a certain size.
These oscillating micro-bubbles can lead to large forces that can damage molecules, and also lead to micro-streaming.
What is inertial cavitation?
also referred to as transient cavitation
more serious than stable cavitation
the gas bubbles grow rapidly and suddenly implode
causes a shock wave that can disrupt and damage molecules
can also cause haemorrhaging.
What is the thermal index?
The measure of the heating effect.
A temperature rise of 1 degree Celsius means that the power input is such that the TI = 1
What is the mechanical index?
This is an indicator of potential cavitation.
If the value of MI is less than 1 then the mechanical cavitation effects are thought to be minimal.
You are performing a carotid colour duplex examination. You are having trouble getting a significant Doppler shift. Using the Doppler equation, list five ways of altering the frequency shift without changing the velocity of the blood.
- changing the frequency of the transducer in use
- changing the transducer to alter the frequency
- angling (heel/toe) the transducer.
- electronically steer the transducer to reduce the angle of insonation.
- changing your window of approach, thus changing insonation angle
What is the cause of spectral aliasing?
Aliasing is the result of insufficient sampling of the signal.
When the size of the Doppler shift recorded exceeds one-half of the PRF aliasing will occur.
This is called the Nyquist Limit.
