When sound is reflected from a moving object, such as blood cells, the returned echoes are at a different frequency to that of the original sound source and the amount of change in the frequency is proportional to the velocity of the interface.
- If the object is moving away from the source, the frequency decreases.
- If the object is moving towards the source, the frequency increases.
As the angle between the transmitter and the interface (insonation angle) nears 90° the accuracy of the estimation of the velocity of the interface decreases. In general use, an insonation angle of less than 60° is used to give accurate estimates of velocity.
Continuous wave doppler
These are usually dedicated handheld devices (e.g. ABPIs, cardiotopograms for fetal heartwave). The Doppler effect is emitted as an audible sound as the Doppler shift is in the audible sound frequency range: the higher the pitch the greater the velocity; the harsher the sound the more turbulent the flow. As they transmit (and, therefore, receive) continuously, they have to contain two separate transmit and receive elements.
- Easy to use
- Sensitive to flow
- Can't measure velocity
- Insonate all vessels in the beam path until the beam is attenuated. This means that as arteries and veins usually lie close together the output often combines arterial and venous signals.
- Can't determine depth
Pulsed wave doppler
In pulsed wave Doppler, the same elements are used for transmitting and receiving and brief pulses of ultrasound energy are emitted. Range gating is used to only accept echoes returning from a specific depth. Duplex involves Doppler imaging overlayed over B-mode imaging.
There are three types of pulsed wave Doppler used in ultrasound machines:
In colour Doppler the sampling volume is set and the mean and variance of the velocity of the moving structures calculated. This velocity is then represented by a scale of arbitrary colours ranging from minus (moving away from the transducer) to zero (no calculated velocity) to plus (moving towards transducer). The pulse frame rate affects the real-time colour Doppler measurement. A lower frame rate results in a stuttering colour Doppler e.g. using a larger Doppler sampling box which requires more Doppler pulses and, therefore, lowers the frame rate.
Power Doppler images map the amplitude only of the Doppler signal without any indication of the velocity. All movement, regardless of phase, contributes to the amplitude. This means that power Doppler emphasises the quantity of blood flow.
- Less dependent on insonation angle
- Can show very low flow rates
- Not subject to aliasing
- No indication of flow direction
- Tissue motion creates artefacts
Spectral Doppler shows the range of Doppler frequencies returned over time and displayed in a sonogram.
Differences in vessel wall resistance produce different spectral traces. The characteristics of the vessel walls can be represented numerically as the Resistive Index (RI) and the Pulsatility Index (PI).
RI = peak systolic frequency - end diastolic frequency
peak systolic frequency
PI = peak systolic frequency - minimum frequency
time averaged maximum frequency
High resistance vessel
Highly pulsatile with sharp upstroke and narrow range of velocities e.g. peripheral vessels such as femoral artery and aorta.
Low resistance artery
Low pulsatility with large range of velocities e.g. in vessels supplying vital organs that need flow even during diastole such as renal artery, internal carotid artery.
Normal RI = 0.6 - 0.7
Abnormal RI = 0.8 - 1.0
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The Nyquist limit states that the sampling frequency must be greater than twice the highest frequency of the input signal in order to be able to accurately represent the image.
Nyquist limit = PRF / 2
If the velocity of the flow is greater than the Nyquist limit, the Doppler shift exceeds the scale and "wrap-around" occurs.
Blood flowing closer the inside of the vessel wall is slower than flow in the middle of the vessel. This large range of frequencies in a particular moment in time produces a widening of the spectral graph and different colours in colour Doppler. This also occurs with turbulent flow (e.g. stenotic vessels) as the turbulence creates flow of different velocities and directions.
Flow velocity estimation requires the flow to be as along the direction of the ultrasound beam. If it is perpendicular, i.e. traveling across the beam, flow is difficult to detect. The angle of insonation should be less than 60° at all times to allow the most accurate estimation of velocity.
An electronic filter is applied to the returning data to eliminate low frequency signals as these are usually produced by low velocity structures such as vessel walls. If the filter is inappropriately applied the real signals from low velocity blood flow are eliminated.
- Flow moving away = decrease in returning frequency
- Flow moving towards = increase in returning frequency
Continuous wave Doppler
- Separate transmit and receive elements for continuous measurement of flow requiring dedicated probe
- Can't differentiate between two structures
- Can't determine depth
Pulsed wave Doppler
- Can display on top of B-mode (called duplex ultrasound)
- Colour Doppler: Mean of velocities represented by colour scale
- Power Doppler: Amplitude of velocities, not direction, displayed
- Spectral Doppler: Range of velocities through time. Resistive index and pulsatility index used to calculate high and low resistance vessels
- Aliasing due to too low a PRF
- Spectral broadening due to turbulent flow and velocity being faster in centre of vessel
- Doppler angle must be <60° for accurate estimation
- Wall filter may remove genuine low velocities