FRCR Doppler 14 - DCPB€¦ · FRCR / MSc - Doppler AJW - 2014 B-mode echoes vs. Doppler echoes In B-Mode we are concerned with the position and the amplitude/intensity of the echoes

FRCR / MSc - Doppler AJW - 2014

Doppler Ultrasound

Amanda Watson

[email protected]


Before we start

Why does blood appear black on a B-mode image?


B-mode echoes vs. Doppler echoes

In B-Mode we are concerned with the position and the amplitude/intensity of the echoes

In Doppler we are concerned with the position and frequency/phase of the echoes


The Doppler effect

Stationary observer moving source of sound. E.g. a police car or motorbike has a high pitched sound when travelling towards you which changes to a lower pitch when it passes you.


The Doppler effectThe change in frequency depends on the velocity of the source


The Doppler effectThe difference in frequency

fd = fo – fr

Is known as the Doppler shift.

If we can measure fd, we should be able to work out the velocity.


c

vCosff

oD

θ2=

fd is the Doppler shift frequency – need to measure thisfo is the frequency of the transmitted signal – know thisΘ is the angle between the ultrasound beam and the direction of flow – can estimate thisc is the calibrated speed of sound – know this

v is the velocity of the moving blood

If the frequency shift can be measured then an estimate of the velocity may be calculated

The Doppler equation - quantities

Can you rearrange this equation to show what velocity is equal to?


Audible

f0 2 MHz 10 MHz v 50 cm s-1 50 cm s-1 θ 45o 45o c 1540 m s-1 1540 m s-1 fd 1.83 kHz 4.58 kHz

The Doppler equation - numbers

c

vCosff

oD

θ2=


Extracting the Doppler signal

For continuous wave, e.g. foetal heartbeat monitors, pocket Dopplers, it is easy to extract the Doppler signal.

BUT – no positional information



Summing Multiplication



Pulsed wave ultrasound –extracting the Doppler signal is more difficult, but the pulses will give us positional information – essential for imaging

Pulsed wave Doppler techniques actually look at the phase shifts between consecutive pulses


Colour Doppler Imaging

Colour Doppler allows us to visualise regions of Blood flow and to give an indication of the mean velocity and direction of flow.

It is a pulsed ultrasound technique so that positional information may be given

Notice that in the colour box there is both colour Doppler information and B-mode information


Colour Doppler Imaging

In addition to the B-Mode image processing, there is extra signal processing carried out in the colour box to identify and encode the areas of flow.

Remember this is in real time – so all the extra processing can potentially slow things down.


Colour Doppler Image ProcessingA first stage in colour Doppler image

processing is to separate out moving signals from stationary signals. This is done using a clutter filter, which combines the received echo with an inverted version of the transmitted signal.

If the echo is from stationary tissue then the two signals will be completely out of phase and will cancel out.

If the echo is from a moving target, then the two signals will be out of phase so will not cancel out – the signals in that pixel can be processed further to create the Doppler image.


Colour Doppler Image Processing

To calculate the mean velocity (and direction) of flow at the colour Doppler pixel location the Doppler statistic estimator uses a technique known as autocorrelation where it sends out consecutive pulses and measures the phase shift.

The phase change as a fraction of the wavelength gives the distance the reflector has moved so using the PRF the mean velocity can be calculated.

More pulses will give a better estimate of the mean velocity but frame rates will reduce.

Phase shift


Colour Doppler Image Processing

The last stage in the process is what is known as the blood-tissue discriminator or the colour write priority.

This determines what will be displayed in the colour box and stops signals from moving tissue being displayed in colour.

Moving tissue will produce “Doppler” signals but the amplitude of the echos from the moving tissue will be much higher than the echoes from blood. Amplitude thresholdingis used to remove these high amplitude signals.

High signals removed

Low signals removed


Colour Doppler User ControlsGain – same as for B-Mode. Too little can give no flow, too much gives noise and colour bleeding.

Velocity scale and baseline– should be set to reflect typical normal values, adjust to avoid aliasing (more on aliasing)

Colour map – how you want the colours to appear

Invert – switch the colour coding of the velocity direction

Colour box size – better sensitivity and frame rates with a small box

Colour box steer/angle – remember the importance of angles on the Doppler equation.

Persistence – frame averaging/compounding technique. Can be useful for slow flow.


Power Doppler

Power Doppler (also known as Angio or Energy mode) uses the same processed information as colour Doppler but maps only the amplitude of the Doppler signal.

There is no velocity or direction information.

It can be useful for assessing perfusion or demonstrating continuity of flow.

Colour Power


Power Doppler

www.medical.siemens.com


Power Doppler



3-D Power Doppler



Spectral Doppler – Making Measurements


Spectral Doppler – Making Measurements

Within a vessel there are a range of velocities characteristic of location and pathology.

Spectral Doppler samples the range or spectrum of velocities at a specific location and maps how they change in time.

The B-mode and/or colour Doppler images serves as a guide to where to sample (place the Doppler Cursor.


Spectral Doppler – creating the spectrum

The Doppler beam is made up of a few elements, just like in B-Mode, but the beam is fixed in position with the position and angle controlled by the user.

In spectral Doppler, we are sampling the echoes from a small area known as the range gate or sample volume. The depth and width of the sample volume can be controlled by the user.


Spectral Doppler – creating the spectrumThe pulsed signal is demodulated as we saw for colour Doppler, and the changes in phase between consecutive pulses are used to create the Doppler signalAdditional timing controls are needed to ensure that only the echoes from the sample volume are used. The next pulse can only be sent after the echoes from the preceding pulse have been received.



The Doppler voltage signal analysed from the received pulses is made up of a range of velocities due to the range of velocities moving through the sample volume.The frequency content is analysed using a Fast Fourier Transform which gives the different frequency components and their amplitudes



For a small time interval (5 to 10 ms) the pulses are collected to get the Doppler signal which is then transformed (FFT) into its frequency components which can be converted into velocities. The different amplitudes for each frequency/velocity are shown with different intensities of greyscale.



Adding the next time interval, and the next , and so on will create the Doppler spectral waveform display which is updated in real time.




Venous flow -Wide band of velocities reasonably steadyin time

Arterial flow -pulsatile.

Spectral Doppler ultrasound


Doppler Ultrasound Systems

Two simultaneous modes – DuplexThree simultaneous modes – Triplex

Don’t forget about frame rates


Doppler Pitfalls - aliasing

2 samples per cycle is the lowest possible sampling frequency to give an accurate Doppler frequency estimation. Any less will give an under estimate (aliasing).

fd < PRF/2

is known as the Nyquist limit



Aliasing can be minimised by adjusting the velocity scale and/or adjusting the velocity baseline



The maximum frequency that can be detected fd(max)=PRF(max)/2

This will limit the maximum velocity that can be detected (Doppler Equation)� A lower transmit frequency will allow for higher velocities

BUT … for a particular depth of interest, the PRF is limited by the time taken for the pulse to complete the “round trip” : PRF = c/2d� Deeper sample volumes will have a lower fd(max) and may

cause aliasing

c

vCosff

oD

θ2=


θCosf

cfv

o

D

2=

At Doppler angles > 60º, any errors in the estimated angle cause a bigger potential error on the estimated velocity.

Doppler Pitfalls – angle effects


Cos 45o =Cos 44o =% error =

0.7070.7192


0.50.5153

0.1740.19110


θCosf

cfv

o

D

2=

Doppler Pitfalls – angle effects 1°error


Sources and Further Reading

Images in this presentation have been taken from:

“Diagnostic Ultrasound Physics and Equipment”, Ed. Hoskins, PR et. al., Pub. GMM Ltd., 2003.

“Peripheral Vascular Ultrasound How, Why and When”, Thrush, A and Hartshorne, T, Pub. Elsevier, 2005.

http://radiographics.rsnajnls.org/cgi/content/full/23/5/1315

http://radiographics.rsnajnls.org/cgi/content/full/24/3/657

http://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm



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FRCR Doppler 14 - DCPB€¦ · FRCR / MSc - Doppler AJW - 2014 B-mode echoes vs. Doppler echoes In B-Mode we are concerned with the position and the amplitude/intensity of the echoes

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