Doppler Ultrasound. Amanda Watson.

Transcription

1 Doppler Ultrasound Amanda Watson

2 Before we start Why does blood appear black on a B-mode image?

3 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

4 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.

5 The Doppler effect The change in frequency depends on the velocity of the source

6 The Doppler effect The difference in frequency f d = f o f r Is known as the Doppler shift. If we can measure f d, we should be able to work out the velocity.

7 The Doppler equation - quantities f D = 2 f o vcos c θ Can you rearrange this equation to show what velocity is equal to? f d is the Doppler shift frequency need to measure this f o is the frequency of the transmitted signal know this Θ is the angle between the ultrasound beam and the direction of flow can estimate this c 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

8 The Doppler equation - numbers f D = 2 f o vcos c θ f 0 2 MHz 10 MHz v 50 cm s cm s -1 θ 45 o 45 o c 1540 m s m s -1 f d 1.83 khz 4.58 khz Audible

9 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

10 Extracting the Doppler signal Summing Multiplication

11 Extracting the Doppler signal 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

12 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

13 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.

14 Colour Doppler Image Processing A 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.

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

16 Colour Doppler Image Processing The last stage in the process is what is known as the bloodtissue 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 thresholding is used to remove these high amplitude signals. High signals removed Low signals removed

17 Colour Doppler User Controls Gain 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.

18 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

19 Power Doppler

20 Power Doppler

21 3-D Power Doppler

22 Spectral Doppler Making Measurements

23 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.

24 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.

25 Spectral Doppler creating the spectrum The pulsed signal is demodulated as we saw for colour Doppler, and the changes in phase between consecutive pulses are used to create the Doppler signal Additional 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.

26 Spectral Doppler creating the spectrum 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

27 Spectral Doppler creating the spectrum 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.

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

29 Spectral Doppler creating the spectrum

30 Spectral Doppler ultrasound Venous flow - Wide band of velocities reasonably steady in time Arterial flow - pulsatile.

31 Doppler Ultrasound Systems Two simultaneous modes Duplex Three simultaneous modes Triplex Don t forget about frame rates

32 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

33 Doppler Pitfalls - aliasing Aliasing can be minimised by adjusting the velocity scale and/or adjusting the velocity baseline

34 Doppler Pitfalls - aliasing The maximum frequency that can be detected f d (max)=prf(max)/2 f D = 2 f o vcosθ c 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 f d (max) and may cause aliasing

35 Doppler Pitfalls angle effects v = 2 f o f D c Cosθ At Doppler angles > 60º, any errors in the estimated angle cause a bigger potential error on the estimated velocity.

36 Doppler Pitfalls angle effects 1 error Cos 45 o = Cos 44 o = % error = v = 2 f o f D c Cosθ Cos 60 o = Cos 59 o = % error = Cos 80 o = Cos 79 o = % error =

37 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., Peripheral Vascular Ultrasound How, Why and When, Thrush, A and Hartshorne, T, Pub. Elsevier, os-html/chapter_01.htm

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