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1

(a) Ultrasound Pulses (i) On a diagram of a B-mode ultrasound pulse as a function of distance, what would the label on the vertical axis be?

i. The vertical y-axis is the pressure or pressure amplitude.

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(ii) State the number of cycles typically found in a b-mode pulse.

ii. A B-mode pulse typically has 3-5 cycles

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(iii) Are the b-mode cycles uniform in amplitude or do they vary in amplitude?

iii. The transmitted pulse should have cycles that builds to a maximum and then decrease again – the received ultrasound pulse shape will still be similar to the transmitted pulse but significantly smaller as it will have been partially absorbed/attenuated by the intervening tissue.

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(iv) How would you describe the spatial pulse length On a diagram of a B-mode ultrasound pulse as a function of distance?

iv. On this diagram, the spatial pulse length would be described from the very start of the first cycle and to the very end of the last cycle. The SPL would therefore be the length/extent of the pulse in space

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(v) State the possible range of the number of cycles in a Doppler pulse and whether they are uniform in amplitude.

v. In pulsed Doppler diagnostic ultrasound, the SPL is increased to about 10 to 20 cycles with a resulting significant decrease in the bandwidth. These cycles builds to a maximum and then decrease again due to the damping required to produce an ultrasound pulse and the range of frequencies found within the damped signal.

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(vi) Explain why the Doppler pulse shape is significantly different from the B-mode pulse shape.

vi. The Doppler pulse is often longer and comprised of lower and more narrow range of frequencies than that in B-mode. As Doppler imaging focuses on the imaging of red blood cells that scatter the beam in all directions, only a fraction of the beam will echo back to the transducer. If the bandwidth is too large the frequency shifts are less easy to determine in the Doppler ultrasound electronic circuitry so the SPL is increased which decreases the bandwidth and as such there is a more narrow range of frequencies. The longer pulse length also increases the power transmitted into the patient by about 100 when compared to b-mode and so obtains adequate echoes from red blood cells that are poor reflectors of ultrasound. Lower frequencies are used to minimize energy absorption by tissues to maximize the return echo signal. Due to these factors the Doppler pulse shape will be longer and comprised of a smaller range of lower frequencies.

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(b) Transducer frequency selection (i) If you wish to significantly increase the depth of a scan, what adjustment are you likely to have to make to the transducer frequency and why?

i) To increase the depth of a scan the transducer frequency must be reduced (and in turn the wavelength increased) – as this results in a beam that is less attenuated, and thus capable of penetrating to greater depths.

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(ii) Explain how the spatial pulse length is affected by the transducer frequency and why.

ii) The transducer frequency has an inverse relation to the wavelength. This is given by c=f λ. The spatial pulse length is related to wavelength in that the SPL is the product of wavelength and the number of cycles. Thus, the SPL is also given as the ratio of the product between the number of cycles with wavespeed, divided by the frequency. The SPL and transducer frequency are inversely proportional. As the frequency increases the SPL decreases and vice versa.

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(iii)Does increasing the transducer frequency improve or degrade the axial spatial resolution? Explain

iii) Increasing the transducer frequency improves the axial spatial resolution since increasing frequency will reduce the wavelength, which means that the difference between the ultrasound wavelength and small features imaged are reduced. As stated above as frequency increases SPL decreases and axial resolution is given by SPL/2. Therefore Increasing the frequency and thus decreasing the SPL will increase the axial resolution.

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(c) Signal processing (i) Describe the effect of rectifying the voltage waveform of a B-mode pulse as a function of time

i) Rectifying a B-mode wave involves altering the negative aspects/phases of the waveform into positive values. The essential effect is that the phase sign/direction is changed into positive, but the magnitude is maintained – the waveform will only have positive values

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(ii) Describe the effect of demodulating a rectified voltage waveform of a B-mode pulse

ii) Demodulation is the conversion of echo voltages from a radio frequency signal to a video signal. The resulting signal represents the amplitude of the detected echoes, but with no cyclic radio frequency variation. This is achieved by filtering the rectified wave. Filtering removes the high frequency variation and converts the RF signal to a smoothed amplitude pulse. In essence this means that the “envelope” of the total signal is retained, but the high frequency oscillations are removed that will then facilitate the calculation of the area bound by this envelope, as this corresponds to the echo intensity.

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(iii) What shape is the final pulse from the signal processor?

The shape of the final pulse is that of a rectangular ‘step pulse’, finite in width and of a given height

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(iii) What shape is the final pulse from the signal processor?

The width of this step is characterised by a time interval with which the pulse-echo time and thus echo depth can be calculated.
The height of this pulse is characterised by a voltage output that is proportional to the returning echo strength/intensity

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(i) Describe what the Doppler spectrum is showing; state its axes.

i) The spectral Doppler display portrays a range of frequency shifts, or velocities, in centimetres per second (cm/s), on the vertical (y) axis, over time (s), on the horizontal (x) axis

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(ii) Explain how the B-mode image is used in spectral Doppler

ii) The b-mode image is obtained by scanning the area of interest and acquiring an appropriate frame from which the spectral Doppler display will be acquired. This image allows the user to adjust the Doppler angle cursor and define the location and size of the sample volume. It is it regularly updated during spectral Doppler scanning to ensure the sample volume is still in the region of interest and hasn’t moved due to patient or probe movement.

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(iii) Describe how reverse flow would appear on the spectral Doppler spectrum.

iii) Flow reversal would appear on spectral Doppler as a crossing of the spectral trace across the velocity axis or baseline which represents 0cm/s blood velocity.

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(iv) On a spectral Doppler spectrum, explain how spectral broadening would appear and give two possible reasons for its occurrence.

iv) Spectral broadening - Spectral broadening is a Doppler artefact signified by a variance in the Doppler signal and it can be either intrinsic or spurious. Spectral broadening manifests as widening of the trace associated with lengthening of the vertical spectral lines and ‘filling’ of the spectral window that should otherwise be dark in appearance.
Intrinsic spectral broadening is caused by the way the machine processes the data it receives, that is, as if there were a single line of sight, a single point of origin and a single Doppler angle. As the Doppler signal received is on a significant area of the transducer, from multiple paths, all with different Doppler angles, there will be an intrinsic range of Doppler shifts in the overall Doppler signal.
Spurious spectral broadening may be caused by: setting a large sample volume, a large Doppler angle or placing a sample volume too close to a vessel wall where blood flow varies in speed. It may also be real and caused by pathology. If vessel disease causes the velocity profile to change and blood flow becomes turbulent the spectrum will have a larger variance in Doppler.

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(i) What is the meaning of the term aliasing? Explain why it occurs in spectral Doppler

i) Aliasing refers to being represented by something other than what it is. – in the context of spectral Doppler, aliasing refers to the Doppler signal and trace being misrepresented where the true signal appears as having a lower frequency than its true value. Aliasing occurs as a consequence of under-sampling, where the detected Doppler shifts fall outside of the range Doppler shift range (- PRF/2 < fD < PRF/2)

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(ii) Explain the meaning of the Nyquist limit and how it is defined.

ii) There is a fixed limit on the range of Doppler shifts that can be correctly displayed and this is called the Nyquist limit and is equal to one half of the Doppler PRF.

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(iii) Describe the appearance of a Doppler spectrum which is exhibiting aliasing.

A Doppler spectrum with aliasing will have a trace that appears to have a sudden discontinuation in the velocity axis, with a trace that is ‘cut off’ and reappearing at the opposite baseline sign.

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(iv) Give two methods of alleviating Doppler aliasing (excluding baseline shifting).

iv) A method of mitigating aliasing is to increase the rate of sampling – i.e. increasing the PRF to allow the Doppler shifts to fall within range. Another method is to increase the Doppler angle by physically altering the transducer and its relation to the imaged vessel. Increasing Doppler angle effectively reduces the Doppler shift.

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(b) Colour Doppler imaging A colour Doppler examination is performed using a phased array transducer. A blood vessel of interest is straight and approximately horizontal with the blood flow from right to left. Describe the shape of the display. Describe the location of the image of the blood vessel and within the image of the vessel, describe the distribution of colour that you would expect to see if the flow is not turbulent and colour settings have been optimised. Ensure that you indicate which regions of the display show a component of motion towards the transducer and which regions show motion away from the transducer. Describe which colour is associated with which flow direction. (Assume standard red/blue Doppler colouring.)

The shape of the display is that of a sector shaped FOV, with a point source (or small footprint) from which the scan lines will emanate.
The image of the blood vessel will be located ideally within the centre of this sector-shaped FOV, and within this FOV the vessel will lie within the smaller colour box that outlines the region for Doppler assessment.

Despite having physiological flow from right to left of the display, the distribution of colour across this horizontally oriented vessel is one that would contain both red and blue hues that indicate directions towards and away from the transducer. This is because of aperture elements having respective Doppler angles that vary from 90 degrees, thus detecting various Doppler shifts and identifying varied flow.

With flow going from right to left, the Doppler angle is therefore defined as the angle formed between the vector of flow direction and the return echo.

In this instance, the left half of the vessel would be assigned blue hues since these Doppler angles would be obtuse and mimic flow away from the transducer. The right half of the vessel would be assigned the red hues, as the Doppler angles from this half would be more ‘acute’, representing flow towards the transducer. In colour Doppler, red hues are conventionally associated with flow towards the transducer whilst blue hues are associated with flow away.

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(a) Write notes on the appearance of range ambiguity in B-Mode Imaging, explaining why it occurs.

Range ambiguity appears as a low-echogenic feature that appears artificially shallow. This artefact occurs when the pulse repetition period is too short in combination with the imaging of lowly attenuating structures at deep depths. Ideally, all echoes should be received prior to the transmitting of the next pulse. In a situation where there is a deep, lowly attenuating structure, it is possible for the echo from this structure to return to the transducer AFTER the next pulse has been sent. Because of this, the feature is assumed to have originated from the 2nd pulse, and with a short pulse-echo time assumed to be from the 2nd pulse, it appears artificially shallow.

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Write notes on the appearance of comet tailing in B-Mode Imaging, explaining why it occurs.

Comet-tailing appears as reverberations of bright lines that appear to rapidly taper and become less bright with depth. Similar to ring down artefact but shorter lived and so a relatively ‘short ‘tail is seen rather than the long bright line as seen in ring down artefact. It is generally caused by small calcifications. They are strong reflectors and so the ultrasound reverberates between the calcifications producing a series of reverberating echoes. What causes the comet tail appearance, is that reflection of US by calcium causes significant loss of energy and hence the reverberation of echoes quickly reduce in amplitude and the artifact fades from the image with depth.

25

Write notes on the appearance of edge shadowing in B-Mode Imaging, explaining why it occurs.

Edge shadowing occurs when the ultrasound beam strikes the edge of a circular structure (such as a cyst or blood vessel in cross section). A combination of reflection and refraction occurs causing the beam to be deflected and broadened. The broadening of the beam due to it’s interaction with the edge of the circular structure leads to a reduction in beam intensity, so echoes from the tissues beyond the edge of the structure are reduced leading to shadowing. This appears as a dark line continuing down from the edge of a rounded structure.

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(b) Discuss the role of the angle correction cursor and the importance of setting it correctly. Discuss in terms of (i) The exact definition of the Doppler angle

i) The Doppler angle (θ) is the angle between the forward blood flow direction and the returning Doppler echo.

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Discuss the role of the angle correction cursor and the importance of setting it correctly. Discuss in terms of (ii) Explain how the Doppler angle is used.

ii) Setting a Doppler angle allows the component of blood flow velocity vector directly toward or away from the transducer to be converted into blood flow velocity in the direction of the vessel. If the Doppler angle has been correctly measured, Doppler shift can be calculated and an accurate blood velocity is acquired from the Doppler spectral display. The Doppler angle is routinely set less than 60° to limit the error in the blood velocity calculation, when the angle is 60° a 5° error in estimating the Doppler angle will mean a 15% error when calculating blood velocity.

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Discuss the role of the angle correction cursor and the importance of setting it correctly. Discuss in terms of (iii) Explain how the Doppler angle may be altered.

iii) The Doppler angle can be altered by changing the scan line orientation in relation to the angle it has relative to the blood vessel and its flow direction. This involves rocking the transducer along the plane that is being investigated or electronically steering the beam. Changing the angle correction cursor does not change the Doppler angle. It only acts to approximate flow direction and thus best approximate of this Doppler angle to calculate blood flow velocity. Once the transducer/ scan line orientation has been changed to achieve a more desirable Doppler angle it will be necessary to set the sample volume and Doppler angle cursor again to properly reflect the new acquisition.

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(c) Define the thermal index, TI, and briefly outline its role in diagnostic ultrasound in terms of (i) Its definition in terms of its defining power ratio

i) The estimated temperature increase is displayed by the ultrasound machine as the thermal index. It is essentially a measure of the likely maximum temperature rise in the patients tissues in degrees Celsius. The thermal index is defined as the ratio of the acoustic power used, W0, to that power required to raise the tissue temperature by 1 Celsius degree, W1°C. For example, of TI of 0.8 means that like likely temperature rise due to US exposure will be 0.8 degrees Celsius.

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Define the thermal index, TI, and briefly outline its role in diagnostic ultrasound in terms of (ii) What does high TI indicate?

ii) The higher the TI the greater the potential for tissue heating that could cause harm. It is widely agreed that a temperature increase of up 1.5 degrees Celsius above normal body temperature (37) can be tolerated by a fetus without harm which is why some machines won’t display the TI unless it is greater than 1. However an increase of 4 degrees or more is considered dangerous even for a short time and should be avoided.