Module 4 questions Flashcards
Explain the difference between laminar flow, eddy flow, reverse flow and turbulent flow. In your answer explain the role of viscosity and vessel diameter in these flow types.
- Laminar flow has parallel stream lines and these follow the general direction of the vessel
- Reverse flow is flow opposite to the predominant flow direction.
- Eddy flow is like small, steady, whirlpools.
- Turbulent flow is flow in random directions and with random speeds.
- Flow becomes more and more disturbed (down the above list) as the vessel gets narrower, the viscosity gets larger and the vessel cross sectional area changes. Vessel branching will also cause the more disturbed flows patterns.
Blood images black in B-mode ultrasound diagnostic imaging but is displayed in colour in colour and power Doppler. Explain.
B-mode uses relatively low strength pulses and the RBCs scatter back at the transducer is below the echo threshold, so the blood in the vessel images black. In Doppler modes the power is increases substantially and this results on RBC echoes at the transducer being strong enough to register and be analysed.
In a spectral Doppler situation the transducer frequency is 5 MHz, and the Doppler angle is 60. If the wall filter is set to 200 Hz what is the minimum blood flow speed that will register in a spectral line?
We have to do the problem in reverse and set the Doppler shift at 200 Hz and solve for the value of blood flow speed, u. The equation we need is: 200 = 2 × 5 × 10o u cos(60°)/1540 with (60o ) = ½ The solution is u = (200 ×1540)/(5 ×106 ) = 0.06 m/s, or 6 cm/s. So blood flow speed at this Doppler angle and transducer frequency must be above 6 cm/s to be above the wall-filter cut-off frequency.
Explain what is meant by the following pairs of Doppler terms, taking care to distinguish between them. (i) range gate and sample volume
(i) The range gate provides the axial limits of the sample volume, the sample volume is also determined by the beam width and slice thickness.
Explain what is meant by the following pairs of Doppler terms, taking care to distinguish between them. (ii) intrinsic spectral broadening and spectrum
(ii) The spectrum is the extent of Doppler shift frequencies at any instant (after 128 samples have been recorded and analysed). This is drawn as a vertical line in the spectral Doppler trace. Intrinsic spectral broadening is the smallest value this spectrum can take, and is governed by variations in the blood flow speed and direction with respect to the somewhat idealised Doppler angle.
Explain what is meant by the following pairs of Doppler terms, taking care to distinguish between them. (iii) Doppler angle and angle correction cursor
(iii) The Doppler angle is the angle between the blood flow and the return echo path. The angle correction cursor is the screen feature that can be oriented along the flow direction in an attempt to calibrate the flow speed axis in spectral Doppler.
Explain what is meant by the following pairs of Doppler terms, taking care to distinguish between them. (iv) resistive index and pulsatility index
(iv) The resistance index is RI = (S – D)/S, (with S = peak systole flow speed and D = diastole speed) while the pulsatility index is PI = 2(S – D)/(S + D). This RI is the difference as a fraction of the peak, while PI is the difference divided by the average.
Explain what is meant by the following pairs of Doppler terms, taking care to distinguish between them. (v) duplex Doppler and triplex Doppler
(v) Duplex is two ultrasound traces on the one display; usually B-mode and spectral Doppler. Triplex is three traces, usually B-mode, spectral Doppler and colour Doppler.
List the advantages and limitations of colour Doppler and power Doppler
Colour Doppler
Allows user to quickly appreciate haemodynaic events in the area of interest due to auto correlation.
Provides a road map for pulsed Doppler spectral analysis
Is qualitative not quantitative – does not provide blood flow velocity information.
Power Doppler (PD) color encodes the echo amplitude (signal strength) and not the flow-velocity or flow-direction as colour Doppler does.
angle-independent so able to detect and display blood movement even at perpendicular angles.
PD is advantageous over conventional CD if the examiner needs to detect a signal from an angle not conducive to Doppler interrogation angles.
PD is much more sensitive in detecting weak or small volume flows such as those found in the false lumen of a dissection.
Disadvantages of PD includes no display of flow direction, reduced frame rates and it is subjective to motion artifacts.
In colour Doppler explain why the colour sequence red-black-blue in the image of a blood vessel is evidence of the lack of aliasing, but the sequence red-white-blue is evidence of aliasing.
Aliasing represents an incorrect colour in colour Doppler. For flow towards the transducer the colour used is red but as the shifts get larger the red colour becomes lighter, and when the shifts hit the Nyquist limit they move to white. For a further increase in blood flow speed and shifts the colour then moves to light blue. Red-white-blue (or reverse) is evidence of aliasing in colour Doppler. If the ‘red’ flow is lowered it goes to a darker shade of red, eventually falling below the wall –filter value and imaged as black. Red-black-blue (or reverse) is evidence of normal (correct) flow in colour Doppler.
When using a linear transducer (particularly) why is it important to be able to steer the colour box direction when using colour Doppler imaging?
Steering the colour box allows a more appropriate Doppler angle (well away from 90o) to be obtained in most imaging situations
Write brief notes on the role of the importance of setting the velocity range correctly in spectral Doppler.
The velocity range adjusts the PRF and sets the maximum frequency shift that can be detected without aliasing. If it is set too low aliasing may occur, and if it is set too high the spectral resolution will be poorer as the displayed spectrum will be too small on the spectral scale.
How does refraction affect the accuracy of angle correction?
If the Doppler beam has non-perpendicular incidence on the vessel wall it will be refracted as it enters the vessel, refracting away from the normal as the velocity of sound in blood is higher than in soft tissue. Hence the Doppler angle is decreased and less than the applied angle correction so the measured velocities will be overestimated.
What factors affect the temporal resolution in Colour Doppler imaging?
The temporal resolution is affected by the width and depth of the colour box, by the line density within the colour box and by the number of pulses transmitted along each line of sight – the sensitivity. i.e whatever may affect the frame rate.
What are the advantages of using continuous wave Doppler rather than pulsed wave Doppler?
The benefits of continuous wave Doppler relate to the reduced frequency bandwidth in the transmitted ultrasound pulse and hence reduced spectral broadening, and the absence of aliasing.
