Doppler Ultrasound Flashcards
What is the Doppler effect used to measure?
Blood speed
What is the ultrasound Doppler shift measured by?
Comparing transmitted and received ultrasound signals (usually pulses)
What is found within a single location?
Spectral Doppler
Over a region: (colour/power)
Doppler Imaging
What is doppler shift proportional to?
Blood speed (m/s or cm/s) for a fixed angle between artery (direction of blood flow) and ultrasound beam direction
Why do you need to calibrate the machine?
Get a good estimate of the speed of blood
How do you calculate the sped in m/s?
Calculate the cross-sectional area and multiply the speed by that cross sectional area
= units of m3/second
What gives an indication of blood flow?
Measure the average of speed and multiple that by cross section
What is B-more image?
Grey-level image that is displayed
Provide anatomical information but doesn’t tell anything about speed or blood
What is blood flow colour coded according to?
Direction
The direction is relative to ultrasound waves
What is veins displayed as?
Shades of blue and green
What is spectral Doppler trace?
Measuring speed of blood at a particular point
Measuring within middle of artery - between 2 little gates
Why is the line angled so that it is in same direction as the artery?
So it will be close to the direction of blood
Where is the ultrasound transducer placed on?
Skin
What is it called when we want to measure at a specific speed?
Sample volume
What is aligned with the vessel?
Angle correction line
What does the spectral Doppler trace tend to measure?
Blood velocity which corresponds to the blue line which is the envelope
What is the envelope in the spectral Doppler trace?
Maximum velocity speed that is being measured
What gives the spectral broadening shape?
Spread of speed within a sample of small volume
Where is there a proportionality between?
Doppler shift and speed of blood
What is kept constant in the Doppler equation?
speed of sound
Around 15-40m/second
What is measured by the ultrasound scanner?
- Doppler shift
- Received frequency
- Transmitted frequency
What is set by the scanner operator?
Doppler angle < = 60 deg
What is known by ultrasound scanner?
- transmitted frequency
2. Speed of sound
What is doppler shift thereotically?
0
Theta=90degree
What is the best practice for doppler velocity measurement?
Theta < 60 degree and fixed
What are doppler frequencies?
Audible - listen to sound
Doppler shifts are in the audible range - 20kHz
What is filtered out using a wall filter?
Signal from vessel walls
- High amplitude
- Low Doppler shifts
Why are measurements inaccurate ?
- Number of sources of error
2. Subject to significant inter- inter observer variability if not performed carefully
What are examples of Doppler ultrasound device?
- Continuous wave (CW)
- Pulsed wave (PW)
- Doppler Imaging
What is continuous wave?
Measures velocities over a wide range of depths
What is pulsed wave?
Gated to measure velocity at a particular depth (I.e. within a sample volume)
- pulse repetition frequency
- pulsed wave Doppler
What is pulse repetition frequency?
~2-8kHz
Determines the maximum velocity that can be measured
What is pulsed wave Doppler?
Measures velocities in just a small region
Change depth of region called gating
Screen out other parts of the signal and just retain signal that is of a particular depth of where you want to measure speed of blood
What is Doppler imaging?
Colour Doppler
Colour = velocity + direction
Power Doppler
Colour= strength of Doppler signal (~flow/no flow)
What is not accurate enough to be used quantitatively?
Colour Doppler
What are examples of colour Doppler artefacts?
- Noise/flash artefact that is due to tissue movement or moving probes of the surface
- Brief of random colour all the way across the image
- Vessel is being obsecured by flash artefacts
- Aliasing
- Very high speeds are actually being mapped as low speed in opposite direction
What does doppler give a good indication of?
Surface of plaque
What represents the power of the received Doppler shift signal?
Pixel colour
What does power Doppler not give?
No velocity information
What are the advantages of power Doppler imaging?
- Relative independence to vessel orientation
- Sensitivity to low blood flow
- Useful for visualising complex vascular geometries (e.g. in brain)
What are the clinical applications of Doppler?
- Extracranial
2. Intracranial
What are features of extracranial clinical applications?
- Assessment of carotid atherosclerosis stenosis
- Arterial dissection
- Carotid body and had and neck tumours
What are features of intracranial clinical applications?
- Assessment of arterial stenosis
- Evaluation of collateral flow and arterial malformations
- Aneurysm detection
- Investigation of dynamic cerebrovascular response
- Embolus detection
- Intraoperative monitoring
What are the main uses of Doppler ultrasound?
Look for disease in the carotid arteries
What does common carotid artery split into?
- ICCA
2. ECCA
What is a common site for atherosclerosis?
Carotid bifurcation
What is the pathology for atherosclerosis?
- The interior of the artery is narrowed
- Histological cross-section slide
- The lumen of the artery is narrowed
- Plaque may become “unstable” rupture and produce emboli, leading to brain infarction
What are the problems of assessing carotid stenosis?
- Several different definitions of stenosis
- Little standardisation in measurement between centres
- Accuracy of methods used for clinical measurement is often limited
What is the NASCET trial?
- One major stroke prevented for every 6 patients treated 2 years after surgery if degree of stenosis is 70-99%
- 1 in 22 of degree of stenosis is 50-69%
What did the ACST trial suggest?
Asymptomatic patients with high grade stenosis may also benefit from surgery
What are the advantages of x-ray/CT/MR angiography
- Invasive: a contrast agent is usually required
- Only the lumen is visualised
- Ionising radiation dose in case of x-ray or CT
What are the problems of 3D diagnostic scanners?
- Not portable in general
- Expensive
- Not suitable for real-time imagine
What is the problem for traditional projection angiography?
Vessel diameter measurements are highly dependent on the projection plane
What are the advantages of Doppler ultrasound?
- Low cost equipment
- Portable
- Real-time imaging
- Visualisation of vessel wall as well as blood
- Multimodal (imaging + velocity measurements)
- Also possible using some MRI techniques, such as black blood MRI
What are the 2 methods to estimate the amount of disease?
- Direct measurements from transverse images
2. Measure velocity
What are the principle of Doppler-based methods?
- Velocity increases with degree of stenosis
- Relate velocity to stenosis
- As lumen narrows, higher velocity - varies with individual patients
When do you measure velocity?
At site of maximal stenosis
Located visually or find location with maximum velocity
What are the problems with Doppler-based stenosis estimation?
- Large variability: PSV only loosely related to stenosis
2. Errors in Doppler velocity measurements
What is doppler clinically useful for?
Discriminating moderate to severe stenosis
why is PSV only loosely related to stenosis?
- Variation in blood velocity between patients unrelated to carotid disease can be due to
- cardiac output
- blood pressure
- contralateral disease - Inter-observer and inter-equipment variability
- Variability in reference measurement adopted
What is transcranial Doppler?
- Ultrasound passes somewhere through the skull
- Most commonly the temporal window on side of skull
- Get spectral Doppler traces
- Characteristics of waveform vary depending on which vessel we are making measurements in
TCD ultrasound
- Measurement of cerebral blood velocities
- Low frequency vs diagnostic ultrasound
- Access via temporal or occipital windows or eye
- Temporal window: MCA > circle of Willis
- Occipital window: basilar arteries
- Stenosis, aneurysm, vasopasm and embolus detection
- Useful for monitoring neurovascular surgical procedures
What are the advantages of TCD?
- Low cost
- Portable
- Real time velocity monitoring
- No constraint agent required
What are examples of real time velocity monitoring?
- Carbon dioxide reactivity
- Dynamic autoregulation investigations
- Cerebral lateralisation
- Intraoperative and bedside monitoring
What are the limitations of TCD?
- Access limited due to limited numbers of numbers
- Not all patients have an acoustic window
- The sample Gate is positioned blind
- Higher end equipment combines imaging and Doppler velocity measurement
- Doppler angle cannot be estimated accurately
- Poor resolution
Real time velocity measurement during surgery
- Usually MCA
- Indicates velocity changes and emboli
- Used to determine co-lateral blood flow and necessity for shunt insertion
- Indicates linking of shunt or poor air seal
