Application of ultrasound Flashcards
What is Doppler effect used to measure?
Speed of blood
How is the ultrasound Doppler shift measured?
Comparing transmitted and received ultrasound signals (usually pulses)
What does ultrasound doppler do?
Send short pulses of sound and receiving back via the transducer equipment
Looks at differences in frequencies within that signal
What is Doppler ultrasound at a single location?
Spectral Doppler
What is Doppler ultrasound over a region?
Doppler Imaging
What do you get with Doppler ultrasound?
A regional map of how blood is flowing
What is Doppler shift (kHz) proportional to?
Blood speed for a fixed angle between artery (direction of blood flow) and ultrasound beam direction (=ultrasound beam direction)
Why do you need to calibrate the machine?
Get a good estimate of speed of blood
How do you calculate the speed in m/s?
Calculate the cross-sectional area and multiple the speed by that cross-sectional area
units of m3/second
What is found on the overlay on the B-mode image?
Dark region within artery
Haven’t measured blood flow within that region
What is the B-mode image?
Grey-level image that is displayed
Provide anatomical information
What is the blood flow colour coded according to?
Direction
What is the direction of the blood flow relative to?
Ultrasound waves
How are veins displayed on the spectral trace?
Shades of blue and green
What does the spectral Doppler trace measure?
Speed of blood at a particular point
Measuring within the middle of artery
Between 2 little gates
How is the line of the spectral trace angled?
It is in the same direction as the artery which should be close to direction of blood
What can the characteristic of waveform tell you?
Which artery is ambiguous
Where is the ultrasound transducer surface placed on?
On the skin
What is the ultrasound tracer sending out?
Ultrasound pulses that are parallel to the white line
What is the path for the ultrasound pulses?
Ultrasound pulses that are parallel to the sides of the green box which looks like a parallelogram
What is the colour Doppler Imaging measuring?
Blood flow at somewhere between 1 and 2 cm
Nearly 2cm into the tissue
What is a sample volume?
Measuring at a specific speed
What is aligned within the vessel?
Angle correction line
What does the envelope of the spectral Doppler trace measure?
Blood velocity
What is the envelope?
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 the Doppler shift?
The difference between received frequency of ultrasound and transmitted frequency and measure angle theta
What is constant?
Speed of sound around 15-40m/s
What is the angle measured by?
The scanner for particular values of theta
What is the Doppler velocity measurement theoretically?
Doppler shift = 0 for theta = 90°
What are errors in theta and velocity significant for?
theta > 60°
What is the best practice for Doppler velocity measurement?
theta < 60° and fixed
What are doppler frequencies?
Audible
Audible range - 20kHz
How is the signal from vessel walls filtered out?
Using a ‘‘wall filter’’
High amplitude, low Doppler shifts
What are the number of sources of error for Doppler velocity measurement?
- Measurements can be inaccurate
2. Subject to significant inter-intraobserver variability
What are the types of Doppler ultrasound device?
- Continuous wave (CW)
- Pulsed Wave (PW)
- Doppler Imaging
What does Continous wave measure?
Velocities over a wide range of depths
What are Pulsed wave (PW) gated to measure?
velocity at a particular depth (i.e. within a sample volume)
What can Pulse repetition frequency (PRF) of Pulsed wave determine?
Maximum velocity that can be measured
~2-8kHz
What can Pulsed wave Doppler measure?
Velocities just in a small region
What is gating?
Changing the depth of the region
Screen out other parts of the signal and just retain signal that is particular depth of where you want to measure speed of blood
What is colour doppler?
Colour = velocity + direction
What is power Doppler?
Colour = strength of Doppler signal (~flow/no flow)
Colour Doppler
not accurate enough to be used quantitatively
What are examples of Colour Doppler Artefacts?
- Noise/flash artefacts
- Brief of random colour all the way across the image
- Vessel is being obscured by flash artefacts
- Aliasing
What is Aliasing?
• The 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 does pixel colour represent?
= amplitude squared
power of received Doppler shift signal
What doesnt Power Doppler Imaging provide?
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 Power Doppler Imaging?
- Extracranial
2. Intracranial
What are the features of Extracranial?
- Assessment of carotid atherosclerotic stenosis
- Arterial dissection
- Carotid body and head and neck tumours
What are the features of Intracranial?
• Assessment of arterial stenosis
- Atherosclerosis, acute stroke, sickle cell, and SAH
• Evaluation of collateral flow and arterial malformations
• Aneurysm detection
- Thin walled enlargement of a blood vessel
• 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 the common carotid artery split into?
- ICA
2. ECA
What is a common site for artherosclerosis?
Carotid bifurcation
Carotid atherosclerosis
~15-20% stroke
What are embolic stroke?
Fragment of disease
Blood clot that is becoming dislodged from the plaque and travels upstream
What is the pathology of artherosclerosis?
- Interior of artery is narrowed
- Thickened region of plaque
- Lumen of artery is narrowed (stenosed)
- Plaques become unstable, rupture and produce emboli –> brain infarction
What are the clinical importance of assessing carotid stenosis?
- Risk factor for stroke and TIA
2. Indication for carotid endarterectomy (surgical removal of plaque)
What the problems for assessing carotid stenosis?
- Several different definitions of stenosis
- Little standardisation i measurement between centres
- Accuracy of methods used for clinical measurement is often limited
What is the aim of influential carotid surgery RCT?
Try and relate the amount of disease [degree of stenosis] to risk of stroke and benefits of surgery
What are the measurements of degree of stenosis?
- Measuring a diameter within a vessel and compare with a reference diameter
What is X REF a diameter of?
- Distal normal ICA (NASCET)
- Carotid bulb (ECST)
- CCA
- Wall at site of maximal stenosis - not possible with angiography
What can measurement of degree of stenosis generalised to?
Cross-sectional area or even volume
What are the disadvantages 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 disadvantages of 3D diagnostic scanners?
- Not portable in general
- Expensive
- Not suitable for realtime imaging
What is the disadvantage 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
- Multi-modal (imaging and velocity measurement)
What are the two 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 (lumen area reduction)
- Relate velocity to stenosis (and in turn clinical significance)
- As lumen narrows, higher velocity – varies with individual patients
Where 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: peak PSV only loosely related to stenosis
- Errors in Doppler velocity measurements
What is Doppler clinically useful for?
Discriminating moderate to severe stenosis
What can variations in blood velocity between patients unrelated to carotid disease be due to?
- Cardiac output
- Blood pressure
- Contralateral disease
What is TCD ultrasound?
- Measurement of cerebral blood velocities
- Low frequency vs diagnostic ultrasound (~2MHz)
- Access via temporal or occipital windows or eye
- Temporal window: MCA –> Circle of Willis
- Occipital window: Basilar arteries
What can TCD ultrasound detect?
- Stenosis
- Aneurysm
- Vasospasm
- Embolus
What is TCD useful for?
Monitoring neurovascular surgical procedures
What are the advantages of TCD ultrasound?
- Low-cost
- Portable
- Real-time velocity monitoring (at multiple sites using modern equipment)
- No contrast agent required
What are examples of real-time velocity monitoring?
- Carbon dioxide reactivity
- Dynamic autoregulation investigations
- Cerebral lateralisation
- Intraoperative and bedside monitoring
What are limitations of TCD?
- Access to limited sites due to limited number of windows
- Not all patients have an acoustic window
- The sample gate is positioned blind (without imaging)
- Higher end equipment combines imaging and Doppler velocity measurements
What is vessel determined by?
velocity waveform characteristics
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 kinking of shunt or poor air seal
