Ultrasound II Flashcards
What is high quality spectral Doppler waveform?
Doppler angle less than 60
What are audible frequencies?
Audible
0.1-15 kHz
What does spectral Doppler cover?
A range of frequencies
Flowing blood cells has a range of velocities
Spectral broadening
What does blood have?
Low amplitude echoes
Low high velocities/frequencies shift
What do vessel walls have?
High amplitude (I.e.) strong echoes Low velocity/frequency shift
How can you discriminate blood and tissue echoes on the basis of frequency?
Wall filter
What is demodulation?
Extracting Doppler frequency shifts information a received ultrasound signal
Separation of the Doppler frequencies from underlying transmitted signals
What is high pass filtering?
Removal of tissue signal
What is the frequency estimation?
Where the Doppler frequencies and amplitudes are calculated
What is continuous wave pulse?
Measures the velocity over a wide range of depth
What are PW devices?
Gated
Only echoes revived within a particular time (depth) range are considered
Where do PW devices transmit pulses?
At a rate determined by pulse repetition frequency (PRF)
~ 2-8 kHZ
What is the region from which Doppler signals are obtained?
By the depth and length of gate which are both controlled by operator
Explain how aliasing applies to PW Doppler measurements?
Relationship between RFP and maximum velocity
PW Doppler demodulation is essentially a sampling process
Limit on max velocity that can be measured
What is the main difference between CW and PW?
Estimation of high velocity (blood)
What happens if the pulse repetition frequency (PRF) is too low?
The Doppler frequency shift cannot be estimated properly - aliasing (not in CW)
What happens when PRF is high?
There are a sufficient number of samples to enable the Doppler system to detect the frequency correctly
As PRF stops there comes a point at which the Doppler signal is only just sampled sufficiently
What happens if there are 2 samples per cycle?
Nyquist limit
The max Doppler frequency shift which can be detected is half of PRF
If PRF drops further the Doppler system can no longer calculate the correct frequency
What is aliasing?
The highest Doppler frequency shift that can be measured is equal to PRF/2
What is the colour Doppler ultrasound?
Colour coded velocity information superimposed on B-mode image (received echoes are processed twice)
The colour of each pixel represents average Doppler shift
Frame rate < B-mode imaging
What are the common artefact encountered in Doppler Imaging?
- Incorrect gain settings
- Shadowing
- Aliasing: high velocities appear as low velocities
- Flash artefact
- Colour spilling outside vessel
What are the advantages of power Doppler imaging ?
- Angle independence
- Sensitivity
- Useful for visualising complex vascular geometries
What are the applications of TCD?
- Evaluation of cerebral blood flow
- Access through temporal window
- MCA > circle of Willis
- Stenosis, aneurysm, and embolus detection
- Useful for monitoring neuro vascular surgical procedures
What are the carotid stenosis measurement?
Peak systolic velocity at site of plaque as a measure of degree of stenosis
Direct measurements from B-more also useful
What are the basic principles of ultrasound safety?
- Only perform scan if clinically required
- Obtain all information required to make a diagnosis
- Minimise time that probe is in contact with patient
- Use Doppler modes sparingly, turn only once site has been found using B mode imaging, minimise size of colour box
- Take Notice if thermal and mechanical indices displayed by scanner and change output settings or limit exposure accordingly
- Ensure scanners are properly maintained
What are at-risk groups and tissues ?
- Embryo during first trimester
- Foetus during second-third trimesters
- Foetus when mother has a fever
- Neonate
- The eye
What are the bio effect mechanism?
Thermal: temperature rise
Non thermal: mainly cavitation.
What are risk of a bio effect indicated by?
Mechanical index (MI) - mechanical damage primarily due to cavitation
Thermal index (TI) - thermal damage from a sustained rise in temperature
What is cavitation?
Gas bubbles in a liquid respond to ultrasound waves
MI > 0.3
- possibility of minor damage to neonatal lung
MI > 0.7
- risk of cavitation if a micro bubble contrast agent is being used. And a theoretical risk even if such contrast agents are not used
What is the range of Doppler frequency shift?
Up to about 15kHz
What is Ft and what is it’s range?
Frequency of transmitted ultrasound signal
Range: 2-15 MHz for medical ultrasound
What is C?
Speed of sound
Range: around 1500 m/s
What happens when PRF is too low?
Aliasing can occur in the blood velocity measurements
Which gives rise to a characteristic aliasing artefact
Low PRF means that is it not possible to measure high velocities
PRF = 2. Fs - the Nyquist limit
What is the equation for maximum measurable blood velocity?
Vmax = c. PRF/2ft
