SVT Day Flashcards
What is acoustic impedance measures in?
Rayls or Pa per cm
What is the equation for acoustic impedance/
Z = pc
What is the intensity reflection coefficient?
Intensity reflected / Intensity incident
- the bigger the difference, the more reflection
Why does bone cause dropout?
Bone has a very high acoustic impedance compared to soft tissue
What is a specular reflection?
Reflection at a smooth, straight boundary
What is Rayleigh scattering?
Occurs with reflectors much smaller than the ultrasound wavelength
- can cause speckle in B-mode
Why is Rayleigh scattering useful for Doppler?
We do not normally scan at 90 degrees in Doppler
Why is the sky blue?
Short wavelength blue light gets scattered more than longer wavelengths
What is Snells law?
The law of refraction
sine(theta2) / sine(theta1) = c2 / c1
What causes attenuation?
absorption, scattering, reflection, refraction, beam divergence
What are the units for attenuation?
dB / cm / Mhz
How is axial resolution calculated?
Axial resolution = 1/2 pulse length
What is Huygen’s Principle?
Array transducers use multiple piezoelectric elements and combine multiple elements to form the ultrasound beam
Why do superficial structures sometimes appear difficult to see?
The are too close for interference and focussing of the beam
What is cavitation?
Ultrasound pressure waves can push fluid out the way - causes rarefaction and expansion.
- can cause tiny microbubbles which can then burst and release energy as heat
- can damage surrounding objects
How can you minimize cavitation?
Reduce MI
What is Ti?
The ratio of total acoustic power to the power needed to raise 1kg of tissue by 1 degree
What are the different ways of measuring ultrasound beam intensity?
- Spatial peak, temporal average
- Spatial average, temporal average
- Spatial average, temporal peak
- Spatial peak, temporal peak
What is the Doppler angle?
The angle between the direction of flow and the transducer lens
Why does aliasing occur due to vessel tortuousity?
Angle becomes closer to zero, so causes a greater Doppler shift
What can strong side lobes do?
Degrade lateral resolution
What is apodization?
Strength of transducer signal becomes weaker towards edges of transducer - to reduce side lobes
What are two advantages of using arrays?
- They enable electronic beam steering
- They enable electronic beam focusing
How is electronic focussing achieved?
By introducing time delays in pulse excitation of different elements
How is receive focus achieved?
By introducing time delays of different durations to the individual signals so the signals will be in-phase when summed
What is frame rate dependent on?
Colour line density
Width and depth of colour box
Number of pulses per length (Ensemble length or packet size)
PRF
What is flow?
Q = mean velocity * Area
How does the total XSA change as arteries branch?
It increases
What is a Newtonian fluid?
Uniform
What is haematocrit?
The percentage of blood occupied by RBCs
What are laminae?
The layers of fluid in blood
What does laminar mean?
Layers of blood moving over each other
What is Pouseilles Law?
Q = change in pressure / Resistance
Q = (change in P * Pi * r**4) / 8 u L
How does resistance add in series?
Sums normally
How does resistance add in parallel?
1/Rt = 1/R1 + 1/R2 ….
What causes reflections of pulse waves?
Changes in impedance e.g. branches
What happens to pulse wave if there is a distal occlusion?
There is a total reflection of pressure = waveforms with no net flow
- High resistance waveform
What happens to CFA flow during exercise?
There is higher diastolic flow as distal arteries have dilated
What are entrance effects?
When there is a change in area, there is a change in flow profile e.g. more plug-like in a stenosis
What is shear of the velocity profile?
The change in velocity across the velocity profile
What does low shear cause?
Thrombosis
How does curvature change flow?
In laminar, fastest flow will be towards outer wall of the curve
In blunt flow, is opposite
What are the 3 components of energy in blood?
- Pressure energy
- Gravitational energy
- Kinetic energy (velocity)
What is boundary layer circulation?
Recirculation of flow due to changes (increases) in area e.g. carotid bulb or post-stenotic dilatation
When should you measure velocities in the carotid bulb?
Never, unless stenosis within
What happens to mean and peak velocity in stenosis?
Mean velocity will increase
Peak velocity might not increase until stenosis becomes more severe
What are viscous losses?
Loss of energy due to friction
What type of losses are entrance and exit losses?
Inertial losses
How do plaques ulcerate?
Pressure differences can cause forces across the plaque
What is pressure drop proportional to?
Velocity
Is pressure loss greater in systole or diastole?
Systole (higher velocity)
What can a damped waveform indicate?
Proximal disease
