Doppler Physics Flashcards
What is the equation for the received doppler frequency as a function of the scatter velocity?
f_d=[2f_tvcos(theta)]/c_0
c_0 is the speed of sound of the ultrasound wave
v is the speed of the scatterer
f_t is the transmitted ultrasound frequency
For rayleigh scatterers how is the scattering power related to the scatterer size and wave frequency?
Scattered power is proportional to (a^6)*(f^4)
How does the backscatter coefficient change with increased haematocrit?
- Initially increases to a peak (25%) at 20% haematocrit, then falls off.
- At physiological levels, the RBCs are closer together, so the scatterers are plasma holes in an RBC continuum.
What is the best shape to model RBCs as for backscatter equations?
Cylinders
How is the scattering different at low velocities?
At low velocities, the cells stick together in a rouleau and so the scattering is more anisotropic.
How does flow type effect the backscatter coefficient?
Turbulent flow produces large-scale density variations which scatter ultrasound more strongly due to their size.
How does frequency affect backscatter coefficient?
- Scattering is weak at normal frequencies but increases with increasing frequency.
How are attenuation and sound speed affected by the haematocrit?
Both increase linearly with haematocrit.
Under what conditions does the equation of continuity hold?
Non-pulsatile flow in a rigid tube.
What is the Bernoulli equation?
E_tot = p_s + p_d + rhogh + 0.5rho*v^2
p_s is the static pressure of the closed system.
p_d is the dynamic pressure due to the heart pumping.
At what point does volume flow decrease in a stenosis?
50% stenosis
What happens to the velocity at 5% stenosis?
Peak velocity doubles.
At what point is there trickle flow?
95% stenosis.
What is the Poiseuille equation?
Q = (8nL/pi*a^4)dp n = blood viscosity a = artery radius dp = change in pressure L = vessel length.
At what point does the Poiseulle equation break down?
When turbulence begins to form.
How do arterial velocity profiles change throughout the vessel?
Flow is fast near the centre, but for steady flow from a reservoir the profile is initially blunt but becomes parabolic further along.
What is the difference between parabolic and blunt flow in a curved vessel?
Parabolic flow - highest velocity is towards the outer edge
Blunt flow - highest velocity is towards the inner edge.
Describe the flow in the carotid bulb.
- Higher velocities toward the inner edge of the bulb.
- Reverse flow near the outer edge, especially during systole.
What is the equation for the pulsatility index?
PI=(S-D)/M
S = Velocity during systolic pressure
D = Velocity during diastolic pressure
M = time average mean pressure of cycle.
What is the equation for the Resistance index?
RI=(S-D)/S
S = Velocity during systolic pressure
D = Velocity during diastolic pressure
