Flashcards in Doppler Physics Deck (20)

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1

## What is the equation for the received doppler frequency as a function of the scatter velocity?

###
f_d=[2*f_t*vcos(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

2

## 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)

3

## 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.

4

## What is the best shape to model RBCs as for backscatter equations?

### Cylinders

5

## 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.

6

## How does flow type effect the backscatter coefficient?

### Turbulent flow produces large-scale density variations which scatter ultrasound more strongly due to their size.

7

## How does frequency affect backscatter coefficient?

### - Scattering is weak at normal frequencies but increases with increasing frequency.

8

## How are attenuation and sound speed affected by the haematocrit?

### Both increase linearly with haematocrit.

9

## Under what conditions does the equation of continuity hold?

### Non-pulsatile flow in a rigid tube.

10

## What is the Bernoulli equation?

###
E_tot = p_s + p_d + rho*gh + 0.5*rho*v^2

p_s is the static pressure of the closed system.

p_d is the dynamic pressure due to the heart pumping.

11

## At what point does volume flow decrease in a stenosis?

### 50% stenosis

12

## What happens to the velocity at 5% stenosis?

### Peak velocity doubles.

13

## At what point is there trickle flow?

### 95% stenosis.

14

## What is the Poiseuille equation?

###
Q = (8nL/pi*a^4)dp

n = blood viscosity

a = artery radius

dp = change in pressure

L = vessel length.

15

## At what point does the Poiseulle equation break down?

### When turbulence begins to form.

16

## 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.

17

## 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.

18

## 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.

19

## 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.

20