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Medical Physics - INIR: Ultrasound Y2 > Ultrasound Scanner Output Measurements > Flashcards

Flashcards in Ultrasound Scanner Output Measurements Deck (15)
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1

What is the equation for the pressure in an ultrasound wave at location x and time t?

p(x,t) = p_A*sin(wt-kx)
w is the radial frequency
k is the wave-number

2

How is the particle velocity related to the pressure?

u(x,t)=p(x,t)/Z

3

What is the equation for the intensity of the sound wave in terms of the energy density epsilon?

I = epsilon*c

4

What is the equation for the instantaneous intensity of the ultrasound wave in terms of the pressure?

I(x,t) = {p(x,t)^2}/Z

5

What is the equation for the time-averaged intensity of the ultrasound wave in terms of the pressure?

I(x,t) = {p_A(x,t)^2}/(2*Z)

6

What is the equation for the power of the ultrasound beam through a surface S?

E = I.S = [{p_A(x,t)^2}/(2*Z)]*S
=F.c
F is the force
c is the speed of sound in the medium.

7

What are the fundamental parameters we want t measure from an ultrasound beam?

- Pressure and its time and spatial derivatives (Spatial Peak Time-Averaged, Spatial Peak Pulse-Averaged, Spatial Peak, Spatial Averaged).
- Intensity and its time and spatial derivatives.
- Power and its time and spatial derivatives.

8

What are the two measurement devices we have to measure an ultrasound beam?

- Hydrophone
- Force Balance

9

What are hydrophones best used for?

- They are special piezoelectric device used for picking up the local pressure in real time.
- They are best used as a point element and for measurements of the interruption-fre ultrasonic field in water.

10

What do hydrophones measure?

- They directly measure the acoustic pressure.

11

What are the advantages and disadvantages of using a hydrophone?

+ Accuracy
+ Detailed field measurements

- Complex devices requiring a high level of competence to use.

12

What are force-balances best used for?

- General measurements of probe/tansducer output.

13

What are the tree types of radiation force balance?

- Reflection
- Absorption
- Combined

14

What is the equation for calculating the power from a force balance reading?

F=(E_0)/c=I_0*cos(theta)*(S/c)
E_0 is the acoustic power
c is the speed of sound in the medium
I_0 is the acoustic intensity
theta is the angle of the reflector (45) or absorber (0) to the horizontal.
S is the surface area of the reflector/absorber.

15

What are the advantages and disadvantages of a radiation force balance?

+ Quick and simple

- Requires calibration with a CW 3.5Mhz check source.
- Curvilinear probes need a conversion factor to account for beam geometry.
- Does not provide detailed wave information