Ultrasound Scanner Output Measurements

Question 1

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

Answer 1

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

Question 2

How is the particle velocity related to the pressure?

Answer 2

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

Question 3

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

Answer 3

I = epsilon*c

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

• 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.

Question 8

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

Answer 8

• Hydrophone

- Force Balance

Question 9

What are hydrophones best used for?

Answer 9

• 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.

Question 10

What do hydrophones measure?

Answer 10

• They directly measure the acoustic pressure.

Question 11

What are the advantages and disadvantages of using a hydrophone?

Answer 11

+ Accuracy
+ Detailed field measurements

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

Question 12

What are force-balances best used for?

Answer 12

• General measurements of probe/tansducer output.

Question 13

What are the tree types of radiation force balance?

Answer 13

• Reflection
• Absorption
• Combined

Question 14

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

Answer 14

F=(E_0)/c=I_0cos(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.

Question 15

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

Answer 15

+ 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