Introduction to US

Question 1

What transducer types are there, what are they used for and what frequencies are typically used? (6 marks)

Answer 1

• Linear array used for vascular and small organs such as thyroid, high frequency to give high resolution, 5-8MHz.
• Curvilinear arrays - used for abdominal and obstetric imaging, lower frequency to offer sufficient penetration (2-5 MHz)
• Phased arrays - used for caridiac applications, small footprint and high penetration to reach through ribs to heart (2-5 MHz)

Question 2

What is meant by CW and PW, and when are they used in US? (3 marks)

Answer 2

Continuous wave and pulsed wave. Most US applications are PW. CW is used in CW doppler imaging.

Question 3

Explain the characteristics of and sketch a CW longitudinal wave? (5 marks)

Answer 3

Longitudinal = compression wave, means that particle motion is in the direction of the wave, sections of compression and rarefraction give rise to the pressure wave. Speed of sound in tissue of the wave is 1540 m/s, particulate velocity is much slower ~ 1-5 m/s.

Question 4

Explain the characteristics of and sketch a CW transverse wave? (5 marks)

Answer 4

Transverse - shear wave, means the particle motion is 90 degrees to the direction of the wave. Like a guitar string. Speed of wave is much slower ~ 5 m/s. Used in shear wave elastography.

Question 5

How is pressure related to intensity? What kind of pressures do scanners go up to? (3 marks)

Answer 5

Do a little sketch… I = k P^2.

5 MPa is some scanners.

Question 6

Give a simple model for the speed of sound? (2 marks)

Answer 6

c = (k/m)^0.5 where k is the stiffness of springs holding the balls together and m is the mass of the balls.

Question 7

What is the speed of sound from the acoustic wave equation? (2 marks)

Answer 7

c = (K/rho)^0.5, K is bulk modulus and rho is density

Question 8

What is the speed of sound in a kidney compared to the assumed speed of sound in tissue? (2 marks)

Answer 8

c is assumed to be 1540 m/s, in a kidney it is around 1590 m/s. Therefore the kidney is actually longer than it appears on the image, by 1590/1540.

Question 9

For a plane circular transducer what is the focal length and divergence equal to? (4 marks)

Answer 9

L = a^2 / lambda

theta = arcsin (0.61 x lamda / a)

a is the radius.

Question 10

What is the beam width for a focussed circular transducer? (2 marks)

Answer 10

W = F lamda / a