Ultrasound: Wave propagation

Question 1

What range of frequencies is normally used in medical ultrasound imaging?

Answer 1

1.5 - 15 MHz

Question 2

What range of wavelengths is normally used in medical ultrasound imaging?

Answer 2

100 - 1000 um

Question 3

Which clinical application uses the lowest ultrasonic frequencies?

Answer 3

Cardiac

Question 4

Which clinical application uses the highest ultrasonic frequencies?

Answer 4

Musculoskeletal

Question 5

Which clinical application uses a phased array transducer?

Answer 5

Cardiac

Question 6

What type of ultrasonic wave is used in B-mode imaging?

Answer 6

Pulsed longitudinal

Question 7

What values of sound speed are typically observed in soft tissue?

Answer 7

1.54 +/- 0.10 km/s

Question 8

Which of these tissue types usually has the lowest sound speed?

Answer 8

fat

Question 9

Which of these tissue types usually has the highest sound speed?

Answer 9

bone

Question 10

What are the shapes of the wavefronts in the near and far fields of a plane, circular transducer?

Answer 10

Plane and convex

Question 11

How do the length of the near field and the angle of divergence of a plane, circular transducer change as the drive frequency is increased?

Answer 11

The former increases and the latter decreases

Question 12

What, roughly, is the best spatial resolution achievable with commercial, non-ophthalmological ultrasound scanners?

Answer 12

300um

Question 13

Calculate the sound speed in a soft tissue with a density of 1050 kg/m3, a bulk modulus of 2.5 GPa, and a shear modulus 5.0 kPa. Answer in units of m/s.

Answer 13

1540

The sound speed is simply c0 = (K/ρ0)1/2 where K is the bulk modulus and ρ0 is the density, giving a value of about 1540 m/s.

Note: the shear modulus is not used in the calculation of ‘sound speed’, which is normally assumed to refer to the longitudinal wave velocity

Question 14

A focused, circular transducer has a diameter of 2 cm and a focal length of 15 cm. If it is driven at a frequency of 2.5 MHz, what is the beam width at the transducer? (Answer in mm).

Answer 14

20

The beam width at the transducer is simply the diameter of the transducer

Question 15

A focused, circular transducer has a diameter of 2 cm and a focal length of 15 cm. If it is driven at a frequency of 2.5 MHz, calculate the beam width in the focal region. (Answer in mm)

Answer 15

9.25
If we assume c0 = 1540 m/s, then the wavelength is λ = c0/f = 0.62 mm.
The beam width in the focal region is w = Fλ/a = 9.3 mm.