Chapter 17: Ultrasound

Question 1

Properties of ultrasound

Answer 1

Freq. > 20kHz
Wavelength < 1mm
Speed of sound in air = 330m/s
Speed of sound in body tissue = 1500m/s

Question 2

How is ultrasound produced

Answer 2

Vibrating source w. freq. of ultrasound

Question 3

Describe the piezoelectric effect

Answer 3

1. Voltage applied in one direction causes crystal to contract, other to expand; i.e. v induces strain
2. Varying stress from ultrasound applied to crystal induces varying e.m.f.

Question 4

When is induced e.m.f. for piezoelectric crystal greatest

Answer 4

When freq. ultrasound = resonant frequency of crystal

Question 5

What’s a transducer

Answer 5

The device that generates and detects ultrasound

Uses a piezoelectric crystal

Question 6

Draw a diagram of a transducer

Answer 6

See page 251

Question 7

How does an ultrasound scan work

Answer 7

Pulses of Ultrasound are directed into the body
Reflected when wave speed changes i.e. at tissue boundary
Time delay used to determine depth
Fraction of reflected signal used to identify tissue

Question 8

What is impedance matching

Answer 8

Use gel w. similar acoustic impedance to skin to reduce ultrasound reflected before entering body
z value of air and skin v. diff. so lots reflected if no gel used

Question 9

Describe an A scan

Answer 9

1. 1 direction only -> find depth
2. Used to work out thickness of bone etc: time between bone boundary pulses = time taken for ultrasound to travel 2x thickness of bone -> thickness = (ultrasound speed x change in time)/2

Question 10

Describe a B scan

Answer 10

1. Detailed image
2. Transducer moved across body (position determined by small sensors
3. Pulse determines depth and nature of reflecting surface
4. 2D image built by positioning dots to represent reflecting surfaces
5. Problems if organ is moving (scanner takes a few seconds to move across body)

Question 11

Describe Doppler ultrasound

Answer 11

1. Doppler effect occurs when waves are emitted/reflected/detected by a moving object
2. If reflector is moving away from source, reflected waves are slightly longer than incident wave
3. If the wavelength is longer but the speed stays the same, freq. will go down
4. Greater the speed of reflector -> larger reflected wavelength -> larger decrease in freq.
5. Speed of reflector deduced from change in frequency of waves
6. Used to monitor blood flow i.e. constant speed or turbulent -> could indicate blockages