Medical Physics: Ultrasound

Question 1

Describe the properties of ultrasound.

Answer 1

A longitudinal wave similar to sound but with a frequency above audible range/above 20kHz.

Question 2

Describe the piezoelectric effect.

Answer 2

When voltage is applied across a piezoelectric crystal in one direction the crystal shrinks slightly, and when the voltage is applied in the alternate direction the crystal expands slightly. The crystal will expand and contract at the same frequency with which the voltage alternates. Voltage induces strain in the crystal.

Question 3

Explain how ultrasound transducers emit and receive high-frequency sound.

Answer 3

• The expanding and contracting of the crystal produces the ultrasound wave.
• The same crystal is then used to detect the signal coming back at the transducer, when the crystal receives the signal it will vibrate, inducing a potential difference which can be measured.

Question 4

Describe the principles of ultrasound scanning

Answer 4

• Pulse of ultrasound transmitted into body where it is reflected at the boundary between different tissues.
• Reflected wave is detected and processed.
• Time for echo to reach detector indicates depth of tissue boundary.
• Intensity of echo gives information about the tissue boundary.

Question 5

Describe the difference between an A-scan and a B-scan.

Answer 5

In a B-scan, the transducer is moved around the body. This produces a two-dimensional outline of an organ, compared to the one-dimensional A-scan giving the depth/thickness of tissue.

Question 6

Describe the importance of impedance matching.

Answer 6

Increases the fraction of I0 that is transmitted. Reduces α (where Ir/I0 = α)

Question 7

Explain why a gel is required for effective ultrasound imaging techniques

Answer 7

• Amount of reflection of a wave at a boundary depends on the difference in acoustic impedance of the two media.
• Acoustic impedance is speed of wave x density of medium.
• Acoustic impedance of air is very different from that for skin. So very little ultrasound energy would pass into the skin at the skin-air boundary.
• Coupling medium has acoustic impedance close to that of skin (and probe) so greatly reduces reflection at skin surface.