Ultrasound

Question 1

What is ultrasound?

Answer 1

High frequency sound (longitudinal) waves, >20kHz. In medical imaging, ~2MHz.

Question 2

How is ultrasound produced?

Answer 2

Piezoelectric effect.
An alternating voltage is applied across a piezoelectric crystal.
This causes it to shrink and expand at the same frequency as the alternating voltage.
These vibrations cause ultrasound PULSES to be produced at the same frequency.

Question 3

How is ultrasound detected?

Answer 3

Piezoelectric effect.
The same crystal vibrates when ultrasound is incident upon it.
The induced e.m.f. has the same frequency as the incident ultrasound.

Question 4

What is an A-Scan?

Answer 4

A pulse of ultrasound is sent into the body and reflections are detected with the time elapsed.
Thickness = (c*t)/2

Question 5

What is a B-scan?

Answer 5

Lots of A-scans are taken by moving the transducer around the patient's body.
The depth (from time) and nature (from amp.) are used to build a 2D image.

Question 6

Principles of ultrasound

Answer 6

Piezoelectric crystal used to send pulses of ultrasound.
Ultrasound reflected at boundary of tissue.
Intensity of reflected pulse depends on acoustic impedances at boundary.
Time delay determines thickness.