Physics of USS

Question 1

How does USS work?

Answer 1

The US probe converts electrical energy into vibration of the piezoelectric crystals.
This generates ultrasound waves which are transmitted through the patient’s skin and soft tissues.
Different tissues have different densities, so they transmit and reflect US waves in different amounts.
The reflected US waves are detected by a parallel row of piezoelectric crystals, which convert the US into electric energy again.
As the US machine knows the velocity of US through tissues, and the time taken for the US waves to be reflected, it can calculate the depth (as the distance form the probe).

Question 2

How is the image created?

Answer 2

As the US machine knows the velocity of US through tissues, and the time taken for the US waves to be reflected, it can calculate the depth (as the distance form the probe).
Different tissues have different densities so transmit and reflect US to differing amount. As the interface between tissues where the density changes, US is reflected.
This effect can be quantified by the reflection co-efficient (e.g soft tissue to air = 99%)

Question 3

What is the average velocity of USS through tissues?

Answer 3

1540 m/s

Question 4

What is the frequency of audible sound and ultrasound?

Answer 4

audible sound: 20Hz - 20kHz

US: 2mHz - 20mHz

Question 5

What factors affect the clarity of image and are there any things/functions that can improve this?

Answer 5

Absorption - Time gain compensation (larger amplitude US waves transmitted progressively to deeper structures in order to penetrate deeper into tissue and more clearly image deeper tissues)

Attenuation

Artefact - e.g. poster acoustic shadowing, comet tail artefact, reverberation artefact etc

Question 6

Describe the ultrasound wave.

Answer 6

It is a longitudinal wave, meaning particles move in the same direction as the wave. It is transmitted by rarefaction and contraction of particles which transmits energy between the movie particles. It has a frequency of 2mHz - 20mHz.

Question 7

What are the 4 ways in which US waves can interact with tissues?

Answer 7

Transmission

Absorption - increasingly absorbed as US penetrates deeper into tissues.

Attenuation - larger particles do not vibrate coherently so USS energy is converted to heat energy and lost/not reflected.

Reflection - essential for image formation.