Ultrasound 1

Question 1

Sound is a longitudinal wave which requires:

Answer 1

• Needs a medium to travel
• Particles in the medium move
  to carry the sound

Question 2

Compression is when

Answer 2

regions in the air where the air particles are compressed together

Question 3

Rarefaction is when

Answer 3

regions where the air particles are spread apart

Question 4

Humans can detect frequencies ranging from

Answer 4

20-20,000 Hz or 20 Hz to 20kHz

Question 4

Infrasound is

Answer 4

f < 20 Hz, below the threshold of human hearing, i.e., earthquakes.

Question 5

Audible sound is

Answer 5

f = 20 Hz-20 kHz, range of frequencies audible to a healthy ear, we lose this range over time.

Question 6

Ultrasound is

Answer 6

f > 20 kHz, above the threshold of human hearing.

Question 7

Wavelength is the

Answer 7

distance between crest to crest or trough to trough

Question 8

Amplitude is the

Answer 8

Magnitude of the disturbance travelling through the medium
The amplitude of a sound wave is proportional to the change in pressure

Question 9

Frequency is

Answer 9

the number of complete wave cycles that pass a fixed-point in 1 s

Question 9

Destructive interference is when

Answer 9

Waves that are out of phase can “cancel out” each other

Question 10

Constructive interference is when

Answer 10

Waves that are in phase can add together

Question 11

How does density and temperature affect the speed of sound?

Answer 11

Density – higher the density the higher the speed of sound. Less distance between molecules.

Temperature – higher temperature results in higher wave speed. Molecules can vibrate faster.

Question 12

Longitudinal waves, such as sound is when particles oscillate…

Answer 12

in the direction parallel to wave propagation

Question 13

Transverse waves, such as light, is when particles oscillate…

Answer 13

in the direction perpendicular to the wave propagation

Question 14

Why are ultrasounds used?

Answer 14

Ultrasound is used in medicine as a fast, cheap & non-invasive diagnostic imaging technique.

Question 15

How do we know which frequency to choose in regards to ultrasound?

Answer 15

In general, the shorter the wavelength, the greater the image resolution. This means a high frequency is desirable, remember 𝑐=𝑓𝜆
The problem with this is that higher frequencies are attenuated (absorbed) quicker.

This means that a compromise must be made, generally frequencies of 1-10 MHz are used. Below are rough guides:

7.5-10 MHz used for superficial structures (tendons, ligaments)
5 MHz for vascular imaging
3.5-5 MHz for deeper structures, bladder, pelvis

Question 16

Lateral resolution indicates the ability to distinguish to points ___________ to the path of the ultrasound beam.

Answer 16

perpendicular

Question 17

Axial resolution is the ability to distinguish between two points _________ to the path of the ultrasound beam.

Answer 17

parallel

Question 18

True or false: The narrower the beam, the higher resolution (in general).

Answer 18

True

Question 19

True or false: A shorter wavelength means longer pulses and worse axial resolution.

Answer 19

False, shorter pulses and better axial

Question 20

Axial resolution =

Answer 20

1/2 pulse length

Question 21

Elevation Resolution is the

Answer 21

The ability to distinguish between two points perpendicular to the scan plan. Depends on depth.

Question 22

Pulse length =

Answer 22

Pulse velocity x pulse duration

Question 23

The key component is the piezo element, which consists of

Answer 23

a piezoelectric crystal that can both produce ultrasound and generate a signal from the ultrasound via the piezoelectric effect.

Question 24

How does the piezoelectric effect work?

Answer 24

A piezoelectric crystal changes dimension if a voltage is applied along one direction (can make it vibrate)

Alternatively, if you apply pressure along one direction of a Piezo-electric crystal it generates a voltage (vibrations hitting the crystal will induce a voltage which is interpreted as a signal)

Question 25

What three things can happen to an ultrasound pulse that meets a boundary between two media?

Answer 25

Transmission – wave passed through
Reflection – waves bounces off
Absorption – wave is absorbed

Question 26

What is acoustic impedance (Z) and what depends on it?

Answer 26

The amount of reflection, and transmission that takes place, depends on the Acoustic Impedance (Z) of the two media.
Acoustic Impedance is a measure of how hard it is for the ultrasound wave to travel through the medium.

Question 27

Why is ultrasound gel used?

Answer 27

If the impedance is matched correctly, most of the ultrasound will travel through the material and won’t be reflected.
Ultrasound gel is used to “couple” the probe and the skin to improve transmission.

Question 28

Briefly explain the A-mode scan. Make sure to highlight the key factors which differ it from other modes of ultrasound:

Answer 28

Largely outdated but the method of scanning is useful to study.
Velocity = distance/time
distance = (velocity)(time)
Speed of ultrasound in the medium is known so the distance from the probe is easily determined from the echo time.
Were used to look at the eye before optical coherence tomography (OCT) became more popular.
1D form of ultrasound imaging

Question 29

Discuss the B-mode scan:

Answer 29

Mode that is most usually associated with ultrasound images.
A 2D image is generated of the slice of the patient immediately below the probe head.