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Biomedical Ultrasound > Acoustic Fundamentals > Flashcards

Acoustic Fundamentals Flashcards

1
Q

How does ultrasound imaging work?

A

Ultrasound pulses in (penetrates through skin with coupling gel)
Sound propagation and reflection (interaction with energy)
ultrasound out
image formation and display

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2
Q

What does an ultrasound machine contain?

A

display
controls
probes
electronics

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3
Q

What does the probe contain?

A

Individual elements which each produce their own A-line

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4
Q

What is the frequency of the ultrasound pulses which are sent into the tissue?

A

a few MHz (3-15 MHz)

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5
Q

What is the wavelength of ultrasound in tissue?

A

0.1-0.5mm

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6
Q

What is the ultrasound detected with when it is reflected?

A

By the same transducer that sent them in

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7
Q

What is ultrasound imaging used for?

A

cardiology
obstetrics
abdominal ultrasound

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8
Q

What type of wave is ultrasound?

A

A mechanical wave, a vibration of tissue

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9
Q

What can shock wave lithotripsy be used for?

A

To break up kidney stones

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10
Q

What does a probe produce in the tissue?

A

Lines which each produce an A line

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11
Q

What is a propagating wave?

A

A mean by which energy can be transferred from one point to another without transfer of matter

Particles oscillate about their mean position

Periodic compression and expansion of an elastic medium

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12
Q

What is a compressional (longitudinal) wave?

A

Particle motion is parallel to wave direction

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13
Q

What is a shear (transverse) wave?

A

Particle motion is perpendicular to wave direction

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14
Q

What type of wave are ultrasound waves?

A

Longitudinal/compressional waves

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15
Q

What is the audible range?

A

> 20Hz
< 20kHz

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16
Q

What is infrasonic?

A

<20Hz

17
Q

What is ultrasonic?

A

> 20kHz

18
Q

What is tissue modelled as in ultrasound imaging?

A

A fluid (as significant fraction of human body is water)

19
Q

How does the piston transducer cause rarefactions and compressions?

A

It squashes tissue which is elastic and pushes back and expands
This causes the next tissue to also expand continue the process

20
Q

What causes rarefaction and compressions?

A

The change in potential and kinetic energy

21
Q

What is the continuum assumption?

A

The medium through which the ultrasound wave is travelling is continuous

(molecular level details can be ignored)

22
Q

When is the continuum assumption valid?

A

The acoustic wavelength&raquo_space; intermolecular distance

23
Q

Why is tissue not treated like a solid (shear wave)?

A

Shear wave speed is very low

Shear waves are not generated efficiently by ultrasound transducers

Shear wave attenuation can be quite high

24
Q

What is a wavelength?

A

The distance between successive points of equal phase

25
Q

What is the wave number/spatial frequency?

A

The characteristic spatial length of a wave (rad m^-1)

k = 2π/λ

26
Q

What is the equation of the speed of compression wave?

A

c = squareroot of bulk modulus/density

27
Q

What is the period?

A

The time between successive points of equal phase

28
Q

What is the frequency?

A

The characteristic temporal length of a wave

ω = 2πf = 2π/T

29
Q

What is particle displacement?

A

vector quantity
Variation of particles about mean position
(x,y,z)

30
Q

What is the total particle velocity?

A

u_T = u_0 + u
total = ambient + acoustic

31
Q

In a plane wave, are the acoustic particle and particle velocity in phase?

A

No out of phase

32
Q

What is the acoustic intensity?

A

The rate of acoustic energy flow per unit area per unit time

W m^-2

33
Q

What is the instantaneous acoustic intensity?

A

Pressure x particle velocity

(force per unit area) x distance per unit time

34
Q

What is the sound power?

A

The rate of acoustic energy flow across a specified closed surface S with a normal n

35
Q

What are decibels?

A

A ratio of two quantities with units of power

36
Q

What happens with an acoustic wave propagates?

A

The pressure, density, temperature and particle velocity all oscillate about their background values

Biomedical Ultrasound (18 decks)

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