Acoustic Fundamentals Flashcards
How does ultrasound imaging work?
Ultrasound pulses in (penetrates through skin with coupling gel)
Sound propagation and reflection (interaction with energy)
ultrasound out
image formation and display
What does an ultrasound machine contain?
display
controls
probes
electronics
What does the probe contain?
Individual elements which each produce their own A-line
What is the frequency of the ultrasound pulses which are sent into the tissue?
a few MHz (3-15 MHz)
What is the wavelength of ultrasound in tissue?
0.1-0.5mm
What is the ultrasound detected with when it is reflected?
By the same transducer that sent them in
What is ultrasound imaging used for?
cardiology
obstetrics
abdominal ultrasound
What type of wave is ultrasound?
A mechanical wave, a vibration of tissue
What can shock wave lithotripsy be used for?
To break up kidney stones
What does a probe produce in the tissue?
Lines which each produce an A line
What is a propagating wave?
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
What is a compressional (longitudinal) wave?
Particle motion is parallel to wave direction
What is a shear (transverse) wave?
Particle motion is perpendicular to wave direction
What type of wave are ultrasound waves?
Longitudinal/compressional waves
What is the audible range?
> 20Hz
< 20kHz
What is infrasonic?
<20Hz
What is ultrasonic?
> 20kHz
What is tissue modelled as in ultrasound imaging?
A fluid (as significant fraction of human body is water)
How does the piston transducer cause rarefactions and compressions?
It squashes tissue which is elastic and pushes back and expands
This causes the next tissue to also expand continue the process
What causes rarefaction and compressions?
The change in potential and kinetic energy
What is the continuum assumption?
The medium through which the ultrasound wave is travelling is continuous
(molecular level details can be ignored)
When is the continuum assumption valid?
The acoustic wavelength»_space; intermolecular distance
Why is tissue not treated like a solid (shear wave)?
Shear wave speed is very low
Shear waves are not generated efficiently by ultrasound transducers
Shear wave attenuation can be quite high
What is a wavelength?
The distance between successive points of equal phase
What is the wave number/spatial frequency?
The characteristic spatial length of a wave (rad m^-1)
k = 2π/λ
What is the equation of the speed of compression wave?
c = squareroot of bulk modulus/density
What is the period?
The time between successive points of equal phase
What is the frequency?
The characteristic temporal length of a wave
ω = 2πf = 2π/T
What is particle displacement?
vector quantity
Variation of particles about mean position
(x,y,z)
What is the total particle velocity?
u_T = u_0 + u
total = ambient + acoustic
In a plane wave, are the acoustic particle and particle velocity in phase?
No out of phase
What is the acoustic intensity?
The rate of acoustic energy flow per unit area per unit time
W m^-2
What is the instantaneous acoustic intensity?
Pressure x particle velocity
(force per unit area) x distance per unit time
What is the sound power?
The rate of acoustic energy flow across a specified closed surface S with a normal n
What are decibels?
A ratio of two quantities with units of power
What happens with an acoustic wave propagates?
The pressure, density, temperature and particle velocity all oscillate about their background values
