Module 6 : Attenuation Flashcards
what is attenuation
- weakening of sound as it travels
what are three reasons attenuation is of great clinical importance
- limits imaging depth
- must be compensated for
- useful in diagnosis
what is 1 bel equal to
- 10 decibels
what does a 3dB drop equate to
- 1/2 original intensity
what does a 10 dB drop equate to
1/10 original intensity
what is the attenuation coefficient
- amount of attenuation that occurs with each one centimetre travels
what is the attenuation coefficient in soft tissue
- 0.5dB drop occurs in every one centimeter travelled per 1 MHz
- 1/2 frequency
how do we calculate the total attenuation
- TA = attenuation coefficient x path length
calculating TA in soft tissue
TA = 1/2f x path length
what is the half value layer
- distance sound must travel in a material to reduce the intensity to half its original value
what three things doe attenuation vary with
- nature of tissue
- frequency of ultrasound
- depth
what are the 5 reasons attenuation occurs
- absorption
- reflection
- refraction
- scatter
- wave front divergence
what is the number one reason attenuation occurs
- absorption
what is absorption
- conversion of sound energy to heat
- dominant factor in attenuation
what three factors affect absorption
- viscosity
- relaxation time
- frequency
what is viscosity
- ease in which molecules can slide past each other
through which process is sound energy converted to heat
- friction
how does viscosity affect attenuation
- increased viscosity provides greater resistance more friction and increased attenuation
what is the relaxation time
- when a mechanical force is applied to a molecule it will vibrate
- the time it takes to come to rest is relaxation time
how does relaxation time affect attenuation
- if molecule can’t come to rest before the next compression phase (increased relaxation time) than more energy is required to reverse its directions leading to increased heat
how does frequency affect attenuation
- number one reason for absorption
- increased frequency then less time available for molecule to recover during relaxation process increasing absorption
what are the two types of reflection
- specular
- non specular
what is a specular reflection
- when sound hits a larger smooth surface, greater than one wavelength in diameter
what in the body is a good specular reflector
- diaphragm
what 2 things does the intensity of the reflected sound depend on
- angle of incidence
- acoustic impedance of two mediums
how does angle of incidence affect the intensity of the reflected sound
- angle of incidence = angle of reflection
- reflections that are more non perpendicular may not return to the probe
what is acoustic impedance
- aka characteristic impedance
- relationship between acoustic pressure and the speed of particle vibrations in a sound wave
what is the impedance equation
- impedance (Z) = density x velocity
do Z values vary between tissue
- yes due to different density and stiffness
does acoustic impedance depend on frequency
- no
what difference between Z values will cause a bigger reflection
- larger Z value difference
how can we calculate the amount of sound reflected at an interface
- intensity reflection coefficient
- IRC = reflected intensity / incident intensity
- IRC = (Z2 - Z1) / (Z2 - Z1) ^2
how do we calculate the intensity transmission coefficient
ITC = 1 - IRC
what is refraction deal with
- deals with the sound transmitted across an interface
- specifically when velocities differ across the interface and angle of incidence non perpendicular
what is snells law
- sound can bend across an interface due to a difference in media velocity
if velocities are equal across the interface what happens to the sound
- no refraction occurs
if the velocity of the first medium is greater than the second what happens to the sound
- bend towards normal
if the velocity of the first medium is less than the second what happens to the sound
- bend away from the normal
what is total internal reflection
- special type of reflection when V1 is less than V2 and angle reaches a critical value
what is scatter a result of
- sound interacting with interfaces smaller than a wavelength and rough
what is scatter responsible for
- internal texture of organs
what does scatter result in
- incident sound beam breaking up into many different echoes
- these echoes will have intensities that are a fraction of the incident beam reflecting in any direction
what 2 things is the amount of scatter dependant on
- frequency
+ higher frequency increased scatter - reflector size
+ smaller reflector increase scatted
what is backscatter
- when sound is directed back to its origin
what is backscatter responsible for
- organ parenchyma
what is acoustic speckle
- brightness non uniformities from interference patterns from echoes that have undergone multi path scattering
what is Rayleigh scatter
- when sound interacts with RBCs that are much smaller than a wavelength that are very very weak
how does wave front divergence affect attenuation
- as sound beam diverges the intensity of the beam weakens and adds to overall attenuation of the beam
