C6: Attenuation Flashcards
Define attenuation
weakening of sound as it travels
1 Bel = how many decibels (dB)
10
how many dB is 100% power in US
0dB
what is the formula for dB?
dB = 10log (new Intensity/original Intensity)
can the dB formula also be used to calculate power and voltage?
yes… substitute intensity in the original formula for power or voltage
how does a 3 dB drop effect intensity?
a drop of 3 dB will half the original intensity
how does a 10 dB drop effect intensity?
a drop of 10 dB is 0.1 of the original intensity
definition of attenuation coefficient
the amount of attenuation that occurs with each one centimetre travelled
how many dB of attenuation occurs in soft tissue for every 1 cm travelled per 1 MHz
0.5 dB
what is the attenuation coefficient for soft tissue
1/2 frequency
whats the formula for total attenuation in soft tissue?
TA = 1/2 (f in MHz) x Path length in cm
the attenuation coefficient if 1/2 (f) in this case
what is the half value layer
the distance that sound must travel in a material to reduce the intensity to half its original value (3 dB loss in equal to 1/2 the intensity)
attenuation varies with which 3 things
nature of tissue
frequency
depth
what are the 5 reasons that attenuation occurs
WARRS:
Wave-front divergence Absorption Reflection Refraction Scatter
what is the most dominant factor effecting attenuation and what % of attenuation does it account for
absorption
80%
define absorption
the conversion of sound into heat
3 factors that effect absorption
which is the most dominant factor
viscosity
relaxation time
frequency
frequency is the most dominant
describe viscosity and how it effects absorption
- the ease at which molecules slide past one another
- increased viscosity provides greater resistance and more friction… this friction converts the sound waves to heat which increases attenuation
describe relaxation time and how it effects absorption
- time it takes a molecule to come to rest after a mechanical force is applied
- if a molecule cant come to rest before it is stimulated again, than more force is needed to stimulate this molecule again which produces heat.
is relaxation time relatively constant
yes
how does increasing frequency effect relaxation time
increased frequency means there is less time for molecules to come to rest because sound waves are being produced at a faster rate… this increases absorption and attenuation
what are the 2 types of reflection, describe them
-specular:
occurs when a sound wave hits a large, smooth surface, larger than 1 wavelength
-non-specular:
occurs when a sound wave hits a small, rough surface, smaller than 1 wavelength
intensity of reflected sound depends on what two things
angle of incidence (90 is best, sound comes straight back)
acoustic impedance of the two media
what is the formula for acoustic impedance
acoustic impedance (Z) = density (p) x velocity (c)
units for impedance
Rayls (z)
does acoustic impedance depend on frequency?
no
what effect does a larger difference in z values of the media have on reflection
larger Z = bigger reflection
what is the intensity reflection coefficient
amount of sound that reflects at an interface
formula for intensity reflection coefficient
IRC = (Z2 - Z1 / Z2 + Z1) ^2
or IRC = reflected intensity/incident intensity
- it does not matter which Z value you choose as Z2 or Z1*
what is the formula for intensity transmission coefficient
ITC = 1 - IRC… the more sound is reflected, the less is transmitted
how does the difference b/w the impedance values of 2 interfaces effect reflection?
greater difference in values = more reflection of sound
what happens to reflection if impedance is equal across interfaces
no reflection
in general, why does reflection occur
due to difference in Z values at an interface
can you have 2 tissues w/ difference velocities but the same z value
yes
Is reverberation a type of reflection
yes
what 2 criteria must be met in order for refraction to occur
- different velocities of 2 media
- non-perpendicular angle of incidence
what is the formula for Snells law
{sin theda (incidence) / sin theda (transmitted)} = velocity (incidence) / velocity (transmitted)
Snells Law: if velocities across an interface are equal will there be any refraction
no
Snells Law: if velocity 1 is grater than velocity 2 how will the refracted sound beam travel
towards the normal
Snells Law: if velocity 2 is grater than velocity 1 how will the refracted sound beam travel
away from the normal
Snells Law: when does total internal reflection occur and what is it
occurs when V2 is greater than V1 and the angle reaches a critical value
-no sound will be transmitted back to the probe
in general, why does refraction occur
if there is a difference in velocities at an interface and there is non- perpendicular angle of incidence
is refraction related to Z values
no
can you have equal Z values and have different velocities at an interface
yes (because density can be different)
what is scatter
why does it occur
- scatter is the breaking up of a sound beam into many different echos, it is responsible for the internal texture of organs and causes non uniform brightness
- it is the result of sound interacting with interfaces that are rough and smaller than 1 wavelength (e.g. non-specular reflectors)
does scatter usually occur with heterogenous or homogenous media
heterogenous (cells or particles)
does scatter depend on sound direction
no
what does back scatter refer to
any scatter that returns back to its origin
the amount of scatter depends on what 2 factors
- frequency
- reflector size
how do frequency and reflector size effect scatter
higher frequency = more scatter
smaller reflector = more scatter
what is responsible for the parenchyma that we see?
backscatter
what phenomenon is responsible for acoustic speckle
scatter
what is acoustic speckle
how does we correct for it
areas of non-uniform brightness seen in the parenchyma
correct by using persistence
what is Rayleigh scatter
a specific type of scatter that occurs when sound waves hit RBCs (b/c they are so small)
-this type of scatter is very weak and keeps us from seeing blood flow at faster velocities
how does wavefront divergence contribute to attenuation
as the sound beams diverge, the intensity of the beam weakens as it covers a larger area
…this contributes to the overall attenuation of sound
why is attenuation clinically important (3 reasons)
limits our imaging depth
it needs to be compensated for (TGC, gain, etc)
can be useful to diagnose (shadowing)
what do gains and TGCs express
the amount of amplification thats required to optimize returning echos (in decibels)
instrument output, dynamic range, and gain and TGCs all use which relative units
decibles
why do Z values vary with different tissues in the body
due to differences in density and stiffness
why does scatter create areas of non-uniform brightness
due to the interference patterns from echoes that have undergone multi-path scattering… resulting in some areas of increased intensity and decreased intensity
