C6: Attenuation Flashcards by M B

Define attenuation

weakening of sound as it travels

How well did you know this?

Not at all

Perfectly

1 Bel = how many decibels (dB)

How well did you know this?

Not at all

Perfectly

how many dB is 100% power in US

0dB

How well did you know this?

Not at all

Perfectly

what is the formula for dB?

dB = 10log (new Intensity/original Intensity)

How well did you know this?

Not at all

Perfectly

can the dB formula also be used to calculate power and voltage?

yes… substitute intensity in the original formula for power or voltage

How well did you know this?

Not at all

Perfectly

how does a 3 dB drop effect intensity?

a drop of 3 dB will half the original intensity

How well did you know this?

Not at all

Perfectly

how does a 10 dB drop effect intensity?

a drop of 10 dB is 0.1 of the original intensity

How well did you know this?

Not at all

Perfectly

definition of attenuation coefficient

the amount of attenuation that occurs with each one centimetre travelled

How well did you know this?

Not at all

Perfectly

how many dB of attenuation occurs in soft tissue for every 1 cm travelled per 1 MHz

0.5 dB

How well did you know this?

Not at all

Perfectly

what is the attenuation coefficient for soft tissue

1/2 frequency

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

attenuation varies with which 3 things

nature of tissue
frequency
depth

How well did you know this?

Not at all

Perfectly

what are the 5 reasons that attenuation occurs

WARRS:

Wave-front divergence
Absorption
Reflection
Refraction
Scatter

How well did you know this?

Not at all

Perfectly

what is the most dominant factor effecting attenuation and what % of attenuation does it account for

absorption

80%

How well did you know this?

Not at all

Perfectly

define absorption

the conversion of sound into heat

How well did you know this?

Not at all

Perfectly

3 factors that effect absorption

which is the most dominant factor

viscosity
relaxation time
frequency

frequency is the most dominant

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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.

How well did you know this?

Not at all

Perfectly

is relaxation time relatively constant

yes

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

what is the formula for acoustic impedance

acoustic impedance (Z) = density (p) x velocity (c)

How well did you know this?

Not at all

Perfectly

units for impedance

Rayls (z)

does acoustic impedance depend on frequency?

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

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

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

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