Module 6 : Attenuation

Question 1

what is attenuation

Answer 1

• weakening of sound as it travels

Question 2

what are three reasons attenuation is of great clinical importance

Answer 2

• limits imaging depth
• must be compensated for
• useful in diagnosis

Question 3

what is 1 bel equal to

Answer 3

• 10 decibels

Question 4

what does a 3dB drop equate to

Answer 4

• 1/2 original intensity

Question 5

what does a 10 dB drop equate to

Answer 5

1/10 original intensity

Question 6

what is the attenuation coefficient

Answer 6

• amount of attenuation that occurs with each one centimetre travels

Question 7

what is the attenuation coefficient in soft tissue

Answer 7

• 0.5dB drop occurs in every one centimeter travelled per 1 MHz
• 1/2 frequency

Question 8

how do we calculate the total attenuation

Answer 8

• TA = attenuation coefficient x path length

Question 9

calculating TA in soft tissue

Answer 9

TA = 1/2f x path length

Question 10

what is the half value layer

Answer 10

• distance sound must travel in a material to reduce the intensity to half its original value

Question 11

what three things doe attenuation vary with

Answer 11

• nature of tissue
• frequency of ultrasound
• depth

Question 12

what are the 5 reasons attenuation occurs

Answer 12

• absorption
• reflection
• refraction
• scatter
• wave front divergence

Question 13

what is the number one reason attenuation occurs

Answer 13

• absorption

Question 14

what is absorption

Answer 14

• conversion of sound energy to heat

- dominant factor in attenuation

Question 15

what three factors affect absorption

Answer 15

• viscosity
• relaxation time
• frequency

Question 16

what is viscosity

Answer 16

• ease in which molecules can slide past each other

Question 17

through which process is sound energy converted to heat

Answer 17

• friction

Question 18

how does viscosity affect attenuation

Answer 18

• increased viscosity provides greater resistance more friction and increased attenuation

Question 19

what is the relaxation time

Answer 19

• when a mechanical force is applied to a molecule it will vibrate
• the time it takes to come to rest is relaxation time

Question 20

how does relaxation time affect attenuation

Answer 20

• 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

Question 21

how does frequency affect attenuation

Answer 21

• number one reason for absorption

- increased frequency then less time available for molecule to recover during relaxation process increasing absorption

Question 22

what are the two types of reflection

Answer 22

• specular

- non specular

Question 23

what is a specular reflection

Answer 23

• when sound hits a larger smooth surface, greater than one wavelength in diameter

Question 24

what in the body is a good specular reflector

Answer 24

• diaphragm

Question 25

what 2 things does the intensity of the reflected sound depend on

Answer 25

- angle of incidence | - acoustic impedance of two mediums

Question 26

how does angle of incidence affect the intensity of the reflected sound

Answer 26

- angle of incidence = angle of reflection | - reflections that are more non perpendicular may not return to the probe

Question 27

what is acoustic impedance

Answer 27

- aka characteristic impedance | - relationship between acoustic pressure and the speed of particle vibrations in a sound wave

Question 28

what is the impedance equation

Answer 28

- impedance (Z) = density x velocity

Question 29

do Z values vary between tissue

Answer 29

- yes due to different density and stiffness

Question 30

does acoustic impedance depend on frequency

Answer 30

- no

Question 31

what difference between Z values will cause a bigger reflection

Answer 31

- larger Z value difference

Question 32

how can we calculate the amount of sound reflected at an interface

Answer 32

- intensity reflection coefficient - IRC = reflected intensity / incident intensity - IRC = (Z2 - Z1) / (Z2 - Z1) ^2

Question 33

how do we calculate the intensity transmission coefficient

Answer 33

ITC = 1 - IRC

Question 34

what is refraction deal with

Answer 34

- deals with the sound transmitted across an interface | - specifically when velocities differ across the interface and angle of incidence non perpendicular

Question 35

what is snells law

Answer 35

- sound can bend across an interface due to a difference in media velocity

Question 36

if velocities are equal across the interface what happens to the sound

Answer 36

- no refraction occurs

Question 37

if the velocity of the first medium is greater than the second what happens to the sound

Answer 37

- bend towards normal

Question 38

if the velocity of the first medium is less than the second what happens to the sound

Answer 38

- bend away from the normal

Question 39

what is total internal reflection

Answer 39

- special type of reflection when V1 is less than V2 and angle reaches a critical value

Question 40

what is scatter a result of

Answer 40

- sound interacting with interfaces smaller than a wavelength and rough

Question 41

what is scatter responsible for

Answer 41

- internal texture of organs

Question 42

what does scatter result in

Answer 42

- 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

Question 43

what 2 things is the amount of scatter dependant on

Answer 43

- frequency + higher frequency increased scatter - reflector size + smaller reflector increase scatted

Question 44

what is backscatter

Answer 44

- when sound is directed back to its origin

Question 45

what is backscatter responsible for

Answer 45

- organ parenchyma

Question 46

what is acoustic speckle

Answer 46

- brightness non uniformities from interference patterns from echoes that have undergone multi path scattering

Question 47

what is Rayleigh scatter

Answer 47

- when sound interacts with RBCs that are much smaller than a wavelength that are very very weak

Question 48

how does wave front divergence affect attenuation

Answer 48

- as sound beam diverges the intensity of the beam weakens and adds to overall attenuation of the beam