Chapter 6

Question 1

Positive decibels

Answer 1

indicate that the signal strengthened (final intensity > initial intensity)

Question 2

Negative Decibels

Answer 2

indicate that the signal has weakened (final intensity < initial intensity)

Question 3

3 dB intensity ratio 2 intensity doubled
6 dB intensity ratio 4 intensity increase 4 fold
9 dB intensity ratio 8 intensity increase 8 fold
10 dB intensity ratio 10 intensity increase 10 fold
20 dB intensity ratio 100 intensity increase 100 fold
30 dB intensity ratio 1000 intensity increase 1000 fold
40 dB intensity ratio 10000 intensity increase 10,000 fold

Answer 3

Intensity ratios

Question 4

Attenuation

Answer 4

The decrease in intensity, power and amplitude as sound travels
expressed in dB

Question 5

Attenuation is determined by two factors

Answer 5

1. Path length or distance traveled by the sound wave
2. Frequency of sound

Question 6

Path length and attenuation are directly related

Answer 6

Path Length ∝ Attenuation

Question 7

Frequency and attenuation are directly related

Answer 7

Frequency ∝ attenuation

Question 8

Three processes contribute to attenuation

Answer 8

Reflection
Scattering
Absorption

Question 9

Reflection

Answer 9

a portion of the sound wave is redirected back towards the sound source when it strikes a boundary between 2 medium

Question 10

There are two forms of reflection

Answer 10

1. Specular
2. Diffuse

Question 11

Specular reflection

Answer 11

is when the sound is reflected off the boundary back towards the probe in only one direction in an organized manner

Question 12

One limitation of specular reflectors

Answer 12

is that once the wave is slightly off axis (non perpendicular to the boundary) the reflection does not return to the transducer

Question 13

An advantage of diffuse reflections

Answer 13

is that the transducer receives reflections from interfaces that it is not perpendicular to

Question 14

a second advantage to backscatter

Answer 14

is that heterogeneous tissue acts as a back scatter and allows us to see tissue texture

Question 15

A disadvantage to backscatter

Answer 15

is that backscattered signals have lower strength than specular reflections

Question 16

Scattering

Answer 16

is the random redirection of sound in many different directions

Question 17

Disadvantage to Scattering

Answer 17

Little to none of the redirected sound wave makes it back to the probe

Question 18

Rayleigh scattering

Answer 18

is a special type of scattering that occurs when the interface/structure diameter is much smaller than the wavelength
Sound is redirected equally in all directions (little towards probe)

Question 19

Disadvantage to Rayleigh Scattering

Answer 19

Little to none of the redirected sound wave makes it back to the probe

Question 20

Rayleigh scattering ∞ frequency4

Answer 20

rayleigh scattering is directly related to frequency quadrupled

Question 21

Absorption

Answer 21

occurs when ultrasonic energy is converted into another energy form such as heat

Question 22

absorption is directly related to frequency

Answer 22

absorption ∝ frequency

Question 23

Attenuation Coefficient

Answer 23

is the number of decibels of attenuation that occurs when sound travels one centimeter; units = dB/cm

Question 24

Attenuation Coefficient is directly related to frequency

Answer 24

attenuation coefficient ∝ frequency

Question 25

Half Value Layer Thickness

Answer 25

is the distance that sound travels in a tissue that reduces the intensity of sound to one-half its original value

Question 26

Acoustic Impedance

Answer 26

it is the resistance offered to sound traveling through the medium
AKA: characteristic impedance

Question 27

Acoustic Impedance

Answer 27

is a characteristic of the tissue, specifically it is the resistance offered to sound traveling through the medium
AKA characteristic impedance
units of rayls
typical values are 1.25-1.75 Mrayls

Question 28

Incidence

Answer 28

the angle at which the wave strikes the boundary

Question 29

Normal incidence

Answer 29

is when the incident sound beam strikes the boundary at exactly 90°
Also called perpendicular, orthogonal, right angle, or 90 degree incidence

Question 30

Oblique incidence

Answer 30

is when the incident sound beam strikes the boundary at an angle other than 90°
Also called nonperpendicular incidence

Question 31

Incident intensity

Answer 31

is the sound wave’s intensity immediately before it strikes a boundary

Question 32

Reflected Intensity

Answer 32

is the intensity of the reflected portion of the sound wave, immediately after it reflects off the boundary

Question 33

Transmitted Intensity

Answer 33

is the intensity of the transmitted portion of the sound wave immediately after it crosses the boundary

Question 34

Intensity Reflection Coefficient

Answer 34

The percentage of the intensity that is reflected back when a sound beam strikes the boundary between two media

Question 35

Intensity Transmission Coefficient

Answer 35

The percentage of the intensity that propagates forward or is transmitted when the beam strikes the interface between two media

Question 36

In oblique incidence if we have reflection the

Answer 36

incident angle = reflection angle

Question 37

Refraction

Answer 37

the soundwave may bend or change direction as it crosses the boundary

Question 38

Refraction occurs only if two conditions are satisfied; you must have

Answer 38

1. Oblique incidence
2. Different propagation speeds between the 2 media

Question 39

the critical angle

Answer 39

The incident angle at which no transmission into the second medium takes place
(>t = 90 degrees)