Part 5-10 (up until midterm)

Question 1

What creates loudness or strength of sound

Answer 1

Amplitude and intensity

Question 2

Attenuation is

Answer 2

The reduction in intensity and amplitude with increasing imaging depth

Question 3

What is absorption

Answer 3

Sound transferred to heat

Question 4

Attenuation increases with

Answer 4

Frequency

Path length

Question 5

Attenuation is caused by

Answer 5

Absorption
Reflection
Scattering

Question 6

Attenuation coefficient (ac)

Answer 6

The attenuation that occurs with each centimeter* the sound wave travels

Unit: dB/cm

Question 7

If sound intensity decreases by 2 what is the attenuation

Answer 7

3 dB

Question 8

If sound intensity decreases by 10 what is the dB

Answer 8

10 dB

Question 9

What does the attenuation depend on?

Answer 9

1. Type of tissue
2. Length/distance of travel
3. Frequency

Question 10

dB vs intensity - what do you do

Answer 10

dB you +

Intensity you x

Question 11

Attenuation coefficient for bone

Answer 11

ac = 6-9 heavy attenuation

Question 12

Attenuation coefficient for water

Answer 12

ac= 0.0022 - weak attenuation

Question 13

Compare attenuation in lung vs. bone vs. soft tissue

Answer 13

Attenuation is higher in lung/air than soft tissue

Attenuation is higher in bone than soft tissue

Question 14

Describe the relationship between imaging depth and frequency

Answer 14

Imaging depth decreases with increasing frequency

Question 15

As frequency ____ attenuation _____

Answer 15

As frequency increases, attenuation increases

Question 16

The average attenuation coefficient for soft tissue is _____ dB/cm for each MHz of frequency

Answer 16

0.5 dB/cm

Question 17

The slope of the TGC in soft tissue is how much? Explain

Answer 17

1 dB/cm * MHz

Soft tissue attenuated at 0.5 dB * MHz, but also attenuated again coming back

To compensate for both directions of attenuation, TGC must give back 1 dB/cm * MHz

Question 18

What can you adjust to compensate for attenuation

Answer 18

Time gain compensation

Question 19

What does TGC slope do?

Answer 19

Amplifies far field echoes to promote uniformity in liver tissue. Displayed graphically with increasing deflection to the right. The slope compensated less at the shallowest portion and more at the deepest portion of the organ

Question 20

What is impedance (formula)

Answer 20

The density of a medium * propagation speed

Unit: Rayls

Question 21

What does impedance determine

Answer 21

How much of an incident sound wave is reflected and how much is transmitted

Question 22

Average soft tissue impedance is how many rayls

Answer 22

1,630,000 rayls

Question 23

There is no reflection when

Answer 23

No reflection when two media have the same impedance with perpendicular impedance

Question 24

IRC

Answer 24

Intensity reflection coefficient - how much of the wave is reflected

IRC = Ir/Ii x 100

Question 25

ITC

Answer 25

Intensity transmission coefficient - how much is transmitted into second medium ITC = It/Ii * 100 OR ITC = 1-IRC

Question 26

The larger the impedance the _____ the IRC and echo intensity

Answer 26

The larger the impedance the greater the IRC and echo intensity

Question 27

If IRC increases, ITC _____

Answer 27

If IRC increases, ITC decreases

Question 28

If impedance is zero, what does that mean

Answer 28

There is total transmission

Question 29

If impedance nears 100%, what does that mean

Answer 29

Almost total reflection Examples: Bone and diaphragm

Question 30

If there is a perpendicular incidence what is the incident angle

Answer 30

Zero

Question 31

What happens with oblique incidence

Answer 31

With oblique incidence, the sound is refracted at a boundary between media for which propagation speeds are different

Question 32

When will no refraction occur?

Answer 32

If propagation speeds are equal | If the angle of incidence is 0 (perpendicular)

Question 33

If C1 < C2, what is the relationship between the corresponding angles?

Answer 33

Incident angle is less than transmission angle

Question 34

Two requirements for refraction

Answer 34

1) oblique incidence | 2) diff propagation speed on either side of medium

Question 35

What does refraction cause

Answer 35

1. Lateral position errors | 2. Duplication of aorta

Question 36

What two angles are always the same at a boundary

Answer 36

Incidence and reflection angles

Question 37

When does scattering occur?

Answer 37

1) not smooth - rough/heterogeneous | 2) small surface

Question 38

Scattering depends on

Answer 38

1. Frequency | 2. Scatterers size

Question 39

Explain the relationship between frequency, wavelength with scattering/reflection

Answer 39

Increase frequency decreases wavelength. Thus the object will be bigger than the wavelength, so there will be less scattering, more reflection, more echoes coming back to the transducer which increase the intensity of echo

Question 40

Explain the condition when increasing frequency increases scattering

Answer 40

Rayleigh scattering - when the object, like red blood cells, are much smaller and increasing frequency causes more scattering

Question 41

Explain how contrast agents work

Answer 41

They produce strong harmonics because air bubbles contact and expand creating non-linear propagation

Question 42

To position the echo on the screen, machine needs 2 pieces of info

Answer 42

1. Direction echo came from 2. Distance to reflector/scatterer - machine measures time travelled

Question 43

What is the pulse-echo round trip time used to determine the distance to a reflector

Answer 43

13 us/cm

Question 44

Constructive interface

Answer 44

In phase, Reinforce - Intensity much stronger than individual sound waves

Question 45

Destructive interface

Answer 45

Out of phase, cancel out

Question 46

Acoustic speckle

Answer 46

A form of acoustic “noise” - the result of constructive and destructive interference of scattered sound waves

Question 47

What is HVLT

Answer 47

The thickness of a medium that decreases the intensity of a beam by 50% or 3dB of the initial intensity

Question 48

When perpendicular bean is transmitted, what happens

Answer 48

The bean moves through the second medium in the same direction as the incident sound beam

Question 49

What does it mean if there is no reflection at a boundary?

Answer 49

All is transmission, no echo, the medium impedances are equal

Question 50

What is acoustic impedance? It’s symbol? It’s unit? Equation with units?

Answer 50

Acoustic impedance is a measure of the resistance of a medium to the transmission of sound. Symbol is z, rayls

Question 51

What must the reflective/transmission coefficients add up to?

Answer 51

1 or 100%

Question 52

What is refraction? What determines it? Why is it important?

Answer 52

Refraction is a change in the direction of sound as it crosses a boundary. Determined by the different acoustic velocity/impedance in the diff mediums. It contributes a great deal to the formation of our image.

Question 53

What is a specular reflector? What anatomy would produce one?

Answer 53

Flat smooth boundary that reflects well (mirror-like) it is produced at the artery walls or diaphragm

Question 54

Describe why TGC is needed

Answer 54

Reflectors with equal reflection coefficient will not result in echoes of equal amplitude arriving at the transducer if their travel distances are different) bc sound weakens as it travels

Question 55

What’s the difference between TGC and overall gain

Answer 55

TGC is depth specific

Question 56

Factors affecting TGC setting

Answer 56

Tissue composition Attenuation coefficient Transducer frequency

Question 57

If a medium has an attenuation coefficient of 2db/cm, what would be the best choice for the slope of the TGC

Answer 57

4db/cm | Slope for TGC should be double the attenuation coefficient

Question 58

How is speckle produced

Answer 58

Constructive/ destructive interference patterns within the returning echoes producing darker and brighter dots on the screen

Question 59

Units for amplitude

Answer 59

Pascal, N/cm2 Kg/cm3 Volts