Midterm Review Notes

Question 1

What is attenuation

Answer 1

The weakening of sound as it propagates through the tissues

Question 2

How does attenuation limit the image

Answer 2

Limits the penetration therefore the depth

Question 3

What are the three contributing factors for attenuation

Answer 3

Absorption, reflection, scattering

Question 4

What is the dominant contributing factor for attenuation

Answer 4

Absorption

Question 5

What is absorption

Answer 5

The conversion of sound into heat

Question 6

What is a decibel

Answer 6

Is a logarithmic unit —> the log of the ratio of the ratio between two quantities such as the ratio between of 2 powers/amplitudes/intensities

Question 7

What is the attenuation coefficient

Answer 7

The attenuation that occurs with each centimetre of sound travel

Question 8

What is the attenuation in soft tissue

Answer 8

0.5 DB/cm x MHz

Question 9

What does hvlt stand for

Answer 9

Half value layer thickness

Question 10

What is HVLT

Answer 10

The thickness of a medium that decreases the intensity of the beam by half

Question 11

In soft tissue there is approx ____dB of attenuation per cm for each MHz of frequency

Answer 11

0.5 dB

Question 12

Attenuation in bone is higher or lower than in soft tissue

Answer 12

Higher

Question 13

A reflected beam is known as

Answer 13

An echo

Question 14

When a perpendicular beam is reflected, what happens?

Answer 14

When the beam hits an interface and is reflected, it goes back to the reciever and is picked up to form our images on the screen

Question 15

When the perpendicular beam is reflected, what happens

Answer 15

The beam moves through the 2nd medium in the same direction as the incident sound beam

Question 16

What is acoustic impedance

Answer 16

Is a measure of the resistance of a medium to the transmission of sound

Question 17

What is the equation to impedance, with units

Answer 17

Z= c (m/s) x p(kg/m3)

Question 18

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

Answer 18

All is transmission and no echo occurs

This happens when the medium impedances are equal

Question 19

What must the reflective/transmission coefficients add up to?

Answer 19

1 or 100%

Question 20

How is oblique incidence different from perpendicular incidence

Answer 20

Oblique is any other angle other than perpendicular

Question 21

What is refraction

Answer 21

A change in teh direction of sound as it crosses a boundary

Question 22

What determines refraction

Answer 22

It is determined by the different acoustic velocity/impedance in the different mediums.

Question 23

Why is refraction important

Answer 23

It contributes to the formation of our image

Question 24

What does snells law tell us about

Answer 24

The law tells us the relationship between angle of incidence/ transmission and the velocity of sound of the 2 media

Question 25

What is a specular reflector

Answer 25

It is a flat smooth boundary that reflects well (mirror like).

Question 26

What anatomy would produce a specular reflector

Answer 26

Artery walls or diaphragm

Question 27

What is scattering

Answer 27

Is the formation of echo in many directions by rough surfaces or by heterogenous media or small objects

Question 28

What does scattering depend on

Answer 28

The surface of the interface and the size of the object compared to the size of the wavelength

Question 29

Describe why tgc is needed

Answer 29

Because echoes do not come back in equal amplitude as they have different travel times

Question 30

What is the TGC slope

Answer 30

TGC displayed graphically with increasing deflection to the righ t, the slope compensates less at the shallowest portion and more at the deepest portion of the organ

Question 31

What does the operator need to keep in mind when setting the TGC

Answer 31

The slope should correspond to the attenuation coefficient in the tissue. Should be about 1db/cm-MHz, because the attenuation coefficient in soft tissue is approximately 0.5db/cm, and the sound must travel “there and back”.

Question 32

What is the difference between TGC and overall gain

Answer 32

TGC is the depth specific; field of view broken into zones and is controlled with a slide bar (pods). The overall gain is the echoes from all depths, its “One” big zone and is controlled with a knob

Question 33

What are the factors affecting TGC setting

Answer 33

Same things that affect the TGC setting Tissue comp Attenuation coefficient Transducer frequency

Question 34

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

Answer 34

4db/cm. TGC slope compensates for attenuation but must do so at twice the rate since it must compensate both for pulse attenuation as the pulse travels to the reflector and for echo attenuation as the echo returns to the transducer. So the slope for the TGC should be approximately double the attenuation coefficient

Question 35

What is speckle

Answer 35

Speckle is a form of acoustic noise and shoes as the graininess of tissue within the image

Question 36

How is speckle produced

Answer 36

It is produced by constructive/destructive interference patterns within the returning echoes producing darker and brighter spots on the screen

Question 37

What is CEUS

Answer 37

Contrast enhanced ultrasound

Question 38

What are contrast agents

Answer 38

Microbubble polymers introduced through IV into the boodstream to highlight particular isoechoic abnormal tissue

Question 39

What are 5 characteristics of contrast agents

Answer 39

Stable, small enough to perfuse, different z to surrounding medium, non toxic, easy admin

Question 40

How does the contrast agent function?

Answer 40

Depending on the type, it will perfuse through the blood into the necessary tissue and due to the difference in impedance (impedance mismatch), will brighten those tissues. This will either brighten the lesion or the surrounding tissue differently and show more contrast and allow better delineation of the lesion. Can also pick out the harmonic frequencies since they produce stronger harmonics.

Question 41

How are contrast agents used to show perfusion

Answer 41

Increased US amplitude can pop the bubbles and we can see the tissues and the rate at which they fill up again with the agent

Question 42

What is the units for amplitude

Answer 42

Pascal, N/cm2, kg/cm3, volts