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Physics > Midterm Review Notes > Flashcards

Midterm Review Notes Flashcards

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1
Q

What is attenuation

A

The weakening of sound as it propagates through the tissues

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2
Q

How does attenuation limit the image

A

Limits the penetration therefore the depth

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3
Q

What are the three contributing factors for attenuation

A

Absorption, reflection, scattering

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4
Q

What is the dominant contributing factor for attenuation

A

Absorption

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5
Q

What is absorption

A

The conversion of sound into heat

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6
Q

What is a decibel

A

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

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7
Q

What is the attenuation coefficient

A

The attenuation that occurs with each centimetre of sound travel

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8
Q

What is the attenuation in soft tissue

A

0.5 DB/cm x MHz

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9
Q

What does hvlt stand for

A

Half value layer thickness

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10
Q

What is HVLT

A

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

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11
Q

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

A

0.5 dB

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12
Q

Attenuation in bone is higher or lower than in soft tissue

A

Higher

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13
Q

A reflected beam is known as

A

An echo

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14
Q

When a perpendicular beam is reflected, what happens?

A

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

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15
Q

When the perpendicular beam is reflected, what happens

A

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

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16
Q

What is acoustic impedance

A

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

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17
Q

What is the equation to impedance, with units

A

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

18
Q

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

A

All is transmission and no echo occurs

This happens when the medium impedances are equal

19
Q

What must the reflective/transmission coefficients add up to?

A

1 or 100%

20
Q

How is oblique incidence different from perpendicular incidence

A

Oblique is any other angle other than perpendicular

21
Q

What is refraction

A

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

22
Q

What determines refraction

A

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

23
Q

Why is refraction important

A

It contributes to the formation of our image

24
Q

What does snells law tell us about

A

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

25
Q

What is a specular reflector

A

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

26
Q

What anatomy would produce a specular reflector

A

Artery walls or diaphragm

27
Q

What is scattering

A

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

28
Q

What does scattering depend on

A

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

29
Q

Describe why tgc is needed

A

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

30
Q

What is the TGC slope

A

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

31
Q

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

A

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”.

32
Q

What is the difference between TGC and overall gain

A

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

33
Q

What are the factors affecting TGC setting

A

Same things that affect the TGC setting

Tissue comp
Attenuation coefficient
Transducer frequency

34
Q

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

A

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

35
Q

What is speckle

A

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

36
Q

How is speckle produced

A

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

37
Q

What is CEUS

A

Contrast enhanced ultrasound

38
Q

What are contrast agents

A

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

39
Q

What are 5 characteristics of contrast agents

A

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

40
Q

How does the contrast agent function?

A

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.

41
Q

How are contrast agents used to show perfusion

A

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

42
Q

What is the units for amplitude

A

Pascal, N/cm2, kg/cm3, volts

Physics (32 decks)

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