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Medical Radiation Sciences 2 > Attenuation and Attenuation Applications > Flashcards

Attenuation and Attenuation Applications Flashcards

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USMLE® Step 1 flashcards
1
Q

What is the term that defines the weakening of the primary beam following penetration through a given thickness?

A

Attenuation

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2
Q

When is there the greatest chance of the photoelectric effect occuring?

A

When the E of the photon is just above the binding E of the material

Can also occur when the energy of the photon is equal to the binding energy of the electron

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3
Q

When is there NO chance that the photoelectron effect will take place?

A

When the energy of the photon is below the binding energy of the material

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4
Q

What is the probability that a given x-ray photon will undergo CS dependant on?

A

The energy of the incident ray

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5
Q

At higher energies, what type of attenuation is more likely to take place?

A

At higher energies, compton scatter is more likely to occur than PE

However, probability goes down for both as the E goes up

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6
Q

At above 80 kVp, what type of attenuation is likely to show differences in the image?

A

Compton scatter

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7
Q

How much of the energy is absorbed by the electron from the incident photon in PE effect?

A

All of it-the photon ceases to exist

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8
Q

Describe the interaction that happens during the photoelectric effect:

A
  1. Photon ionizes atom and a K shell electron in the body is ejected, called a photoelectron
  2. All of the energy is absorbed-none left over from photon
  3. An outershell electron drops down to vacancy and secondary charecteristic radiation is produced
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9
Q

What is the energy of the secondary charecteristic radation and what happens to it?

A

The energy is very low, therefore, it is absorbed by the body and never escapes

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10
Q

What happens to the photoelectron after being ejected from the inner shell?

A

They interact with other atoms within the tissue and never penetrate outside of the body

All of the energy of the photon goes to the photoelectron

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11
Q

What effect does photoelectrons have on the final radiographic image?

A

None, they cannot reach the image receptor

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12
Q

If the energy of a photon is greater than the binding energy of an electron, what happens to the remaining energy in the PE. Effect?

A

The remaining energy is given to the photoelectron

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13
Q

How is the energy divided with compton scatter?

A

-An amount equal to binding E ejects e- from orbit
-Small amount of E given to the speed of the ejected e-
-Remainder is reemitted as compton scatter (photon)

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14
Q

Describe the interaction that takes place during compton scatter:

A
  1. Photon ionizes atom (ussually outershell)
  2. During ionization, it knocks out e- called a compton e-
  3. The photon gives random amount of kE to compton e-
  4. The x ray photon changes direction and has less energy due to how much it gave up
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15
Q

What particle carries most of the energy after a compton scatter interaction?

A

The scattered photon

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16
Q

How much energy does the scattered photon retain after a compton interaction?

A

2/3 of its initial energy

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17
Q

Are lower or higher energy photons more likely to hit the image receptor?

A

Higher energy

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18
Q

What type of attenuation contributes most to personnel?

A

Compton scatter

19
Q

Generally, in which direction are the scattered photons with higher energy deflected?

A

Deflected less from the original direction(straight)

20
Q

What effect does compton scatter have on the final image?

A

-Reduces image contrast
-Provides NO useful information

21
Q

With more energy lost during a compton scatter interaction, is the photon more likely to scatter?

A

Yes

22
Q

What is the angle of deflection of a photon in compton scattering dependant on?

A

Dependant on how much E of the photon is lost

23
Q

If a photon in CS retains 76% of its E, at what angle does it travel?

A

At a 45 degree angle

24
Q

If a photon in CS retains 84% of its E, at what angle does it travel?

A

At a 90 degree angle

25
Q

If a photon in CS retains 92% of its E, at what angle does it travel?

A

Travels at a 135 degree angle

26
Q

If a photon in CS retains 68% of its E, at what angle does it travel?

A

Travels at 180 degrees (back scatter)

27
Q

If a photon in CS retains 100% of its E, at what angle does it travel?

A

Straight (0 degrees)

28
Q

What is inherent subject contrast in the image generated by?

A

Generated by the differrence between areas where more x rays are attenuated and areas where fewer x rays are attenuated

29
Q

As kVp is reduced, does differential attenuation increase or decrease? What is the downside?

A

-Differential absorption increases
-Downside=dose increases

30
Q

Define mass density:

A

The quantity of matter per unit volume; how tightly atoms of a substance are packed together

31
Q

Is CS and PE effect more likely to happen at higher or lower energies?

A

Lower energies

32
Q

As the number of x-rays being transmitted increases, does image contrast increase or decrease?

A

Image contrast decreases

33
Q

What is the k-edge?

A

The binding energy of the k shell electron of an atom

34
Q

When is absorption most likely to take place with k-edge elements?

A

When the E is just above the BE

35
Q

What does attenuation coefficants tell us?

A

How much attenuation (scatter and absorption) takes place per unit thickness.

36
Q

Are attenuator coefficants linear or exponential?

A

Linear because it is talking about the thickness of the material

37
Q

What does total linear attenuation depend on in relation to the attenuator?

A

Depends on the probability of interactions between the photons and the material

38
Q

What does the probability of x ray interactions with an attenuator depend on?

A
  1. KVp
  2. Atomic number
  3. Denisty
39
Q

What is total linear attenuation represented by?

A

u (mew)

40
Q

What does TMAC stand for?

A

The total mass attenuation coefficient

41
Q

What interactions is TMAC a result of?

A

Scattering AND absorption

42
Q

What does PEMAC stand for?

A

The photoelectric mass attenuation coefficent

43
Q

When does u increase an decrease?

A

Increase: When attenuation is higher
Decrease: When energy increases

Medical Radiation Sciences 2 (13 decks)

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