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1. Medical Radiation Sciences > Attenuation Process > Flashcards

Attenuation Process Flashcards

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USMLE® Step 1 flashcards
1
Q

What type of interaction of energy (high, medium, low) with electrons is used in diagnostic x-ray?

A

Medium energy

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2
Q

Define attenuation:

A

Any process that has a reduciton in radiation intensity

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3
Q

What is the shape of the graph when depicting attenuation

A

Semilogarithmic

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4
Q

Describe the interaction happening during the photoelectric effect:

A
  1. Photon ionizes atom and a K shell electron in the body is ejected
  2. All of the energy is absorbed-none left over from photon
  3. The outershell electron drops down to vacancy and secondary charecteristic radiation (photoelectron) is produced
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5
Q

What are the end products of the photoelectric effect?

A
  1. Ion pair (free radical of the atom)
  2. Photoelectron, which can cause new ionizations
  3. Secondary charectoristic radiation (ussualy K shell)
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6
Q

When is there no probability that the photoelectric effect will take place?

A

If the energy of the photon is just below the BE of the atom

Or reallyyyyy high

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7
Q

When is there the greatest probability that the photo will occur?

A

Greatest probability if the photon E is just above the BE

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8
Q

What is the relationship between the photoelectric effect density, E, ect.?

A

PE—> density, Z^3
————
E^3

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9
Q

What happens when secondary charecteristic radiation is produced?

A

Will have lower E and will get absorbed by outer shell e-

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10
Q

What is the effect on the image from the photoelectric effect?

A
  1. No scattter radiation produced
  2. Magnifies subject contrast
  3. Shows difference in both Z and density
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11
Q

How do you see greater attenuation with the photoelectric effect?

A

Lower kVp closer to the binding E of what you want to see absorbed

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12
Q

What is the effect on the patient from the photoelectric effect?

A
  1. Is a high dose interaction
  2. All of the energy of the photon is absorbed by the body
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13
Q

What are the other names for coherent scatter?

A
  1. Thomson scatter
  2. Rayleigh scatter
  3. Unmodified scatter
  4. Classical scatter
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14
Q

Descibe the interactions that occurs during coherent scatter:

A
  1. The photon strikes the whole atom and excites the entire thing
  2. Once it calms down, all of energy is released
  3. All of the E released adds up to the energy of a photon which is the same as the photon that hit it and will have the same wavelength
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15
Q

What type of interaction occurs with coherent scatter?

A

Low energy interaction

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16
Q

It what circumstance is it more likely that coherent scatter will occur?

A

Increased probability with high atomic number, and low energy photons

Doesnt happen often in radiography

17
Q

What is the relationship between coherent scatter and E, denstiy, ect.?

A

Coherent scatter—>Z^2/E

18
Q

What is the effect on the patient dose with coherent scatter?

A

No affect on dose because it doesnt damage the tissue

19
Q

What is another name for compton scatter?

A

Modified scatter

20
Q

Describe the interaction that occurs during compton scatter:

A
  1. Photon ionizes atom (ussually outershell)
  2. During ionization, it creates 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
21
Q

What is the angle of change in direction dependant upon in compton scatter? How much initial energy does it retain?

A

-Angle of change depends on how much energy was lost
-It retains 2/3 of initial energy and can interact with other tissues in the body

22
Q

What are the end products of compton scatter?

A
  1. Ion pair (positive atom)-free radical
  2. Compton e-
  3. . Scattered photon with a longer wavelength
23
Q

What is the relationship between Compton scatter and E, density, ect.?

A

Compton scatter–> (density)/E

24
Q

When is compton scatter most likely to occur?

A

Occurs most when photon E is much higher than the binding energy of the atom

25
Q

With higher kVp’s do we get more scatter?

A

At high kVp’s, theres less scatter OCCURING, but SCATTERED PHOTONS have more energy so they can go out of the body and reach the IR

26
Q

What is the effect on the image from compton scatter?

A
  1. Degrades image qualilty
  2. Reduces contrast
  3. Creates noise
27
Q

What are ways we can decrease compton scatter?

A
  1. Grids
  2. Collimation
  3. Compression
28
Q

What is the effect on the patient from compton scatter?

A
  1. Limited dose to patient
    -minimal photon energy loss
  2. Compton electron
    -Further ionizations and absorption
  3. Staff safety hazard-becuse high Es scatter can reach others in room
29
Q

Describe the interaction during pair production:

A
  1. Photon comes in and interacts with the nucleus
  2. The electric field interacts with the photons and takes the E, and converts it into mass
  3. Pair of subatomic particles (e- and positron) is created and they fly off
  4. Two photons are created by the positron called inialtion radiation
30
Q

When is pair production used in radiography?

A

Used in pet scans

31
Q

What energy does pair production require?

A

Greater than 1.02 MeV

If it was higher than this, E would split between 2 inialation radiation

31
Q

What happens in photodisintigration?

A
  1. A photon interacts with the nucleus and excites it
  2. It ejects nuclear fragments
  3. It is how universe creates heavy metals (ei;supernova)
31
Q

What is the minimum amount of energy required for photodisintigration?

A

Greater than 10 MeV

1. Medical Radiation Sciences (18 decks)

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