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Medical Radiation Sciences 1 > Unit 4 > Flashcards

Unit 4 Flashcards

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USMLE® Step 1 flashcards
1
Q

Radiation interacts with matter of similar size

A

low energy with atoms/molecules
Diagnostic X-ray energy with electrons
Higher energy with nuclei

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2
Q

What is attenuation?

A

reduction in radiation intensity
5 types of interactions for x-rays
not all occur with diagnostic x-ray imaging

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3
Q

What is the interaction of x-ray photons?

A

photon ionizes the atom
- k-shell electron ejected
- all energy is absorbed
outer shell electron drops down
- secondary characteristic radiaiton

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4
Q

What is an ion pair?

A

Photoelectron
- KE=hf-BE (binding energy)
- Can cause new ionizations
Secondary characteristic radiation
- K characteristic

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5
Q

What is the probability of photon interactions?

A

PE is proportional to atomic #^3
PE is inversely proportional to E^3
PE is directly proportional to electron density
No probability is the photon E is >BE of that e-
Greatest probability is photon E just above the BE

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6
Q

What is the effect of photon interactions on image?

A

No scatter radiation produced
Magnifies subject contrast
- shows difference in both Z and density
lower kVp shows greater contrast because more absorption

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7
Q

What is the effect on patient of attenuation?

A

high does interaction
- all energy is absorbed by body
- photoelectron
- secondary characteristic radiation

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8
Q

What is the coherent scatter interaction?

A

low energy interaction
photon strikes atom
- excitation -> energy released
- same wavelength
The tissue damage is negligible
a very rare interaction

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9
Q

probability of coherent scatter?

A

increased probability with high Z, low energy
Coherent scatter is proportional to Z^2/E
Very rare in diagnostic radiography <5%

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10
Q

What is the interaction of compton scatter?

A

photon ionizes atom - usually the outer shell
X-ray photon changes direction - angle of change depends on energy loss, retains at least 2/3 of initial energy

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11
Q

what are the end products of compton scatter?

A

ion pair - positive atom and compton electron (recoil electron)
Scattered photon - longer wavelength, can be in any direction

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12
Q

probability of compton scatter?

A

compton scatter is proportional to electron density and inversely proportional to energy
occurs most when photon E much higher than BE
At high kVp’s - less scatter, but more scatter than absorption

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13
Q

Compton scatter effect on image?

A

Degrades image quality
- reduces contrast
-creates noise

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14
Q

Compton scatter dose effect?

A

limited dose to patient
- minimal photon energy loss
- compton electron - further ionizations and absorption
Staff safety hazard
- high E scatter can reach staff, volunteers and visitors
- Require lead walls and shielding
- be aware of distance and positioning for portables

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15
Q

What is pair production?

A

requires >1.02MeV - anything over 1.02 will be kinetic energy
Photon interacts with nuclear field
- creates an electron and positron
- both can cause further interactions
very high energy interaction

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16
Q

What is a PET scan?

A

positron emission tomography

17
Q

What is photodisintegration?

A

Photon > 10MeV
interacts with nucleus
Excitation - nucleon or nuclear fragment ejected
Ex. super nova

18
Q

Does Photoelectric Effect or Compton Scatter occur more at a low kVp?

A

Photoelectric Effect - more absorption

19
Q

Does Photoelectric Effect or Compton Scatter occur more at a high kVp?

A

Compton Scatter

20
Q

At what KeV do you get more scatter and less absorption in soft tissue?

A

20 KeV

21
Q

At what KeV do you get more scatter and less absorption in bone?

A

40 KeV

22
Q

What interaction would showcase a difference in atomic number?

A

Photoelectric Effect

23
Q

What interaction would showcase a difference in density?

A

Compton Scatter

24
Q

Image formation at low energy shows differences in?

A

More PE than CS
Z and density - more subject contrast

25
Q

Image Formation at High Energy Shows?

A

Mostly CS, no appreciable CS
Only differences in density - less subject contrast

26
Q

Chest Imaging Technique Choice rationale?

A

Tissues present: air, soft tissue and bone - have high natural contrast
Reduce subject contrast with high energy - 110 to 130 kVp for more CS

27
Q

Mammography imaging technique rationale?

A

Tissues present: soft tissue, fat, glands and tumours
Low natural subject contrast
Increase SC with low energy - 30 kVp
Mostly PE

28
Q

L-Shell in K-edge Absorption

A

Greatest probability with very low E photons
Probability drops off drastically as E increases

29
Q

K shell in K-edge Absorption

A

Zero probability if less than BE
greatest probability when just above BE
Probability drops off drastically as E increases

30
Q

Absorption and Low Z Matter

A

Human tissue
Carbon K-shell: 284 eV
Calcium K-shell: 4 keV
Drops off shortly after this
No K edge because BE is so low

31
Q

What elements do K-edge absorption apply to in radiography?

A

K-edge filters
Iodine
Barium
IR phosphors
Lead shielding
Does not apply to tissue

32
Q

What are heavy element filters?

A

Selectively transmit a narrow range of photon energies
- absorb low energy photons
- transmit photons below BE
- high absorption above BE

33
Q

What are contrast studies?

A

To better visualize anatomy: vessels and GI tract

High atomic numbers: absorb more radiation, appear lighter on images

34
Q

Contrast Media

A

Iodine and Barium both have a K-edge in the 30’s keV
kVp ranges for these agents would be around 68 to 102 kVp for iodine, multiply keV by 2 and 3; for barium 76 to 114 kVp

35
Q

Image Receptors and absorption properties

A

Need to absorb radiation to produce image
Diagnostic X-ray beams are high energy: need high Z
Faster IRs need less mAs

36
Q

Scatter Protection

A

Lead: high atomic number and high absorption
Shielding: aprons, walls, glass, etc.
Grids

Medical Radiation Sciences 1 (4 decks)

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