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Medical Radiation Sciences 1 > Unit 4 > Flashcards

Unit 4 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 in the photoelectric effect?

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

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17
Q

What is photodisintegration?

A

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

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18
Q

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

A

Photoelectric Effect - more absorption

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19
Q

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

A

Compton Scatter

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20
Q

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

A

20 KeV

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21
Q

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

A

40 KeV

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22
Q

What interaction would showcase a difference in atomic number?

A

Photoelectric Effect

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23
Q

What interaction would showcase a difference in density?

A

Compton Scatter

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24
Q

Image formation at low energy shows differences in?

A

More PE than CS
Z and density - more subject contrast

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25
Q

Image Formation at High Energy Shows?

A

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

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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

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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

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28
Q

L-Shell in K-edge Absorption

A

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

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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

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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

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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

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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

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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

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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

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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

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36
Q

Scatter Protection

A

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

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37
Q

Breast cancer

A

most common cancer among women
2nd leading cause of cancer death
majority are infiltrating duct carcinomas
in the upper lateral quadrant of the breast

38
Q

Screening for breast cancer

A

recommended for women of risk aged 40-74 (every 2 years)
half of new cases are found in this cohort
begin screening earlier if at risk

39
Q

risk factors for breast cancer

A

age, family history, genetics, breast architecture (denser), menstruation, menopause, prolonged use of estrogen, late age at birth of first child or no children, education (increased with higher education), socioeconomics (higher status), previous radiation therapy to chest at early age

40
Q

benefits of the screening program

A

rate of breast cancer decreasing since 2002
death rate decreasing since mid-1980’s

41
Q

Breast tissue

A

adipose, glandular, fibrous and tumors
similar densities and atomic numbers

42
Q

Beam quality - spectrum (mammography)

A

high energy beams differentiate density only
use lower kVp and molybdenum target for breast imaging
With approximately 30 kVp we will get 20-30% of the beam as k-characteristic radiation
large amount of 17-19 keV photons
- best for breast imaging is 17-24 keV

43
Q

Beam quality - filtration (mammography)

A

want a narrow range of energy for good subject contrast
use a molybdenum filter with a molybdenum target
- preferentially filters all but 17-19keV photons

44
Q

Denser tissue and Rhodium (mammography)

A

denser breast tissue requires more penetration
solution: use rhodium target and filter, BE: 23 keV

45
Q

Beam quality - anode to cathode distance

A

Recall: space charge compensating circuit - with lower kVp, space charge will be larger
change anode to cathode distance accordingly - less attenuation of beam

46
Q

Beam quality - equipment composition

A

normal glass filters out low energy beam - use beryllium window instead
mirror
compression paddle

47
Q

Shorter SID of mammography

A

60-80cm
pros: higher mAs, shorter exposure time
Cons: higher dose, less sharpness, and greater heel effect

48
Q

Anode size and angle in mammography

A

effective focal spot size: 0.3/0.1 mm - circular
variable anode angle and tube tilt
- approx 20-24 effective angle
- reduce anode-heel effect while maintaining spatial resolution

49
Q

Automatic selection and AEC of mammography

A

Compression device - measures thickness of breast
AEC - located after IR, 100ms burst of radiation before exposure
both combine to select optimal technique and target material

50
Q

spatial resolution of mammography

A

small focal spot
image receptor design
grounded metal tube
compression
AEC location

51
Q

Magnification in mammography

A

increased OID
More pixels per area of tissue - increased spatial resolution
More photons per area of tissue - increased contrast resolution
Entrance skin dose - decreased SOD, no grid

52
Q

contrast resolution in mammography

A

affected by the compression, beam energy grid, tube tilt, and tube orientation

53
Q

contrast resolution - digital tomosynthesis

A

tube moves during exam
many images
can see different “slices”
higher dose
higher contrast resolution

54
Q

what is different about the grids used in mammography?

A

usually use a crossed grid, not lead, use something with a lower atomic number but differs from manufacturer to manufacturer

55
Q

Film screen resolution

A

higher spatial resolution
lower attenuation cassettes

56
Q

digital resolution

A

higher contrast resolution
no phosphor

57
Q

Is film or digital a better option for mammography?

A

digital

58
Q

factors that affect patient dose

A

compression decreases patient dose
beam energy, image receptor, tomosynthesis, short SID and grid increase patient dose
magnification doses remains the same because not using grid

59
Q

How do the interactions of EMR with matter differ with differing energy levels? Why does this occur?

A

EMR is more likely to react with matter with energy levels close to the energy level if the EMR. This occurs because it is like a ghost, will just go right through someone, but a person can’t walk through a person.

60
Q

List the three types of interactions that occur between diagnostic x-ray photons and tissue. Explain where photon energy goes, and any by-products created, for each of them. What are the factors affecting the probability of each of them occurring.

A
  1. Photoelectric effect – photon ionizes the atom, and a k-shell atom is ejected. Probability is proportional to atomic number^3 and inversely proportional to energy^3.
  2. Coherent scatter – photon strikes atom, energy is released in the same wavelength it was before hitting the atom. Probability is proportional to atomic number^2 and inversely proportional to energy.
  3. Compton scatter – photon will ionize the atom – usually the outer shell. Produces an ion pair (positive atom and Compton or recoil electron) Probability is proportional to electron density and inversely proportional to energy.
61
Q

Explain the conditions under which no photoelectric effect occurs between a photon and a bound electron, and the condition where there is the highest likelihood/probability of it occurring.

A

There is no probability of this happening if the photon energy is lower than the binding energy of the electron it is trying to displace. The condition where there is the highest probability of photoelectric effect occurring is when the photon energy is just above the binding energy.

62
Q

What are the main similarities and differences between the production of characteristic radiation at the tungsten target and photoelectric interactions?

A

Both involve interactions with inner shell electrons, that is then filled by an electron from a higher energy shell, resulting in released energy. Characteristic radiation is caused by a high energy electron striking the tungsten target, whereas the photoelectric effect is caused by an incoming photon interacting with a bound electron. Tungsten target produces primary characteristic radiation while the photoelectric effect produces secondary characteristic radiation.

63
Q

Which attenuation interaction is a potential hazard to hospital personnel in the vicinity and why? How is the hazard related to kVp? What can be done to protect staff from this? What effect does this have on the image?

A

Compton scatter is a potential hazard to hospital personnel in the vicinity because the scattered photon can go in any direction. This hazard is related to kVp because the higher the energy the shorter wavelength, and the higher the energy the farther it can go. Lead walls and shielding can be used to protect staff. This degrades the image quality by reducing contrast and creating noise.

64
Q

Which attenuation interactions are ionizing events? Why is ionization of tissue dangerous?

A

Photoelectric effect and Compton scatter, ionization of tissue is dangerous because it can cause tissue damage.

65
Q

Compare what imaging parameters you might change when imaging something like the lumbar spine versus imaging the abdomen. Explain your reasoning.

A

When imaging the lumbar spine I would use photoelectric effect because it showcases differences in atomic number. For the abdomen I would use Compton scatter because it showcases a difference in density.

66
Q

Describe pair production and photodisintegration. Why are these not prevalent in general x-ray imaging?

A

Pair production and photodisintegration are not relevant in general x-ray because they only occur at very high kVp. Pair production is when a photon interacts with a nuclear field and creates an electron and a positron. Photodisintegration is when a photon interacts with a nucleus, and excites it creating a nucleon or a nuclear fragment ejected.

67
Q

What attenuation interaction is more likely to occur at high energies in diagnostic imaging?

A

Compton Scatter

68
Q

Compare the relative occurrence of photoelectric effect (PE) interactions between 100keV photons and 75keV photons. Do the same for Compton scatter (CS) interactions.

A

Photoelectric effect is 2.2x more likely to occur at 75keV than 100keV. Compton scatter is 1.3x more likely to occur at 75keV than 100keV.

69
Q

Compare the relative probability of PE occurring between bone and fat based on differences in their atomic number and density. Use the values from your slides.

A

PE is only affected by atomic number. Probability of PE occurring between bone and fat based on atomic number is (13.8/7.42)^3 = 6.43 times higher in bone than fat

70
Q

Compare the relative probability of CS occurring between bone and fat based on differences in their atomic number and density. Use the values from your slides.

A

CS is only affected by electron density. Probability of CS occurring between bone and fat based on electron density is (1.85/1)= 1.85 times higher in bone than fat

71
Q

Why are soft tissues not likely to be differentiated from each other at high kVp? Why are they more likely to be differentiated at low kVp?

A

Because they have very similar densities, however they have different atomic numbers

72
Q

Why can soft tissue and air be differentiated in images at both high and low kVp settings?

A

Because they have very different densities.

73
Q

Why is there always significant contrast between bone and other tissues at any diagnostic kVp?

A

Because bone has a very different atomic number and electron density.

74
Q

Describe the concept of k-edge absorption.

A

k-edge absorption is when the amount of absorption is slowly decreasing due to the difference in energy between L-shell and K-shell, then when the photons have enough energy to overcome k-shell electrons the amount of absorption spikes.

75
Q

Listed below are 3 different items used in diagnostic radiography. Explain for each, the relevance of the material’s k-edge to image production and patient dose.

A

a. Heavy element filter
Selectively transmit a narrow range of photon energies – absorb low energy photons, transmit photons below BE, high absorption above BE
b. Image receptors
Need to absorb radiation to produce image – faster IR’s need less mAs
c. Contrast media
To better visualize anatomy – high atomic numbers absorb more radiation and appear lighter on images

76
Q

Increasing kVp always causes increased penetration within body tissues. Why?

A

Because the higher energy the beam the more energy it has to go through the tissues of the patient.

77
Q

Which average beam energy, 32 or 34 keV, will produce the greatest contrast between iodine and patient tissue? Use the image above showing the chance of absorption happening in various materials.

A

34keV will produce the greatest contrast between iodine and patient tissues.

78
Q

Using the image above, what approximate range of photons would be transmitted through a holmium filter?

A

The approximate range of photons would be transmitted through a holmium filter is 52-56 keV

79
Q

Iodine and barium have similar k-edge. The GI tract containing Ba is usually imaged using a higher kVp setting than what is used to image blood vessels containing iodine. Why? Hint: it’s not just because iodine’s binding energy is slightly lower.

A

Because the GI tract is deeper in the body than the blood vessels

80
Q

Describe how the following factors produce a suitable beam for soft-tissue subject contrast in mammography:

A

a. Low kVp with Mo target
Low kVp shows difference in atomic number between similar tissues, Mo target ensures low kVp
b. Combo of Mo target and Mo filter
Mo has a low kVp produced and shows difference in atomic number between similar tissues, Mo target ensures low kVp
c. Shorter spacing of anode-cathode
There is less attenuation of the beam with the anode to cathode distance, meaning less anode heel effect
d. Different materials used for the tube window
Regular glass absorbs low energy beam, use beryllium window instead

81
Q

What would the approximate energy of a 100kVp beam produced by a W target and W filter be? What type of radiation production process produces the majority of this beam?

A

Approximate energy of this beam would be 30-50 keV majority of this beam would be bremsstrahlung, characteristic would have energies of around 60-67 keV.

82
Q

What is the optimal beam energy used in breast imaging?

A

30 kVp

83
Q

Apart from beam energy, what factors improve contrast resolution?

A

Digital radiography, magnification, compression, beam energy, grid, tube tilt, tube orientation, digital tomosynthesis

84
Q

We use slightly different grids in mammography compared to in general x-ray. What is different about them? Do you think they would be moving or not moving during the exposure?

A

The grids are not made of lead, they are made with something with a lower atomic number but differs based in manufacturer. The grids in mammography are crossed grids. They will not move during the exposure.

85
Q

What are some reasons for using a shorter SID in mammography?

A

Shorter exposure time and higher mAs

86
Q

How and why is the x-ray tube tilted in mammography?

A

Reduces the anode heel effect while maintaining spatial resolution

87
Q

What effect does magnification have on the image in mammography? How does it affect dose?

A

More pixels per area of tissue – increased spatial resolution. More photons per area of tissue – increased contrast resolution. Dose remains the same because the decreased SOD increases it, however, not using grid decreases it.

88
Q

Discuss the role of compression in mammography.

A

Compression decreases patient dose by increasing the SOD, it also helps create a better image

89
Q

What factors improve spatial resolution in mammography?

A

Explain why each helps.
Small focal spot – less blur (penumbra), image receptor design – pixel size and density, grounded metal tube – reduces off focus radiation, compression – less OID, AEC location – less OID

90
Q

How do we ensure proper penetration and exposure levels in mammography? Why is this so important?

A

Using low energy beam, filters, AEC and compression. This is important for diagnostic accuracy of the images and patient dose.

91
Q

Compare digital tomosynthesis and traditional mammography in terms of contrast resolution and diagnostic accuracy. What are the potential benefits and drawbacks of each technique?

A

Digital tomosynthesis is where the tube moves during the exam taking many images, this creates increased contrast resolution and can create better images for diagnostic accuracy and gives a better chance at eliminating the need for biopsy, or wait and recheck, however, it comes with a higher patient dose.

92
Q
  1. A patient presents with a palpable lump but has dense breast tissue. How would you approach the imaging process to ensure accurate diagnosis?
A

I would use the rhodium target and filter, then from there maybe digital tomosynthesis depending what is seen on the first image using the rhodium target and filter.

Medical Radiation Sciences 1 (4 decks)

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