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Radiology VCS845 > Radiology Physics > Flashcards

Radiology Physics Flashcards

1
Q

What are X-rays

A
  • Ionizing, electromagnetic radiation
    • have enough energy to create ions by ejecting electrons
  • Act as a wave and a particle
  • Creates free radicals and can damage cells/DNA
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2
Q

How are x-rays produced?

A

High energy electrons strike a high anatomy weight metal, and energy is released as X-rays

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3
Q

How x-rays are produced?

A
  1. electrical current is applied to the cathode filament → thermionic emission of electrons
  2. A voltage (potential difference) is applied to make the cathode negatively charged and the anode positively charged
  3. Negatively charged electrons fly across the vacuum and strike the positive anode
  4. Energy is released form the anode in the form of x-rays
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4
Q

How electrons make x-rays

A
  • 2 types of radiation produced in the x-ray tube
    • bremsstrahlung (breaking) and characteristics
  • 99% of electron energy lost as heat, 1% lost as x-rays
  • Wide range of different energy level x-rays
    • many are too low energy to be useful and increase patient dose and scatter
    • Filters used to remove low energy x-rays and increase the average (no max) energy of the beam.
      • usually copper or aluminum
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5
Q

Collimator

A

Limits the beams area

Reduces scatter

reduces radiation dose

imporves image quality

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6
Q

Grid

A
  • Plate with thin lead strips between patient and detector
  • Absorbs scattered x-rays to improve image quality
  • Use when imaging patients/structures greater than 10cm
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7
Q

Detector

A
  • Film (analog)
    • Single-use film in cassettes
  • Computed Radiography (CR)
    • multiple-use cassettes
    • Plate reader
  • Digital Radiography (DR)
    • wired/wireless plate
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8
Q

Detectors: Films

Pros

A

highest spatial resolution

inexpensive upfront cost

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9
Q

Detectors: films

Cons

A

More technique sensitive

Higher dose

Slow

Caustic development chemicals

Large space needed for storage

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10
Q

Detectors: Computed Radiography

A

Older form of digital radiography

Detector in cassette

Plate reader scans imaging plate inside cassette

Digital processor converts into digital image

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11
Q

Detectors: Digital Radiography

A
  • Direct:
    • x-rays → digital data
  • Indirect:
    • x-rays → visible light → Digital data
  • Digital systems are able to “correct” to some extent technique related errors
    • makes previously non-diagnostic imagaes diagnostic
    • Decreases dose due to decreased re-takes
    • Conversely, overexposure is less easily identified
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12
Q

Detectors:

Direct / Indirect Digital Radiography

Pros

A

excellent contrast

less sensitive to technique

Smaller space required

Pre/post-processing

Image transfer

Lower dose

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13
Q

Detectors:

Direct / Indirect Digital Radiography

cons

A

expensive

lower spatial resolution

can inadvertently cause overexposure

requires modern technology infrastructure and know-how

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14
Q

How to take a radiograph

A
  1. Postition and Collimate
  2. Set technique
  3. Expose
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15
Q

What is technique

A

Adjustments made to determine number and energy of x-rays produced

kVP

mA

mAs

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16
Q

Technique: Elecrrical Basis

mA

A

mA = milliampere

  • Current applied to cathode
  • Affects how many electrons in the “cloud”
  • Keep high as possible
  • Not frequently adjusted
17
Q

Technique: Electrical Basis

mAs

A

mAs = milliampere seconds

  • mA * seconds = mAs
  • current over as period of time
  • How long the current is applied to the cathode
  • Also affects number of electrons in the “cloud”
18
Q

Technique: Electrical Basis

kVp

A

kVp = Kilovoltage Peak

  • Magnitude of voltage (potentail difference) between the anode and cathode
  • Affects primary speed and energy of electrons
  • Determines maximum energy and electron or photon can possess
19
Q

Technique: Clinical Basis

mAs

A

mAs = milliampere seconds

Number and density of photons in the beam

Primarily controls image exposure/blackening

Minimize to reduce sensitivity to motion

20
Q

Technique: Clinical basis

kVp

A

kVp = Kilovoltage peak

Peak energy of photons in the beam

“How hard” the beam “pushes” to get through the patient

Primarily controls image contrast and scatter

21
Q

Technique: Contrast

A

15% CHANGE IN KvP = ½ change in mAs

Contrast: difference between parts of an image

High contrast: black and white

Low contrast: lots of grey

22
Q

Technique: Contrast

Scale

A
  • Short Scale:
    • high contrast
    • black and white
    • musculoskeletal
    • High mAs, low kVp
  • Moderate Scale:
    • Moderate contrast
    • Many shades fo grey
    • abdomen
    • Moderate mAs, moderate kVp
  • Long Scale:
    • Low contrast
    • more shades of grey
    • Thorax
    • Low mAs, High kVp
23
Q

How do x-rays interact with tissue?

A
  1. Absorption
  2. Transmission
  3. Scatter

Changes to the x-ray beam via any of the above is termed attenuation

24
Q

How do z-rays interact with tissue?

Absorption

A
  • Results form photoelectric effect
    • x-ray ejects an electron form a tissue atom
    • Creates low-energy radiation and an ion pair
    • Increases with moderate to lower kVp
  • x-rays do not strike the detector
25
Q

How do x-rays interact with tissue?

Transmission

A
  • x-ray passes through patient unaltered or with a lower energy
  • Strikes detector and contributes to image darkening
  • Transmission and Adsorption good: Contribute to image quality
26
Q

How do x-rays interact with tissue?

Scatter

A
  • Common in soft tissue
  • Compton scatter = most important type
    • photon loses energy, changes direction, and ejects an electron
  • Exposes personnel = safety issue
  • Reduces image contrast
27
Q

How to decrease scatter

A

Increase collimation

Decrease kVp

Decrease tissue thickness or use a grid

Remember, technique must be increased with a grid

Grids do not decrease personnel exposure to scatter, only imporve image quality

28
Q

Imaging Artifacts

A

Errors in technique, processing, digital reconstruction

Things that make the image look odd that are not related to pathology

29
Q

Imaging Artifacts:

Underexposure

A
  • Film:
    • poor contrast, lack of darkening
  • Digital:
    • quantum mottle (grainy texture)
  • How to fix?
    • add 15% kVp for soft tissue.
    • Double mAs for bone
30
Q

Imaging artifact:

Overexposure

A
  • Film:
    • overdarkening
  • Digital:
    • clipping, planking, paradoxical overexposure effect
  • How to fix?
    • Decrease kVp by 15% for bone
    • ½ the mAs for soft tissue
31
Q

Imaging Aftifacts:

Motion

A

Common problem

Results: blurring

Fix:

Lower mAs

sedate patient

retake

32
Q

Imaging Artifacts:

Shape and Size Distrotion

Foreshortening

A

z-ray tube and detector in the same plane, radiographed part angled

33
Q

Imaging Artifacts:

Shape and Size Distortion:

Elongation

A

Radiographed part and detector in the same plane, x-ray tube angled

Radiographed part and x-ray tube in the same plane, detector angled

34
Q

Imaging Artifacts:

Shape and Size Distortion

Magnification

A

Radiographed object elevated away from detector

Object is enlarged and blurry

35
Q

What about other modalities

A
  • Computed tomography (CT)
    • x-rays
  • Magnetic resonance imaging (MRI)
    • radio waves and magnetic fields
  • Ultrasound
    • sound waves
  • Nuclear Scintigraphy
    • gamma rays

Radiology VCS845 (6 decks)

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