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75 Cards in this Set
- Front
- Back
In diagnostic range of x-ray energies, target interactions produce _____ heat and _____ x-ray photons
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99%; 1%
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Characteristic Interactions (inside the tube)
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- electron from cathode interacts w/ inner shell electron on focal track of anode
- removes electron from atom - loses the energy necessary to remove the electron |
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What is the energy of a characteristic interaction equal to?
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The difference in the binding energy of the orbital electrons involved
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X-rays generated from the K Shell of __________ atoms are the only energies of strength that can be used for characteristic radiation.
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Tungsten
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Bremsstrahlung Interactions (inside tube)
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- incident electron from cathode passes near nucleus of the tungsten atom on anode, completely bypassing all electrons
- electrostatic attraction between incident electron & nucleus occurs |
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Braking Radiation
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When Bremsstrahlung's interact w/ tungsten, it's Z number makes it slow down, lose some energy, & change direction
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The closer the electron goes to the nucleus, the _________ (more/less) energy the electron loses, the ___________ (higher/lower) amount of Bremsstrahlung produced.
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more; higher
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Polyenergetic
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X-rays produced at many different energies
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What's an x-ray emission spectrum?
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A graph showing the numbers of photons produced at particular ranges of energies
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Characteristic X-ray Spectrum
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Discrete energy levels representing the characteristic of each electron shell
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Bremsstrahlung X-ray Spectrum
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Bell-shaped curve ranging from 0 to max kV setting
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Factors Affecting Emission Spectrum - change in mA
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A change in mA results in a directly proportional change in the amplitude of emission spectrum
- increasing mA will increase the number of incident electrons flowing from cathode to anode |
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Factors Affecting Emission Spectrum - change in kVp
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- changes will affect amplitude & position of x-ray emission spectrum
- increase in kVp = increase amplitude w/ a curve shift right |
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Factors Affecting Emission Spectrum - Target Material Effect
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As atomic number increases, efficiency of the production of Bremsstrahlung radiation increases
More high energy x-rays are produced |
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Factors Affecting Emission Spectrum - Changes in Voltage Waveform
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- operation w/ 3-phase power is equivalent to a 12% increase in kVp over single-phase (almost doubles the mAs)
- more efficient waveform = increased amplitude |
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Factors Affecting Emission Spectrum - Filtration
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- increases effective energy of x-ray beam by attenuating low-energy photons
- improves overall quality of x-ray beam |
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Filtration
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Process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials into primary beam
Reduces patient dose |
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What's the standard filtering material?
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Aluminum
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Aluminum Equivalent
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- any other material other than aluminum that's going to attenuate at the same rate
ex. Copper is slightly more dense, less of it is needed to attenuate the same amount |
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Half Value Layer (HVL)
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- amount of absorbing material that will reduce the intensity (# of photons) of the primary beam to one-half its original value
1 HVL = 2.5 mm Al/Eq for 100 kVp beam |
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Inherent Filtration
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- filtration that's a result of composition of tube & housing
- between target & exit port of glass envelope - can't be altered, remains constant - typical tube has 0.5-1.0 mm Al/Eq of filtration |
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Added Filtration
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- any filtration occurring outside the tube & housing and before patient is considered to be added
- ex. filtration occurring in collimator |
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Compound Filtration
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- composite filters using 2 or more materials to absorb photons
- layered w/ highest atomic number material on top; subsequent layers absorb characteristic photons producing by previous layer |
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Thoreaus Filter
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- form of compound filter
- highest atomic number = tin - middle atomic number = copper - lowest atomic number = aluminum - see this in radiation therapy |
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Compensation Filtration
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- external filter used to compensate for unequal absorption due to part density/thickness
- produces more uniform density - does NOT reduce patient dose - ex. wedge filters |
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Total Filtration
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- equal to the sum of inherent & added filtration
- doesn't include compound or compensating filters |
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Recommended Minimum Total Filtration Levels - kVp below 50
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0.5 mm aluminum
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Recommended Minimum Total Filtration Levels - 50-70 kVp
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1.5 mm aluminum
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Recommended Minimum Total Filtration Levels - kVp above 70
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2.5 mm aluminum
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Increasing filtration will _________ (increase/decrease) radiographic density
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Decrease
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Significant soft tissue penetration occurs between _________ kVp
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30-40
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Prime Factors
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milliampere (mA), time, kilovolt peak (kVp), distance
factors the radiographer controls that affect x-ray emission |
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Roentgen (R)
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Unit of measurement of x-ray quantity or intensity
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X-ray Quality
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- measurement of penetrating ability of x-ray beam
- represented by HVL - factors that directly affect x-ray quality are kVp & filtration |
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Millampere (mA)
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determine number of x-rays produced; radiation QUANTITY
Primary controller radiographic density; doubling the mAs doubles the density produced |
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Law of Reciprocity
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An increase in mA requires a decrease in exposure time if the same degree of blackening on the film is desired
Blackening on the film remains the same as long as the total energy exposing the film is constant |
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Equation for Law of Reciprocity
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mA1/mA2 = S2/S1
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Kilovoltage Peak (kVp)
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- primary controller of beam QUALITY (penetrability)
- controls the scale of contrast on a finished radiograph (relates to number of shades of gray & difference between them) |
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High kVp / _____ contrast (high/low) / ______ scale of contrast (short/long)
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low; long
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Low kVp / _______ contrast (high/low) / ________ scale of contrast (short/long)
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high; short
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As we increase kVp, the emission spectrum is shifted ________ (right/left) because of the increase in energy
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Right
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The higher the kVp, the ___________ (higher/lower) the energy of the electrons, the _________ (higher/lower) the optical density
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higher; higher
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15% Rule
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An increase in kVp by 15% will approx. double the overall density/exposure on film
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While trying to maintain density, an increase in kVp by 15% forces you to ________ (double/halve) the mAs
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Halve
Reverse is also true |
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Increasing kVp increases.....
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Penetrability
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Beam intensity decreases with a(n) ___________ (increase/decrease) in distance
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Increase
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X-ray beam intensity/exposure is measured in...
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Roentgens (R) or milliroentgens (mR)
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Inverse Square Law
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- deals with x-ray beam intensity
- as distance increases the amount of exposure received by patient or tech decreases |
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There is a(n) _________ (direct/inverse) relationship between distance & radiation exposure
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Inverse
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Inverse Square Law Formula
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I1/I2 = D2^2/D1^2
To determine amount of exposure... I2 = I1 x (D1^2/D2^2) |
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Density Maintenance
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- provides a mathematical method to compensate for changes in SID
- determines the amount of mAs necessary to provide enough photons to create an image after a change in distance |
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Density Maintenance Formula
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Old mAs/New mAs = Old Distance^2/New Distance^2
Use this formula when you're asked to solve for mAs |
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Doubling the distance will require ___ times the original mAs to maintain density
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4
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Attenuation
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Reduction in the number of x-ray photons in the beam
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5 Basic Interactions between X-rays & Matter
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- Coherent Scattering
- Photoelectric Absorption - Compton Scattering - Pair Production - Photodisintegration |
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Photoelectric Absorption
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- incident photon energy is completely absorbed by inner shell electron
- x-ray photon should have a slightly higher energy than a K shell electron - causes an ion pair |
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Photoelectron Energy Equation
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Ei = Eb + Eke
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What does the ionized atom cause?
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Characteristic Cascade
- atom electrically unstable after losing electron - when another electron falls down to fill a hole, it emits a characteristic photon when they change energy levels |
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Secondary Radiation
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- radiation that originates from irradiated material outside x-ray tube
- production similar to characteristic x-ray production within target - amount is directly proportional to atomic number |
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As photon energy increases, chance of photoelectric interaction ___________ (increases/decreases) dramatically
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Decreases
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If the atomic number is doubled, the chance of photoelectric absorption increases by a factor of _____
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8
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Coherent Scatter
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- low energy photons (below 10 keV)
- electrons get excited from incident photon & vibrate @ photon frequency - no electrons ejected; no ionization |
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Thompson Scatter
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Single outer-shell electron used
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Rayleigh Scatter
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All electrons of atom used
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Product of Coherent Scatter
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Photon released with equal energy to incident photon but traveling in a different direction
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Compton Scatter
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- incident photon interacts w/ outer shell, loosely bound electron & ejects it
- ion pair is formed - photon transfer some energy to the recoil (Compton) electron & continues in a different direction - most relevant to diagnostic imaging |
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Compton Scatter Equation
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Ei = Es + Eb + Eke
Eke = recoil electron Es = scattered photon |
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What is the source of the most occupational exposure & radiation fog?
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Compton Scatter
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Backscatter
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Scatter emitted in the exact opposite direction as incident beam
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Pair Production
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- incident photon energy must be 1.02 MeV or higher
- photon energy absorbed by nucleus, causing it to be unstable - it release a positron & negatron - negatron will join with another atom; positron is unstable antimatter |
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What happens when a positron combines with an electron?
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Annihilation reaction
Results = 2 photons of .511 MeV traveling at 180 degrees to each other |
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Pair production is more effective in....
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Radiation Therapy
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Photodisintegration
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- extremely high energy photon (10 MeV or greater)
- nucleus absorbs photon; release alpha particle |
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Which interactions have a less likelihood of occurring with an increase in kVp?
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Photoelectric absorption, Compton interactions
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What 2 circumstance would photoelectric absorption likely happen?
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1) Lower energy ranges (40-70 kVp)
2) In elements with higher Z numbers Image will have short scale contrast (low kVp, high mAs) [opposite for Compton] |