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APPH 226 > Density Part II > Flashcards

Density Part II Flashcards

1
Q

Latent image

A
  • occurs immediately after exposure, but before processing
  • invisible
  • this happens to all images, regardless of how it is acquired (film, digital DR, CR)
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2
Q

Image formation

A
  • how do we get it? Result of differential absorption of the x-ray beam
  • differential absorption is: process where some of the beam is ABSORBED and some is TRANSMITTED
  • it is called differential absorption because the beam is not absorbed to the same degree over the entire area exposed
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3
Q

Differential absorption

A
  • creates an image that represents the structures of anatomy (results from the variation between absorption and transmission
  • when an image is created this way, several processes are required
    • occurs in beam ATTENUATION
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4
Q

Beam attenuation

A
  • when the primary x-ray beam passes through tissue and loses some of its energy (this reduction is known as attenuation)
  • attenuation (loss of energy) consists of:
  • absorption
  • scattering
  • photon transmission
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5
Q

Absorption

A
  • interaction called photoelectric effect
    • complete absorption of x-ray photon
    • inner shell electron ejected (by x-ray photon)
      • photoelectron
    • outershell electron drops to fill the spot
    • characteristic radiation emitted
      • low energy (usually remains in the body)
    • atom is ionized
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6
Q

Probability of PE interactions increases if:

A
  • energy levels are CLOSER together
    • binding energy of inner shell electron and energy of incoming x-ray photon
    • the closer they are, the higher the odds of PE occurring
    • complete absorption of the x-ray photon
  • what other interactions are classified as absorption? Pair production and photo disintegration
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7
Q

Scattering

A
  • what interaction would this be? Compton
    • incoming x-ray photon is not absorbed
    • ejects outer shell electron
      • compton electron, secondary electron
    • x-ray photon loses energy, changes direction
      • can go onto interact with IR
      • can leave body
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8
Q

Scattering cont

A
  • what else would be in this category? Coherent
    • not significant
    • occurs with very low energy x-rays
    • can interact with IR but very minimally
    • In diagnostic ranges, very little contribution from coherent scatter
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9
Q

Scattered photons

A

Why are they an issue?

-if they reach the IR they degrade image, contribute nothing useful to the image, AKA image fog

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10
Q

KVp and image fog

A
  • when kVp is increased there are: fewer interactions in total, increased amount of x-rays transmitted, increased compton scattering
  • scattered radiation creates unwanted exposure on the image called image fog
    • does not offer any useful information
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11
Q

Scatter and image fog

A
  • when kVp is INCREASED the amount of fog is INCREASED
  • how is density affected? Increased fog will increase overall image density
  • how is contrast affected? Increased fog leads to a decrease in contrast
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12
Q

Transmission

A
  • defined as: the x-ray photon passes through to the image receptor
    • direct transmission
    • indirect transmission
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13
Q

Interactions and kVp

A

⬆️ kVp ⬇️interactions

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14
Q

Exit radiation

A

= transmitted and scattered radiation
=(direct and indirect transmission)
-amount of absorption and transmission are varied (differential absorption)

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15
Q

On the image

A
  • WHITE (low radiographic density, areas of absorption, photoelectric interactions)
  • BLACK (transmission, high radiographic density)
  • GREY (mix of both absorption and transmission)
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16
Q

All about compromise

A

⬆️ kVp ⬇️ patient dose

17
Q

Exponential absorption

A
  • as x-rays pass through tissue, there is no fixed range to describe how far they go
  • attenuated exponentially
  • for every increment of thickness, the x-rays are decreased in #, by a certain percentage
  • never reaches 0
18
Q

Distance and density

A

SID

  • source to image distance aka FFD (focus to film distance)
  • intensity changes with distance
  • inverse square law
19
Q

SID

A
  • as the distance between the focal spot and the IR changes, the intensity of the radiation will change also
    • you’ll notice as the distance is increased, the light covers a larger area
  • what happens to the # of photons as the area is increased? Same # of photons covering a larger area, so less density
20
Q

SID cont

A
  • What happens to the image as the area is increased? Because it is receiving less radiation, there will be less radiographic density
  • what if we have a very short SID? The reverse is true, more photons in a smaller area -> increased intensity, more radiographic density
21
Q

Density maintenance formula

A
  • derived from the inverse square law

- when do we use it? Portables (especially chests), to fit larger parts to a receptor, patient condition

22
Q

OID

A
  • object to image distance
  • increased OID allows less scatter to reach the IR (coming off in all directions and may just miss the IR)
  • how would less scatter affect radiographic density? It would decrease it
  • increased OID, results in decreased radiographic density
23
Q

Body habitus

A
  • sthenic (average, muscular)
  • hyposthenic (thin, slim, healthy)
  • hypersthenic (large)
  • aesthenic (small and frail)
24
Q

Thickness

A
  • calipers
  • with increased thickness, how would we adjust our exposure factors? MUST INCREASE EXPOSURE FACTORS
  • can change mAs or kVp
  • in most situations would choose to adjust mAs
25
Q

General rule of technical factors

A
  • chest (high kVp)
  • soft tissue (low kVp, high mAs)
  • bony extremities (low kVp)
26
Q

Pathology

A

Radiographic technique needs to be adjusted in cases with known pathology

  • destructive (increases radiolucency of tissue)
  • constructive (increases radiopacity of tissue)
27
Q

Filtration

A

Three types

  • added
  • inherent
  • compensating
28
Q

Inherent filtration

A
  • glass or metal envelope
  • collimator usually provides some additional filtration
  • tech has no control over this
  • built into the unit
29
Q

Added filtration

A
  • can be adjusted or selected
  • aluminum is the material most commonly used
  • technique charts are based on units using the lowest allowable added filtration
  • when you add more, you must adjust your technique to compensate
    • usually increase filtration for areas with high subject contrast
    • can decrease dose
30
Q

Compensating filters

A
  • sole purpose is to balance the intensity of the x-ray beam to deliver a more UNIFORM exposure to the IR
  • can be aluminum and attached by collimator
  • can be placed on the patient, to even out density
31
Q

Filtration

A
  • Why do we use it? Increased x-ray beam quality, increased penetrability (due to losing lower energy photons)
  • what is the result on our image? More scatter, decreased image contrast, compensating -> more uniform density
32
Q

Grids

A
  • device placed between patient and IR
  • purpose: absorb scatter exiting the patient
  • why do we use it? Improve image quality, high ratio grid can absorb 80-90% of scatter
33
Q

Grids and density

A
  • how will a grid affect radiographic density? It will decrease the amount of density on the image
  • so then: mAs must be adjusted when adding, removing, or changing a grid
  • this is to maintain appropriate density
34
Q

Beam restriction

A
  • collimation (adjusted by tech)
  • what does this affect? The amount of tissue being irradiated, the amount of scatter reaching the IR
  • what does this mean for density? Less scatter reaching the IR results in decreased radiographic density
35
Q

Generators and density

A
  • exposure factors that we use are developed based on the type of generator in the room
  • generator factors:
  • if the output is more efficient, LOWER technique settings
  • if the output is less efficient, HIGHER technique settings

APPH 226 (23 decks)

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