Image Formation Flashcards
what are the 4 things that may happen to photons when they are directed at matter?
Pass through the object
- too high energy or soft tissue
Stopped and absorbed by the object
- due to high density - eg metal
Redirected from the object as scatter
- likely to travel across the room
Losses energy before reaching the object
- soft photons
what is the absorption of photons impacted by?
if the object is radiopaque or radiolucent
the atomic number of the object
- eg. lead = 82, muscle/soft tissue = 6/7, bone = 20
- the higher the number the more absorption of photons
- more moving faster if a high atomic number
- less if low otherwise pass through
the specific gravity of the tissue
- density
- how packed the mass of an object is
- soft tissue less dense than muscle - so less compacted so less absorption
the thickness of the tissue
- quad vs neck
- species, age, breed
higher the atomic number, density and thickness = more photons absorbed by the tissue
what are the 5 identifiable shades in an x-ray?
black/dark = air
dark grey = fat
grey = water
light grey = bone
very light grey/white = metal
signs an image is under or over-exposed?
white areas (not metal) = underexposed
- all photons absorbed
- none passed through to plate
very dark areas (not air/water) = overexposed
- all photons passed through
- none absorbed by object
think about toast = dark means overdone
what are the different exposure factors?
what happens if you alter them?
Kilovoltage (kV)
- controls the power of x-ray beam through speed
- faster = more energy
- too high = too much energy = dark film with no contrast
- too low = not enough energy = too much contrast = soot and whitewash
Milliamperage (mA)
- controls the quantity of photons
- higher = more intensity
- altering effects of radiographic density as changes number of photons that will penetrate the tissues
Seconds
- with mA gives the total number of x-rays in one exposure
- increasing the time increases the exposure OR same level of exposure over longer period (depends on needs)
- will not change the penetrating ability or contrast of image
how should you work with kV and mAs?
they should be used together to create best possible image for the situation
- eg. conscious patient - increase kVs, reduce mAs - less but faster
- eg very dense - more mAs - more x-rays but moderate how quick depending on the density
a high mAs and low kV should produce the same image as low mAs and high kV
what is FFD?
Film Focus Distance
- the total distance between the focal spot and the x-ray beam
the intensity of the primary beam is reduced by a 1/4 every time the FFD is doubled
- to stop this happening you must alter the mAs
- long FFD = higher mAs
must keep the same for best comparisons
what is a grid?
a flat plate positioned between the patient and the x-ray cassette
- made up of thin alternating strips of radio-dense material (eg lead) with radiolucent material between
- often encased in aluminium
designed to stop oblique angles after reflecting from patient
- controls scatter - doesn’t impact patient exposure
- reduces blurring and displacement of the image
two main groups
- stationary
- moving
what is the Grid Parameter : The Grid Ratio?
what does it mean?
the ratio of the height of the strips to the width of the radiolucent interspace
the higher the ratio
- the morse efficient at absorbing scatter
- the more expensive
- the higher the grid factor
typically a 5:1 or 10:1
what is the air gap technique?
used during images of high radiation
a gap is made between the patient body and the cassette
- photons through the patient - lose energy
- excess scatter die-off
- less reliance on the grid
how would you prepare a patient for x-rays?
what is important and why?
type of restraint
- manual - only if too sick for chemical
- mechanical - will it cause further damage? may have resistance due to pain
- chemical - ideal - GA/sedation risks (especially if compromised) - have they been starved?
cleanliness
- if booked in advance ask to not have walked prior
- or to have groomed
- dirt can impact the image
