Transcript Technical Aspects: the machine the image

Technical Aspects:
the machine
the image
LeeAnn Pack DVM
www.upei.ca/~vetrad
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Exposure settings: 4
components
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kVp
mAs
Time
Focal film distance (FFD)
– All can be changed alone or in combination
www.upei.ca/~vetrad
Make it just right
• Make a film more black
– Increase the kVp, mAs or time
– Decrease the FFD
• Make a film less black
– Decrease the kVp, mAs or time
– Increase the FFD
www.upei.ca/~vetrad
Questions to Ponder
• Is there adequate penetration of the part
imaged?
• How much contrast is needed on the
film (some inherent?)?
• Is motion a strong possibility during the
exposure?
• Does the tube move?
www.upei.ca/~vetrad
Inverse Square Law
• Decrease distance
by ½ intensity is
increased by 4 times
• Increase distance by
½ intensity is
increased by 4 times
• Technical
applications
• Radiation safety
www.upei.ca/~vetrad
Cassettes
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Contain single or double screen
Plastic, cardboard, metal
Must be durable
Film placed between the screens in the
cassette
• Foam between cassette and screen
• Keeps out unwanted light
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Intensifying Screens
• Absorb X ray photons and convert them
to light
• Reduces amount of radiation needed
• Contain phosphors
– Calcium tungstate  blue light
– Rare earth phosphors  green light
• More efficient
www.upei.ca/~vetrad
Screen Speed
• Determined by the efficiency of X ray
absorption and conversion
• Thicker phosphor layer and larger crystal
produce a greater amount of light  faster
• Slow  detail   photonsresolution
• Medium  par  average
• Fast less resolution but not as many
photons needed
www.upei.ca/~vetrad
Screen Speed/Resolution
• 100,200,400,800
• Smaller number being a slower high
resolution screen and the higher
numbers are a faster screen
• Screen speed and resolution are
inversely proportional
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Screen Cleaning
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Keep free of debris
Sharp white artifact
Screen cleaner
Soft lint free cloth
Never use soap and
water
• Stand on end to dry
www.upei.ca/~vetrad
Radiographic Film
• Composition
– Base supports the emulsion – blue
– Adhesive – base to emulsion
– Emulsion – gelatin with silver halide
crystals which is sensitive to radiation
– Protective coat protects from damage
during handlin and processing
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Radiographic Films
• Single or double emulsion
• Light sensitivity
• Film speed
– Film speed is inversely related to exposure
needed to produce a given degree of
blackness
– 50 speed is half as fast as a 100 speed
www.upei.ca/~vetrad
Radiographic Film
• Film latitude
• Range of exposures which can be used
to achieve an acceptable film density
• Wide range = high latitude
– Image contrast suffers
• Narrow range = low latitude
– Good image contrast
www.upei.ca/~vetrad
Radiographic Density
• Subject density
– Additive – thick parts absorb more
• Summation – know definition
– Relative
• What are the surroundings?
• Silhouette sign – know definition
• Over exposed = film to black
• Under exposed = film not black enough
www.upei.ca/~vetrad
Factors that affect film density
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Subject density – we cant change this
mAs – biggest factor
kVp – increases the penetrability
Distance – ISL
Development time and temp
Scatter and fog
www.upei.ca/~vetrad
Radiographic Contrast
• Difference of densities of the different
parts on the image
• Subject contrast
• Film contrast
• Scatter and fog
www.upei.ca/~vetrad
Turn that dial: Density
• The 16-20 rule
– kVp by 20% will double film density
– kVp by 16% will half film density
– Double mAs  double density
– Half mAs  half density
– Combos:
• Inc kVp by 20% and dec mAs by half
• Dec kVp by 16% and double mAs
www.upei.ca/~vetrad
Contrast
• Short scale  lot of contrast few grey
– High mAs and low kVp
• Long scale  many shades of grey
– Low mAs and high kVp
• Low contrast = long scale = high latitude
= low mAs = high kVp
• High contrast = short scale = low
latitude = high mAs = high kVp
www.upei.ca/~vetrad
Magnification
• Enlargement of the image relative to the
actual size
– Causes loss of detail and blurring
• Subject film distance
– Dec SFD  decreases magnification
• Focal film distance
– Inc FFD  decreases magnification
www.upei.ca/~vetrad
Distortion
• Image which does not represent the
true shape of the object
– Unequal projection of an object
• Femurs
– Location of an object within the beam
• spine
www.upei.ca/~vetrad
Scatter radiation
• What is scatter?
– Bounce around undergo numerous
interactions
• Scatter increases with:
– Increased thickness of the patient
– Increased X ray beam energy
– Amount of patient exposed (collimate)
www.upei.ca/~vetrad
Beam Limiting Devices
• Smaller fields = less scatter and less
radiation expsoure
• Cones, cylinders, shuttered collimators
• They do not shape the beam only
exclude the part we don’t want to use
• Light to illuminate field
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Filtration
• Inherent
– Glass housing, oil and tub housing window
– Filter only low energy photons
• Added
– Aluminum filter
– Will not remove the high energy photons
• Polychromatic beam
• Reduces patient dose
www.upei.ca/~vetrad
Grids
• Used to keep scatter from hitting film
• Perpendicular radiation gets through, off
angle photons do not
• Used on parts > 10-11cm
• Film contrast is improved
• Required more X ray exposure
www.upei.ca/~vetrad
Grid ratio
• Height of lead strips:width between the
strips
• The higher the GR, the greater the
ability of the grid to remove scatter
• 4:1 – 16:1
• Lines per inch
– Good and bad
www.upei.ca/~vetrad
www.upei.ca/~vetrad
Grid Cutoff
• Grid must be precisely under the center
of the central ray of the X ray beam
along the central axis of the grid
• Know what the types of grid cutoff look
like
• Bucky-Potter moving grid
www.upei.ca/~vetrad
Fog
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Light fog
Storage fog
Safelight fog
Chemical fog
www.upei.ca/~vetrad
www.upei.ca/~vetrad
www.upei.ca/~vetrad