Lecture 2 Flashcards
Modern radiation photography
Digital imaging
Image quality
Digital film faults
Image receptors
Storage phosphor cassette - CR (computed radiograph)
Flat panel detector- DR (direct radiography)
Computed radiography CR
Uses cassettes that contain a ‘storage phosphor’ plate
energy from interactions with the x-ray beam is stored in the cassette until it is put in a reader where lasers excite and releases the energy as light. this is caught by a photomultiplier tubes in order to form an image but converting it to an electrical signal.
- can erase phosphor plate by bright light to be reused
- if plate in sat around for period of time erase before use and may have picked up scattered x-rays
Direct digital system DR
Electron detector directly capture the x-ray image
wireless and cable machine available - connect to computer
x-ray will interact with chemicals in the reader - image is displayed immediately on the screen
Both systems will display a numerical value for exposure after the exposure
as exposure factor increase more radiation to the patient
tell system which but of anatomy you are radiographing so it can match the exposure to how black or white the image is
Pros of digital imaging
can manipulate the image
- filters e.g edge enhancement, contrast enhancement
- contact/ brightness
- size (zoom in and out)
- Orientation
-Annotation and measurement
Storage
Images stored as computer files
need regular daily back up
can be printed but would loose resolution in the process
dicom format = .dcm
Digital radiography advantages
- Decreased running costs (unless printing images)
- Time saving - reduced repeat rate (as less likely to over expose) -may lead to decreased radiation dose
- improved images in some cases
- Easy retrieval and storage - space saving
- Easy communication of images - referrals etc.
- Portable - instant images in the field
Digital radiography disadvantages
- Set up costs
- Viewing images is limited by availability of computers - only in consulting rooms theatres etc.
- Specific artefacts
- Ensuring adequate back up of files
Image quality
Depends on= radiographic technique, patient factors, exposure, digital processing etc.
image quality will assess contrast, sharpness and distortion
Contrast
The difference in density (blackening) between 2 adjacent areas in the radiograph
will depend on…
- subject contrast (the area of animal you are radiographing)
- kV
- Scattered radiation meeting film
Subject contrast continued
is the range of radiographic densities within an animal
depends on the tissues
- atomic number
- density
- thickness
Effects of kV
higher kV = overall rise in x-ray penetration through all tissues
will decrease the differences within tissues = lower contrast image
note: abnormal radiographs use lower kV as less subject contrast in tissues so will help see the contest in areas
Scattered radiation
Will cause overall blackening on the film so reduces contrast
doesn’t help for a useful image
important to be considered in radiation protection as scatter results in radiation outside the primary beam
- will give ghosting image around the edge
Reduce scatter by…
- Use of a grid (for thicker areas)
- collimation - reducing size of primary beam - use a series of plates on machine to restrict are of beam that it covers - beam will get absorbed on lead
Grids
Reduces the amount of scattered radiation reaching the cassette film so improves image quality
- flat plates with a series of thin lead strips alternating with radiolucent strips (plastic or aluminium)
- primary through radiolucent - scattered absorbed by lead
- proportion of the primary beam will be absorbed/ removed by the grid so a higher exposure needed (
mAs)
Grid factor
indicates the multiples of mAs needed when using the grid
typically 2-6 (so will need to increase mAs by 2-6 when using a grid)
factors increasing grid factor = grid ratio (hight:width of lead strips), number of lead lines per cm
- if these are higher then more primary beam is taken out so mAs needs to be increased more
Types of grids
Parallel
- centring of beam and distance aren’t important
- will grid cut of. at the edge of the image as ray in at an angel to grid
Focussed
- strips are angles so entering of beam and distance are important
no cut off occurs as angle of strips will correspond to the angle of the beam as it spreads out
- grid must be used right side up - upside down will cause extreme grid cut off - radiation will head in in wrong way so will hit lead
Grid lines
some grid lines will always be present but will be exacerbated if a focused grid isn’t accurately aligned
Collimation
Limiting the x-ray beam to the area of the patient you are interested in
- will reduce production of scattered radiation
- improves image quality
- reduces unnecessary radiation to patient and scattered to staff
Collimation continued
Done by a light beam diaphragm
metal lead shutters will alter aperture size
will shine through gap to specific res
edge of the light is where the primary beam is limited to
Poor collimation =
lots of empty (black) space - lots of scatter, lack of detail
Sharpness
effected by movement blur, focal spot size and distances
Distances = film/ focal spot distance (FFD) plate and focal spot so xray machine and table
= Object/film distance (OFD) animal and table
Movement blur
from animal. tube, cassette/ table
x-ray machine may be wobbly
hand held machine in the field
Focal spot size
Smaller focal spot = sharper image as less tendency for them to spread out
Bigger area of focal spot = more divergence of the beam
Distances
FFD =smaller leads to a larger penumbra and less sharpness - machine would be closer to the table - the divergence would be significant
OFD= larger would lead to a greater penumbra and less sharpness- animal is further from the table
- closer to the table = sharper lines
what is a penumbra
The partially outer shaded outer region of the shadows cast by an opaque object
Magnification
Result from the diverging beam - the more spread out the bigger the image will be
calculated by FFD / (FFD - OFD)
need to position area of interest as close to table/ cassette as possible
Reducing magnification effects?
Longer FFD- more distance between x-ray machine and table
Smaller OFD - less distance between animal and table
orthopaedic implants
calculate the real size of something by using a radiography ruler to calculate the magnification
Distortion
occurs when object is not parole to film
increased by poor positioning
e.g. long bone one one will look enlarged
higher from table = more magnified
film faults
- Poor radiographic technique
- Radiation safety issues
- Double exposure
- Grid cut off (upside down grid etc.)
- Movement blur
- Inappropriate exposure factors
Double exposure
CR plates
press button twice so 2 images on one cassette or reuse a cassette
not usually possible with DR
Overexposure
CR = image too dark/ black
DR= doesn’t happen too much as is manipulated by computer to give a satisfactory image
Underexposure
CR= image too pale/white
DR= image is very grainy and pixilated - correct it by increasing exposure factors (kV &/or mAs)
Digital film faults
- “uberschwinger” or rebound artefact
- Ghost images
- Moire artefact
- Dirt along light guides in CR systems
Uberschwinger artefact
Happens when there is a large density difference between adjacent objects
commonly seen around orthopaedic implants
radiolucent halo around place of high contrast
Arises due to excessive edge enhancement of the image
Note: Important to recognise this artefact as can mimic implant loosening
Ghost artefact
From incomplete erasure of a CR plate before use - remnant of an image
Can also occur is CR plate is exposed to light - sat out in background radiation
Moire artefact
Bands seen across image (CR)
result of interference between the frequency of the laser reader and the number of lines per cm in the grid
Solutions - turn grid 90* or use higher frequency grid
Dirt along light guide
CR
dust means light can’t get up into reader - end up with a radiopaque line across
