film imaging and processing Flashcards
(31 cards)
image receptor (film)
Protective coat emulsion adhesie coating base 'repeat'
protective coat
transparent layer (usually gelatine) to protect emulsion
emulsion
sliver halide crystals held in gelatine suspension
-reacts to light
geletine
increases sensitivity of crystals
provides even suspension for crystals
adhesive layer
sticks emulsion to base
base
translucent blue plastic tp make the structure sturdy
image receptor screen
protective layer reflective layer phosphor layer protective layer x ray film ...
protective layer
transparent layer to protect phosphor
reflective layer
reflects light back towards film
- increases sensitivity
- allows reduction of the dose due to scatter
image receptor screen interection steps
An x ray photon will likely interact in the phosphor layer above or below the film converting its energy to may light photons
- the photons are emitted in all directions (might hit reflective layer or not)
- those that head towards the film may activate the silver halide in the emulsion
- as the photons spread out from the interaction point they will cause a larger area of blackening
- this reduces the resolution of the system
what does speed depend on
thickness of phosphor layer in screen
crystal size in emulsion
conversation efficiency
light absorbing dyes within screen
image formation steps
When an x ray photon hits a crystal in the emulsion an electron from an Br ion gets enough energy to escape
- the electron moves to a defect in the crystal or to a Ag2S impurity known as a sensitivity speck
- this becomes – charged and attracts a mobile silver ion from within the crystal and to form an silver atom
- this Ag atom then acts as a trap for a second electron and the process is repeated
- by this small deposits of silver will have formed those crystals hit by an xray photon
- these crystals will then be more sensitive to the developer and the whole crystal will be blackened (not just the silver atoms)
can vary the crystal size – more dose by using smaller more crystals compared to larger crystals
developer characterisics
alkali
acts as reducing agent
converts remaining silber ions to silver atoms (adding e-)
what does washing do
stops developer working on film
fixing
acid
- removes untreated silverhalid crystals
also hardens the gelatine suspension
fixes crystals in position
latent image fading
Silver atoms may revert to silver ions
- can happen if not processed straight away
- optical density may fall by 10-20 percent in 24 hours
factors which affect radiographic image
1) contrast
- depends on subject and film
2) characteristics of the x ray beam
3) image geometry
4) image sharpness and resolution
factors which affect subject contrast
1) different target/filter combinations
2) reducing the beam penetration/tube voltage
- how easy to get through substance
- harder beam goes through everything lose contrast
- too soft a beam, give lots of radiation dose to soft tissues close, will need even bigger to reach deeper tissues
3) using contract media
- can be used
- not common
- something into soft tissue so it can be imaged
4) use of scatter rejection grids or air gap
- allows scatter from patient to spread out before interacting with the film
- should get less scatter of what you don’t want
film contrast
dependant on sensitivity on filter ray
narrow latitide, small change in dose big change in optical density
what is fog and what does it lead to
blackening of film from natural processes
fog leads to a smaller ratio, therefore reduces the contrast by having great layer over image
film speed
speed defined as change in optical density for change in exposure
ideal characteristics of x ray beam
Ideal characteristics
- parallel
- produced from a point source to reduce penumbra
- sufficiently penetrating to pass through to a varying degree
what is the beam held at and why
parallel to avoid distortion
focal spot size
can’t have a small focal point as you need to dissipate the heat or a long time to do it slow (patient will move)
