Lecture 6: image recording & processing Flashcards
What does an X-ray film pack consist of?
1- outer cover: water and light proof
2- black card: primary function is to add physical strength, so card doesn’t bend in patients mouth. Secondary function is to absorb light.
3- lead foil: to maintain a clear image by preventing any ‘backscatter’ from other tissues (tongue, teeth) affecting the image
4- film with photographic emulsion: this produces the image.
What does the x-ray film consist of?
- polyester base (maintains shape and form) this has a slight colour dye to it
- on each side of the base is a subbing layer (fixes emulsion to the base)
- active emulsion
in dental x-ray film, what is the active emulsion?
silver-bromide
What causes differences in absorption?
- high kV (low attenuation due to less scatter)
- density
- atomic number
- thickness
Why is it called an emulsion?
because the silver-bromide molecules are suspended in gelatine
What does the film be dupitized mean?
Why is this done?
- it means the active emulsion is on both sides of the base
- this arrangement allows a diagnostic level of density to be achieved at half the dose that would otherwise be needed
What are imaging steps involved in producing radiographs?
1- latent image formation
2- developing the latent image
3- fixing the image
what is a latent image?
It is an image that already exists, but is invisible.
It needs to be developed.
How is a latent image formed?
- Upon interaction with an x-ray the silver or bromine atom of the molecule is ionised and this weakens the bonds between silver and bromine causing them to dissociate.
- The number of molecules dissociating is proportional to the number of x-ray photons hitting that part of the emulsion.
What would happen when photons pass through the pulp cavity?
A lot of photons would pass through and hit the silver-bromide crystals causing many of the molecules to dissociate
What would happen when photons pass through enamel?
Few photons would pass through and hit the silver-bromide crystals causing no molecules to dissociate
How is an image developed?
- The crystals in the film emulsion are surrounded by a negative charge that repels the electrons of the developer
- After exposure, the crystals with dissociated molecules have a defect in this negative charge shield and allows electrons from the developer to enter.
- The developer enters exposed crystals and donates electrons.
- These electrons exert a force of attraction on the positive charged silver atoms, causing them to clump together and this darkens the exposed area.
What is used as a developer for x-ray images?
a reducing agent (donates electrons to the material)
What 2 functions does the fixer have?
Functions:
1-The fixer dissolves the unexposed silver bromide crystals so no further exposure can take place: image is made permanent
2- The fixer hardens the emulsion to protect it and prevent scratching during storage
-after fixing, the image is washed and dried
Why is an image fixed?
-To make the image permanent and the protect it from damage
What are the 6 stages in full processing of radiographs?
1- Development 2- Rinse 3- Fixing 4- Rinse 5- Wash 6- Dry
in manual tanks, how long does the processing take?
over an hour
EXAM QUESTION:
in manual tanks, what are the labels of each process starting at film handling?
1-film handling 2-developer 3-fixer 4-wash 5-dryer
What would a yellow stain on the film be caused by?
- fixer left in the emulsion
- it breaks down and deposits sulphur crystals, due to inadequate washing
why would automatic processing be faster?
- the chemicals are more concentrated and faster acting
- the process takes place at higher temperatures
What is a disadvantage of automatic processing? (high temperature and higher concentration?
it will reduce the contrast of the image
What are intensifying screens?
fluoroscopic materials that convert high energy x-ray photons into an increased number of light photons
What is the purpose of the intensifying screen?
to reduce the radiation dose to the patient
What would cause the film to be fine in the morning and progressively get darker throughout the day?
thermostat is broken and the film overheats
What would cause the film to be fine in the morning and progressively get lighter throughout the day?
heater is broken and the film doesn’t get warm enough
What are the 2 types of Digital Image Recording?
1- Direct (DDR: Direct Digital Recording)
2- Indirect (CDR: Computer Digital Radiography)
What does direct digital imaging use?
charge coupled devices (wired system)
This is totally instant
What does indirect digital imaging use?
photo phosphor plates (need to process the plate)
What 3 types of recording medium are there?
1- phosphoplates
2- film (best image quality)
3- direct image radiography, using sensors
What is the process of digital imaging?
1- analysis: collect numerical information based on the image
2- enhancement: improve appearance of image
3- encoding:
How does the digital system produce an image?
the digital system reduces the analogue image to average values in the form of pixels (less information as it leaves out subtle differences in shade)
How does indirect imaging work?
-uses phosphor plates
1- phosphorescence absorbs radiation and the electrons are raised in energy but they are delayed in falling back down to their original energy level
2- the release of the lower energy photons is controlled by external forces
What is phosphorescence?
is the absorption of high energy photons followed by the later release of lower energy photons.
How does direct imaging work?
1- x-ray photons passing through phosphor plate excite electrons (raised to higher energy level)
2- The electrons are held in a suspended state within ‘traps’ known as energy anomalies
3- the image is stored as a pattern of raised energy electrons
4- The laser light scans and allows the electrons to drop back down.
5-The energy released is detected and the values recorded.
What are the advantages of film imaging?
- records a wider range of grey scales
- contains more image information
What are the disadvantages of film imaging?
- vulnerable to extreme environments
- processing faults and artefacts are common
- COSHH regulations
What are the advantages of digital imaging?
-image manipulation (density and contrast)
-ease of image transfer
-good for teaching purposes
no COSHH regulations
What are the disadvantages of digital imaging?
- sensors/plates are expensive
- sensors/plates easily damaged
What does film processor quality assurance check for?
1- fall in fluid levels
2- fall in temperature
3-exhaustion of chemicals
