L2 Image Receptors and Processing Flashcards
What are the 3 major imaging types used for dental radiographs?
- Solid state detector (14%)
- Photostimulable phosphor plate (54%)
- Film (28%)
What are the 2 types of solid state detector?
- Charged coupled device (CCD)
- Complementary metal-oxide semiconductor active pixel sensor (CMOS-AP)
Can solid state detectors be autoclaved?
No, for infection control purposes they are kept in a polyethene bag instead.
What are the disadvantages of solid state detectors?
- Bulky, 3-4mm in depth
- More likely to encounter positioning errors
- Delicate equipment, easily broken if dropped, expensive to repair
Describe how a solid state detector produces an X-ray image.
- Scintillation layer above the sensor produces light when hit by X-rays
- Light activates the solid state detector underneath to produce a voltage
- Voltage transmitted to PC
- Voltage converted to digital signal by an analogue digital converter
- Then converted to a number which corresponds to a grey scale value shown on screen
What are the benefits of a photostimulable phosphor plate system?
- Less bulky than solid state
- Better tolerated by patient
- Great number of sizes, can be used for intraoral, occlusal and panoramic
Describe how the photostimulable phosphor plate system produces an X-ray image.
- Phosphor plate exposed to X-rays
- Plate placed in scanner and scanned with laser beam
- Releases stored energy within the plate in the form of light
- Light detected by photomultiplier layer which amplifies signal and produces a voltage
- Voltage converted to digital signal by analogue digital converter
- Converted to number correlating to grayscale value
What are the disadvantages of the photostimulable phosphor plate system?
- Light exposure, if plate exposed to light image will be erased or artefacts produced
- Plates are relatively flexible, may bend in mouth and cause distortion of image
- Phosphor layer is soft and easily scratched, creates artefacts
- Phosphor plate must be completely free of dust or debris
How does the discrete number produced from the digital signal produce an image?
- Image is made of pixels
- Range of 256 shades of grey
- Each pixel assigned a shade of grey depending on the amount of radiation that has reached the sensor
What is the effect of adjusting image brightness?
Adds equal amount of numbers to all of the numbers already assigned to each pixel.
What is the effect of adjusting image contrast?
Adjusts the difference between black and white elements.
- High contrast useful to see caries
- Low contrast useful to see bone levels
How can image enhancement alter the X-ray image produced?
- Digital systems allow images to be enhanced
- You can rescue under/over exposed images
- Can optimise images for diagnostic purposes
- Image can be inverted (useful in endodontics)
- Image can be embossed (also useful in endodontics)
- All systems allow you to accurately measure (if properly calibrated)
- Image can be magnified
What layers are present in conventional film?
- Supercoat (gelatine layer to prevent emulsion being scratched)
- Emulsion
- Polyester base
- Emulsion
- Adhesive
What is the difference between film packets and cassettes?
- Film packet: contains direct action film
- Film cassette: contains indirect action film
Describe the structure of a film packet.
- Contained in polyethene packet for cross infection purposes
- White side facing X-ray beam
- Film
- Black paper to protect film from light
- Lead foil with embossed pattern on back of film (absorbs X-ray beam, prevents back splatter, prevents film being placed back to front in patient)
What are the differences between F- E- and D-speed film packets?
- F-speed is the fastest, requires less X-rays to produce an image
- D-speed is the slowest and requires the most X-rays to produce an image, not used clinically
Describe the structure of a film cassette.
- Thin cassette front made of metal
- Sponge rubber to push intesifying screen against film
- Front intensifying screen
- Film
- Back intensifying screen
- Sponge rubber
- Thick cassette back made of metal (absorbs X-rays passed through film)
When are film cassettes used?
Generally used for extraoral radiography.
Describe the structure of an intensifying screen.
- Protective layer (closest to film)
- Phosphor layer
- Reflective layer
- Screen support layer (furthest from film)
What is the main benefit of a film cassette?
Dose saving, less radiation exposure to patient.
Describe what happens when an X-ray hits the phosphor layer of a film cassette.
- Light is produced (photoelectic effect)
- The reflective layer reflects this light back to the surface of the film
Define film speed.
Exposure required to produce an optical density of 1 above background fog.
Define film contrast.
The difference in optical density between two points on a film that have received different exposures.
What type of image does coventional film produce?
A latent image.
How is the latent image processed?
- Developed
- Fixated
Describe how film is developed.
- Film is covered in silver bromide crystals (green) due to latent image centres being produced due to exposure
- Development: silver bromide crystals reduced to black metallic silver
- Fixation: black metallic silver fixed onto film surface as gelatine hardened
Describe how film is fixated.
- Image placed in fixing solution
- Removes silver bromide from emulsion
- Hardens the gelatine
What are the 3 types of film processing?
- Manual processing: film placed on rack, dipped in developing solution, washed, dipped in fixing solution
- Automatic processing: hands placed through sleeves, packet opened, film placed into processor (quick)
- Self-developing film
