5. How Do You Create An X-ray Image? Flashcards
LOs
Types of radiographic film? What they’re used for?
- Direct action / wrapped packet film for intraoral
- Indirect action film used in conjunction with intesifying screens inside extra oral cassete
Intra oral radiographic film main sizes?
What part of mouth they’re used for?
A = 22 x 35 mm»_space;» anterior periapical + childrens bitewings
B= 31 x 41 mm»_space;> posterior periapicals + adult bitewings
C = 57 x 76mm»_space;> occlusals
- packaged in barrier envelopes to control infection
Intra oral radiographic film packaging, speed, colour etc
Surface facing X-ray beam = white
Reverse surface = white + colour
3 speeds - D,E,F
Different film speeds packaged using different colours
Single or double film - different coloured
No standardisation between companies
- hence, always ensure correct orientation, speed, etc
Intra oral radiographic film packet contents?
A = clear plastic barrier envelope
B = outer wrapper»_space; white + colour coded
C = Plastic film with embossed dot in corner to indicate orientation , dot aims towards operator
D = protective black paper on either side of film , protect from mechanical damage
E = thin sheet of lead foil
How to hold an intra oral film
- Hold the black paper to protect the film from damage from your fingers
- once you’ve removed the black paper, hold the film on its edges
- NOT on the emulsion
Structure of the film itself?
- clear plastic base is covered by a thin layer of adhesive
- both sides have thin layer of green silver halide emulsion
(sensitive to x-ray photons + light) - Both sides of emulsion covered by a thin layer of protective gelatine
What is the emulsion sensitive to?
- designed to be primarily sensitive to x-ray photons,
- sensitive to light, so the packet must only ever be opened up in the dark.
Why is the lead foil inside the film packet embossed with a pattern?
Embossed with a pattern
Allows easy identification of error in placing film the wrong way round
- film would be pale + show pattern of foil instead
Lead foil prevents residual radiation passing into the patient
Prevents some scattered photons coming backwards + hitting film
- will degrade image
Why is the lead foil in the packet? (EXTRA INFO)
- prevents some residual radiation passing on into the patient
→ The x-ray machine generates the x-rays which pass into the patient. Some of these then hit the film. The lead foil is positioned behind the film, so the x-ray photons will have to pass through it before passing on into the patient. Therefore, the sheet of lead will prevent some, but not all, of these photons from going on into the patient. - to prevent some scattered photons from coming backwards and hitting the film
→ the x-ray machine generates the x-rays, which pass into the patient and hit the film, as the lead foil is positioned behind the film it will prevent some of those photons which have hit the patient’s tissues beyond the film, from being scattered backwards onto the film. If not stopped, this radiation would scatter back onto the film and degrade the image
What is instant process / self developing film?
Film packaged in a plastic sachet
Attached to a developer and fixer solution tab
Exposed film causes developer tab to be pulled , solution will melt down through the sachet to affect the film
After 15 seconds , fixer tab is pulled to allow fixer to contact the film
Once fixed, chemicals are discarded and film is washed in water
No dark room / processing facilities required
Poor image quality
No protective lead foil
Used in emergency
How does indirect action film work ?
Used in conjunction with 2 intensifying screens
- one if front of film+ one behind film
Sandwiched together for extra-oral radiography in a cassette
Silver halide emulsion is sensitive to colours of light , not X-ray
Must be used with intensifying screens emitting the correct light colour
No orientation embossed for
- R and L lettering needed on outside of cassette
What is the function of intensifying screens inside the cassette?
Consist of fluorescent phosphors embedded in plastic matrix
- emit light when hit by X-rays
Phosphors include :
- calcium tungstate
- rare earth materials = gadolinium + lanthanum
- Yttrium
Different screens emit different colours of light
- blue
- green
- UV
- Red
Close contact between 2 intensifying screens and film m must be maintained
- avoids image blurring
What is the action of a conventional calcium tungstate intensifying screen ?
Calcium tungstate screen emits blue light
- Incoming X-ray photon
- Activates phosphor crystal that emits light
- Light diverges and hits film emulsion
- Cross over in clear film base
- Hits second rear emulsion
- Reverse effect of photons that hit the rear screen
What is the action of a UV system intensifying screen ?
Incoming X-ray photons activates phosphor crystal to emit light
Difference: little crossover effect through film base
Image produced has sharper edge definition / resolution
Summary of intensifying screens
Visible light emitted when screens are hit by X-ray
- via photo electric effect
Reduced number of X-ray photons needed
- dose to patient reduced
What happens when there is a gap between intensifying screens and film ?
Larger area of film will be affected
Image will be less sharp + blurred
Advantage of using rare earth green sensitive screens
Initial amount of X-ray needed to generate light less than conventional tungstate screen
Dose to patient reduced even further
How to maintain intensifying screens?
Must be kept clean
- dirt / marks prevent emitted light from hitting the film
Must avoid scratching + damage
How are cassettes constructed ?
Film is sandwiched tightly between front and back intensifying screens
Layer of sponge rubber - helps to compress contents of cassette together
Consist of a light-tight aluminium or carbon fibre box
Available in different sizes for extra oral radiography
Three main variables on control panel
Kv
- determines energy of photons + quality of X-ray beam
-Determines penetration of photons
- affects film contrast
- lower kV = better contrast between black and white
MA + time
- both determine quantity of X-ray photons
- affect degree of blackening of film
- overexposure = too black
What is chemical processing ?
Events required to convert invisible latent imagine into visible radiographic image
Traditionally done manually
- film clipped into holder + dipped into chemicals
Automatic processors
How does processing convert green silver halide emulsion ion film into radiographic image ?
Exposed side:
- green silver halide emulsion becomes sensitised
- film placed in developer
- developer causes green silver halide to be reduced to black metallic silver
Unexposed side:
- remains green
Film inserted into a fixer to remove residual green emulsion
- reveals white base underneath
Importance of development of radiographic film
Developer = alkaline solution
Reduced sensitised silver emulsion goes black metallic silver
Oxidised by air
- effectiveness reduced over time
Ability to blacken film should be monitored
Dependent on time + temp
Over developed = too dark
Under developed = too pale
Importance of fixation of radiographic film ?
Film = acidic solution
Removes unsensitisied silver halide emulsion to reveal clear/white base
Films should be fixed for twice the clearing time
Inadequately fixed films = greenish yellow / milky appearance due to residual emulsion
- can become brown with time
Two main types of digital receptors
- Intra + extra oral solid state receptors
- Intra + extra oral phosphor plate receptors
Features of solid state digital receptors
Real-time = produced a image quickly
Use charge coupled device sensors or complementary metal oxide semiconductor sensors
Sensors are directly linked to a computer
- instantaneous image
Disadvantage :
Intra-oral sensors are bulky , difficult to position§
What is the basic design of charge couple device solid state sensors ?
CCD = charged coupled device
Array of CCD pixels
Scintillation layer
- above CCD pixels
- fluoresces when hit by photons
Each pixel has N and P type silicon
How does the CCD solid state receptor work?
X-rays ht scintillation layer
- works like an intensifying screen
- emits light
Light interacts with silicon in each CCD
- creates a charged packet , concentrated by electrodes and converted into analogue voltage signal
Passes into computer analog digital converter
- digital signal will have discrete numbers
Each pixel has voltage measured and given a numerical value , 0-255
Numerical values converted onto a grey scale , black - white
Features of phosphor plate system receptors
Cordless
Different sizes of photostimulable phosphor storage plates available
Not directly linked to computer = no instantaneous image
Reader required to read plate + produce image via laser beam
Time consuming
Easy to use clinically
How do the phosphate plate system receptors work ?
Plates have layer of phosphor layer on a backing plate
X-rays pass through patient + hit phosphor layer
- can be stored in phosphor plates
Plate is then placed in reader
Laser beam shone on plate to release X-ray energy stored in form of green light
Green light detected in reader by photomultiplier tube
- converts light into voltage
Laser beam travels down plate + releases energy
Voltage passes into computer analogue digital converter
Creates digital signal
Converted into grey scale
How is a digital image constructed ?
Made of millions of pixels
2d image produced
265 shades of grey
Each pixel is allocated a shade of grey depending on amount of radiation that has reacher that part of sensor
Technical issues of digital imaging
Not accurate
Overlapping pixels has grey shade averaged out
Smaller the pixels = more accurate representation
How can image resolution be measured ?
Digital images
- measured by pixel size
Storage phosphor plates
- pixel sizes range from 60-70 microns
CCD
- pixel size ranges from 20-70 microns
- should have a better resolution than phosphor plates
Resolution measured in line pairs/mm = spatial resolution
- how many line pairs seen in a mm
- more line pairs = better resolution
Spatial resolution for direct action varies between 10-20 LPs/ mm
Spatial resolution for indirect action is about 5 LPs / mm
Spatial resolution for digital systems varies 7-25 lp / mm
How do we process / enhance digital images?
Doesn’t involve chemicals
Uses computer software
Brightness altered
- increased pixel value
- 8 will be unchanged = max pixel value
- increases towards white end of scale
Altering contrast
- pixel value from middle of scale either increased towards 8 or decreases towards 1
- makes black/ white more evident
- role of kV in film captured imaging is irrelevant for digital imaging
Image inversed
- black + white alter
Image embossed / pseudo 3d
Image pseudo coloured
- may not be clinically useful
Printing of digital hard copies
Several different printers
Ink jet printers mainly used
Can’t produced 256 shades of grey
Reduces quality
Thermo-sublimation printers
- expensive
Adv of digital imaging
Lower radiation dose ( particularly using solid state receptors )
Elimination of film processing
Rapid image production
Image manipulation
Automated image analysis
Electrical storage
Patient education
Disadv of digital imaging
Expensive
Large storage capacity
Sensors are bulky
Difficult to use
Easily damaged
Image manipulation
- loss of evidence of disease
PC crash
Current recommendations for digital imaging
Sensor sizes should be available should be available in a range that is comparable to dental film
Sensitivity of detector system should be compatible with X-rays et being used
- ideally DC / constant potential
- short exposure times
Retakes are properly justified , recorded and included in QA stats
