Image Receptors Flashcards
What is an image receptor?
The item the x-ray image is formed on.
What are the 3 different types of image receptor?
- Film
- Plate
- Digital sensor
What are X-rays produced from?
X-rays are produced from a source (ie. x-ray tube)
Where do X-rays pass through?
The air
What happens to the weaker x-ray photons?
They bounce off the patient and scatter
What happens to the stronger photons?
The stronger photons interact with the patient.
Some will be absorbed by dense structures in the body, and some will continue on to interact with the receptor and ultimately produce the x-ray image.
What causes film fogging?
When photons don’t have enough energy to make a useful interaction with the receptor, they scatter. This can degrade the image causing fogging of the film.
How do you process film?
By placing it in chemicals in darkened conditions to fix the image
How do you process digitally?
The information is read and translated into an image by a computer.
When was the first permanently fixed image taken?
What was the exposure time that was required for this image?
1827
8 hours (not very practical)
When was the exposure time, for a permanently fixed image, reduced to several minutes?
What was introduced during this time that aided this exposure time reduction?
By 1839
Metal based daguerreotype process was introduced.
What did paper reducing exposure time reduce the exposure time to?
A couple of minutes
What did film reduce the exposure time to?
A few seconds.
When were X-rays first discovered?
1895
What are the 2 ways of capturing an X-ray image?
Digital and Film
What are the 2 types of film?
- Direct action film
2. Indirect action film
What are the 2 types of digital receptors?
- Solid state sensors
2. Photo - stimulable phosphor
What is a direct action film?
Direct action film is a piece of film which is sensitive to x-rays and wrapped in a packet.
What is indirect action film?
A film inside a cassette which contains intensifying screens.
How does an indirect action film work?
The X-ray photons react with the intensifying screens
This produces light photons which expose the film to generate the image.
The light produced is in direct proportion to the x-rays that hit the screens.
What are the indirect action film cassettes used for?
Extra-oral radiography.
What are the two categories that fall under solid state sensors?
- CCD: charge-coupled device
2. CMOS: complimentary metal-oxide semiconductor.
How do solid state sensors work?
They work by converting light into electrons.
The information is then collected by a computer programme and transformed into the image
What size do solid state digital sensors usually come in?
What issue arises with this size of sensors?
Solid state sensors usually come in size 2 only
This can make x-raying anterior teeth more difficult due to the large size of the receptor.
What types of sensors do DDH usually use in the radiology department?
Photostimulable phosphor plates (PSP)
How are images produced with PSP sensors?
Images are produced by the interaction of x-rays with the phosphor layer on the plate, which is then scanned by a laser to produce the image.
What are the EO plates in the department?
The extra-oral cassettes in the department are PSP plates.
What are the main differences between solid state sensors and PSP plates?
PSP:
- Can be used with Rinn film holder whereas solid state sensors can’t
- More tolerated than solid state
- Damaged easily: can all affect the final image quality;
- Patients’ teeth
- Operator handling errors
- Processor wear and tear
- Relatively expensive:
- size 0 = £40
- size 4 = £140 - Sensitive to background radiation (daylight) - must be cleared every 24 hours if not been used.
Solid state:
- Capture images in the same way as a digital camera, and are not compatible with any other system.
- Intra-oral sensors are large and bulky - difficult for patients to fit them in their mouths.
- Very expensive.
- Easily damaged - irreversible damage so needs replacement.
- Image resolution (detail) is better than with phosphor plates.
What are most OPT and lateral cephalograms most likely taken on?
They are most likely taken on machines that are solid state sensors.
What are the 2 silver plates on black cables?
Solid state sensors for intra-oral use.
What processor is used for extra-oral radiography.
The large floor standing processor for the cassettes.
How many sides does a phosphor plate have?
What colour is each side?
2 sides
- Front of the plate - pale
- Back of plate - black
What coating does the front of the plate have?
Phosphor coating
What does the back side of the plate show?
The size of the film and the orientation dot.
Where is the plate placed before being used?
The plate is placed in a protective bag before being used.
What should you be able to see through the window of the bag?
You should be able to the see the black side of the plate
How does the PSP system work?
- After being placed into a protective bag, the plate is then placed into the patients mouth and the phosphor coating is exposed to X-rays which cause a reaction within the phosphor - this is known as the stable excited state
- The plate is then dropped into the vista scan unit where it is scanned by a red laser - this is known as the unstable excited state
- The blue light which is then emitted is collected and converted to an image.
Summary:
- Laser scans plate as it passes down through scanner.
- This triggers stored energy to be released as light.
- This is converted to image seen on monitor
What is latent image?
When the phosphors store the radiation in local electron energies.
What happens to the electrons when the phosphor layer is scanned by a red laser?
The electrons drop to lower energy levels by emitting blue light.
What is the emitted blue light detected by?
Where is it sent to?
What is this conversion known as?
Detected by a photodetector
It is sent to a signal digitiser.
This is known as an Analogue to Digital converter (ADC)
What does the ADC do?
The ADC converts the light to a digital signal
What is the grey scale?
Each pixel’s numerical value corresponds to a different shade of grey which forms the image on the monitor.
The different shades form the grey scale.
How is the plate is erased in order to be reused?
It is erased by exposing the phosphor to bright light which releases any remaining trapped energy in the phosphor electrons, and the plate is then ready to reuse.
How many layers does the detector have in a solid state Charge Couple Device system?
What are these layers?
4 layers:
- A front cover
- A scintillator layer
- A silicon wafer
- A back cover incorporating a cable
What does the Scintillator layer do?
Converts x-ray energy to light
What does the Silicon wafer layer do?
Converts the light into an electrical signal
What is the cable on the back cover responsible for?
Carrying the signal to a PC.
How do solid state Charge Couple Device systems work?
- X-rays hit the scintillator layer, which is either caesium iodide or gadolinium oxysulphide
- The reaction gives off light which hits the photosensitive cells within the silicon wafer.
- Electrons within each cell are released and these form a charge (voltage) which is converted to the image.
- Each cell is connected to its neighbour and the signal converted to the image is from a line of neighbouring cells.
How does the solid state CMOS system differ from the CCD system?
The only difference is that in CMOS the photocells are electrically isolated.
Therefore a signal is sent from each individual cell rather than a line of cells
Comparing CCD and CMOS, which system is better?
CCD:
- CCD possibly gives a better image but CMOS is catching up.
- CCD’s have been around longer so you could argue they are more reliable.
- CCD is comparable to a high quality digital camera.
CMOS:
- CMOS is cheaper to manufacture.
- CMOS uses less power.
- CMOS is comparable to a mobile phone camera
What are the advantages of digital imaging?
- WIRELESS:
– Detectors are wireless
– Easier for patient, image appears in seconds.
- NO CHEMICALS:
– No chemicals required to process digital images
– Less storage facilities to stock the chemicals and boxes of films.
– Don’t require excellent ventilation if chemicals aren’t present
– Don’t have to do QA on processor to check for off chemicals
– Don’t have to dispose of the used chemicals.
- FASTER PROCESSING:
Film - min of 90 seconds
Digital- <15 seconds.
- EASY ARCHIVING:
– Referring clinician has almost instant access to the image
– Don’t have to wait for the patient to bring film back to the clinic
– Won’t get lost in transit or misfiled.
- EASY IMAGE TRANSFER
– you can send the image to anyone anywhere in the world, which has great benefits for teaching and also if you require expert diagnosis. - IMAGE MANIPULATION:
– Can make it bigger, shrink it, flip it, darken it, lighten it, write on it.
- DOSE REDUCTION:
– Exposure factors can be reduced
– This means the need for repeats due to over/under exposure is reduced
What are the disadvantages of digital imaging?
NO ORIENTATION DOT:
- Can get disorientated easily
Suggest a common sign of damage on a Solid state image (digital).
Dark area - suggests sustained damage possibly from being dropped
Suggest some common signs of damage on a PSP (digital)
Scratches - wear and tear from passing through scanner
White wavy edge - phosphor layer starting to peel from base.
What are the signs of too high an exposure?
Why does this occur?
- Dark
- Flat
- Very few shades of grey
- No contrast.
This is because the high radiation has caused too much light to be produced in each pixel
This means there is little variation in the electrical signals which are converted to form the image.
What are the signs of too low an exposure?
Why does this occur?
- Pale
- White mottling on image
Not enough radiation hit receptor = No reaction to produce light
Therefore no signal from that pixel will be produced and the computer cannot guess and make better what is just not there.
Why do we aim to select optimal exposure factors ?
So the resultant image gives you the best diagnostic information.
How is image manipulation useful
Allows you to focus on a specific area (eg. soft tissues).
Where must the images be stored in a dental practice?
On the local server
Although must back up elsewhere to allow access to images in case something goes wrong.
What local image storing server is used in DDH?
Picture Archiving and Communication System (PACS)
Where are the images transferred after being uploaded to local PACS?
They are then transferred to the National PACS server which is located at Livingstone.
What must you be careful about when uploading to PACS?
That you upload the image to the correct patient profile.
What are the 3 different types of intra-oral (direct) radiographs?
- Periapicals
- Bitewings
- Occlusal
What are the 4 layers that make up the film packet?
- Outer plastic wrapping
- Black paper
- Lead foil
- Film
Whatis the purpose of the outer plastic wrapping layer in the film packet?
- Acts as a barrier for infection control.
- Keeps the film dry (saliva free)
- Protects the film from light damage (fogging)
- Indicates the correct orientation of the film in the mouth – white side towards x‐ray tube
Whatis the purpose of the black paper layer in the film packet?
- Protects film from light leakage
2. Stiffens & supports film - prevents crimp/pressure marks on the film
Whatis the purpose of the lead foil layer in the film packet?
- Readily absorbs x‐rays
2. Prevents back scatter
What is the lead foil layer composed of?
A mixture of lead and aluminium
What is the direct action film layer composed of?
- A plastic polyester or cellulose base layer
- Adhesive layer containing anti-glare tint on either side
- Emulsion layer on either side
- Protective layer of gelatine
What is film emulsion?
The emulsion is the layer that reacts with the x-rays to form the image.
What is film emulsion comprised of?
Silver halide crystals, usually silver bromide, suspended in gelatin.
What are the 2 functions of the gelatine?
- It allows even distribution of the crystals
2. It absorbs liquid easily which helps during processing
What is a latent image?
The pattern produced within the emulsion by the sensitizing of silver bromide crystals, either by light or x-rays.
Is a latent image visible to the naked eye?
No
However, it can be made visible by the chemical reactions during processing.
What reaction during processing is the main chemical reaction in making a latent image visible to the naked eye?
The reduction of the silver bromide crystals to black metallic silver.
What is the film speed?
The sensitivity of film to x‐rays.
- how big an exposure is required to produce a certain degree of blackening on an image.
What is the sensitivity of the film dependent on?
The size of the crystals in the emulsion
Larger the crystals = faster the film speed = the lower the amount of x-rays required to produce an image.
How does the crystal size affect the detail?
Larger crystals = reduced detail = lower resolution.
What are the 4 most common film sizes?
0, 1, 2, 4
What sizes are used for periapicals and bitewings?
0, 1, 2
What size is used for anteriors and children?
0
What size is used for canines?
1
What size is used for posteriors, bitewings, and children older than 10?
2
What size is used for occlusals?
4
What is Indirect Action Film used for?
Extra-Oral radiography:
- DPT
- Lateral cephs
- Other head and neck radiography
How does indirect action film differ to direct action film?
Indirect action film is sensistive to light rather than X-rays
What does and indirect action film allow you to do to the exposure?
Allows you to reduce exposure required to produce an image and therefore the dose received by the patient.
What are the 4 different layers of an intensifying screen?
- Base layer - plastic or polyester
- Reflective layer - reflects all the light produced back towards the film.
- Phosphor layer - reacts with x-rays to produce the light, the light produced being in direct proportion to the energy in the x-ray photon.
- Supercoat - protects the phosphor layer from damage.
What does the use of intensifying screens allow?
Allows a significant reduction in the radiation exposure as the screen in front absorbs and converts the low energy photons to light and the screen at the back absorbs the high energy photon and converts them to light, making the system extremely efficient.
However, why would the use of intensifying screens cause a reduction in detail?
As one x-ray photon will produce several light photons.
This in turn exposes large areas of the film leading to a reduction in the fine detail seen on the film.
Why must you be careful not to bend the film?
As applied pressure sensitises the film.
Why must you never hold a film by one edge ?
As the weight of it will make it bend.
Why should you not use an out of date film?
As the emulsion starts to degrade over time and can start to stick to the intensifying screens.
How do we process the latent image?
By the reduction of the exposed silver bromide crystals to black metallic silver, and then making this image permanent.
In intra-oral films, what are the 3 methods of processing?
- Automatic processor
- Manual processor
- Instant processing
In the automatic processor where would you place the film?
Between 2 rollers which would then feed the film through the tanks and pop out developed from the slot on the top.
Which method of processing is least reliable and only used in disaster situations?
Instant developing
Where are the chemicals contained?
The chemicals are contained in bubbles in the pouch which you burst and slide down around the film.
What are the 5 stages of image development?
- Develop- is where you make the latent image visible.
- Wash – is where you stop development and remove the excess developer. This first wash only happens in manual processors.
- Fix – is where you make the image permanent.
- Wash – is where you stop the fix and remove the residual fixer.
- Dry – stops the image feeling sticky and prevents damage.
Inside a tank of developer, what 2 developing agents will you find?
Phenidone and Hydroquinone .
Besides the 2 developing agents, what else is found inside a tank of developer?
- Activator
- A restrainer
- A preservative
- A solvent
What activator is present inside a tank of developer and what is its function?
Activator: Calcium Carbonate
Function: controls the activity of the developing agents.
What restrainer is present inside a tank of developer and what is its function?
Restrainer: Potassium bromide
Function: stops the developer working on unexposed crystals.
What preservative is present inside a tank of developer and what is its function?
Preservative: Sodium Sulphite
Function: slows down oxidation
What solvent is present inside a tank of developer and what is its function?
Solvent: water
Function: dilutes the chemicals.
What is the action of the developing agents on the silver halide crystals dependant on?
- Time:
Film stays in the developer too long = too dark.
Not long enough = too light
- The developer temperature:
Too hot = film will be dark
Too cold = film will be too light
- The concentration:
– If you mix the developer wrong there will be consequences!
What is fixing?
The process by which the image is made permanent.
What is the role of the fixing agents?
The fixing agents change the unexposed silver halide crystals to a soluble compound so they can be washed away.
Acid in the fixer maintains the pH and neutralises the developer.
What components make up the fixer?
- Clearing agent - Ammonium thiosulphate.
- An acidifier - Acetic acid
- A hardener - usually Aluminium Chloride
- A preservative - usually sodium sulphite.
- A solvent - water.
What is the purpose of the acidifier in the fixer?
Maintains pH
What is the action of the fixing agents on the silver halide crystals dependent on?
Like developer, the action is dependant on;
- time
- temperature
- concentration.
What happens if an image is over fixed?
It will be completely removed from the film
What happens if an image is under fixed?
If it is under fixed the image will have a green/brown tinge and will not archive well.
The image will degrade over time until it is undiagnostic
Where does the film next pass to after leaving the fixer tank?
it will pass into the wash tank.
In a manual processor, when will the film be washed?
In a manual processor the film will be washed between developing and fixing
In an automatic processor, when will the film be washed?
In an automatic processor the film is only washed after fixing.
What does the wash do to the unexposed silver halide crystals?
The wash removes the unexposed silver halide crystals once they have been made soluble by the fixer.
How will the film appear if it hasn’t been washed properly?
What does this mean in terms of quality?
It will feel a sticky and have this cloudy appearance.
This means it will not archive well and the image will deteriorate.
What is the final process that the film will undergo?
Drying
Out of the 3 types of film processing systems available, what type is the best system to use?
Automatic processors
However manual processing if done well can be as good.
When is instant film beneficial?
If you have to process images away from a power source.
In a 4 step summary, how do you correctly process a film?
- Accurately develop
- Fix the image
- Wash away the residue
- Dry the final product
What is the main health and safety regulation you need to be aware of?
COSHH – Control of substances hazardous to health
What legislation was put in place that meant employers had a legal duty to ensure their staff were safe and risk free at work?
The Health & Safety at work act 1974
How could employers ensure their staff were safe and risk free at work?
By preventing or reducing their workers exposure to substances that are hazardous to health (eg. processing chemicals)
What is a risk assessment?
An exercise where you estimate the risks involved in carrying out a certain job or task, then come up with a system of work that will eliminate the risk
If the risk cannot be eliminated personal protective equipment must be supplied.
What must all manufacturers of processing chemicals supply?
Material safety data sheets for all potentially hazardous substances.
This will let you know how dangerous your developer and fixer are.
How does your employer protect you from the effects of processing chemicals?
- By situating processor in a well ventilated room where there are 10 or more room volume exchanges of air every hour
- By supplying PPE - Goggles to protect the eyes, respirator to prevent chemical inhalation, and a chemical resistant apron and gloves to protect the skin from irritation
- By changing the chemicals in the processor every 2 weeks
- By stripping and cleaning the processor to prevent build up of residue and cross contamination between the tanks.
