Image Receptors Flashcards
What are the 2 types of image receptors
Film and digital
What is film split Into
Direct action and indirect action
What is digital split into
Solid state and phosphor plate
What is solid state split into
CCD and CMOS
What is direct action film
•Used for intra oral radiographs
•Shows fine anatomical detail
•Film sensitive to X-ray photons
What is the construction of direct action film
•Plastic base
–Clear and transparent
–Cellulose acetate
•Thin layer of adhesive
–Fixes emulsion to base
•Emulsion
–On both sides of base
–Silver halide (usually bromide) crystals
–Sensitive to xray photons
•Protective layer
–Clear gelatin
–Shields emulsion from mechanical damage
Describe Direct action film packet
•Outer packet
–Sealed to prevent saliva ingress
•Black paper lining
–Protects film from light
–Prevents damage when opening packet
–Absorbs saliva
•Lead foil
–Absorbs radiation passing through film
–Prevents backscatter back onto film.
Describe Direct Action Film Orientation
•Embossed dot on one corner
•Raised dot placed towards X-ray beam
•Raised dot at occlusal plane for periapical
•When mounted, raised dot towards the viewer.
What is indirect X-ray film used in combination with
intensifying screen in a cassette
What is indirect action film
Intensifying screen sensitive to x-ray photons and emits light photons
What extra oral projections use indirect action film?
–Oblique lateral radiographs
–Skull radiographs
–Panoramic radiographs
What is the construction of indirect action film
•Silver halide emulsion sensitive predominantly to light
•Made to be sensitive to different colours
–Standard silver halide sensitive to BLUE light
–Modified silver halide emulsion with ultraviolet sensitizers sensitive to UV light
–Orthochromatic emulsion sensitive to GREEN light
–Panchromatic emulsion sensitive to RED light.
What is a intensifying screen
Fluorescent phosphors convert x-ray energy into light photons by the photoelectric effect
How many light photons are made from one X-ray photon
One X-ray photon will produce many light photons
–Light photons will expose a larger area of film emulsion
–Amount of radiation to expose film (dose) reduced at cost of resolution (fine detail)
What must be matched to the sensitivity of film and why
Phosphor material must be matched to sensitivity of film as different phosphors will emit different wavelength (colour) light
What are cassettes
•House the image receptor and intensifying screen together
•Light-tight container
•Film sandwiched between two intensifying screens
What are the 5 stages in film processing
•Development
–Sensitized silver halide crystals in emulsion are converted to black metallic silver
•Washing
–Removes residual developer solution
•Fixation
–Unsensitized silver halide crystals removed
•Washing
–Removes residual fixer solution
•Drying
2 things that image receptors allow us to do
•Allow us to describe the way in which the film responds to the x-ray or light photons incident on it
•Allow us to compare features of different imaging systems
Describe speed in image receptors
–How ‘quickly’ the image receptor responds to the x-ray photons incident on it
–Related to number and size of silver halide crystals in the emulsion
How does speed effect image size?
Faster speed = larger crystals
How does speed effect X-ray dose
Faster speed = lower dose of x-rays
How does crystal size effect resolution
larger crystals = lower resolution
Describe optical density in image receptors
–Degree of blackening in a radiograph
–OD= log (incident light/transmitted light)
OD = 0, 100% of light is transmitted through the film
OD = 1, 10% of light is transmitted
OD = 2, 1% of light is transmitted
–In diagnostic radiology the range of optical densities is usually 0.25-2.5 (no units for OD)
–Affected by beam quality and quantity (mA, kV, exposure time and filtration), thickness of object and background fog
Describe fog in image receptors
–Degree of blackening with zero exposure
–Caused by
•Density of plastic base
•Development of some unexposed crystals
–Fog density should be < 0.2
What is a characteristic curve
Variation in optical density with different exposures
What is the latitude
–Range of exposures that produce distinguishable differences in OD
–Linear portion of characteristic curve
What is the average gradient
–If high, good contrast but narrower range of different ODs displayed
–If low, poor contrast but wider range of different ODs displayed
Uses of digital receptors
•Periapical and bitewing
–Solid state or phosphor plates
•Extraoral radiography (excluding lateral oblique)
–Solid state or phosphor plates
•Occlusal and lateral obliques
–Phosphor plates only
–Too expensive to manufacture sufficiently large solid-state sensors.
What are digital images captured in
Pixels
Describe the pixels in digital imaging
•Each 2D pixel represents a 3D cube (voxel) of the patient
•Each pixel measures the total X-ray absorption through the whole voxel
•Each pixel is given a grey scale value to represent the absorption
How many numbers are allocated to the grey scale
How many black, how many white?
There are 256 numbers allocated to the grey scale
0 = black
255 = white
What are solid state sensors (direct digital) CCD
Charge coupled device (CCD)
–Silicon chip embedded in an electrical circuit
–Pixels of silicon chips are arranged in rows and columns called an array or matrix
–Surface scintillation layer of rare earth type material, similar to an intensifying screen
–Most have a wire/cable linking it to the computer
•X-ray photons hit scintillation layer and are converted to light
•Light interacts with silicon via the photoelectric effect
•Creates a charge packet which is concentrated by electrodes
•Charge packets represent the latent image
•To read image, the charge is transferred to a readout amplifier then transmitted as an analogue signal to the computer’s analogue-to-digital converter
CCD vs CMOS
What are solid state sensors (direct digital) CMOS
Complimentary metal oxide detectors (CMOS)
–Array of pixels, but unlike CCDs, each pixel is isolated from it’s neighbour
–Charge packet from each pixel is transferred directly to a transistor as a voltage
–Enables each individual pixel to be assessed separately
Drawbacks of solid state sensors (direct digital)
•Allow almost immediate viewing of image
BUT
•Not autoclavable so require protective barrier
•Smaller detector surface than film
•Bulky detector and wire
•Wireless CCDs have no wire but bulkier due to extra electronics
Extra oral sensors in solid state sensors direct digital
–Contain CCDs in long thin linear arrays
–A few pixels wide and many pixels long
–Pixel array is aligned with a slit-shaped X-ray beam and the equipment scans across the patient
–Scanning motion takes time (a few seconds)
Describe Photostimulable Phosphor Storage Plates (Indirect Digital)
•Range of sizes of plates, similar to film
•Wireless
•Once cleared, plates are reusable
•Protective barrier envelopes needed for cross-infection control
Photostimulable Phosphor Storage Plates (Indirect Digital)
How do they work?
•Barium fluorohalide phosphor on a flexible plastic backing
•Phosphor layer absorbs and stores x-ray energy
•Image plate placed in a reader where it is scanned by a laser beam
•Light is released which is detected by a photo-multiplier tube and converted into a voltage to be relayed to the computer
From black to white how many distinct shades are monitors able to display
256 distinct shades from black to white
Each shade has a what value
Greyscale
What does brightness and contact relate to
to how greyscale values are assigned to relative absorption values in each pixel
Brightness=
Window level
Lower number =
Lighter
Higher number =
Darker
Contrast =
Window width
Lower number (narrow window) =
fewer shades of grey, more contrast
Higher number (wide window) =
(Range of numbers =window width)
more shades of grey, less contrast
What can image processing artefacts mimic?
Pathology, particularly at high contrast boundaries
What are the main advantages of digital dental radiology
•Virtually instant images
•Computer can compensate for incorrect exposure
•Lower doses
•Phosphor plates have a wide latitude for exposure
•Image processing/enhancement/archiving
•Avoid chemical processing faults and hazards
•Easy storage
•Easy transfer of images
Disadvantages of digital dental radiology
•Direct digital:
–Physically large detectors compared to film for intraoral use
–Connecting wire to the computer for intraorals
–Low doses but narrow exposure latitude
•Monitor quality and ambient light issues
•Software issues/failure
•Risk of dose creep
•Manipulation- can be misleading
What makes a good quality image
•Proper visual characteristics
•Geometric accuracy
•Anatomical accuracy
•Absence of artefacts
•Adequate coverage
What does contrast tell
-Describes the range of densities on a radiograph
-Difference in OD between two points
Low contrast =
Large number of grey shades
High contrast =
Few shades of grey between black and white
Contrast is dependant on
–Subject thickness and density of the tissue
–kVp of x-ray beam
–Scatter
–Characteristic curve of image receptor
What’s an example of a thing that can reduce contrast
Increased tissue thickness in large patient
What does contrast look like in film?
•Illumination should be uniform
•For low contrast details high illumination required
What does contrast look like in digital viewing on monitor
•Viewed on bright, high-resolution monitors
•Reduce ambient light
What is resolution
Ability of a system to differentiate objects close together
What is resolution measured in
line pairs (lp) per mm
In a digital system, what is resolution dependant on?
Pixel size
Movement of what produces a blurry image?
–Patient
–Receptor
–X-ray beam
What is distortion
Unequal magnification of different parts of the same object
When dies distortion arise
Arises when the film is not parallel to the object or when the X-ray beam is not perpendicular to the object and film plane
What does distortion cause
elongation and foreshortening
