5. How Do You Create An X-ray Image? Flashcards

(39 cards)

1
Q

LOs

A
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2
Q

Types of radiographic film? What they’re used for?

A
  1. Direct action / wrapped packet film for intraoral
  2. Indirect action film used in conjunction with intesifying screens inside extra oral cassete
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3
Q

Intra oral radiographic film main sizes?
What part of mouth they’re used for?

A

A = 22 x 35 mm&raquo_space;» anterior periapical + childrens bitewings

B= 31 x 41 mm&raquo_space;> posterior periapicals + adult bitewings

C = 57 x 76mm&raquo_space;> occlusals

  • packaged in barrier envelopes to control infection
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4
Q

Intra oral radiographic film packaging, speed, colour etc

A

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

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5
Q

Intra oral radiographic film packet contents?

A

A = clear plastic barrier envelope

B = outer wrapper&raquo_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

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6
Q

How to hold an intra oral film

A
  • 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
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7
Q

Structure of the film itself?

A
  • 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
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8
Q

What is the emulsion sensitive to?

A
  • designed to be primarily sensitive to x-ray photons,
  • sensitive to light, so the packet must only ever be opened up in the dark.
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9
Q

Why is the lead foil inside the film packet embossed with a pattern?

A

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

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10
Q

Why is the lead foil in the packet? (EXTRA INFO)

A
  1. 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.
  2. 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
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11
Q

What is instant process / self developing film?

A

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

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12
Q

How does indirect action film work ?

A

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

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13
Q

What is the function of intensifying screens inside the cassette?

A

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

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14
Q

What is the action of a conventional calcium tungstate intensifying screen ?

A

Calcium tungstate screen emits blue light

  1. Incoming X-ray photon
  2. Activates phosphor crystal that emits light
  3. Light diverges and hits film emulsion
  4. Cross over in clear film base
  5. Hits second rear emulsion
  6. Reverse effect of photons that hit the rear screen
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15
Q

What is the action of a UV system intensifying screen ?

A

Incoming X-ray photons activates phosphor crystal to emit light

Difference: little crossover effect through film base

Image produced has sharper edge definition / resolution

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16
Q

Summary of intensifying screens

A

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

17
Q

What happens when there is a gap between intensifying screens and film ?

A

Larger area of film will be affected

Image will be less sharp + blurred

18
Q

Advantage of using rare earth green sensitive screens

A

Initial amount of X-ray needed to generate light less than conventional tungstate screen

Dose to patient reduced even further

19
Q

How to maintain intensifying screens?

A

Must be kept clean
- dirt / marks prevent emitted light from hitting the film

Must avoid scratching + damage

20
Q

How are cassettes constructed ?

A

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

21
Q

Three main variables on control panel

A

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

22
Q

What is chemical processing ?

A

Events required to convert invisible latent imagine into visible radiographic image

Traditionally done manually
- film clipped into holder + dipped into chemicals

Automatic processors

23
Q

How does processing convert green silver halide emulsion ion film into radiographic image ?

A

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

24
Q

Importance of development of radiographic film

A

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

25
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
26
Two main types of digital receptors
1. Intra + extra oral solid state receptors 2. Intra + extra oral phosphor plate receptors
27
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§
28
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
29
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
30
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
31
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
32
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
33
Technical issues of digital imaging
Not accurate Overlapping pixels has grey shade averaged out Smaller the pixels = more accurate representation
34
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
35
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
36
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
37
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
38
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
39
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