Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 10
• Physical Characteristics of X-Ray Film Film
  Processing

George David Associate Professor Medical College
of Georgia Department of Radiology
2
X-Ray Film Construction

• Film base
• Adhesive layer
• attaches emulsion to base
• Emulsion layer
• Supercoating

.007
.0005
Supercoating
Adhesive Layers
Emulsion Layers
Film Base
3
Film Base

• structural support for fragile emulsion
• low light absorption
• no visible pattern
• flexible, thick, strong
• processing
• handling
• viewbox insertion / removal abuse
• dimensional stability
• in processing
• For archival
• varying humidity

Film Base
4
Film Base Materials

• early films used cellulose nitrate
• Flammable
• safety base
• cellulose triacetate used until 1960s
• polyester
• .007 inches thick
• Base color
• Clear
• Blue dye added
• Requested by radiologists
• reduces eye strain

Film Base
.007
5
Emulsion

• Most films use two emulsions
• each emulsion lt.5 mil thick
• thicker emulsion less light penetration
• Gelatin
• keeps silver halide grains dispersed / prevents
  clumping
• allows penetration of processing solutions
  without compromising strength or permanence
• made from cattle bones

Emulsion Layers
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Supercoating

• Thin supercoating covers emulsion
• protects from mechanical damage
• makes film smooth slick for use with processors

Supercoating
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Silver Halide

• light sensitive
• 90 - 99 silver bromide
• 1 - 10 silver iodide
• increases sensitivity
• small crystals (grains) precipitated and
  emulsified in gelatin
• crystal has lattice structure
• Ag / Br- / I-
• silver nitrate added

8
Silver Halide

• precipitation determines crystal size
  concentration
• typical size 1 - 1.5 microns
• 1 grain averages 1 - 10 million silver ions
• chemical sensitization of crystal
• sulfur-containing compound added to emulsion
• silver sulfide formed
• usually located on crystal surface
• called sensitivity speck
• traps electrons to begin formation of latent
  image centers

9
Light Image on Film

• light photon allows escape of electron in bromine
  ion (Br -)
• neutral bromine atoms leave crystal, go into
  emulsion gelatin
• electron travels to, fixed in sensitivity speck
• negative sensitivity speck attracts mobile silver
  (Ag) ion forming silver atom
  Ag electron Ag
• repeated trapping of electrons results in growth
  of silver

10
Latent Image Centers

• one light photon produces one silver atom
• silver atoms collect at sensitivity speck
• no visible change in grain
• visible amounts of silver deposited at latent
  image centers during processing
• one or more latent image centers per grain
• 3 - 6 centers required for grain to be
  developable
• centers may contain 100s of silver atoms

11
Direct X-Ray Exposure of Film

• photoelectric Compton interactions in film
• liberated electrons have long ranges
• each electron reacts with many grains
• electrons strip other electrons from Br-
• Bromine atoms free electrons produced
• electrons captured at sensitivity speck as before

12
Direct X-Ray Exposure

• Efficiency
• most photon energy lost
• much energy lost in gelatin
• only 3 - 10 of photon energy produces silver
• sensitivity varies with
• kVp
• processing
• film as a dosimeter
• 20 accuracy
• badge include filters of variousthicknesses
• allows estimate of x-ray spectrum

13
Processing

• Amplifies latent image by 100,000,000!
• forms visible silver
• reduces silver ions into neutral black metallic
  silver atoms which remain on the film after
  processingAg electron Ag
• processing initiated at latent image speck
• grain either develops entirely or not at all

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Processing

• Silver atoms at latent image center act as
  catalyst
• Grains with no latent image also develop much
  more slowly
• Developer time is fundamental in development
• processing should stop when maximum difference
  between exposed unexposed crystals

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Processing Developing Solution

• developing agent
• hydroquinone
• phenidone or metol
• combination yields development rate greater than
  sum of each
• alkali
• adjusts pH
• preservative (and oxidation preventative)
• sodium sulfite
• restrainers
• antifoggants (reduces development of unexposed
  grains)

16
Developing Time

• Controlled by
• Speed of transport
• Film path in develop rack
• System of rollers chains which direct film
  through developer tank
• 90 second processor
• 90 seconds from film in to film out
• Time in developer 20 second
• Time in fixer wash tanks controlled by size of
  fixer wash racks
• Transport speed does not change because at any
  time films may be in any or all tanks

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Developing

• Temperature
• controlled to 0.5 degrees
• 90 - 95 degrees for 90 second processor
• 100 degrees for 60 second processor
• Replenishment
• automatic addition of fresh chemistry to replace
  chemistry depleted in development
• even with replenishment, chemistry must be
  completely replaced periodically

18
Fixing

• Function
• removes remaining silver halide / silver ions
  without damaging metallic silver
• hardens gelatin
• composition
• cyanides (poisonous not usually used)
• thiosulfates
• sodium or ammonium salt
• hypo
• buffers to maintain pH

19
Washing

• Removes fixer chemicals
• Fixing leaves milky appearance on film
• unwashed film turns brown with age