physical characteristics of x- ray film & film processing george david associate professor...
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Physical Characteristics Physical Characteristics of X-Ray Film & Film of X-Ray Film & Film ProcessingProcessing
George DavidAssociate ProfessorMedical College of GeorgiaDepartment of Radiology
Film baseAdhesive layer
attaches emulsion to base
Emulsion layerSupercoating
FilmBase
AdhesiveLayers
EmulsionLayers
Supercoating
.007”
.0005”
structural support for fragile emulsion
low light absorptionno visible patternflexible, thick, & strong
processing handling viewbox insertion / removal abuse
dimensional stability in processing For archival
varying humidity
FilmBase
early films used cellulose nitrateFlammable
“safety” basecellulose triacetate used until 1960’spolyester
.007 inches thickBase color
ClearBlue dye addedRequested by radiologists
reduces eye strain FilmBase
.007”
Most films use two emulsionseach emulsion <=.5 mil thick
thicker emulsion = less light penetration
Gelatinkeeps silver halide grains dispersed /
prevents clumpingallows penetration of processing
solutions without compromising strength or permanence
made from cattle bones EmulsionLayers
light sensitive90 - 99% silver bromide1 - 10% silver iodide
increases sensitivity
small crystals (grains) precipitated and emulsified in gelatincrystal has lattice structure
Ag+ / Br- / I-
silver nitrate added
precipitation determines crystal size & concentrationtypical size: 1 - 1.5 microns1 grain averages 1 - 10 million silver ions
chemical sensitization of crystalsulfur-containing compound added to
emulsionsilver sulfide formed
usually located on crystal surfacecalled sensitivity specksensitivity specktraps electrons to begin formation of latent
image centers
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
one light photon produces one silver atomsilver 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 100’s of silver atoms
photoelectric & Compton interactions in filmliberated electrons have long rangeseach electron reacts with many grainselectron’s strip other electrons from Br-
Bromine atoms & free electrons produced electrons captured at sensitivity speck as
before
Efficiencymost photon energy lost
much energy lost in gelatinonly 3 - 10% of photon energy produces
silversensitivity varies with
kVpprocessing
film as a dosimeter20% accuracybadge include filters of various
thicknessesallows estimate of x-ray spectrum
Thin supercoating covers emulsionprotects from mechanical damagemakes film smooth & slick for use with
processors
Supercoating
Amplifies latent image by 100,000,000!
forms visible silverreduces 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
Silver atoms at latent image center act as catalyst
Grains with no latent image also develop much more slowlyDeveloper time is fundamental in
developmentprocessing should stop when maximum
difference between exposed & unexposed crystals
developing agenthydroquinonephenidone or metolcombination yields development rate greater than sum
of eachalkali
adjusts pHpreservative (and oxidation preventative)
sodium sulfiterestrainers
antifoggants (reduces development of unexposed grains)
Controlled bySpeed of transportFilm path in develop rack
System of rollers & chains which direct film through developer tank
“90 second” processor90 seconds from film in to film outTime in developer ~ 20 secondTime in fixer & wash tanks controlled by
size of fixer & wash racksTransport speed does not change because
at any time films may be in any or all tanks
Temperaturecontrolled to ~0.5 degrees90 - 95 degrees for 90 second processor100+ degrees for 60 second processor
Replenishmentautomatic addition of fresh chemistry to
replace chemistry depleted in development
even with replenishment, chemistry must be completely replaced periodically
Functionremoves remaining silver halide / silver
ions without damaging metallic silverhardens gelatin
compositioncyanides (poisonous & not usually used)thiosulfates
sodium or ammonium salthypohypo
buffers to maintain pH