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

Removes fixer chemicalsFixing leaves milky appearance on

filmunwashed film turns brown with age