Physical Characteristics of X- Ray Film & Film Processing George David Associate Professor Medical College of Georgia Department of Radiology

X-Ray Film Construction Film base Adhesive layer attaches emulsion to base Emulsion layer Supercoating .007” .0005” Supercoating Adhesive Layers Film Base Emulsion Layers

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

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

Emulsion Most films use two emulsions Gelatin each emulsion <=.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

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

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

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

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 100’s of silver atoms

Direct X-Ray Exposure of Film photoelectric & Compton interactions in film liberated electrons have long ranges each electron reacts with many grains electron’s strip other electrons from Br- Bromine atoms & free electrons produced electrons captured at sensitivity speck as before

Direct X-Ray Exposure Efficiency film as a dosimeter 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 various thicknesses allows estimate of x-ray spectrum

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

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 processing Ag+ + electron Ag processing initiated at latent image speck grain either develops entirely or not at all

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

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)

Developing Time Controlled by “90 second” processor 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

Developing Temperature Replenishment 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

Fixing Function composition 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

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