• Physical Characteristics of Physical Characteristics of X-Ray filmX-Ray film

Double coated film

Film constructionFilm construction

X-Ray Film ConstructionX-Ray Film Construction• Film base• Adhesive layer

– attaches emulsion to base

• Emulsion layer

• Supercoating

FilmBase

AdhesiveLayers

EmulsionLayers

Supercoating

.007”

.0005”

Film BaseFilm Base• structural support for fragile emulsion• low light absorption• no visible pattern• flexible, thick, & strong

– processing– handling– viewbox insertion / removal abuse

• dimensional stability-• It should maintain its Size & Shape • in processing & use

– For archival» varying humidity

FilmBase

• Should be inert

• Should be of uniform lucency

Film Base MaterialsFilm Base Materials• early films used cellulose

nitrate– 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

FilmBase

.007”

• Polyster –

• More resistant to wraping with age and is stronger.

• The polyster bases are similar in composition to the polyster fibres in clothing.

EmulsionEmulsion• It is heart of xray film.

• It is material in which xray or light photons from screen interact & transfer information.

• It consists of homogenous mixture of gelatin and silver halide(silver bromide & silver iodide) crystals.Beside this it also contains various additives like wetting agents,

• hardner,antifoggants,sensitizers etc

• Most films use two emulsions– 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 performance

– made from cattle bones

• Emulsion should be clear and porous so that it transmits light and alows chemicals to peneterare to the crystals of silver halide during processing.

• To be suitable in radiography emulson should have to important characteristics.-

• (a)speed or sensitivity –The relative ability of an emulsion to respond to light or xrays.

An emulsion is said to be fast or high speed if small exposure produces a radiograph of adequate density(blackening)

• Ordinary xray films is rated as being fast.

• (b)Latitude – The ability of emulsion to display the radiographic image

Another aspect of latitude is margin of error permissible with any given technique.

Silver iodobromide crystal lattice In emulsion

Silver iodobromide crystal lattice In emulsion

Br-

I-

Ag+

SupercoatingSupercoating

• Thin supercoating covers emulsion

• protects from mechanical damage

• makes film smooth & slick for use with processors

Supercoating

Silver HalideSilver 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 HalideSilver 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 specksensitivity speck

» traps electrons to begin formation of latent image centers

Light Image on FilmLight 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 CentersLatent Image Centers

• one light photon produces one silver atom

• silver atoms collect at sensitivity speck and forms latent image.

– 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

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 ExposureDirect 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

Types of filmTypes of film

(1)Screen film-

• most often used

• Has three characteristics- contrast,speed and light absorption.

• High contrast fim produces very black and white image while a low contrast film is more gray.

• Screen films are available with different speed.

• In general thicker the emulsion the more sensitive the film & therefore higher the speed.

• If emulsion is too thick than it is not properly exposed.

• Lare grain emulsion are more sensitive than small one.

• Most imp. Consideration in selecting a screen film is spectral absorption characteristics.

• The film sensitivity to various colors of light should match with spectrum of light emitted by the screen.

• Calcium tungstate screens emit blue and blue voilet lights.

• The film sensitive to these light is k/a blue sensitive film.

• If rare earth screen is used it should be matched with a film that is sensitive not only to blue light but also to green light.Such film is orthochromatic & K/a green sensitive film.

• If there is improper match of fim & screen –image receptor speed

• Will be greatly reduced & pt. dose is increased.

• In darkroom- for blue sensitive film amber filter & for green sensitive film red filter is to be used.

• (2)Direct exposure or Nonscreen film.- has thicker emulsion.

• Mainly used in industries.

• Manually processed.

• (3)Mammography film- is a fine grain single emulsion film designed to be exposed with a single intensifying screen.

• (4)Duplicating film.-Single emulsion film.

• Used to copy radiographs.

• (5)Substraction film- single emulsion.

• Used in angiography.

• (6)Dental films –are doble emulsions film. Exposed without screens.

• (7)Medical imaging film.- used in CT,Digital radiography,Nuclear magnetic resonance,PEET etc.

• These films are single coated

• (8)Other films- cine film ,spot film. laser film.

SUMMARYSUMMARY

• Xray film is a photographic film coated with emulsion on both sides of the film base.

• Xray film has got –

• Base

• Emulsion

• Adhesive –between base & emulsion.

• Supercoating.

• The light sensitive material in the emulsion is silver iodobromide crystal.

• Latent image-is siliver atoms deposited at sensitivity speck.