Download - physical characteristics of an x ray film
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• Physical Characteristics of Physical Characteristics of X-Ray filmX-Ray film
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Double coated film
Film constructionFilm construction
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X-Ray Film ConstructionX-Ray Film Construction• Film base• Adhesive layer
– attaches emulsion to base
• Emulsion layer
• Supercoating
FilmBase
AdhesiveLayers
EmulsionLayers
Supercoating
.007”
.0005”
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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
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• Should be inert
• Should be of uniform lucency
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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”
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• Polyster –
• More resistant to wraping with age and is stronger.
• The polyster bases are similar in composition to the polyster fibres in clothing.
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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,
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• 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
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• Emulsion should be clear and porous so that it transmits light and alows chemicals to peneterare to the crystals of silver halide during processing.
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• 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)
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• 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.
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Silver iodobromide crystal lattice In emulsion
Silver iodobromide crystal lattice In emulsion
Br-
I-
Ag+
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SupercoatingSupercoating
• Thin supercoating covers emulsion
• protects from mechanical damage
• makes film smooth & slick for use with processors
Supercoating
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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
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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
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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
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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
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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
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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
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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.
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• 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.
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• 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.
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• 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
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• 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.
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• (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.
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• (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.
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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.
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• The light sensitive material in the emulsion is silver iodobromide crystal.
• Latent image-is siliver atoms deposited at sensitivity speck.