physical characteristics of x ray film

8/3/2019 physical characteristics of x ray film

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Physical Characteristics ofX-Ray film

Dr.Amrit RajanPG-1st yr.Dept. of radiologyNMCTH-Birgunj


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Double coated film

Film construction


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X-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 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 itsSize & 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 Materials early films used cellulose

nitrate Flammable

safety base cellulose triacetate used until 1960s

polyester .007 inches thick

Base color Clear

Blue dye added

Requested by radiologists

reduces eye strain

FilmBase

.007
http://www.uwosh.edu/archives/images/fire.jpg


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Polyster

More resistant to wraping withage and is stronger.

The polyster bases are similar in

composition to the polyster fibresin clothing.


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Emulsion It is heart of xray film.

It is material in which xray or lightphotons from screen interact &transfer information.

It consists of homogenousmixture of gelatin and silverhalide(silver bromide & silver

iodide) crystals.Beside this it alsocontains various additives likewetting agents,


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hardner,antifoggants,sensitizers

etc Most films use two emulsions

each emulsion


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Emulsion should be clear andporous so that it transmits lightand alows chemicals topeneterare to the crystals ofsilver halide during processing.


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To be suitable in radiographyemulson should have toimportant characteristics.-

(a)speed or sensitivityTherelative ability of an emulsion torespond to light or xrays.

An emulsion is said to be fast orhigh speed if small exposureproduces a radiograph ofadequate density(blackening)


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Ordinary xray films is rated asbeing fast.

(b)Latitude The ability of

emulsion to display theradiographic image

Another aspect of latitude is

margin of error permissible withany given technique.


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Silver iodobromide crystal latticeIn emulsion

Br-

I-

Ag+


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Supercoating

Thin supercoating covers emulsion

protects from mechanical damage

makes film smooth & slick for usewith processors

Supercoating


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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


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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


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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 growthof silver


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Latent 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 latentimage centers during processing

one or more latent image centers per grain 3 - 6 centers required for grain to be developable

centers may contain 100s of silver atoms


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Direct X-Ray Exposureof Film

photoelectric & Compton interactions infilm

liberated electrons have long ranges

each electron reacts with many grains

electrons strip other electrons from Br-

Bromine atoms & free electrons producedelectrons captured at sensitivity speck as before


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Direct 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 film

(1)Screen film- most often used

Has three characteristics-

contrast,speed and lightabsorption.

High contrast fim produces

very black and white imagewhile a low contrast film ismore gray.


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Screen films are available with

different speed.

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

If emulsion is too thick than it isnot properly exposed.

Lare grain emulsion are moresensitive than small one.


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Most imp. Consideration in

selecting a screen film isspectral absorptioncharacteristics.

The film sensitivity to variouscolors of light should match withspectrum 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 itshould be matched with a filmthat is sensitive not only to bluelight but also to green light.Suchfilm is orthochromatic & K/a

green sensitive film. If there is improper match of fim

& screenimage 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 finegrain single emulsion filmdesigned to be exposed with asingle intensifying screen.

(4)Duplicating film.-Singleemulsion film.

Used to copy radiographs. (5)Substraction film- single

emulsion.

Used in angiography.


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(6)Dental filmsare dobleemulsions film. Exposed withoutscreens.

(7)Medical imaging film.- used inCT,Digital radiography,Nuclearmagnetic resonance,PEET etc.

These films are single coated (8)Other films- cine film ,spot film.

laser film.


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SUMMARY

Xray film is a photographic filmcoated with emulsion on bothsides of the film base.

Xray film has got

Base

Emulsion

Adhesivebetween base &emulsion.

Supercoating.


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The light sensitive material in theemulsion is silver iodobromidecrystal.

Latent image-is siliver atomsdeposited at sensitivity speck.

physical characteristics of x ray film

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