flim screen physics

Physics overview of Film Screen Radiography

Dr Muhammad Wasim Awan Head Of Radiology Department

Diagnostic Rad Dept: KRL Hospital

Objectives• Film and intensifying screen structure• Image Formation• Characteristic curve• Film-screen sensitivity

Radiographic FilmFilm

The radiographic images are recorded on a sheet of X-ray film.

Composition of Film:Film consists of polyester base which is coated on both side by emulsion.

Emulsion is suspension in gelatine of silver iodobromide crystals.Bromide (90%) Iodide (10%)

Film is affected by storage and handling implications as

• Mechanical pressure• Creasing • Static electricity• Chemical Liquids• Vapours

To Protect against curling and abrasions films are coated with antistatic super coat.

Polyester Base

Silver iodobromide

Silver iodobromide

FILM STRUCTURE

Intensifying ScreenA photographic emulsion is more sensitive to ultraviolet and visible light than X-raysX-ray films are relatively insensitive to x-rays and if used alone require unacceptably high dose of radiation to produce satisfactory image.

To avoid this intensifying screens are used.

Structure of Intensifying Screen

An intensifying screen consists of 4 layers:

1 A Base “for support” polyester “plastic” 0.25mm thick.

2 A Phosphor layer: 0.1-0.5mm thickDense layer of fine phosphor crystal (3-10 micrometer) bound by a transparent resin.

3 A Reflective layer “not in all screens”Thin white reflective layer- b/w the base and phosphor Some of the light is directed towards the back of the screen reflective layer reflect that light again towards the film.4 A Protective layer (outer most) Physical protection to phosphor layer

Polyester base

Reflective LayerPhosphor

Protective Layer

Intensifying Screen

• Old traditional phosphor used was Calcium Tungstate.

• Most commonly use phosphor is Lanthanum oxybromide/oxysulphide because of increased efficiency.

• Phosphor contain impurities (activator) which trap energy.

• The intensity of light emitted by a screen depends of phosphor and color of it depend on the activator.

• Films that are coated with emulsion on both sides needs screens on both side.

INTENSIFYING SCREENSFunction:Intensifying screens absorb the energy in the x-ray beam that penetrated the patient

Convert it into light pattern

Advantages of intensifying screen

Film expose directly to X-rays

• More exposure is required if no screen are used

• At same density , contrast is always lower for a film expose to x rays only rather than for the same film exposed to light from intensifying screen

How this actually work --- as intensifying screens are less sensitive than film to lower energy x-rays--- effect of lower energy (scattered radaition----- contrast

Film exposed with screens to X-rays

Use of screens• Reduce the Dose to the patient

• Reduce the Loading of tube and generator

• Allow the use of Short exposure time-- motion blur

• Allow the use of Small focal spot-- geometrical blur

SCREEN SLECTION• Screen materials should not emit delayed

fluorescence because these would retain “memory” of previous images .

• On reloading the cassette the pervious image would be recorded on the new film and superimposed on the image

Film CassetteIt is flat light tight box which consists of pressure pads to keep film in uniform contact with intensifying screen.

Front of the cassette is near to the tube is made up of low atomic number (carbon fiber) to increase attenuation of beam so reduce dose for patient.

The back cassette is made up of thin sheet of lead to minimize back scattered.

Pressure pads

Pressure pads

IMAGE FORMATION

Exposure to LightTwo features of silver iodobromide crystals accounts for photographic process:

• Small proportion of iodide as compare to bromide distort the lattice and allows the silver ions to move through the lattice.

• Silver iodobromide crystals posses sensitivity specks on their surfaces

A crystal absorbs a light photon an electron liberated which reach the sensitivity speck and its potential energy is reduced.

When hundreds of photons have been absorbed and enough electrons accumulated at sensitivity speck , these electrons attract mobile silver ions to join them and be neutralized.

Now this forms a submicroscopic speck of silver metal on the surface of crystal. Distribution of silver speck in the emulsion form a latent image in the film which is now awaited for development process

SILVER IODOBROMIDE CRYSTAL

Sensitivity Speck

Silver ions

PHOTON

E

Changes occur with Exposure

PROCESSINGThe invisible pattern of latent images made visible by processing.

Processing is of two types :

Manual (Old Technique)Automatic ( Recent widely used now a days)

Manual ProcessingThere are 4 steps of processing

1. DEVLOPMENT

2. FIXING

3. WASHING

4. DRYING

Step 1 Development :

Film is developed by immersion in alkaline solution of an reducing agent which enter into the crystal and reduce the positive silver ions to silver atoms latent image grows into grains of metallic silver.

Crystals which carry no latent image remain unaffected initially as bromide prevent entry of developer. But when enough time given developer penetrate the crystals and results in background fog in the image.

Step 2 Fixing : Film is now fixed with acidic solution (Thiosulfate) which dissolve out unaffected silver ion , so that image remain stable and unaffected by further light

Step 3 Washing:The film is now washed and dried under hot air . Washing is important so that acid doesn't turn film yellow brown as time pass by.

Automatic Processor

Automatic processor use roller feed system to transport film through different solutions

Characteristics of Film• Film now caries a pattern of silver grains

corresponding to X-rays leaving the patient.

• It is a negative image

• Portion of the film where X-rays are intense appear darkest and vice versa

• Image is viewed on illuminator

OPTIC DENSITY

The term optic density (D) means BLACKENING of an area depends on the number of silver grains per unit area

The degree of film blackening is : Directly related to intensity of radiation Directly related to silver grains/unit area Inversely related to light transmision

The measurement of film BLACKNESS is called optic Density

It is a defined as log of the ratio of incident and transmitted light

D = log10 (incident light/transmitted light)

It is measured by Densitometer.

Why we use LOG to express density?Because Logarithms express large differences in number on small scale Physiological response of human eye to different light intensities is logarithmic

If an unexposed x-ray film is processed , it will demonstrate the density of about 0.12 . This density is refered as Base plus Fog Level

Base density (0.07) caused by plastic material and dye used to make film base

Fog (0.05) : the density result form developed unexposed silver grains

CHARACTERISTICCURVE

Characteristic curveThe response of a film and screen combination to X-ray is described by characteristic curve.

It is measured using instrument sensitometer

Characteristic curve shows the relationship between optic density and relative exposure

Primary function of characteristic curve is to describe the contrast characteristics of the film throughout a wide exposure range.

The area of correct exposure: • Straight line portion • Steepest• Densities within the area of diagnostic interest should lie

within this range• Density is proportional to log exposure.Shoulder Region• Shallow curve• High exposure region• Dense image• Dark imageToe Region• Shallow curve• Low exposure region• Low density• Bright image

Area of high exposure in which slope is decreased is referred as SHOULDER. The two significant characteristics of image areas receiving exposure within this range are that the film is quite dark (dense) and contrast is LOW.

The part of the curve associated with relatively low exposures is designated the TOE, and also corresponds to the light or low-density portions of an image . It contribute to LOW contrast.

 The highest level of contrast is produced within a range of exposures falling between the toe and the shoulder. This portion of the curve is characterized by a relatively straight and very steep slope in comparison to the toe and shoulder regions. In most imaging applications, it is desirable to expose the film within this range so as to obtain MAXIMUMcontrast.

Latitude The exposure range over which a film produces useful contrast is designated the LATITUDE. 

 Exposure values above the latitude range also produce areas with very little contrast and have the added disadvantage of being very dark or dense.

An underexposed film area contains little or no image contrast.

Over Exposure

UnderExposure

Four Properties of film screen combination derived from Characteristic Curve

1.Film speed 2.Gamma 3.Film Latitude4 Fog Level

Speed

Speed here is loosely defined as the reciprocal of exposure required to produce net density 1.

It is said to be High speed film when low exposure required to produce useful density .

Factors Speed • Increase grain size • Modern emulsions with flat crystals• X-ray photon energy 30-40 keV

Film Gamma (Y): Film gamma is average slope of characteristic curve .

• It influences the amount of contrast on the radiographic image.

• Steeper the curve higher the gamma therefore display more contrast.

• Slope of curve increased with increased exposure and density.

• Generally average slope is between densities 0.25-2,• Average Film Gamma value is in range 2-3

Film Latitude: It is range of exposure that produce net useful densities ranging from 0.25-2.

Below 0.25 gradient is too low.

Above 2 too dark to visualize on illuminator

Effects of developing Conditions

Developer temperature cause increase rate of chemical reaction:

• Speed • Fog level• GammaIncreasing developer concentration and developing time cause similar effects

Film Screen Sensitivity

Film exposed alone without screen is relatively insensitive to X-rays. In such case only 2% of incident photon will interact with film emulsion and form single latent image.

While using Film with intensifying screen about 30% of the photon interact which is greater than the film alone.

During interaction X-ray photon generate 1000 light photon (intensification factor) and only half of them reach the film.

Film and Screen Film

X-ray absorbed by phosphor, 30

Light photon produced, 18000

Light photon reaching film, 9000

Latent images produced, 90

Absorbed by film, 2

Latent images produced , 2

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