Intensifying screens & films

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CASSETTE

Protection:•Exposure to light•Bending and Scratching•Film in close contact with screen.

FRONT: Radiolucent material like aluminum, carbon fiber.

BACK: Lead foil to prevent backscatter

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INTENSIFYING SCREENS

WHAT IS AN INTENSIFYING SCREEN?

-IT’S A PART OF CASSETTE-CONVERTS X RAY ENERGY INTO VISIBLE LIGHT SPECTRUM.

HISTORY:•FIRST DEVELOPED BY THOMAS EDISON IN 1897•INITIAL SCREENS USED CaWO4

ADVANTAGES:

•REDUCES X RAY DOSE TO THE PATEINT•SHORT EXPOSURE TIMES- REDUCED MOTION BLUR

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PRINCIPLE BEHIND INTENSIFYING SCREENS?

LUMINISCENCE : Emission Of Light By A Substance

• Light is emitted instantaneously(<10-8 sec)

• Stops after the stimulus is removed

• Conventional

• Emission of light is delayed beyond 10-8 sec.• Continues to emit light (after glow).

• Digital radiography.

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BASE

PROTECTIVE COATING

REFLECTING LAYER

PHOSPHOR LAYER

CONSTRUCTION

High grade cardboard or polyester plastic

Titanium dioxide

Cellulose compoundPrevents static electricityPhysical protectionSurface Cleaning

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PHOSPHOR LAYER : FUNCTION OF THIS LAYER IS TO CONVERT FEW ABSORBED X RAY PHOTONS INTO MANY LIGHT PHOTONS BY PHOTOELECTRIC EFFECT.

PHOSPHOR

X - RAYSLIGHT PHOTONS

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PHOSPHOR CRYSTALS-SUSPENDED IN PLASTIC

Most frequently used:- Calcium Tungstate(CaWO4)-Superseded by various Rare earths:

Lanthanum oxybromide. Lanthanum oxysulfide. Gadolinium oxysulfide. Yttrium oxysulfide. Barium lead sulfate. Barium fluorochloride.

RARE EARTH PHOSPHORS: - THE TERM IS USED NOT BECOS THEY ARE RARE BUT BECOS THEY ARE DIFFICULT TO SEPARATE FROM THE EARTH. - 97% OF THE RARE EARTHS ARE FROM CHINA.

• High X-Ray absorption efficiency

• High X-Ray to light efficiency

• Emission spectre matched to film sensitivity

• Fast light emission

• Absence of afterglow

• Uniform light output i.e. uniform dispersion in suspension media.

DESIRABLE FEATURES OF PHOSPHORS:

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DEPEND ON:

1)Thickness of the phosphor layer.

2) Size of the phosphor crystals.

3) Presence or absence of light-absorbing dye in the phosphor layer.

4) Phosphor conversion efficiency.

SPEED OF THE SCREENS

Speed ranges from 100(slow) to 1200(fast)

100-------------Reference screen

200-------------Requires half the exposure of the reference screen to produce the same level of luminescence.

Greater efficiency = less exposure = faster

Speeds for routine work: 200 – 800

Speeds for high detail: 50 – 100

Conversely, any measure taken to increase the intensification factor of the screen increases the unsharpness of the image.

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DECREASE IN CLARITY IS PRIMARILY DUE TO DIFFUSION OF LIGHT LESS SHARP BORDERSINCREASED THICKENSS CAUSES INCREASED DIFFUSIONDECREASED THICKENESS CAUSES DECREASED DIFFUSION

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SCREEN – FILM CONTACT:

Cassette: light tight containerHolds the film in tight contact with the screens over its entire surface

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WIRE MESH TEST

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Standard Film sizes 8” x 10”10” x 12”11” x 14”14” x 17”14” x14”12”x12”

• Film is a media that makes a permanent record of the image

• The energy of the x ray beam is converted into light by intensifying screens, and this is used to expose the film

X RAY FILM

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Film construction:Film consists of a photographically active or radiation sensitive, emulsion that is usually coated on both sides of a transparent sheet or base.

EMULSION

EMULSION

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FUNCTION : Support for the emulsion

3 characteristics:

•No visible pattern or should not absorb light•Flexibility, thickness and strength-ease of processing•Dimensional stability-shape and size should not change over time

Originally base was made from CELLULOSE NITRATE – flammable

Now CELLULOSE TRIACETATE BASE and POLYESTER (DMT and Ethylene glycol)

Base is adhered to emulsion using a thin layer of glue.

FILM BASE

22EMULSION

1.GELATIN:

Its is a translucent, colorless, flavourless solid substance, derived from the collagen mainly inside pig skin (hide) and cattle bones.

It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing.

Purpose ?Keeps silver halide grains well dispersed and prevents clumping of grains

Photographically active layer of the film.

Emulsion can coated on a single side or 2 sides

Most important ingredients : 1. Gelatin 2. Silver halide

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Light sensitive material

Silver halide is in the form of small crystals suspended in gelatin

HALIDES Bromine, chlorine and iodine.

90-99% - silver bromide1-10% - silver iodide

2. SILVER HALIDE

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Ag Br

Cubic lattice/ crystal size: 1.0-1.5 microns

One cubic cm emulsion: 6.3x109 grains

Each grain: 1,000,000 to 1,000,000 silver ions

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GUNREY MOTT HYPOTHESIS

Sensitivity speck or defect

X RAY Photon

Silver Atoms Pileup

Latent imagesite

The clumps of silver can be seen with electron microscopy and are termed LATENT IMAGE CENTERS.At these sites developing process will cause visible silver deposit.

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PHOTOGRAPHIC CHARECTERISTICS OF AN X RAY FILM

What is an exposure?

How the film responds to the exposure ?

Exposure = mA x Secs= mAs

Exposure of the x ray films produces Film blackening or Density.

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PHOTOGRAPHIC DENSITY

Measurement of film blackness is called PHOTOGRAPHIC DENSITY.

Degree of blackening depends upon :

1.Intensity of radiation2.Silver grains/unit area

Increased intensity = increased blackening

Density = Log Io/ It

Io = Intensity of radiation reaching the filmIt = Intensity of the radiation transmitted.

Density= Log Io/It= Log 10/1 = 1

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Useful densities in diagnostic radiology range from 0.3 to 2

0.3 ------- 50% of light transmitted’2 ------- 1% of light transmitted

Density increase of 0.3 - The opacity is doubled. - Transmitted light is halved.

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BASE DENSITY AND FOG

Base density: 0.07

Caused by the plastic material & the blue dye used to make film base.

Fog: 0.05

the density resulting from developed unexposed silver grains.

BASE DENSITY + FOG: 1.12

Why is density expressed as Logarithm?

•Large differences can be expressed on a small scale•Physiologic response of the eye to intensities is logarithmic•Superimposition of densities can be best described as logarithmic.

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CHARECTERISTIC CURVE(H &D CURVE)

RELATIONSHIP BETWEEN DENSITY AND EXPOSURE

A film is given a series of Exposures.

Developingthe film and plotting the resulting density against known exposure gives the H&D curve.

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4 properties can be derived:

1.Fog level2.gamma3.Speed4.Latitude

FOG LEVEL:

•DECREASED CONTRAST•INHERENT FOG LEVEL: 0.12•ADDITIONAL FOG LEVEL CAN OCCUR DURING STORAGE.

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Film Gamma

Defined as maximum slope of charect-eristic curve.

Formula

Gamma : D1 - D2

Log E2 – Log E1

Ranges from 2.0 to 3.5.

In radiology gamma is of little value:

-short slope-slope of the curve over entire range of densities is useful

D1

D2

Log E1

Log E2

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SPEED

Film B is 0.3 log relative exposure units to the right of the Film A

Both the films show identical contrast

Film B requires twice the exposure as film A

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LATITUDE

Range of log relative exposure(mAs) that will produce the density within the acceptable range for diagnostic radiology

Film B is having a greater lattitude than Film A

The greater the lattitude the less the contrast.

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Radiographic Contrast

Contrast is the difference in luminance and/or color that makes an objectdistinguishable.

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Radiographic contrast depends on

1.Subject contrast2.Film contrast

Subject contrast is affected by:

1. Thickness of the subject.2. Density and atomic differences of the subject.3. Radiation energy (kVp).4. Contrast material.5. Scatter radiation

Film contrast is affected by:

1.Film gamma2.Screen or Direct exposure3.Film density4.Film processing

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FILM STORAGE

• Clean and dry location, • light tight location• 40 – 60 % Humidity and 70 º Fahrenheit• Away from chemical fumes• Safe from radiation exposure

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Why x ray image is viewed as a transparency?

Why can’t it be printed on a paper and viewed ?

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References:

Christensen’s Physics of Diagnostic Radiology

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