Intensifying screens & films

Intensifying screens and films in diagnostic radiology

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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 compound
Prevents static electricity
Physical protection
Surface 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.
P
H
O
S
P
H
O
R
X - RAYS
LIGHT PHOTONS

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PHOSPHOR CRYSTALS
-SUSPENDED IN PLASTIC
Most frequently used:
- Calcium Tungstate(CaWO
4
)
-Superseded by various Rare earths: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.

12. •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

14. 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 BORDERS
INCREASED THICKENSS CAUSES INCREASED DIFFUSION
DECREASED THICKENESS CAUSES DECREASED DIFFUSION

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SCREEN – FILM CONTACT:
Cassette: light tight container
Holds 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

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EMULSION
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 bromide
1-10% - silver iodide
2. SILVER HALIDE

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Ag Br
Cubic lattice/ crystal size: 1.0-1.5 microns
One cubic cm emulsion: 6.3x10
9
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 image
site
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 radiation
2.Silver grains/unit area
Increased intensity = increased blackening
Density = Log I
o
/ I
t
I
o
= Intensity of radiation reaching the film
I
t
= Intensity of the radiation transmitted.
Density= Log I
o
/I
t
= 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.
Developing
the 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 level
2.gamma
3.Speed
4.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 : D
1
- D
2

Log E
2
– Log E
1
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
D
1

D
2
Log E
1
Log E
2

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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 object
distinguishable.

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Radiographic contrast depends on
1.Subject contrast
2.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 gamma
2.Screen or Direct exposure
3.Film density
4.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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