CASSETTES

&

INTENSIFYING SCREENS

&

FILMS

LECTURE 6 (b)

KAMARUL AMIN BIN ABDULLAH

KAAB ©

REVISION

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Creating the IMAGE

• When x-rays pass through a patient's body, three things can happen:

• (1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film;

• (2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; and

• (3) the x-ray photon is scattered and reaches the film causing an overall gray fog.

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Radiographic Images

• RADIOLUCENT - dark on image

–AIR, CO2 , Lungs

• RADIOPAQUE - white on image

–BARIUM, IODINE, Bones

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• Primary Radiation exit

from tube

• 100 % enters patient

• 1% exits for form

image on cassette

below

• REMNANT Radiation

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CASSETTES

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Cassettes

Cassettes serve 3 important functions:

1. Protect film from exposure to light

2. Protect film from bending and scratching

during use.

3. Contain intensifying screens, keeps film

in close contact to screen during

exposure.

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Image formation

Remnant x-ray photons converted

to light photons

Image before processing = Latent image

Made visible by chemical processing =

Manifest image

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CASSETTE or FILM HOLDER

• The CASSETTE is

used to hold the film

during examinations.

It consist of front and

back intensifying

screens, and has a

lead (Pb) backing.

The cassette is light

tight

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• ALWAYS KEEP

THE

COLLIMATED

AREA SMALLER

THAN THE SIZE

OF THE

CASSETTEKAAB (C)

Cassette Features - Front

• Exposure side of cassette is the “front”.

• Has the ID blocker (patient identification)

• Made of radiolucent material – easily penetrated by x-rays, lightweight metal alloy or plastic material made of resin.

• Intensifying screen mounted to inside of front.

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Cassette Features - Back

• Back made of metal or plastic

• Inside back is a layer of lead foil –

prevents backscatter that could fog

the film

• Inside foil layer is a layer of padding –

maintains good film/screen contact

• Back intensifying screen mounted on

padding

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Cardboard Cassettes

Direct x-ray exposure to film required

• 25 to 400 times more radiation to create an image on the film

• BETTER DETAIL THAN FILM SCREEN (NO BLURRING OF IMAGE FROM LIGHT)

• ALL EXPOSURE MADE FROM X-RAY PHOTONS

• BIG DOSE TO THE PATEINT

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INTENSIFYING SCREENS

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INTENSIFYING SCREEN

• INTENSIFYING SCREEN is a device that

converts X-rays to visible light. It converts

a higher energy electromagnetic radiation

to a lower energy electromagnetic

radiation.

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INTENSIFYING SCREEN

• Flat surface coated with

fluorescent crystals

called phosphors that

glow, giving off light

when exposed to x-

rays.

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• ADVANTAGES OF USING SCREENS

– reduces the dose required for a particular

examination.

– short exposure time

– less movement unsharpness

– Reduce patient exposure

– Increase x-ray tube life

INTENSIFYING SCREEN

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INTENSIFYING SCREENS

DISADVANTAGES:

• less detail than direct exposure (detail better

with rare earth than calcium tungstate screens)

• introduces the screen unsharpness

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INTENSIFYING SCREEN

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INTENSIFYING SCREEN

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CONSTRUCTION OF

SCREENS1) BASE

– This acts as a support for all other layers of

intensifying screen.

– Made of polyester plastic 1mm thick.

– Characteristics:-

a) Must be flexible yet tough – good contact

b) Rigid – stay in place

c) Chemically inert – not react with phosphor

d) Radiolucent – transmission of x-ray photonsKAAB (C)

2) REFLECTIVE OR ABSORPTIVE LAYER– modern technology has already incorporated the reflective or

absorptive layer in the upper part of the base.

– Reflective layer intercepts light going away from the film and redirects it towards the film.

• Increases speed but increases also the amount of unsharpness.

• Made from thin (30m thick) coating of titanium dioxide (TiO2) or magnesium oxide (MgO2)

– Absorptive layer absorbed the light travelling away from the film.

• This layer is made of dye incorporated on the base material.

• This slows down the speed of the system, but has the advantage of improving the sharpness of the image.

CONSTRUCTION OF

SCREENS

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3) PHOSPHOR LAYER

– The active layer.

– Materials that capable to absorb x-ray photons to emit light photons.

– Thickness: 150 - 300m but depending on speed and resolving power.

CONSTRUCTION OF

SCREENS

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Contd’

– Phosphor crystal is a metallic crystalline solid, naturally occurring or artificially made, that exhibits the property of fluorescence when exposed to X-rays and can be manufactured in useful form to produce high image quality.

• Calcium tungstate (CaWO4)

• Rare earths

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Contd’

Phosphors must have:-

• High atomic number

- To increase the interaction of incident x-ray

photons. X-ray photons (high energies)

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Contd’

• Appropriate Spectral Emission

- Is an indication of the precise wavelength

of light emitted by the phosphor.

- The spectral emission match the

sensitivity of the film to ensure maximum

latent image formation.

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Spectrum Emission/Matching

• Calcium Tungstate

emits a broad blue

spectrum.

• Rare earth emits a

green spectrum.

• The film, screens and

safelight must match.

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• Luminescence

- The emission of visible light

- Two types of visible light

– Fluorescence (instantaneous emission)

• Emitted only during stimulation of phosphors

– Phosphorescence (delayed emission)

• Continues to emit light after stimulation

• Afterglow or screen lag

• A flaw with early screens

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Contd’

RARE EARTH – (emits green light)

• Developed in 1980’s

• Most efficient – most common in use today

CALCIUM TUNGSTATE (blue light)

• Not as efficient

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Rare Earths

1. Gadolinium

2. Lanthanum

3. Yttrium

• Found in low abundance in nature

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RARE EARTH

PHOSPHORS

Absorp Conv

Phosphor Z K-Edge Effic Effic

CaWO4 74 69.5 20-25% 3.5%

GdO2S:Tb 64 50.2 45-50% 15%

LaO2S:Tb 57 38.9 45-50% 12%

Y2O2S:Tb 39 17.1 20-25% 18%

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4) SUPERCOAT/PROTECTIVE COAT

– This is the top protective layer of the screen.

– It is approximately 25m thick.

– It is made of cellulose acetobiturate, or other

polymer.

CONSTRUCTION OF

SCREENS

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– It serves three functions:

• protects the delicate phosphor layer from

mechanical damage,

• provides a surface which can be cleaned without

damaging the phosphor layer, and

• provides a smooth evacuation of entrapped air

resulting in a good film-screen contact.

– It must be a poor conductor of static

electricity.

– The surface of protective coating can be

made with varying “roughness”.

CONTINUE…

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Characteristics/Properties of

Screens 1. X-ray absorption

2. Screen efficiency

3. Image noise

4. Spatial resolution/ Image Blur

5. Screen speed

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1. X-Ray Absorption

• The percent absorption of x-rays in the

phosphor layer of intensifying screens

• The Rare Earth elements produce higher

x-ray absorption than calcium tungstate.

• Thus, increase the interaction to produce

light photons.

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2. Screen Efficiency

• Absorption Efficiency

Fraction of incident x-ray photons captured

(this is “information capturing step: those x-

rays not absorbed cannot contribute to image

formation)

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2. Screen Efficiency

• Conversion Efficiency– Fraction of absorbed x-ray energy converted to light (to which

the film is sensitive)

– The ability of phosphors to emit as much light per xray photons

interaction.

– Increase CE, Decrease Patient Dose

• Screen Efficiency

Fraction for emit light reaching film

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Conversion Efficiency

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3. Image Noise

• Deterioration of the image

• Affected by

– mAs - or number of x-rays used

– limited absorption efficiency

– randomness of conversion

• Quantum mottle

– Noisy appearance of an image

– More apparent in fluoroscopy

– Raising mAs tends to overcome Q.M.KAAB (C)

4. Spatial Resolution/ Image Blur

• Measured in line pairs/mm

• Direct exposure film has highest lp/mm

• The slower the speed, the more lp/mm

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5. Screen Speed

• Efficiency of a screen in converting x-rays to

light is Screen Speed.

• The relationship between screen speed and

detail is a reciprocal one: as the speed of the

screen increases, the amount of detail

decreases.

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5. Screen Speed

• Also depends on speed of film used

• Assigned speeds - 100, 200, 400, 800,

and 1000

• Screen speed depends on:

– Number of x-rays interacting with phosphor

layer

– Conversion of x-ray energy to visible light

– This is interrelated with phosphor distribution

and sizeKAAB (C)

5. Screen Speed

• Greater efficiency = less exposure = faster

-Standard screen speed class of 100

-200 screen speed is twice as fast

• Speeds for routine work: 200 – 800

• Speeds for high detail: 50 - 100

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IMAGE QUALITY

• Resolution:

– Screen thickness

– Light diffusion (light absorbing dyes)

– Screen Asymmetry

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Asymmetric Screens

• Screens in the cassette can be of two

types or speeds. Some people use two

different speeds in cassette for full spine

radiography.

• When types of screens are different, they

are referred to as Asymmetric screens.

One side may be high contrast and the

other side wide latitude. The combined

image is superior.KAAB (C)

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RADIOGRAPHIC FILMS

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FILM Standard Sizes

in Inches

• 14 X 17

• 14 X 14

• 12 X 15

• 10 X 12

• 8 X 10

Metric:

• 18cm x 24cm

• 24cm x 30cm

• 30cm x 40cm

• 35cm x 40cm

• 35cm x 43cmKAAB (C)

X-Ray Film

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

• Image recorded on film is caused by exposure to photons:

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RADIOGRAPH

• PERMANENT RECORD MADE USING

RADIATION

– RADIO- RADIATION (usually x rays)

– GRAPH PERMANENT RECORD

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• FIRST “FILM”

• GLASS PLATES

• WW 1

• CELLULOSE ACETATE

• HIGHLY FLAMMABLE

• EASILY TORN

• RESPONSIBLE FOR

MANY FIRES IN

HOSPITAL

BASEMENTS

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IMAGE ON FILM

• SINGLE EMULSION = BETTER DETAIL

• DOUBLE EMULISON = LESS DETAIL

• PARALLAX

With double emulsion – an image is

created on both emulsions – then

superimposed – slight blurring of edges

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PARALLAX –each emulsion has an image

single image overlaped – edges

less sharp

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Film Characteristics

• Size of silver halide crystals &

emulsion thickness determine speed

of film and degree of resolution

• Speed – the response to photons

• Resolution – the detail seen

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Film Speed / Crystal size

• Larger crystals or Thicker crystal

layer

Faster response= less detail, and

less exposure (chest x-ray)

• Finer crystals / thinner crystal layer

=Slower response, greater detail,

more exposure (extremity)

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LIGHT VS DARK AREAS ON FILM

• DARK SPOTS – SILVER HALIDE

CRYSTALS THAT HAVE BEEN

EXPOSED TO PHOTONS – TURN TO

BLACK METALLIC SILVER AFTER

PROCESSING

• LIGHT AREAS – NO CRYSTALS

EXPOSED – SILVER HALIDE IS

WASHED AWAY WITH PROCESSING

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FILM STORAGE

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FILM BIN - STORAGE

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Film Storage

• Clean, dry location

• 40 – 60 % Humidity 70 º Fahrenheit

• Away from chemical fumes

• Safe from radiation exposure

• Standing on edge

• Expiration date clearly visible

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X-ray Film Sensitivity

• Light

• X-rays

• Gamma Rays

• Gases

• Fumes

• Heat

• Moisture

• Pressure

• Static

Electricity

• Age

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EXAMPLES OF POOR

STORAGE AND HANDLING

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FILM FOG!!!!

• Unintended

uniform optical

density on a

radiograph

because of x-rays,

light, or chemical

contamination that

reduces contrast &

affects density

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POOR SCREEN CONTACT

• FOAM BACKING HELPS TO PLACE

INTENSIFYING SCREENS IN DIRECT

CONTACT WITH THE FILM – NO GAPS

• IF GAPS – MORE LIGHT CAN BE

EMITTED IN SPACE, CAUSING THE

IMAGE TO BE OF POOR DETAIL

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FILM/SCREEN

COMBINATION

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Spectral Sensitivity OR

SPECTRAL MATCHINGFilm is designed to be sensitive to the color of light

emitted by the intensifying screens

• Blue – UV light sensitive film –

CALCIUM TUNGSTATE screens

• Green, Yellow-Green light

sensitive film -

RARE EARTH screens

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• The light photons generated in the

intensifying screen are emitted by

phosphor crystals.

• These crystals are significantly larger

than the silver halide crystals in the

film

• use of a screen reduces image

sharpness somewhat

• Some examinations requiring

extremely fine detail use screens with

small crystals. KAAB (C)

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Intensification Factor

• The intensification factor of a screen is the

ratio of the x-ray exposure needed to

produce the same density on a film with

and without the screen

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Intensifying Screen & Film

Cross Section

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Factors Affecting Image

QualityFactor Noise (QM) Resolution

• Screen Reduces-more decreased-more

Thickness x-rays absorbed light spread

• Absorption Reduced-more No effect

Efficiency x-rays absorbed

• Conversion Increased-use less No direct effect

Efficiency mAs for proper

film density

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Factors Affecting Image

Quality-con’t

Factor Noise (QM) Resolution

• Absorption Reduces-use Increased-absorbs

of light in more mAs for most diffuse light phosphor density preferentially

• Asymmetry Minor Increased-light

of screens effects emitted closer to

film on average

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