darkroom & supplies - xraykamarul · hdr 102 chapter 4 physics for radiographers 1 cassettes...

HDR 102

CHAPTER 4

PHYSICS FOR RADIOGRAPHERS 1

CASSETTES AND INTENSIFYING

SCREENS

PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH

SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCE

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CHAPTER 4: Cassette and Intensifying Screen

LEARNING OUTCOMES

At the end of the lesson, student should be able to:-

Cassette

Define of cassette

Describe function, design, and construction of cassette

Explain problem associated with cassettes and ways of overcoming them

Explain quality assurance on cassettes

Describe artifacts associated

Intensifying Screen

Define fluorescence

Explain function and advantages of IS

Describe design, functions, and characteristics of layers of IS

Explain quality assurance on IS

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CASSETTES

&

INTENSIFYING SCREENS

&

FILMS

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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

KAAB (C)

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KAAB (C)

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Primary Radiation exit from tube

100 % enters patient

1% exits for form image on

cassette below

REMNANT Radiation

KAAB (C)

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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.

KAAB (C)

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KAAB (C)

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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

KAAB (C)

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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

KAAB (C)

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KAAB (C)

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KAAB (C)

ALWAYS KEEP THE

COLLIMATED AREA

SMALLER THAN THE

SIZE OF THE

CASSETTE

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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.

KAAB (C)

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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

KAAB (C)

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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

KAAB (C)

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KAAB (C)

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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.

KAAB (C)

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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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INTENSIFYING SCREEN

ADVANTAGES OF USING SCREENS

reduces the dose required for a particular examination.

short exposure time

less movement that cause image unsharpness

increase x-ray tube life

improve image contrast

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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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CONSTRUCTION OF SCREENS

1) 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 photons

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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.

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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.

Two types of phosphor:-

1. Calcium Tungstae (CaWo)

2. Rare Earth Phosphor

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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

1. Fluorescence (instantaneous emission)

Emitted only during stimulation of phosphors

2. Phosphorescence (delayed emission)

Continues to emit light after stimulation

Afterglow or screen lag

A flaw with early screens

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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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KAAB (C)

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RARE EARTH PHOSPHORS

1. Gadolinium

2. Lanthanum

3. Yttrium

Found in low abundance in nature

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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.

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”.

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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)

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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0

2

4

6

8

10

12

14

Series 3

Series 2

Series 1

2. Screen Efficiency

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.

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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 size

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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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Cont’d

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.

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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.

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KAAB (C)

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FILM/SCREEN COMBINATION

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Spectral Sensitivity OR Spectral Matching

Blue – UV light sensitive film – CALCIUM TUNGSTATE screens

Green, Yellow-Green light sensitive film - RARE EARTH screens

Film is designed to be sensitive to the color of light emitted by the intensifying 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.

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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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0

2

4

6

8

10

12

14

Series 3

Series 2

Series 1

Intensifying Screen & Film Cross Section

KAAB (C)

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0

2

4

6

8

10

12

14

Series 3

Series 2

Series 1

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Factors Affecting Image Quality

Factor 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

KAAB (C)

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Factors Affecting Image Quality-cont’d

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

KAAB (C)

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Questions?

KAAB (C)

darkroom & supplies - xraykamarul · hdr 102 chapter 4 physics for radiographers 1 cassettes...

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