RTEC A Spring 2009

FILM CASSETTES

& INTENSIFYING SCREENSWEEK 9

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.

Radiographic Images

• RADIOLUCENT - dark on image

–AIR, CO2 , Lungs

• RADIOPAQUE - white on image

–BARIUM, IODINE, Bones

• Primary Radiation exit from tube

• 100 % enters patient• 1% exits for form

image on cassette below

• REMNANT Radiation

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.

Image formation

Remnant x-ray photons converted to light photons

Image before processing = Latent image

Made visible by chemical processing = Manifest image

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

• ALWAYS KEEP THE COLLIMATED AREA SMALLER THAN THE SIZE OF THE CASSETTE

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.

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

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

Intensifying Screens

• Flat surface coated with fluorescent crystals called phosphors

• that glow, giving off light when exposed to x-rays.

Image creation

• 1% of xray photons that leave patient

• Interact with phosphors of intensifying screens

• 100’s of light photons created to make image on film

• Light photons expose silver halide crystals in the film emulsion –

• Turn black metallic silver after procession

Intensifying Screens Phosphors

• RARE EARTH – (emits green light)

• Developed in 1980’s

• Most efficient – most common in use today

• CALCIUM TUNGSTATE (blue light)

• Not as efficient

Rare Earth Screens

• Gadolinium

• Lanthanum

• Yttrium

• Found in low abundance in nature

INTENSIFYING SCREENSDISADVANTAGES:

• less detail than direct exposure

• (detail better with rare earth than

• calcuim tungstate screens)

ADVANTAGES:

1. Reduce patient exposure

2. Increase x-ray tube life

Screen Construction

• Polyester plastic base – support layer

• Phosphor layer – active layer

• Reflective layer – increases screen efficiency by redirecting light headed in other directions

• Protective coating

Intensifying screens

Phosphor Layer

• Active layer – x-ray photons converted to light photons

*Photoelectric Effect

Screen Speed

• Efficiency of a screen in converting x-rays to light is Screen Speed.

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

SCREEN SPEEDS

• FASTER SPEED – REDUCES PATIENT EXPOSURE

• FASTER SPEED - REDUCES IMAGE DETAIL (LIGHT BLURING AROUND IMAGE)

FILM Standard Sizesin Inches• 14 X 17• 11 X 14• 10 X 12• 8 X 10

Metric:• 18cm x 24cm• 24cm x 30cm• 30cm x 35cm• 35cm x 43cm

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:

RADIOGRAPH• PERMANENT RECORD MADE USING

RADIATION

– RADIO- RADIATION (usually x rays)

– GRAPH PERMANENT RECORD

• FIRST “FILM”• GLASS PLATES• WW 1 • CELLULOSE ACETATE• HIGHLY FLAMMABLE• EASILY TORN• RESPONSIBLE FOR

MANY FIRES IN HOSPITAL BASEMENTS

X-ray Film cont’d

• Radiographic film is/was most common image receptor

• Two parts:

1. Base

2. Emulsion

Film Construction - BASE

• Made of a polyester plastic• Must be clear, strong, consistent

thickness• Tinted pale blue or blue-gray (reduces

eye strain)• COATED ON 1 OR 2 SIDES WITH

EMULSION

Film Construction - EMULSION

• Film emulsion can be on one side or both sides of base (single emulsion / double emulsion)

• Protective overcoat layered on top of emulsion

• Emulsion is a gelatin containing the film crystals

FILM COMPOSTION

• SINGLE OR DOUBLE EMULSIONCOATED ON A BASE

EMULSION : GELATIN

• WITH SILVER HALIDE CRYSTALS

• BASE: SUPPORT

POLYESTER

Film Emulsion

• Made of mixture of gelatin & silver halide crystals (fluorine, chlorine, bromine, & iodine)

• Most x-ray film emulsions made of :

silver bromide (90%) silver iodide (10%)

• Photographically active layer – activated by light & radiation to create image

X-Ray Film Cross Section

FILM CONSTRUCTION

• BASE WITH EMULSION

• CAN BE ON 1 (SINGLE EMULSION)

• OR 2 SIDES (DOUBLE EMULSION)

• MUST BE MATCHED WITH 1 OR 2 SIDED INTENSIFYING SCREENS

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

PARALLAX –PARALLAX –each emulsion has an image

single image overlaped – edges less sharp

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

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)

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

FILM BIN - STORAGE

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

X-ray Film Sensitivity

• Light

• X-rays

• Gamma Rays

• Gases

• Fumes

• Heat

• Moisture

• Pressure

• Static Electricity

• AgeSo what happens??

FILM FOG!!!!

• Unintended uniform optical density on a radiograph because of x-rays, light, or chemical contamination that reduces contrast & affects density

POOR SCREEN CONTACT

• FOAM BACKING HELSP 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

Intensifying Screen & Film Cross Section

Spectral Sensitivity ORSPECTRAL MATCHING

Film 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

• 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.

Questions?