Intensifying ScreensRT 104

Cassettes

Cassettes serve 3 important functions:1. Protect film from exposure to light2. Protect film from bending and

scratching during use.3. Contain intensifying screens, keeps

film in close contact to screen during exposure.

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

Cassette Features - Front

Exposure side of cassette is the “front”.

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

Has the ID blocker (patient identification)

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

Flat surface coated with fluorescent crystals called phosphors

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

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

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

DISADVANTAGES: less detail than direct exposure (detail better with rare earth than calcuim tungstate screens)ADVANTAGES:

1.Reduce patient exposure2. 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

A relative number that describes how efficiently x-rays are converted into usable light

Ranges from 100 (slow) to 1200 (fast)

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 Increasing speed also increases image

noise

SCREEN SPEEDS

FASTER SPEED – REDUCES PATIENT EXPOSURE

FASTER SPEED - REDUCES IMAGE DETAIL INCREASES NOISE (LIGHT BLURING AROUND IMAGE)

Technique Changes

Relative Speed = Film speed & Screen speed

mAs 1 = RS 2 mAs 2 RS 1

They are inversely related

Image Noise

Speckled background on the image Caused when fast screens and high

kVp techniques are used. Noise reduces image contrast

The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE)

The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)

SCREEN SPEEDS

Quantum Mottle causing a grainy, mottled or splotch image

Often results of using very fast-speed screen-film systems

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.

Image Quality

Rare Earth Screens

Have higher DQE (detective quantum efficiency). Higher x-ray absorption abilities.

Have higher CE (conversion efficiency). More light emitted per x-ray absorbed by the screen.

Spatial resolution

The use of intensifying screens lowers spatial resolution compared with direct-exposure radiographs.

Spatial resolutionexpressed by thenumber of line pairsper millimeter (lp/mm)

The higher the lp/mm the smaller the object that can be imaged

Very fast screens = 7 lp/mm

Fine-detail screens= 15 lp/mm

Direct-exposure screens = 50 lp/mm

Screen speed vs Spatial resolution

Wire mesh test – check for screen-film contact. Good contact

Wire mesh test – check for screen-film contact.

Warped cassette –poor contact

Care of Screens

Image artifacts can appear if screens are modified

Small scratches can leave artifacts Dirty screens can leave artifacts Screens should be cleaned once each

month with manufacturer’s cleaner with antistatic compounds

Questions?