Intensifying screen and cassette

Most of the images recorded during

conventional radiography are obtained with

film/screen combination image receptors. Which in lessens the patient

dose due to the conversion of x-rays to

light called luminescence.

Luminescence

Can occur in two processesFluorescencephosphorescence

Fluorescence

Fluorescence – is light of certain crystals emitted within 10-8

seconds after the crystals are exposed to radiation. This means that light is emitted promptly. This is the type of luminescence that is desired for use in intensifying screen

Phosphorescence Phosphorescence – Is the light of certain

crystals emitted sometime after 10-8 seconds after the crystals exposure to radiation, resulting to delayed emission of light. This delayed emission is sometimes called

after glow or lag. This not desired for use in intensifying

screens because the delayed emission of light fogs the film in the cassette before the radiographer can get it to the processor.

Who developed the intensifying screen?

Thomas Edison developed the intensifying screen in 1896

Later that year Michael Pupin first used a film/screen combination in radiography.

Thomas Edison

Michael Pupin

Did you know?

The study of phosphorescence materials led to the discovery of radioactivity in 1896.

Henri Bequerel discovered radioactivity while studying different glow in the dark materials, which led him to think that the light emitted in the cathode rays tube are connect.

The effect

The fluorescence light from the crystals in the in the intensifying screen is used to expose the film and creates 95% - 98% of the optical density.

Because only a relatively small number of x-rays are necessary for the screens to emit a relatively large quantity of light.

Which in tern lower patient dose is required.

Intensifying screen speed

It refers to the amount of light emitted by the screen for a given amount of x-ray exposure.

A Screen that is designated as fast, creates an increased amount of light compared with a screen designated as slow when both are exposed to identical kVp, and mAs.

Intensifying Factor

Screen speed can be measured by intensification factor, relative name or speed value.

Intensification factor The exposure required to create a

certain optical density without a screen is divided by the exposure required with a screen to create the same optical density, w/c determines the intensification factor.

Intensification Factor

Intensification factor = exposure w/o screens

exposure w/ screensExample:If a 100mAs creates an optical density of 1.0 on a direct exposure film and a 5mAs creates the same optical density value with a film/screen combination.•Then that screen has an intensification factor of 20. The larger this value, the faster the speed of the screen.

Relative speed value

Is the most common method of designating screen speed and is used for all screens with rare earth phosphors.

When one speed is changed to another, a change in mAs is required to maintain optical density.

Relative speed value

New mAs = Old mAs x Old relative speed value

New relative speed value

•Answer: 5 mAs

Example:If 10 mAs is used with a 100-speed screen, when using a 200-speed screen. What is the new mAs?

Name of screen

Older, non-rare earth screen used specific names, such as fast or slow, to designate screen speed.

A Listing Of These Older Names, Along With Their Relative Speed Values, Is Presented.

Name of Screen Relative Speed Value

Ultra high or hi-plus

300

High or fast 200Medium, par, or standard

100

Detail, slow, or high resolution

50

Ultra – detail 25

Factor Affecting Screen Speed

Type of phosphor material Thickness of phosphor layer Size of phosphor crystals Reflective layer Light-absorbing dyes Ambient temperature Kilovolt (peak) selection

Type of Phosphor Material

Many different phosphor materials have been used in screens since 1896.

They are generally divided into two categories•Rare earth•Non-rare earth phosphor

Non – earth phosphors

They are the original type of screen material and emit light in the blue-violet portion type of color spectrum. Calcium tungstate Barium strontium sulfate Barium fluorochloride

Rare – earth phosphors

They were developed in the early 1970’s and are currently the most common type of intensifying screen materials.

The name rare earth is used because these materials have atomic numbers ranged 57-71 in the lanthanide or rare earth in the periodic table of elements.

Rare earth

These materials possess a greater quantum detection efficiency

(the ability to interact with x-rays) Greater conversion efficiency ( the

ability of screens to convert x-ray energy into light energy)

Compared to older screens Older calcium tungstate screens have a

conversion efficiency of 4%-5% Rare earth screens have values ranging

from 15%-25%. Rare earth are much faster than non-

rare earth phosphors. The rare earth are mixed with materials

called activators (terbium, niobium, or thulium) that determines the intensity and color of light emitted.

Emission color of common rare earth phosphors.

Rare earth phosphor

Color of emission

Gadolinium oxysulfide

Green

Lanthanum oxysulfide

Green

Yttrium oxysulfide Blue-greenYttrium tantalate Blue-greenLanthanum oxybromide

Blue

Lutetium tantalate blue

Thickness of phosphor layers

A thicker layer of phosphor material causes the screen to emit more light, because the extra material can absorb more x-rays.

This decreases the resolution of the resulting image because of increased light diffraction or diffusion.

True ImageUnsharpness

Rare earth

Rare earth screens are generally has better resolution because of their greater conversion efficiency therefore they do not have to be placed in as thick a layer.

Size of the phosphor crystals

Using larger-sized phosphor crystals increases the spread of screen but decreases image resolution because of light diffusion.

Reflective layer

Faster speed screens add a layer of titanium dioxide to reflect light back toward the film.

This increases the speed but decreases the resolution because of the angle of the reflected light.

Reflected light

X-ray photon

Base

Phosphor layer

Reflective layer

Light – absorbing dyes

Slower speed screens have light absorbing dyes added to the phosphor layer to control reflected light.

This dye decreases speed but increases image resolution.

Ambient Temperature

When the ambient temp. of intensifying screen increases significantly above room temperature (above 850 F or 300 C) the screen may function slower than usual.

The higher temperature gives the phosphor crystal more kinetic energy.

It does not cause more light but increases the energy (color) of the light.

The film may not be sensitive to the new color so the image may appear underexposed.

Kilovolt (peak) selection

The phosphor material in a screen must interact with the x-ray photons for luminescence to occur.

The greatest absorption of x-rays occurs when the x-ray photon energy and the binding energy of the k-shell electron are almost the same. K-edge effect.

K-edge effect

kVp must be match to the k-edge value

If values are not match for example of a dedicated mammography cassette usually has a lower k-edge value (15-20 keV) if the kVp used is at 100kVp it function much slower than if used at its proper kVp.

K-shell binding energies for some phosphor materials

Element Atomic number

K-shell binding energy (keV)

Yurium 30 17.05Barium 56 37.40Lanthanum 57 38.90Gadolinium 64 50.20Tungsten 74 69.50

Summary