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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 processes
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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 processes • Fluorescence • phosphorescence
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.
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/ screens • Example: • 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 • Example: • If 10 mAs is used with a 100-speed screen, when using a 200-speed screen. What is the new mAs? • Answer: 5 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.
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.
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.
Unsharpness True Image
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.
X-ray photon Base Reflected light 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.
