Intensifying Screens Kyle Thornton DMI 50B

History Of Intensifying Screens First developed by Thomas Edison Calcium tungstate was used as the phosphor Converted x-rays to ultraviolet-blue light First screens were flawed in design Screen design was perfected in the 1920’s

What Is An Intensifying Screen? Part of the radiographic cassette Screens contain phosphors that emit visible light when struck with x-ray photons Depending on the phosphor used, the screen will emit light in the blue or green spectrum Usage of screens result in lowered patient doses

Screen Construction Protective coating Phosphor layer Reflective layer Layer closest to film about 10 - 20 micrometers thick Phosphor layer Active layer emits light Reflective layer 25 micrometers thick Intercepts light and redirects it to film Base Provides support Farthest from film

Screen Construction

Cassette Construction

Luminescence The emission of visible light Two types of visible light Fluorescence Emitted only during stimulation of phosphors Phosphorescence Continues to emit light after stimulation Afterglow or screen lag A flaw with early screens

Intensifying Screen Characteristics X-ray absorption Screen conversion efficiency Image noise Spatial resolution Image blur Screen speed

Intensification Factor Compares patient dosage with screens to without screens A reduction in dose is expected Technical factors must be modified to accommodate changes in screen speed

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

Manufacturer’s Design Of Intensifying Screens Phosphor composition Rare earth elements are most efficient Phosphor thickness The number of x-rays converted to light increases with phosphor thickness Reflective layer Increases conversion efficiency but adds to image blur Dye Controls spread of light Improves spatial resolution Crystal size Larger phosphors produce more light Concentration of phosphor crystals Higher crystal concentration results in higher conversion efficiency

Penumbra

X-Ray Absorption The percent absorption of x-rays in the phosphor layer of intensifying screens

Conversion Efficiency The efficiency of converting x-ray to visible light Increase in conversion efficiency will make a screen brighter and affects image receptor speed

Conversion Efficiency

Image Noise Deterioration of the image Affected by Quantum mottle 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.

Spatial Resolution/Image Blur Measured in line pairs/mm Direct exposure film has highest lp/mm The slower the speed, the more lp/mm

Resolution Test Tool

Compatibility Film must match the screen Each screen exposes the emulsion that it is nearest

Advantages Of Screen Film Increased Adjustment of radiographic contrast Spatial resolution w/small focal spots Ability for magnification radiography Flexibility of kVp selection X-ray tube life Decreased Patient dose Occupational exposure Heat production Exposure time Tube mA Focal spot size

Elements Used Rare earth Gadolinium - Green Lanthanum - Blue Yttrium - Blue Very high conversion efficiency 15 - 20%

Screen Variations Asymmetric cassettes Advantages in: Chest Pediatric Portable radiography

Care Of Intensifying Screens Handle with care Must be cleaned periodically Special cleaning fluid must be used Must be tested for screen-film contact Wire mesh test

Wire-Mesh Test Results