RAD 254 Chapter 13 Intensifying Screens

Physical purpose: to convert x-ray photons into light photons (done

at the phosphor layer)

Most – if not all in use are “rare earth”

• Rare earth crystals include (but are not limited to):– Gadloinium– Lanthanum– Yttrium

Desired Physical Properties of crystals

• High atomic number = high absorption (DETECTIVE QUANTUM EFFICIENCY {DQE} )

• Phosphor should emit a LARGE # of light photons for each x-ray photon – CONVERSION EFFICIENCY (CE)

• Color of light should match the color light the film is sensitive to – SPECTRAL MATCHING

• ZERO/next to zero afterglow

Important terms

• Luminescence – process of giving off light when stimulated

• Fluorescence – giving off light ONLY when stimulated

• Phosphorescence – continuing to give off light after stimulation

• Intensification factor – amount of radiation reduction with screens vs NO screens

Screen speed

• Can be judged by intensification factor (IF)

• Increasing speed increases noise

• Increasing speed REDUCES spatial resolution

• Increasing speed INCREASES quantum mottle (line-pair test pattern)

Technologist CONTROLABLEitems

• Screen attributes the tech can control: radiation quality; image processing and temperature (also screen care and cleaning)

Cassette Construction

• Rigid, light proof protective housing for the film and screens.

• Felt/rubber/sponge “compression” layer to assure good film-screen contact

• K-edge of crystals determines light spectrum

Screen cleaning

• Compare and contrast screen cleaning solutions (home made vs commercially made)

• Cotton vs 4 X 4’s

Screen – Film Contact Test

• Wire mesh test for screen-film contact and proper resolution/visibility of detail