Intensifying screens are used in the x-ray cassette to intensify the effect of the x-ray photon by producing a larger number of light photons. It decreases the mAs required to produce a particular density and hence decreases the patient dose significantly. It also reduces motion blur and x-ray tube loading by reducing exposure time.
In cassettes, which use double emulsion films, two screens are used, mounted on both sides of the cassette. In mammography, however, a single screen on the back side and a single emulsion film is used.
The thickness of an intensifying screen is about 0.4 mm. The thickness of the screen affects the screen speed and spatial resolution: thicker screen improves speed but reduces spatial resolution (increased diffusion of light before image formation).
The intensity factor is a measure of efficiency of intensifying screen, and is the ratio photon exposures with and without intensifying screen to achieve a designated film density 4,5. The absorption and conversion efficiency of the screen affect the intensity factor.
Film-screen combination typically used for chest radiography has a limiting resolution of 6 line pairs per mm, which is two times better than a typical digital radiography of 3 line pairs per mm 6.
Layers
base
reflecting layer/absorptive layer
-
luminiscent layer - absorbs the x-ray photon and converts it to visible light that is recorded by the film. Contains fluorescent materials called 'phosphors':
calcium tungstate (CaWO4): blue light
lanthanum oxybromide (LaOBr): blue light
gadolinium oxysulfide (Gd2O2S): green light
protective layer
Rare earth elements are used in present-day screens as they are faster and have higher absorption and conversion efficiency:
gadolinium: green light
lanthanum: blue light
yttrium
Spectral matching
It is important to note that the color of the light emitted (wavelength) must match the light sensitivity of the film used. This is known as spectral matching:
conventional films: sensitive to ultraviolet and blue lights
orthochromatic films: sensitive to ultraviolet, blue and green lights
If the wavelength of light emitted by the screen is not in the sensitive range of the film, it will not be absorbed by the film.
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