·
Intensifying screens decrease
the dose/ exposure to the patient but still afford proper film exposure
·
Intensifying screen allows
shortening of exposure times
·
Luminescence = emission of
light from a substance e.g. inorganic salt crystals (phosphors)
o
Fluorescence = instantaneous
emission (<10-8 sec)
o
Phosphorescence = delayed
emission (>10-8 sec)
·
Base; Reflecting coat; Phosphor
layer; Protective layer
·
Screen and film spectra must
match e.g. blue – green light emission spectrum must match film absorption
spectrum of the film emulsion
·
The efficiency of a screen of a
screen to convert XR into light is the screens conversion efficiency :
o
l = 12.4/ keV
·
The ability of light emitted by
the phosphor to escape from the screen and expose the film is the screen
efficiency: See fig 6-7 BB: 152
·
Diffusion of light in the
screen affects the resolution - LSF
·
The intensification factor is
the ratio of the XR exposure needed to produce the same density on a film with
and without the screen
o
Increases with kVp
o
Also filtering which increases
the average beam kVp increases Intensification factor
·
Speed (sensitivity/ efficiency)
of screen:
o
Thickness of screen (thicker
increases absorption, but increases defusion)
o
Size of crystals (increased
absorption)
o
Conversion efficiency (increase
conversion efficiency – increase speed – rare earth crystals[gadolinium;
europium])
o
Higher absorption - rare earth crystals k-absorption matching
o
Presence of light absorbing dye
(decrease speed)
o
Speedµ
1/resolution (ability to record detail)
·
Increase screen speed:
o
Increased noise/ mottle – with
phosphor with higher conversion efficiency
o
No increase in noise/mottle –
with increased phosphor thickness and higher absorption coefficient for x-rays
·
Screen film contact:
o
Light tight cassette
o
Must have intimate contact with
film (otherwise decrease resolution)
·
Must be clean – dust creates
unexposed points on film (high-spots)
