topic 1

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.
Can occur in two processes
 Fluorescence
 phosphorescence
 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 – 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.
 Thomas Edison developed the
intensifying screen in 1896
 Later that year Michael Pupin
first used a film/screen
combination in radiography.
The study of phosphorescence materials
led to the discovery of radioactivity in
 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 fluorescence light from the crystals in the
in the intensifying screen is used to expose
the film and creates 95% - 98% of the optical
Because only a relatively small number of xrays are necessary for the screens to emit a
relatively large quantity of light.
Which in tern lower patient dose is required.
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.
Screen speed can be measured by
intensification factor, relative name or speed
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 = exposure w/o screens
exposure w/ screens
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.
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
New mAs = Old mAs x Old relative speed value
New relative speed value
If 10 mAs is used with a 100-speed
screen, when using a 200-speed
screen. What is the new mAs?
•Answer: 5 mAs
Older, non-rare earth screen used specific
names, such as fast or slow, to designate
screen speed.
Name of Screen
Ultra high or hi-plus
High or fast
Medium, par, or
Detail, slow, or high
Ultra – detail
Relative Speed Value
Type of phosphor material
Thickness of phosphor layer
Size of phosphor crystals
Reflective layer
Light-absorbing dyes
Ambient temperature
Kilovolt (peak) selection
Many different phosphor materials have been
used in screens since 1896.
They are generally divided into two
•Rare earth
•Non-rare earth phosphor
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
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.
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)
Older calcium tungstate screens have a
conversion efficiency of 4%-5%
 Rare earth screens have values ranging from
 Rare earth are much faster than non-rare earth
 The rare earth are mixed with materials called
activators (terbium, niobium, or thulium) that
determines the intensity and color of light
Rare earth phosphor Color of emission
Gadolinium oxysulfide
Lanthanum oxysulfide
Yttrium oxysulfide
Yttrium tantalate
Lutetium tantalate
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.
True Image
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.
Using larger-sized phosphor crystals
increases the spread of screen but decreases
image resolution because of light diffusion.
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
Reflected light
Reflective layer
Slower speed screens have light absorbing
dyes added to the phosphor layer to control
reflected light.
This dye decreases speed but increases image
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.
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.
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.
energy (keV)

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