X-ray Film-Intensifying Screen
System: The x-ray film by itself is relatively insensitive to
x-rays. This means that it takes a substantial number of x-rays to
expose the film. X-ray film is actually much more sensitive to light.
This phenomenon forms the basis of the use of intensifying screens in
conjunction with x-ray film. The intensifying screen consists of a base
layer on which are coated crystals of a fluorescent compound. These
crystals absorb the x-rays and convert the x-ray energy to light; the
light emitted from the screen exposes the x-ray film emulsion. For every
x-ray absorbed by the screen multiple crystals are stimulated to give
off light. Thus the screen amplifies or intensifies the effect of the
x-rays. Also because of this effect fewer numbers of x-rays are needed
to create a given degree of blackness to the processed film.
Intensifying screens vary by 5 main factors :
-
Chemical
composition;
-
size of
fluorescent crystal;
-
Thickness of
crystal layer;
-
Efficiency of
absorbing x-rays and
-
The efficiency of
converting the absorbed x-ray to light.
The chemical
composition of the screen is either calcium tungstate (CaWO4) or one of
the several rare-earths phosphor combinations (gadolinium oxysulfide,
lanthanum oxybromide, yttrium oxysulfide) the major difference in the
chemical composition effects the absorption and conversion efficiency.
The rare earth phosphors have both a higher absorption and conversion
efficiencies than calcium tungstate. This means that the rare-earth
screens require fewer x-rays to produce the same amount of screen light.
This obviously indicates that reduced x-ray exposure is needed to
produce film blackening. Translated even more is that a shorter exposure
time can be used to deliver the x-ray exposure. Of course this is all
based on a "compared to” Basis. Screens are typically rated with a
relative speed. Words/and or numbers are used to designate the speed. A
screen with 100 or par speed rating is the reference standard. A screen
with a 200 speed rating would be twice as fast as the 100. This means
that the 200 speed screen requires 1/2 the x-ray exposure to produce the
same amount of light as was needed by the 100 speed. The 200 speed
screen may be referred to as a fast screen. The speed properties just
discussed are based on the inherent characteristics of the different
fluorescent compounds. The other way to change the speed of the screen
is to change the size of the fluorescent compounds. The other way to
change the speed of a screen is to change the size of the fluorescent
crystal or the thickness if the crystal layer.
X-ray film is composed
of a layer of sensitive emulsion coated onto a plastic base. The
emulsion can be coated on a single side or on both sides of the base.
The former is called single emulsion or sometimes detail film; the
latter called double emulsion film. The sensitive layer of the emulsion
is a silver compound usually silver bromide ( AgBr). When there is an
interaction with x-rays or light the silver bromide is put into an
excited state. During chemical processing the excited silver bromide is
reduced to elemental silver which is black.
X-ray film can be used
alone (non-screen film) in a light proof container or more typically it
is used with intensifying screens. The best example of film being used
without screens is Dental film. When used without screens the film is
referred to as screen film. The film + screen holder is called the x-ray
cassette. The screens are mounted to the inside front and inside back of
the cassette by adhesive. The film is then placed between the screens.
When the cassette is closed a tight contact is created between the
screens and the film, It is very important that this tight contact be
maintained. Any increase in the distance between the screen and the film
results in blurring of the image and degradation in image contrast.
Any increase in the screen-film separation distance
allows the light from the screen to spread. This result in exposure of
film that is undesired. A common cause of poor film-screen contact is
warping of the cassette from dropping or other rough handling.
Another very important feature of the screen-film
combination is that of matching the absorption spectrum of the film to
the emission spectrum of the screens. Silver Bromide is naturally
sensitive to light in the ultraviolet blue wavelengths. This allows for
an excellent match with the emission of calcium tungstate, which emits a
continuous distribution of blue light. The rare-earth screens emit
stronger narrower bands of light. Depending on the chemical composition
of the rare-earth phosphors the strong bands of emissions range from
blue to green. The silver bromide crystal. Films with this dye that are
sensitive to green light are called orthochromatic film. If you are to
take full advantage of the increased light emission from the rare-earth
screens, it is very important to match the screen emission to the films
absorption sensitivity.
| 1-800-346-9729 |
www.vetxray.com |
Sales@vetxray.com |
Diagnostic Imaging Systems,
Inc.
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