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INTRODUCTION
Photographic film can be exposed
directly to X-rays but its sensitivity is very low and prohibitively
large patient exposures would result if this appraoch was
implemented on its own. Therefore, almost all conventional
radiographic examinations require that the image be converted to
light by an intensifying screen before being recorded by the film.
We will consider pertinent features
of both Intensifying Screens and X-ray film below.
FLUORESCENCE
We have seen previously that
luminescence refers to the stimulated (by light, ionising radiation,
chemical reactions etc.) emission of light by certain materials. If
the light is emitted instantaneously, that is within 10 nanoseconds,
the phenomenon is called fluorescence.
If the emission is delayed somewhat, it is called
phosphorescence. More
particularly, in radiology, fluorescence is the term used to
describe the ability of certain inorganic phosphors to emit light
when excited by X-rays.
Until the early 1970s the only
phosphor of note was calcium tungstate (CaWO4), but since
then a plethora of rare-earth phosphors with improved efficiency
have appeared on the scene. No matter what type of phosphor
material is used, the conversion of a relatively small number of
X-ray photons of high energy to a large number of light photons of
low energy is due predominantly to X-ray absorption via the
photoelectric effect in the high Z components of the phosphor.
The incident X-ray photons are
absorbed either totally or partially in the phosphor layer. The
absorbed energy is transferred to electrons which in turn deposit
their energy by ionisation and excitation. The energy added to the
atoms of the phosphor raises the atomic electrons to excited
states. Most of this added energy is then dissipated as heat but a
fraction (5% - 20%) is radiated as electromagnetic radiation in the
visible or near visible wavelengths and it is this radiation which
is utilised in the production of the latent image on the X-ray film.
INTENSIFYING SCREENS
The use of intensifying screens has
three major benefits:
- Reduction of patient dose
- Reduction of tube and generator
loading and
- Reduction of patient motion
artifacts.
However, there is one disadvantage
that is occasionally relevant to radiology which is that the image
clarity is degraded in comparison with a directly exposed film.
Figure 1 gives a schematic of a
typical screen. The thin protective layer provides protection for
the phosphor and can easily be cleaned. In some screens, the
reflecting layer is not included. In a typical situation, two
screens are used, one on either side of a double emulsion film To
compensate for the absorption of some X-rays by the front screen,
the back screen may be thicker than the front screen.
Figure 1:
Cross-section of a typical intensifying screen. 1 micron = 1 mm.
The isotropic emission and
scattering of light photons in the phosphor results in the lateral
diffusion of the scintillation pulse before it escapes the screen.
This results in a loss of resolution or sharpness and becomes
increasingly important as the screen thickness is increased. This
can be compensated for by using light absorbing dyes in the screen
which will preferentially absorb the photons that travel the
greatest distances.
RARE
EARTH SCREENS
We have already noted that the
interaction of diagnostic X-rays with screens occurs primarily via
the photoelectric effect. Therefore we can say that we need our
phosphors to have K-edges appropriately matched to the X-ray photon
energies. More explicitly, this means that we want a phosphor whose
K-edge is between 25 and 50 keV.
You may recall that the
photoelectric effect interaction probability is a maximum at
energies just above the K-edge. A look at Figure 2
establishes that Gd2O2S
has a significant advantage over calcium tungstate for photon
energies between 50 and 70 keV. The same is true of other
rare-earth type screens such as BaSrSO4 to a slightly
lesser extent. It is also useful to note that Gd-based phosphor
screens are more favourably disposed to the detection of primary
radiation than scatter radiation as a greater proportion of the
primary spectrum is above the K-edge of Gd than of the scatter
spectrum.
Figure 2:
Approximate Screen Absorption as a Function of Photon Energy for
pairs of CaWO4,
Gd2O2S
and BaSrSO4
screens. The spectrum from an X-ray tube operated at 80 kVp
with 12.5 cm of perspex as phantom is also illustrated.
Most inorganic phosphors (calcium
tungstate is an exception) do not emit light efficiently unless
doped with a small quantity of activator. For example, the
activator in the rare-earth oxysulphides is terbium (Tb). The
concentration of the activator not only affects the amount of light
emitted but the spectral emission as well. This can be used to
advantage to achieve better spectral matching between the phosphor
and the film response. Certainly, the use of these activators is
the reason for the substantially improved conversion efficiency of
the rare-earth screens compared with the old calcium tungstate
screens.
X-RAY
FILM
The major recording medium used in
radiology is X-ray film - although the situation is changing with
the introduction of new technologies in recent years. The film can
be exposed by the direct action of X-rays, but more commonly the
X-ray energy is converted into light by intensifying screens and
this light is used to expose the film, as described above. The
basic structure of the film is outlined in Figure 3 below.
Figure 3: Cross-section
through a double emulsion film
The film
base provides the structural strength for the film.
However, the base must be flexible for ease of processing,
essentially be transparent to light and be dimensionally stable over
time. Early base materials were glass and cellulose nitrate, but
more recently cellulose triacetate and polyester have been adopted.
A thin layer of adhesive is then
applied to the base and this binds the
emulsion layer. Covering the emulsion is a thin
supercoat that serves to protect
the emulsion from mechanical damage.
The two most important ingredients
of a photographic emulsion are gelatin
and silver halide. With most
X-ray film the emulsion is coated on both sides of the film but its
thickness varies with the nature and type of the film, but is
usually no thicker than 10 mm. Photographic gelatin is made from
bone and is ideal as a suspension medium in that it prevents
clumping of grains. In addition, processing chemicals can penetrate
gelatin rapidly without destroying its strength or permanence.
Silver halide is the light sensitive
material in the emulsion. In X-ray film, sensitivity is increased
by having a mixture of between 1% and 10% silver iodide and 90 to
99% silver bromide. In photographic emulsion the silver halide is
suspended in the gelatin as small crystals (called
grains). Grain size might average
one to 2.3 mm in diameter with up to a billion silver ions per grain
and billions of grains per ml of emulsion. In its pure form the
silver halide crystal has low photographic sensitivity. The
emulsion is sensitised by heating it under controlled conditions
with a reducing agent containing sulphur. This results in the
production of silver sulphide at a site on the surface of the
crystal referred to as a sensitivity speck. It is the sensitivity
speck that traps electrons to begin formation of the latent image
centres.
Silver bromide is cream coloured and
absorbs ultraviolet and blue light, but reflects green and red
light. Historically, this was fine since the principle emission
from calcium tungstate screens is blue light. Films for photography
of image intensifier images and films for use with rare earth
screens need to have their spectral sensitivity broadened to
encompass the longer wavelengths associated with the emissions from
these screens. This is accomplished by the addition of suitable
dyes. Thus, we have green sensitive
orthochromatic film and red sensitive
panchromatic film.
FILM
PROCESSING
Film processing is a multi-stage
process involving development, fixing, washing and replenishment (Figure
4). In development, the exposed grains are preferentially
reduced to black metallic silver. In fixing the remaining unexposed
grains are dissolved so that they can be removed from the emulsion
by washing. Replenishment ensures that chemical balance is
maintained with usage of the processing solutions.
Figure 4: Schematic
of an automatic film processor, showing the pathway followed
by film as it is guided by roller mechanisms through the
processing solutions.
PHOTOGRAPHIC CHARACTERISTICS OF X-RAY FILM
When the X-ray beam passes through
body tissues, variable fractions of the beam will be absorbed,
depending on the composition and thickness of the tissues and the
quality (kVp & filtration) of the beam. The magnitude of this
variation in intensity is the mechanism by which the X-ray beam
emanating from the patient produces diagnostic information. The
information content of this X-ray image must be transformed into a
visible image on the X-ray film with minimal information loss.
In general radiography, the X-ray
image is first converted to a light image using intensifying
screens, which in turn produce a visible pattern of metallic black
silver on the X-ray film. Ultimately, the degree of blackening is
related to the intensity of the radiation reaching the intensifying
screen. The amount of blackness on the film is called the
optical density,
D, which is defined in
Figure 5. For example, if 100 light photons are
incident on a film and only one is transmitted the film density
would be log10(100) or 2. Useful densities in diagnostic
radiology range from about 0.2 to about 2.5. High density means
black films.
Figure 5: The
definition of optical density, D.
If the relationship between the
logarithm of the radiation exposure and the optical density is
plotted we obtain a curve known as the
Characteristic Curve. For film exposed with an
intensifying screen, this curve is essentially sigmoidal in shape (Figure
6). It is characterised by:
- a toe or region of low gradient
at low exposures,
- a region of relatively steep
increase in density for minimal exposure increases, and
- a third relatively flat region
called the shoulder at high exposures.
The important part of the curve
diagnostically is the approximately linear region between the toe
and the shoulder where the density is proportional to the logarithm
of the exposure.
Figure 6: The
Characteristic Curve of X-ray film.
The information content resulting
from the radiograph arises from differences in the film density,
which we can define as radiographic
contrast. Radiographic contrast depends on
subject contrast and
film contrast. For the moment you
should recall that subject contrast depends on the differential
attenuation of the X-ray flux as it passes through the patient and
is affected by thickness, density and atomic number of the
irradiated parts of the subject, the kVp, the presence of contrast
medium and scattered radiation. For example, relatively few X-ray
photons pass through bone compared with soft tissue but care must be
taken in selecting the correct kVp in order to produce an X-ray
image of high information content for the screen-film to record.
That is, the kVp influences the magnitude of the subject contrast.
Film contrast depends on four
factors:
- the characteristic curve of the
film,
- the film density,
- use of intensifying screens or
direct exposure and
- the film processing.
The slope of the straight line
portion of the characteristic curve tells us how much change in film
density will occur as exposure changes. The slope or gradient of
the curve may be measured and the maximum gradient is called the
film gamma, which tells us how
well the film will amplify the subject contrast.
X-ray film will fog slowly with
time, the extent depending markedly on how well it is stored. This
fogging, along with the optical density of the film base, will
generate a low density in the toe section of the Characteristic
Curve.
The shoulder region of the curve
indicates over exposure.
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