1 Paleoradiology: History and New Developments - Academia.dk

material strips). The strips are arranged to transmit
only the x-rays directed toward the receptor that have
not been scattered. One disadvantage of using a grid
for medical applications is increased x-ray dose to the
patient, as a higher tube currents and exposure times
are required to make up for the x-rays lost to the grid.
A disadvantage applicable to bioarcheological applications
is the production of grid lines on the radiographic
film, caused by the absorption of x-rays by the
grid. This is a shortcoming that can be minimized by
use of a reciprocating grid, which moves back and
forth rapidly throughout the x-ray exposure, thereby
decreasing grid lines (Bushong 2004).
2.2.4.2
Radiographic Film
An image receptor is any medium that converts incident
x-rays into an image. Film remains the most
commonly used image receptor in radiography, although
it is largely being replaced by computed and
digital radiography (CR and DR, respectively) in the
hospital environment (see sections 2.4.1 and 2.4.2, respectively).
Most radiographic film consists of a base,
which causes the film to be rigid, and an emulsion
layer on both sides. The emulsion layer is a mixture
of gelatin and silver halide crystals; this is the part of
the film that creates the image. The main purpose of a
dual emulsion film is to limit patient dose, a consideration
less important for bioarcheological specimens.
Most radiographic films are used in conjunction with
an intensifying screen, a sheet of crystals of inorganic
salts (phosphors) that emit fluorescent light when excited
by x-rays. This serves to intensify the effect of
x-rays during exposure of the radiographic film. Figure
2.6 is a schematic cross-section of a screen-film
image receptor. For portability and durability, these
are usually permanently mounted in cassettes (Bushong
2004).
When selecting a film-screen combination for bioarcheological
radiography, it is important to consider
the film speed. The faster the speed of the film, the
thicker it will be, allowing for improved x-ray absorption
and reduction in the necessary x-ray dose. However,
this benefit is not as critical for bioarcheological
specimens and it comes at the expense of resolution,
therefore slow speed (thinner) film is optimal. Other
important parameters to consider are single-emulsion
films, to maximize resolution, and uniform small
crystal size in the intensifying screen to provide high
contrast and maximum resolution.
2.2 Radiographic Production
Fig. 2.6. Simplified cross-section of a screen-film image receptor
system. The phosphor screens serves to absorb x-rays and
emit visible light photons, which is recorded on the film emulsions.
The emulsion-base-emulsion layers comprise the film
Proper handling and storage of radiographic film
is very important. Film should be kept free of dirt,
and bending and creasing films should be strictly
avoided. The film is sensitive to light and radiation,
so it must be stored and handled in the dark, away
from sources of radiation, such as the x-ray imaging
system. The storage area should also be dry and cool,
preferably less than 20°C (68 F).
2.2.5
Geometry Factors
The geometric arrangement of x-ray equipment is an
important determinant for image quality. Geometry
factors include the size of the focal spot and its distance
from the object and image receptor.
2.2.5.1
Focal Spot Size
Most general x-ray tubes are equipped with a small
and a large focal spot. Recall from section 2.2.1 that
the focal spot is the x-ray source, the area on the anode
where the electron beam interacts to produce xrays.
A small focal spot provides greater image detail
than its large counterpart because it casts the smallest
penumbra, which is the area of blur at the edge of
the image (Schueler 1998) (Fig. 2.7). One might wonder
when a large focal spot would be ever required.
It is used because the greater surface area for x-ray
production minimizes heat production and the risk
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