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to detect photons, but with differentiation that the detector is designed for the
detection of X-ray spectral wavelength.
There are a few fundamental and characteristic differentiating factors between
the creation of images in photography as compared to radiography. One primary
difference, again, is the variation in the characteristic frequency of the radiation
being used in the image formation process. A corollary of this follows, in that
photons in photography re
ect from the surface of the object being recorded, and
are absorbed by the detector in the camera unit, creating an image of the subject
fl
s
surface. X-rays photons, on the other hand, with higher frequency and energy than
optical photons, pass through the tissue of the body more easily, with a lower
proportion of re
'
fl
ected photons. Thus, the X-ray photons which are not scattered,
re
ected, or absorbed, pass through the patient and are absorbed by a detector plate
placed on the opposite side of the patient from the X-ray source. The image formed
as a result of this process is thus a transmission image.
The X-ray photons passing through the patient in this transmission imaging
process penetrate through a variety of body tissues, unique to each photon pathway.
There are varying X-ray attenuation factors of the internal anatomic tissues of the
patient, and structures of higher attenuation (as in the case of bones, for instance)
preferentially attenuate the beam, while lower attenuation structures (such as the
lung) allow a higher proportion of photons to pass. The result is an image created
by the variant pathways of the individual photons and so variant density combi-
nations of tissue through which the photon passed in its beamline to the detector.
Each density stack is formed by a unique cumulative superposition of the anatomy
encountered, or equivalently a unique total attenuation, illustrated in Fig.
1
. The
resultant radiographic image consists of multiple boundary shadows created by the
internal anatomy tissue planes. A simpli
fl
ed analogy to this phenomenon encoun-
tered in everyday life is the shadowing of light on a wall created by intervening
structures in a room. This X-ray beam attenuation occurs as a result of
five fun-
damental tissue densities composing the body, and in so doing scales the brightness
of the resulting images and delineates the internal anatomic structures. The usual
highest naturally-occurring tissue density and correlated highest X-ray beam
attenuation is due to calci
cation or bone. Muscle density follows this, and then in
descending order,
fl
fluid. Lipoid, or fatty tissue, continues on the decreasing density
scale,
finally reaching the low end of the density and beam attenuation spectrum
with air or gas [
1
,
2
]. Some of these densities are shown in Fig.
2
. Metallic
appliances such as orthopedic and dental hardware demonstrate higher beam
attenuation than any naturally occurring tissue, Fig.
2
. Again, resulting
final image
is thus a map of the
of internal anatomy created by density differences
between organs and internal structures of the body created by the transmitted
photons.
In areas where no signi
“
edges
”
cant different in density difference exists between
adjacent normal tissue structures or between normal tissue and pathology, these
structure may be not be con
dently distinguishable as unique entities, and so may
necessitate alternative modalities of visualization. Additionally, since three-
dimensional objects are collapsed into two-dimensional data by the imaging process
