Biomedical Engineering Reference
In-Depth Information
the inner, tightly bound electrons of an atom. The
Compton coefficient, by contrast, is relatively in-
dependent of the atomic number of the tissue atoms,
because x rays Compton-scatter almost exclusively off
the outer, loosely bound atomic electrons. Both co-
efficients increase linearly with the tissue density. To
achieve contrast between soft tissues that differ only
slightly in Z, one must use low-energy x rays, because
they interact predominantly by the photoelectric effect.
An example is in mammography, which employs x rays in
the range of 15-30 keV. For chest x-ray images, which
involve tissue of greater intrinsic contrast, clinicians use
x rays with energies ranging from 50 to 150 keV.
X-ray images represent a combination of four kinds of
resolution: spatial, contrast, temporal, and statistical.
Improving any one of these resolution factors degrades
one or more of the others. The compromise among them
represents a balance such that no single factor dominates
the degradation of the image.
detector. This type of projection discards a lot of in-
formation about tissue variation along the beam direction.
For many years, techniques of analog tomography were
used in attempts to overcome this limitation, but they
were restricted to certain applications, and images were
difficult to interpret.
An important development was achieved in 1972 with
the introduction of x-ray transmission computed to-
mography (CAT). This technique was brought to clinical
medicine through the effort of Godfrey Hounsfield and
Allan Cormack, who shared the 1979 Nobel Prize in
medicine.
Consider a highly collimated x-ray pencil beam in the
plane of a slice of the body only a few millimeters thick.
X rays transmitted all the way through the slice are
measured with a collimated detector on the opposite side
of the patient. The signal from the x-ray detector is
converted to digital output. The tight collimation of
source and detector prevents scattered radiation from
degrading image contrast. The number of x-ray photons
recorded by the detector at one position constitutes
a single pencil-beam projection of x-ray transmission data
at a specific angle through the tissue slice. This process is
repeated many times at slightly different angles to create
a set of multiple projections of the entire tissue slice
(
Figure 6.2-20
).
6.2.5.1 Sectional imaging
A limitation of conventional planar x-ray imaging is the
projection of a three-dimensional distribution of atten-
uation coefficients as a shadow onto a two-dimensional
a
b
X
X
X
X
X
Tissues
Tissues
D
D
D
D
D
DD
D
D
D
c
d
X
X
Tissue
Tissue
Figure 6.2-20 Evolution of geometries of x-ray CT scanners.
