Biomedical Engineering Reference
In-Depth Information
located at points within a 3D Cartesian grid whose geometry is
accurate to within the mechanical accuracy of the scanner, namely at
the sub-millimeter level. This spatial accuracy is of particular value
in designing radiation treatments as the beams need to be located in
space relative to the anatomy to within millimeters and, in certain
cases, to within a fraction of a millimeter.
The quantitative information which computed tomography provides is
of great use in computing the dose distribution within a patient for a
given radiation beam insofar as the influence of tissue heterogenei-
ties is concerned. However, the CT numbers cannot be used directly
because they are measured at kilovoltage energies while therapeutic
X-rays are in the megavoltage range. As the X-ray absorption
coefficient varies with the X-ray energy, the CT values need to be
corrected for the energy
difference. This is mainly
a problem for bone and,
secondarily, for fat.
3
The
practical solution is to use
a look-up table, based on
empirical
measurements,
which
converts
from
Hounsfield numbers (the
unit given to the CT
values) to megavoltage X-
ray absorption coefficients
relative to water, as seen in
Figure 3.5.
Figure 3.5. CT scanner calibration curve
for 135 kVp X-rays. Figure based on
data from Battista (1980).
The interpretation of CT images
A discussion of the clinical interpretation of CT or any other images
is beyond the scope of this topic and beyond the competence of its
author. However a few points are in order.
3
Bone contains a high proportion of calcium which, because of its higher
atomic number, causes photoelectric collisions more frequently than do
soft tissues. As the probability of a photoelectric collision is highly
energy dependent, decreasing roughly as the fourth power of energy as
energy increases, the X-ray absorption coefficient of bone is also energy
dependent - much more so than soft tissues. Fat is more hydrogenous than
most other soft tissues and so, too, has an anomalous effective atomic
number.
