Biomedical Engineering Reference
In-Depth Information
tomography are higher than routine radiographs, and because the exposures are longer,
patient motion may degrade the image content.
Computerized tomography represents a completely different approach. Consisting pri-
marily of a scanning and detection system, a computer, and a display medium, it combines
image-reconstruction techniques with x-ray absorption measurements in such a way as to
facilitate the display of any internal organ in two-dimensional axial slices or by reconstruc-
tion in the Z axis in three dimensions. The starting point is quite similar to that used
in conventional radiography. A collimated beam of x-rays is directed through the section
of body being scanned to a detector that is located on the other side of the patient
(Figure 15.7c). With a narrowly collimated source and detector system, it is possible to send
a narrow beam of x-rays to a specific detection site. Some of the energy of the x-rays is
absorbed, while the remainder continues to the detector and is measured. In computerized
tomography, the detector system usually consists of a crystal (such as cesium iodide or
cadmium tungstate) that has the ability to scintillate or emit light photons when bombarded
with x-rays. The intensity of these light photons or “bundles of energy” is in turn measured
by photodetectors and provides a measure of the energy absorbed (or transmitted) by the
medium that is penetrated by the x-ray beam.
Since the x-ray source and detector system are usually mounted on a frame or “scanning
gantry,” they can be moved together across and around the object being visualized. In early
designs, for example, x-ray absorption measurements were made and recorded at each
rotational position traversed by the source and detector system. The result was the genera-
tion of an absorption profile for that angular position. To obtain another absorption profile,
the scanning gantry holding the x-ray source and detector was then rotated through a small
angle and an additional set of absorption or transmission measurements was recorded.
Each x-ray profile or projection obtained in this fashion is basically one-dimensional. It is
as wide as the body but only as thick as the cross section.
The exact number of these equally spaced positions determines the dimensions to be
represented by the picture elements that constitute the display. For example, in order to gener-
ate a 160 by 160 picture matrix, absorption measurements from 160 equally spaced positions
in each translation are required. It will be recalled, however, that each one-dimensional array
constitutes one x-ray profile or projection. To obtain the next profile, the scanning unit is
rotated a certain number of degrees around the patient, and 160 more linear readings are taken
at this new position. This process is repeated again and again until the unit has been rotated a
full 180
. When all the projections have been collected, 160
180, or 28,800, individual x-ray
intensity measurements are available to form a reconstruction of a cross section of the patient's
head or body.
At this point the advantages of the computer become evident. Each of the measurements
obtained by the preceding procedure enters the resident computer and is stored in memory.
Once all the absorption data have been obtained and located in the computer's memory, the
software packages developed to analyze the data by means of image-reconstruction algo-
rithms are called into action. These image reconstruction techniques, which are based on
known mathematical constructs developed for astronomy, were not used routinely until
the advent of the computer because of the number of computations required for each recon-
struction. Modern computer technology has made it possible to fully exploit these reconstruc-
tion techniques.
