Biomedical Engineering Reference
In-Depth Information
15.4.1 Basic Concepts
All x-ray imaging systems consist of an x-ray source, a collimator, and an x-ray detector.
Diagnostic medical x-ray systems utilize externally generated x-rays with energies of
20-150 keV. Since the turn of the twentieth century, conventional x-ray images have been
obtained in the same fashion: by using a broad-spectrum x-ray beam and photographic
film. In general, x-rays are produced by a cathode ray tube that generates a beam of x-rays
when excited by a high-voltage power supply. This beam is shaped by a collimator and
passes through the patient, creating a latent image in the image plane. Depending on the
type of radiographic system employed, this image is detected by x-ray film, an image inten-
sifier, or a set of x-ray detectors. Using the standard film screen technique, x-rays pass
through the body, projecting an image of bones, organs, air spaces, and foreign bodies onto
a sheet of film (Figure 15.7). The “shadow graph” images obtained in this manner are the
results of the variations in the intensity of the transmitted x-ray beam after it has passed
through tissues and body fluids of different densities.
This technique has the advantages of offering high-resolution, high-contrast images
with relatively small patient exposure and a permanent record of the image. On the other
hand, its disadvantages include significant geometric distortion, inability to discern depth
information, and incapability of providing real-time imagery. As a result, conventional
radiography is the imaging method of choice for such tasks as dental, chest, and bone imag-
ery. Since bone strongly absorbs x-rays, fractures are readily discernible by the standard
radiographic technique. When this procedure is used to project three-dimensional objects
into a two-dimensional plane, however, difficulties are encountered. Structures represented
on the film overlap, and it becomes difficult to distinguish between tissues that are similar in
density. For this reason, conventional x-ray techniques are unable to obtain distinguishable/
interpretable images of the brain, which consists primarily of soft tissue. In an effort to over-
come this deficiency, attempts have been made to obtain shadow graphs from a number of
different angles in which the internal organs appear in different relationships to one
another and to introduce a medium (such as air or iodine solutions) that is either translu-
cent or opaque to x-rays. However, these efforts are usually time-consuming, sometimes
difficult, sometimes dangerous, and often just not accurate enough.
In the early 1920s, another x-ray technique was developed for visualizing three-dimensional
structures. With this technique, known as plane tomography, the imaging of specific planes
or cross sections within the body became possible. In plane tomography, the x-ray source is
moved in one direction, while the photographic film (which is placed on the other side of the
body and picks up the x-rays) is simultaneously moved in the other direction (Figure 15.7b).
The result of this procedure is that while the x-rays travel continuously, changing paths
through the body, each ray passes through the same point on the plane or cross section of
interest throughout the exposure. Consequently, structures in the desired plane are brought
sharply into focus and are displayed on film, whereas structures in all the other planes are
obscured and show up only as a blur. Such an approach is clearly better than conventional
methods in revealing the position and details of various structures and in providing three-
dimensional information by such a two-dimensional presentation. There are, however, lim-
itations in its use. First, it does not really localize a single plane, since there is some error in
the depth perception obtained. Second, large contrasts in radiodensity are usually required
in order to obtain high-quality images that are easy to interpret. In addition, x-ray doses for
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