Biomedical Engineering Reference
In-Depth Information
resolution. The density resolution reflects the sensitivity of the method to tissue change,
while spatial resolution indicates the fineness of detail possible in the image produced by
the CT scanner. These two characteristics of resolution are related to each other and to
the radiation dose received by the patient. The clarity of the picture and thus the accuracy
with which one can measure absorption values are often impaired by a low signal-to-noise
ratio that is caused by a reduction in the number of photons arriving at the detectors after
penetrating the body. Fewer available photons, in turn, result in a reduction in the sharp-
ness of the image, since the resultant differences between the absorption coefficients of adja-
cent pixels are reduced. This is a situation that must be accepted as long as low-radiation
doses are used. Despite this limitation, the reconstructed image can be enhanced by “display”
units that constitute another major component of the CT system.
Display units usually have the capacity to display a given number (16, 32, etc.) of differ-
ent colors or gray levels at one time. These different colors (or gray levels) can be used to
represent a specific range of absorption coefficients. In this way, the entire range of absorp-
tion values in the picture can be represented by the entire range of available gray tones or
colors (or both). To sharpen the resolution or detail of the display, the range of gray tones
between black and white can be restricted to a very small part of the scale. This process
of establishing a display “window” that can be raised or lowered actually enhances the
imaging of specific organs. For example, if an image of the tissue of the heart is desired, this
window is raised above water density, but if an image of the detail in the lung is desired, it
is lowered. The overall sensitivity can be further increased by reducing the window width.
This permits ever-greater differentiation between absorption coefficients of specific organs
and the surrounding tissue.
Various other techniques are available to make the picture more easily interpretable.
For instance, a portion of the picture can be enlarged, or certain absorption values can be
highlighted or made brighter. These methods do not add to the original information but
enhance the image made possible by the data available in the computer's memory. Since
a human observer can see an object only if there is sufficient contrast between the object
and its surroundings, enhancement modifies the subjective features of an image to increase
its impact on the observer, making it easier to locate and precisely measure obscure details.
There are a number of distinct advantages of CT scanning over conventional x-ray tech-
niques. For example, computerized tomography provides three-dimensional information
concerning the internal structure of the body by presenting it in the form of a series of slices
(Figure 15.11). Because of its contrast sensitivity, it can show small differences in soft tissue
clearly, which conventional radiographs cannot do. It accurately measures the x-ray absorp-
tion of various tissues, enabling the nature of these tissues to be studied, and yet the
absorbed dose of x-rays given to the patient by CT scanning is often less than the absorbed
dose by a conventional x-ray technique such as tomography.
In comparing the CT approach with conventional tomography, which also images a slice
through the body by blurring the image from the material on either side of the slice, it
becomes apparent that in the tomographic approach, only a thin plane of the beam pro-
duces useful information as the slice to be viewed. The remainder of the beam, which is
by far the larger component, passes through material on either side of the slice, collecting
unwanted information that produces artifacts in the picture. On the other hand, the x-ray
beam in computerized tomography passes along the full length of the plane of the slice,
thereby permitting measurements to be taken that are 100 percent relevant to that slice
