Biomedical Engineering Reference
In-Depth Information
FIGURE 15.6
(a) Multihole collimator used in conjunction with scintillation detector. Event x is represented at
only one location in the crystal, whereas event y has multiple sites associated with its occurrence. (b) The pinhole
collimator allows magnification and is used for viewing small organs at short range.
Consider the multihole collimator shown in Figure 15.6a. In this case, the collimator
consists of a flat lead plate through which narrow holes are drilled. As can be seen, only
a gamma event occurring directly under each hole will penetrate the collimator, and it will
be represented at only one location on the face of the crystal. If this gamma event occurred
much farther away from the collimator, such as in region
, then it would be represented by
more than one location on the face of the crystal. When this occurs, the resolution, which
may be defined as the ability of the detector to distinguish between two sources at various
distances from the collimator, is greatly decreased. For the multihole collimator, then, the
best resolution occurs when the area of interest is close to the collimator. Thus, as the sub-
ject is moved from point
Y
toward the detector, the resolution is improved. Obviously,
when viewing an organ that lies beneath the surface of the skin, it becomes quite important
to closely approximate its distance from the probe relative to the degree of resolution
required.
There are also other types of collimators—for example, the pinhole collimator shown in
Figure 15.6b. The pinhole collimator permits entry of only those rays aimed at its aperture.
These gamma rays enter the collimator and proceed in a straight line to the crystal, where
they are detected in inverted spatial correspondence to their source. When the source is
located at a distance from the collimator equal to that of the pinhole to the crystal, then
the source is represented on the crystal in exactly the same size as it exists. However, by
proper positioning of the subject, it is possible to actually magnify or decrease the field of
view of the detector. That is, as the source is moved closer to the aperture of the collimator,
magnification occurs. A pinhole collimator, therefore, enlarges and inverts the image of the
source located beneath it.
From this brief discussion, it can be seen that collimators in conjunction with a scintilla-
tion detector essentially “focus” the radioactivity occurring at a particular point in space
within the organ to a particular point on the surface of the crystal. The radiation passing
through the collimator then impinges on or interacts with the scintillation crystal.
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