Collimator
In gamma cameras, a collimator is attached to the face of the NaI(Tl) detec-
tor to limit the field of view so that g-radiations from outside the field of
view are prevented from reaching the detector. Collimators are normally
made of material with high atomic number and stopping power, such as
tungsten, lead, and platinum, among which lead is the material of economic
choice in nuclear medicine. They are designed in different sizes and shapes
and contain one or many holes to view the area of interest.
Collimators are primarily classified by the type of focusing, although
other classifications are also made based on septal thickness and the
number of holes. Depending on the type of focusing, collimators are classi-
fied as parallel-hole, pinhole, converging, and diverging types; these are
illustrated in Figure 9.3.Pinhole collimators are made in conical shape with
a single hole and are used in imaging small organs such as the thyroid glands
to provide magnified images.Converging collimators are made with tapered
holes converging to an outside point and are employed to provide magni-
fied images when the organ of interest is smaller than the size of the detec-
tor. Images are magnified by converging collimators.Diverging collimators
are constructed with tapered holes that are divergent from the detector face
and are used in imaging organs such as lungs that are larger than the size
of the detector. The images are minified with these collimators.Parallel-hole
collimators are made with holes that are parallel to each other and per-
pendicular to the detector face and have between 4000 and 46,000 holes
depending on the collimator design. These collimators are most commonly
used in nuclear medicine procedures and furnish a one-to-one projected
image. Because pinhole and converging collimators magnify and the diverg-
ing collimators minify the image of the object, some distortion occurs in
images obtained with these collimators. Because LFOV cameras are readily
available now, diverging collimators are not used in routine nuclear medi-
cine studies.
Gamma Cameras 111Fig. 9.3. Different designs of collimators.