monly used collimators, the most preferable photon energy being 150 keV.
At energies below ~50 keV, photons are absorbed in the body tissue,
whereas at energies above ~300 keV septal penetration of the photons
can occur. Current collimators are made with appropriate septal thickness
for specific photon energies to limit septal penetration. Parallel-hole colli-
mators are classified as low-energy collimators with a few tenths of a milli-
meter septal thickness (for up to 150-keV g-rays) and medium-energy
collimators with a few millimeter thickness (up to 400-keV photons)
(Cherry et al., 2003). Currently very high energy collimators are available
for counting 511-keV photons. It is understandable that for a given diame-
ter collimator, the number of holes are greater in low-energy collimators
than in high-energy collimators. Normally, high-energy collimators have
poorer efficiency and resolution than low-energy collimators.
In another classification, collimators are termed high-sensitivity and high-
resolution collimators. Often, these collimators are made with an identical
number of holes with identical diameters but with different thicknesses.
Thus, the collimator with longer holes is called the high-resolution colli-
mator and that with shorter holes is called the high-sensitivity collimator.
The spatial resolution for the high-sensitivity collimator deteriorates
sharply with the source-to-collimator distance. All-purpose, or general-
purpose, collimators are designed with intermediate values of resolution
and sensitivity. Typical values of resolution (FWHM at 10 cm) are about
8 mm for low-energy high-resolution and all-purpose parallel-hole collima-
tors and about 13 mm for high-sensitivity collimators.
The collimator resolution for pinhole, diverging, and converging colli-
mators is expressed by similar but somewhat complex equations, and their
details are available in reference books on nuclear physics and instrumen-
tation. For pinhole and converging collimators, best resolution is obtained
when the object is at the focal plane. The overall system resolutions of
different collimators are illustrated in Figure 10.2. Fan-beam collimators
give better spatial resolution but poorer sensitivity than parallel-hole
collimators.
Scatter Resolution
Radiations are scattered by interaction with tissue in patients and with the
detector. It is possible that some of these radiations are scattered without
much loss of energy and fall within the field of view, resulting in pulses of
acceptable amplitude set by pulse-height analyzer (PHA). This degrades
the overall spatial resolution. This component is called the scatter resolu-
tion (Rs) and depends on the composition of the scattering medium, the
source configuration, and PHA discriminator settings. Scatter increases in
heavy patients, and decreasing the PHA window reduces the scatter con-
tribution. The effect of scatter resolution is essentially the same for all col-
limators (Rollo and Harris, 1977).
Spatial Resolution 121