from the photocathode and dynodes in the photomultiplier (PM) tubes
have significant effects on the intrinsic resolution. In gamma cameras, the
X,Y positioning of the pulses is improved by increasing the number of PM
tubes, thus improving the intrinsic resolution. Also, PM tubes with greater
quantum efficiency and their improved optical coupling to the detector for
greater light collection provide better intrinsic resolution.
Intrinsic resolution improves with higher g-ray energy and deteriorates
with lower energy because greater statistical fluctuations occur in the
production of light photons by lower energy photons and vice versa. For
example, the 140-keV photons of 99mTc produce almost twice as many light
photons in the detector as the 69- to 80-keV photons of^201 Tl and thus result
in better intrinsic resolution. However, there is little improvement in intrin-
sic resolution with photon energy above 250 keV because of multiple scat-
tering of photons within the detector that can result in photoelectric
absorption (see below). Intrinsic resolution improves with narrow PHA
window settings, because scattered radiations are avoided.
Multiple Compton scattering of a g-ray photon followed by absorption
of all scattered photons in the detector causes uncertainty in the X,Y loca-
tion of the original g-ray interaction and makes the intrinsic resolution, and
hence spatial resolution, worse. This effect is worse with thicker detectors
and high-energy photons (>250 keV) because of the increased chances of
multiple scattering. For this reason, only thinner detectors (0.63–1.84 cm)
are used in gamma cameras.
Most modern cameras have intrinsic resolution of the order of 4-mm full
width at half maximum (FWHM) for 140-keV photons of 99mTc.
Collimator Resolution
Collimator resolution, also termed the geometric resolution (Rg), constitutes
the major part of the overall spatial resolution and primarily arises from
the collimator design. In general, collimator resolution is worse than intrin-
sic resolution. As already mentioned in Chapter 9, there are four major col-
limators: parallel-hole, pinhole, converging, and diverging. Of these,
parallelhole collimators are most commonly used in nuclear medicine.
The different parameters of a typical parallel-hole collimator are shown
in Figure 10.1. The spatial resolution for this collimator is given by the geo-
metric radius of acceptance,Rg:
(10.2)whered is the hole diameter of the collimator,b is the distance between
the collimator face and the source of radiation,c is the distance between
the back face of the collimator and the midplane of the detector, and te
is the effective length of the collimator holes. The teis empirically given by
te=t − 2 m−^1 , where mis the linear attenuation coefficient of the photons in
R
dt b c
t
ge
e=
()++
Spatial Resolution 119