Statistical variations of the count rates give rise to noise that
increases with decreasing information density or count density (counts/cm^2 )
and is given by (1/ ) ×100, where Nis the count density. For a given
imaging setting, a minimum number of counts need to be collected for rea-
sonable image contrast. Even with adequate spatial resolution from the
imaging device, lack of sufficient counts may give rise to poor contrast due
to increased noise, so much so that lesions may be missed. This count density
depends on the amount of activity administered and the uptake in the organ
of interest. Contrast is improved with increasing administered activity and
also with the differential uptake between the normal and abnormal tissues.
However, due consideration should be given to the radiation dose to the
patient from a large amount of administered activity. Sometimes, high count
density is achieved by counting for a longer period of time in the case of
low administered activity. It should be emphasized that spatial resolution is
not affected by the increased count density from increased administered
activity or longer counting.
Background in the image increases with scattered radiations and thus
degrades the image contrast. Maximum scatter radiations originate from
the patient. Narrow PHA window settings can reduce the scatter radiations,
but sensitivity, that is, counting efficiency, is reduced by narrow window set-
tings. A 15–20% PHA window centered on the photopeak of interest is
most commonly used in routine imaging.
As discussed above, at high count rates, pulse pileup can degrade the
image contrast.
Image contrast to distinguish a lesion depends on its size relative to
system resolution and its surrounding background. Unless a minimum size
of a lesion larger than system resolution develops, contrast may not be suf-
ficient to appreciate the lesion, even at higher count density. The lesion size
factor depends on the background activity surrounding it and on whether
it is a “cold” or “hot” lesion. A relatively small-size “hot” lesion can be well
contrasted against a lower background, whereas a small “cold” lesion may
be missed against surrounding tissues of increased activities.
Patient motion during imaging reduces the image contrast. This primar-
ily results from the overlapping of normal and abnormal areas by the move-
ment of the organ. It is somewhat alleviated by restraining the patients or
by having them in a comfortable position.
Quality Control Tests for Gamma Cameras
To ensure high quality of images produced by imaging devices, several
quality control tests must be performed routinely on gamma cameras. The
frequency of tests is daily, weekly, and, for some tests, monthly or even
quarterly. The most common tests are the positioning of the photopeak
N
Quality Control Tests for Gamma Cameras 133