Effects of High Counting Rates
As discussed in Chapter 8, the scintillation cameras suffer count losses at
high counting rates due to pulse pileup. Pulse pileup results from the detec-
tion by the camera of two events simultaneously as one event with ampli-
tude different from that of either original event. If one or both of the events
are photopeaks, then the amplitude of the new event will be outside the
pulse-height window setting and so the event will be rejected resulting in a
loss of counts. If, however, two Compton scattered photons are processed
together to produce an event equivalent to the photopeak in amplitude,
then the event will be counted within the window setting. But the X,Y posi-
tion of the event will be misplaced on the image somewhere between the
locations of the two events. This causes image distortion. Both count rate
loss and image distortion at high count rates must be taken into consider-
ation in evaluating the performance of different cameras.
Several techniques are employed to improve the high count rate
performance of a gamma camera. In modern cameras, buffers (or deran-
domizers) are used in which pulses are processed one at a time, and over-
lapping events are kept on “hold” until the processing of the preceding
event is completed. In some cameras, the dead time is shortened by short-
ening the time to integrate the signals from the PM tubes. Other cameras
use pulse pileup rejection circuits to minimize the count loss and image dis-
tortion and thus to improve images, although they tend to increase the dead
time of the camera. Recent developments include high-speed electronics
that reduce the number of misplaced events and improve the image quality
significantly.
Contrast
Contrast of an image is the relative variations in count densities between
adjacent areas in the image of an object. Contrast (C) gives a measure of
detectability of an abnormality relative to normal tissue and is expressed
as
(10.8)where Aand Bare the count densities recorded over the normal and abnor-
mal tissues, respectively.
Lesions on the image are seen as either “hot” or “cold” spots indicating
increased or decreased uptakes of radioactivity in the corresponding areas
in the object. Several factors affect the contrast of the image, namely, count
density, scattered radiation, pulse pileup, size of the lesion, and patient
motion, and each contributes to the contrast to a varying degree.
C
AB
A
=
−
132 10. Performance Parameters of Gamma Cameras