spatial resolution [see Eq. (10.2)]. Thus,for a given collimator, as the spatial
resolution of a system increases, its sensitivity decreases, and vice versa. Note
that collimator efficiency,Eg, for parallel-hole collimators is not affected by
the source-to-detector distance for an extended planar source; that is, it
essentially remains the same at different distances from the detector. The
low-energy all-purpose (LEAP) parallel-hole collimators have efficiencies
of about 2 × 10 −^4. Collimator efficiency varies with different types of colli-
mators, and the values are shown as a function of source-to-collimator dis-
tance in Figure 10.8.
Uniformity
It is always expected that a gamma camera should yield a uniform response
throughout the field of view. That is, a point source counted at different
locations in the field of view should give the same count rate by the detec-
tor at all locations. However, even properly tuned and adjusted gamma
cameras produce nonuniform images with count density variations of as
much as 10%. Such nonuniformity adds to the degradation of the spatial
resolution of the system.
The nonuniformity in detector response arises from several factors: (a)
variations in PM tube response, (b) nonlinearity in X,Y-positioning of
pulses across the field of view, and (c) edge packing. Although factor (c) is
preventable as discussed below, factors (a) and (b) are the leading causes
of deterioration in uniformity and special attention is needed to remedy
them.
Pulse-Height Variation
As already mentioned in Chapter 8, there are variations in the light pro-
duction from g-ray interaction in the detector, light transmission to PM
tubes, and in the pulse formation in PM tubes. These variations result in
photopeaks of different amplitude. Because there are many PM tubes
across the detector, the amplitude of the photopeaks will vary from loca-
tion to location (i.e., from PM tube to PM tube) across the detector even
with a fixed energy window. These variations in PM tube response con-
tribute significantly to the nonuniformity of the detector response.
This part of nonuniformity is corrected by acquiring an image using a
well-tuned gamma camera and a 99mTc point source that is placed at a dis-
tance of five times the detector’s FOV. An intrinsic image (without the col-
limator) is acquired using the appropriate window and stored in a 128 × 128
matrix. The pulse height in each (X,Y) pixel is determined and stored in a
128 ×128 look-up table. In subsequent patient studies, a microprocessor
compares the pulse height in each pixel of the patient image with the cor-
responding value in the look-up table, and then either moves the energy
Uniformity 129