Performance of SPECT Cameras
Performance of SPECT cameras is assessed by evaluating several parame-
ters such as spatial resolution, sensitivity, and contrast, which are discussed
below.
Spatial Resolution
Spatial resolution of a SPECT camera is affected by all the factors discussed
in Chapter 10. Typically, it consists of intrinsic resolution, collimator reso-
lution, and scatter resolution. Because of 3-D orientation, SPECT images
have both axial and transaxial spatial resolutions. The axial spatial resolu-
tion refers to the resolution along the axis of rotation of the camera heads,
whereas the transaxial spatial resolution indicates the resolution across the
transverse FOV perpendicular to the axis. Normally, planar images have
better spatial resolution than SPECT images. Typical values of extrinsic res-
olution for SPECT cameras with low-energy, high-resolution collimators
are 7 to 10 mm at a radius of rotation of 10 cm. Spatial resolution deterio-
rates but sensitivity increases with increasing slice thickness. As a trade-off
between spatial resolution and sensitivity, an optimum slice thickness
should be chosen.
Sensitivity
The sensitivity of an imaging system is always desired to be higher for better
image contrast. All factors that affect the conventional cameras also affect
the SPECT systems in the same manner, as discussed in Chapter 10. The
sensitivity of gamma cameras varies from 200 to 500 kcps/mCi. The SPECT
systems are designed for greater sensitivity so that high counts can be accu-
mulated for images of thin slices of an organ in a reasonable time. For con-
ventional two-dimensional planar images of good contrast, about 500,000
counts are required. Thus, if each sectional image (i.e., slice) of an organ
requires 500,000 counts for the same contrast as in a conventional image,
and if there are, for example, 20 sectional images of an organ of interest,
then 10 million counts would be needed for the entire organ. For most
SPECT systems using low-energy all-purpose collimators, 5 to 20 million
counts are acquired. Total counts may be increased by either counting for
a longer period or by administering more activity. However, long counting
is inconvenient for the patient and administering a larger amount of activ-
ity increases the radiation dose to the patient.
Other Parameters
Other important parameters to evaluate include effects of high-count rates,
uniformity, and contrast of images. These parameters have been discussed
Performance of SPECT Cameras 177