photons in the patient do not interfere with the CT scan, because of the
high intensity of the CT photon beam. Fusion of the two sets of images is
performed by using commercial software as discussed under SPECT/CT. As
described later, CT transmission data is used to calculate the attenuation
factors for the PET emission data. The principle of attenuation correction
is the same as described in SPECT/CT, based on the ratios of pixel data of
a blank and the patient’s CT transmission scans. Factors affecting the atten-
uation corrections are discussed later.
Because the position of the patient on the table does not change, both
CT anatomical and PET functional images can be fused with accurate align-
ment providing better accuracy in detection of diseases. The overall accu-
racy of diagnosis increases by 20 to 25% with PET/CT compared to either
modality alone. Because CT scanning is fast, the total scanning time is sig-
nificantly reduced, and the patient throughput is increased. Because of the
increased reimbursement, whole-body imaging with PET/CT for accurate
diagnosis of various cancers has become the standard of practice. The sale
of PET/CT units worldwide has outpaced that of PET scanners alone by
almost nine to one.
Mobile PET or PET/CT
Largely because of the low patient volume and high cost, many community
hospitals cannot afford PET or PET/CT scanners, but can take advantage
of mobile PET or PET/CT that provides PET scanning services to differ-
ent locations. PET or PET/CT scanners and necessary accessories are
installed in sturdy vans, along with nuclear pharmacy facilities. The mobile
unit moves to different clients’ facilities on different days depending on the
schedule. The patient’s schedule and delivery of PET tracers must be well
coordinated to provide efficient services. The owner of the mobile unit must
have a license from appropriate authorities to operate the mobile PET/CT
and also a letter of agreement between the client and the licensee to provide
the service. The van must meet the Department of Transportation’s over-
load regulations, and the rules and regulations of fire safety and security of
local authorities.
Micro-PET
For research animal imaging, clinical PET scanners with a large bore give
poor spatial resolution. Micro PET scanners with a smaller bore and, hence,
smaller in overall size (to be fitted in small rooms) have been developed by
several manufacturers. The typical bore diameter is about 16 cm. The spatial
resolution can be obtained as small as 1 mm with the use of LSO detectors.
These scanners are useful for drug evaluation in animals.
190 13. Positron Emission Tomography