PET/CT Scanners 189PET/CT Scanners
Parallel to SPECT/CT systems, PET/CT has been developed, in which func-
tional PET images are fused with anatomical CT images to provide more
accurate diagnosis of human diseases. In a PET/CT unit, both scanners are
mounted on a common gantry with the CT unit in the front and PET unit
in the back attached to the CT unit. Typically, this is opposite to the
SPECT/CT arrangement in which the SPECT camera is placed in the front
and the CT unit in the back. Both units use the same scanning table. A
commercial PET/CT is shown in Figure 13.5. As in SPECT/CT, the centers
of the PET FOV and CT FOV are separated by a fixed distance, called the
displacement distance. The horizontal travel range of the scanning table
varies with the designs of the scanners. The actual scan field is the maximum
travel range of the scanning table minus the displacement distance. Typi-
cally, a CT scan is taken first of the patient on the scanning table and in the
CT scan field. The table with the patient in the same position is moved to
the PET scan field and imaging is performed. Note that^18 F-FDG was
administered 60 to 90 minutes prior to scanning, and low-intensity 511-keV
Fig. 13.5. A commercial PET/CT scanner, Biograph 16. (Courtesy of Siemens
Medical Solutions, Hoffman Estates, IL.)