Efforts are made to co-register the two sets of images, in which the matrix
size, voxel intensity, and rotation are adjusted to establish one-to-one spatial
correspondence between the two images. Various techniques of such align-
ment are employed, and co-registered images are displayed side by side
with a linked cursor indicating spatial correspondence, or may be overlaid
or fused using the gray or color scale. The major drawback of these align-
ment techniques arises from positional variations of the patient scanned on
different equipment and at different times. Furthermore, patient motion,
voluntary or involuntary, adds to the uncertainty in the co-registration.
Even with the sophisticated algorithm, a misalignment of 2 to 3 mm is not
uncommon.
To overcome the problem of positional variations in alignment of images
from different equipment, a dual-modality system has been introduced, in
which a SPECT camera and a CT scanner are combined into a single system
for imaging the patient in the same clinical setting. Both units are mounted
on the same gantry, with the SPECT camera in the front and the CT scanner
in the back, and use a common imaging table (Fig. 12.15). The two units are
mounted fixed, therefore the centers of the scan fields of SPECT and CT
scanners are separated by a fixed distance, called the displacement distance.
The axial travel range of the scanning table varies with different designs of
the manufacturers. The scan field is limited by the maximum travel range
of the table minus the displacement distance.
The details of CT scanners are found in standard reference books on CT
and are not given here. The CT scanner consists of an x-ray tube that pro-
jects an intense beam of x-rays (of energy ~70 to 140 keV) through the
170 12. Single Photon Emission Computed Tomography
Fig. 12.15. A SPECT/CT camera, Symbia model (Courtesy of Siemens Medical
Solutions, Hoffman Estates, IL).