Corrections for Scanner-Related Factors - Correction Techniques in Emission Tomography - page 113

point source positioned at a distance of 4{5 times the gamma camera head di-
ameter. Extrinsic ood-eld images are acquired with the collimator in place
using a disk or thin flood phantom that covers the area of the detector.
A number of non-uniformity patterns is typically observed. One of them is
called edge-packing, and is related to geometric factors enhancing the PMT
output for edge portions of the gamma camera head. For this reason a distinc-
tion is made between the total field of view (TFOV) and the useful field of
view (UFOV), where the latter excludes the edge region of the gamma camera
head (typically 5 cm). Since the PMT response to incoming radiation is the
main source for image non-uniformities, the uniformity is energy dependent.
Geometric sources of image non-uniformities may be compensated for by
the use of appropriate lookup tables, based on high statistic scans that are
acquired 2{4 times per year by the user. In practice, image non-uniformity is
dealt with in a regular quality control program. Acceptable values are <3%
in the UFOV.
Per point 2 above, misalignment between the electronic and mechanical
axes of rotation will result in artifact generation and image degradation dur-
ing reconstruction of projections obtained from one or more rotating gamma
camera heads. Image reconstruction in which the center of rotation (COR)
error for a given slice is larger than 0.5 pixels will lead to image degradation
and artifacts [5]. Since alignment of these axes is assumed during image recon-
struction, possible (mechanically induced) misalignments need to be corrected
for. Most SPECT cameras inhere a quality control measurement that allows
for an evaluation of the presence and magnitude of a possible misalignment.
If the electronic and mechanical axes of rotation are aligned, then a single
COR measurement is applicable for the complete FOV, but for verification of
the alignment multiple (4{5) point sources need to be measured under various
angles. The consistency of the resulting linograms can be assessed. Correc-
tions for small misalignments are implemented in image reconstruction by
shifting certain projection profiles before incorporation of the data into the
reconstruction process.
Per point 3 above, whole body gamma camera imaging can usually be
done in one run, given that a typical gamma camera head measures 50 x 40
cm. Whole body imaging makes uses of an electronic opening and closing of
the gamma camera head FOV as well as physical movement of the table. The
various actions need to be timed correctly, and the velocities need to be tuned
such that for a constant radioactivity concentration on the table a constant
radioactivity concentration is visible in the reconstructed whole body image.
By placing a flood source on a gamma camera head the radioactivity concen-
tration profile obtained from a whole body acquisition should be constant.
A recent trend in SPECT hardware incorporates a CT scanner, which al-
lows for delineation of structures, improves localization of uptake and provides
information needed for attenuation correction [13, 4]. Since both components
of such a scanner contain established technology, the major factor that needs
to be taken into account in the context of scanner-related correction factors is