Basics of PET and SPECT Imaging - Correction Techniques in Emission Tomography - page 58

outside the image boundaries, while it is for the scatter-corrected case and for
the SPECT images (Figure 4.4, bottom).
Besides photon scatter in the object to be measured, scatter can also ap-
pear in the scintillation crystals. Due to the higher atomic number of the
crystal material even for PET energies, the photoelectric effect plays an im-
portant role. Especially in small animal tomographs intra-crystal scatter can
lead to relevant reduction of the spatial resolution [6] [8].
4.1.4 Variation in detector eciency, normalization
PET and SPECT systems do not have a uniform sensitivity for photon
detection over their field of view. For SPECT systems there are two typical
reasons for such nonuniformities:
1. Nonuniform detection eciency either due to the fact that the eciency
for detecting light impulses is higher at locations directly over the pho-
tomultipliers compared to areas between different photomultiplier tubes
or due to differences in the pulse-height spectrum for different photo-
multipliers.
2. Nonlinearities, describing the effect that straight line objects appear
as curved-line images. A well-known example is that when a source is
moved toward the center of a photomultiplier tube the light collection
eciency of the photomultiplier increases more quickly than the distance
the source is moved. Hence the image of the line source is distorted
towards the center of the photomultiplier tube. This typically causes
hot spots in the planar image at the location of the photomultiplier
tubes.
Other reasons for nonuniformities can be collimator defects, crystal crack-
ing, nonuniformities in the light guide, drop-out of a photomultiplier tube,
or defects in the electronics. In contrast to planar scintigraphy, where small
nonuniformities may be acceptable, in SPECT nonuniformities can lead to
major artifacts, often with the shape of rings. The effect of nonuniformities in
peripheral areas of the detector is distributed over large areas in the images,
whereas nonuniformities close to the center of the rotation leads to strong ar-
tifacts close to the center of the image: the intensity of the artifact is inversely
proportional to the distance between nonuniformity and center of the rotation
of the detectors. Actual PET scanners, using the block-detector concept, con-
sist of several thousands of detector elements. Each of these detector elements
can have its own photon detection eciency due to inhomogeneities in the
detector block. Besides differences between scintillation crystals there can be
differences in the eciency of the light guide and of the photomultiplier tubes
as described for SPECT systems before. Additionally in the axial direction,
PET systems are built using three to four rings of detector blocks. Between
the rings there are gaps without the ability to detect incoming photons. If