Biomedical Engineering Reference
In-Depth Information
axially angled LOR are taken together into segments (segment 0 com-
prising the non-axially angled LOR). Within a segment various axial an-
gles are combined. For an increasing axial angle a decreasing z-coverage
of the FOV results.
The newest generation PET scanners acquire raw data in list-mode format.
It is possible to convert list-mode data into sinograms, but newer approaches
are explored that might not rely on the stringent requirements of a complete
set of projection data that accounted for some of the trade-offs mentioned
above [15]. However, these techniques did not enter clinical practice yet and
will not be discussed.
After a prompt sinogram has been obtained, corrections need to be made
in order to allow for a reconstruction of the measured data that yield quan-
titatively accurate data. Most corrections are carried out in the form of mul-
tiplications or summations/subtractions of estimated or measured sinograms
containing correction factors (i.e., randoms, attenuation, and scatter as dis-
cussed in chapter 5).
6.1.2 Data normalization
In image reconstruction it is assumed that every photon that is incident
on a crystal surface has the same probability of being detected. In practice,
this probability differs from crystal to crystal, for various reasons:
the growth of single crystal ingots that are being cut subsequently into
detector blocks is a dicult process that does not allow for a complete
absence of differences in crystal characteristics (light output, decay time,
etc.).
•performance tolerances on the signal processing chain (e.g., photomulti-
pliers, ADCs, and possible drift in these components) will lead to small
changes in the energy spectrum (number of channels in the energy win-
dow of interest) and (stability of the) calibration of the energy scale.
the amount of light detected for a photon interacting in the middle of a
block detector is higher than for a photon interacting at the edge of a
block detector (not to speak of scatter within the detector).
the angle of incidence between the incoming photon and the crystal sur-
face normal. For larger angles a smaller path length and, subsequently,
a smaller interaction probability exists.
a sinogram bin relating to a direct plane will have a lower sensitivity
than a sinogram bin relating to an indirect plane (roughly twice as low
for a span 3 sinogram).
timing calibration and its stability determine whether a photon pair is
 
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