Biomedical Engineering Reference
In-Depth Information
FIGURE 11.8
Simultaneous structural (transmission) and functional (emission) cardiac scan on a modern
SPECT camera using scanning line sources of
153
99m
Gd for the transmission scan and [
Tc]-
tetrofosmin for the emission scan.
The structural images of electron density and the emission data need to
be spatially coregistered before correcting for attenuation and scattering. If
the transmission data are acquired on the gamma camera simultaneously
with the emission data, registration is virtually guaranteed. However, data
from x-ray CT will usually be recorded by a separate scanner on a different
occasion and therefore needs to be registered to the emission data before
generating the correction factors to be applied to the emission data. In
addition, as x-rays are (a) usually of lower energy than the gamma rays
used in nuclear medicine imaging, and (b) polychromatic (i.e., composed
of a continuum of photon energies rather than a discrete single energy),
some adjustments are needed to scale the attenuation coefficent (
) to the
29
Once the data have been
spatially registered and scaled to the appropriate
appropriate energy for the emission nuclide.
value they can be incor-
porated into the reconstruction process. An example of simultaneously
acquired SPECT emission and transmission data is shown in Figure 11.8.
The transmission scan not only provides a means for generating attenuation
correction factors, but also an anatomical framework to aid interpretation of
the emission data.
More recently, attenuation data have been incorporated into scatter cor-
rection techniques in both PET and SPECT. Coregistered SPECT attenua-
tion and emission projection data have been used to estimate the amount of
scatter (
) with which to scale the scatter distribution estimate
(the emission projections convolved with an appropriate scatter kernel).
scatter fraction
30,31
The data are usually acquired in a simultaneous emission-transmission scan.
A similar approach has been used with reconstructed emission and attenuation
data.
32
The use of information about the body density as well as the distri-
bution of the radiotracer permits more sophisticated estimations of the scat-
ter, including methods based on Monte Carlo realizations
33
or direct
calculation of the scatter using the physics of photon scattering cross-sections
