Hi,
My question relates to the order of processing images from different time points in the longitudinal stream.
The base-line images I have were collected in 2006, but many of these images are of poorer quality than those collected in 2008 at follow up.
Given this description from Han et al., 2006:
"The longitudinal scheme differs from the original procedure in
three
major steps: preprocessing, intensity normalization and
surface
deformation. Assuming that a series of scans of the same
subject
are obtained at different time points, the data from time
point one is first processed using the original procedure. For the
processing
of later time points, the three major steps mentioned
above are
modified. First, a linear registration is computed between
the image volume of a later time point and that of the first time
point.
Note that the volume itself is not transformed, but only the
registration
matrix is stored and will be used in later steps. Second,
at the
intensity normalization step, instead of re-computing the
WM control points, the control points (automatically computed)
from
time point one are mapped to the current volume using the
previously
computed linear registration, which are then used to
estimate the
bias field. Third, at the surface deformation step,
instead of starting the deformation from the topology-corrected
WM
tessellation, the gray/white and pial surfaces reconstructed
from
time point one are first transformed using the linear
registration
and then used to initialize the deformation for the
gray/white and pial surfaces respectively for the current time point.
Such
an initialization scheme reduces the problem of deformable
model
methods being sensitive to initialization and local optimality
(Dale
et al., 1999; Xu et al., 1999; Han et al., 2005a). It also
eliminates variation in initial surface tessellation and topology
correction
since these two steps are no longer needed in the
processing of
later time points."
Would it make sense to use the better quality images, which in this case are those collected at time point 2, as the "baseline" when processing in the longitudinal stream to get a more accurate result for time point 1?
Additionally, is there any consensus about the degree to which the longitudinal processing stream assists in overcoming variance due to multi-site scanning?
Thanks in advance for any help on these issues,
Kind Regards,
Meg Dennison
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PhD Candidate / Masters Clinical Psychology
Psychological Sciences
Melbourne University
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