Some parameters are associated only with a small partial volume
effect, normally at a single voxel, which can occur when a shape of
interest or the area of no interest partially overlaps a measured
voxel. The parameters will be called pure partial volume parameters,
here. In this case the associated column of
has a
norm less than 1.0, and the finite range of
can no longer be
ignored. Hence the approximation of using equation 13
(in appendix A) must be replaced by the more accurate
integral of equation 14. However, in this case the
region of integration also becomes important. In the preceding
sections the variables are often transformed in order to compute the
integrals (the substitution
which is done in
appendix A).
For typical models, most columns of , apart from those associated
with
, have norms that are much larger than 1.0, and are
almost orthogonal with the columns associated with these partial
volume parameters. Consequently, the effect of the change of
variables is to induce a slight rotation in the effective region of
integration. However, this effect is small and will be ignored here.
Applying the previous marginalisations of other parameters
first, leads to a posterior of the form
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Using equation 14 and assuming that
has negligible off-diagonal terms gives
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(9) |
Note that it is now no longer possible to easily marginalise over
as it appears in the complimentary error functions.
Consequently, the posterior has been left in the form
which can either be used with a pre-specified value of
(e.g. by estimating the SNR from the image) or numerically
marginalised.
Using the fact that
and
gives
Furthermore, as
then
, and as
then
Also, as the partial volume overlap becomes small (
)
then the posterior assumes the form that would result by increasing
by 1 and including the residual intensity mismatch from
voxel
back into the general residual term. This is what would
happen as this voxel makes the transition fully into the measured area
of interest, resulting in an extra effective voxel in the measurement
(the increase in
) and the full inclusion of the residual
intensity error.
In between these extreme values the contribution becomes partial. This can be compared to the form of de-weighting used for voxels at the edge of the valid field of view, in [3], which can be expressed in this notation as
Note that the careful use of the finite range of and the
introduction of the complimentary error functions in the integrations
is crucial for making the posterior a continuous function of the
spatial transformation.