There are several tricks-of-the-trade, which can be used to speed up the process of registration assessment and model evaluation in 3-D. A few examples follow.
Multi-scale. One strategy, for example, involves resampling the data, then dealing with smaller (downsized) versions of the whole, as shown in Figure . The basic concept is that, if the scale of the problem is reduced, it can be handled at a coarse level and then iteratively handled at finer levels, until the original unscaled data is reached.
|
Improving optimisation and reducing complexity. Chapter 5
described a method for registering sets of images and refining a model
using optimisation, whereas subsequent chapters described a method
for assessing a registration by repeated computation. In both cases,
an answer is available only once the process is completed and further
analysis takes place. Some of the information derived relates to locality
of variation, so it's worth taking advantage of it.
A strategy worth employing and exploring is one which uses figures
of merit, e.g. model properties or Specificity, to also affect decisions
that are made by the algorithm and place focus on areas of greater
significance (containing increased variation for example). In practice,
it is possible to divide the problem into a set of smaller ones. This
means that a box of voxels may be sliced into 8 () equal-sized
boxes which are then used in the analysis, or even more usefully,
the box should be rescaled to become 8 times smaller in terms of volume
and then dealt with separately.
Selective assessment of slices. In general, there is not much which distinguishes the method's use in 2-D and in 3-D, other than efficiency factors. However, several possibilities emerge owing to the fact that 3-D data can be interpreted differently once its dimensionality is reduced. For example, one can choose one representative slice from a larger volume and refine the evaluation by considering more slices, one at a time. This suffers from the fact that voxels whose position varies in the third dimension, i.e. it moves between the slices due to warping, will not be treated appropriately. All these issues, along with other pitfalls, will be addressed in the near future.
Roy Schestowitz 2010-04-05