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Ideas for Using Tags

Stepping aside from literature, an important problem to overcome will be use of tags or analysis in their absence. One potential way to approach this problem is to assume we get a maximally-spread diamond/square when its furthest two edges are far apart, usually before it gets compressed by movement or localised stress. To demonstrate this point imagine a complete blurring of edges and think of what would happen if you measured the radius of a circle passing through/within (or conversely containing) the diamond. Depending on what corners it passes though, its size can tell us something about the level of localised compression and we have code that calculates this. If put on top of the image a radiologist may be able to assess the level of pressure applied by nearby motion that flattens a diamond. If circles are too messy as annotation, then colour too can be used. We want to explore this idea of visualising a surrogate of stress because we have not seen it in other papers yet.

If a distance or other general measure is used for calculating the stress at one point in the tagged image, a method is still needed for identifying the boundaries of the blocks which tagging yields. Cross-frame analysis is required here and one method for finding a match amid movement is a shuffle transform based on MSD/SSD. Initial tagging is just a grid, which is simple to set up. Having initialised a grid layout (be it diagonal or upright wrt xy), which is essentially just a board of blocks, we can then apply shuffle map wherein two frames (or more) are taken at a time and then for each pixel/voxel (or junction in the tagging, for speed and avoidance of interpolation) searches for a best match after the movement, using available information in all 3 axes (shuffling within a cube of fixed size, where vicinity/neighbourhood is defined and limited by a window size). From this we obtain simple tagging which is quite resistant to blurring or can at least estimate best match when the tagging is blurred. The size of the shuffle window ensures that the grid does not move too far, too fast (although it can be adjusted to explore a wider window in particular areas of the image) and to avoid folding or tearing there can be constraints that need not rely on any splines which are diffeomorphic, such as clamped-plate splines. The merit of this approach is that it uses information in three dimensions without relying on heavy computation (shuffling can be encoded as a $4^{3}$=64 bit mask), it explores frame correlation, and it is resistant/sensitive to blurring/noise because of a `best fit' approach. We have begun implementing this for testing on low-resolution data that we got.

We are exploring a way of utilising tagged cardiac data as means of improving segmentation and perhaps - if this goes further - statistical analysis of tags/segmented parts can take place. The nature of the imaged subject/s does not matter to us; it is just that many hours were spent in vain trying to find tagged images and the most/best we found was a single low-resolution (under 300 pixels) sequence of $\sim$10 images. Now, back to the literature.

Figure: Extraction of a 3-D slice from the heart for strain analysis of the box/cube
Image heart-cube-slice

Roy Schestowitz 2010-12-25