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Projections and MR Angiography Projections and MR Angiography

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Submitted by bpittman. on 2006-01-03 08:59.
It is common in MRA to display the 3D results by a Maximum Intensity Projection (MIP) for 'white blood' imaging, and a minimum Intensity Projection (mIP) for 'black blood' imaging.

The direction of projection is rotated around, and the result is a series of 2D images that show in each pixel the brightest (or darkest) voxel along the projection ray. The vessels stand out as 1D tubular structures in this type of animation. It is a very useful way to look at MRA data.

This technique could be used as a way of detecting vessels automatically as well. The idea is to do many such projections from a lot of directions covering the ½-sphere, and for each voxel, keep track of from how many directions it is the max (or min) along a projection. Voxels that are quite often the extreme value are likely to be in a vessel. Then some sort of connectivity criterion can be used to exclude isolas and/or spackle over gaps.

Each projection, on a 256x256x128 volume, should take less than 0.5 s, so in a couple minutes the basic process could be carried out. The 'extract_' functions in AFNI's cox_render.c could be modified to carry out this computation relatively easily. Someday ....

Projections and MR angiography

Posted by dglen at 2006-02-15 09:46
In AFNI, one can easily see this with the display panel options. Using projections of several slices, the more the better, and a maximum or minimum intensity projection, blood vessels stand out. The "Project" and "Slab" buttons are used here. Similarly, one can create datasets with 3dLocalstat and set the neighborhoods to generate these datasets. 3dLocalstat has sphere and rect options. Perhaps a ray option could be added to accomplish the goal here. Then a kind of polled maximum (the median of the maxima?) might be even another option to 3dLocalstat as the result of the various projections.
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