Dear SIENA users,

We have a paper under review which uses a SIENA voxelwise analysis. We have been asked by a reviewer to report the peak boundary shift of a significant change we detected in part of the lateral ventricles.

I had a look at the A_to_B_flow_to_std.img images for each subject at the co-ordinates in question. I hoped to get the displacement of brain boundary in mm from the voxel intensity at that point. However the mean displacement over all subjects seemed too small (about a 50th of a 1mm). I went back to the individual A_to_B_flow.img (flow image in native space) and the flow values were larger and more reasonable.

I was wondering if values in the flow_to_std image are usually lower than the corresponding flow images. As the data we are working only shows very small displacement, to test this further we looked at 3 subjects who had been scanned 2 years apart where there was evidence of significant brain atrophy. For each subject we recorded voxel values at 10 'randomly' chosen points in the lateral ventricle brain/CSF boundary both in the A_to_B_flow_to_std.img and A_to_B_flow.img file. In the 3 subjects the voxel intensity in the A_to_B_flow.img seemed to be about right (1.5mm) while the voxel intensities in the A_to_B_flow_to_std.img file seemed about 10 times too small (0.15mm).

Has anybody else seen this and know why this happens? and is this deliberate or is it a 'side effect' of the dilation and smoothing steps… or have I just done something wrong?

Best wishes,

Matthew Kempton

Institute of Psychiatry

London