[Neuroimaging] downsampling without an anti-aliasing filter

[Satrajit Ghosh]

hi folks,

on the list and other places people often ask for scripts/tools for
resampling or downsampling imaging data. my training in signal processing
tells me that:

1. upsampling and downsampling are different operations
2. if i downsample data without low-pass filtering at an appropriate cutoff
i can introduce aliasing if there are frequencies present in the original
data above the new nyqist rate.

for epi sequences where data are acquired in planes, i can easily
understand doing this for slices. but i don't know why in imaging, many
software downsample the in-plane samples without an anti-aliasing filter,
especially as we are getting to higher sampling frequencies (smaller voxel
sizes).

i would love to understand if i am missing something fundamental about
imaging data or signal processing applied to imaging data.

cheers,

satra
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