Nonzero residuals from 3dTproject?

AFNI Message Board
1

I’ve been using 3dTproject to run denoising with bandpassing. However, I have noticed that the voxels in the detrended outputs do not quite have a temporal mean of zero, as would be expected of residuals from a linear regression. Is this just due to the accumulation of floating-point error? Or is there something about the mathematics of this process that I am misunderstanding?

A minimal working example (given an input image “input.nii.gz”)

3dTproject -input “input.nii.gz” -prefix “detrended.nii.gz” -ort “design.tsv” -passband 0.009 0.080
3dTstat -mean -prefix “mean_detrended.nii.gz” “detrended.nii.gz”

2

There will certainly be truncation errors. What sorts of means are you seeing?

Also, what is the output of:

3dinfo -prefix -datum -fac mean_detrended.nii.gz

  • rick
3

I’m getting intensities of around 0.1 to 0.4 in the mean residual image with intensities of around 20,000 to 90,000 in the original image. Very small deviation from expectation overall, but I wanted to make sure the deviation is just error (e.g., cumulative truncation error) rather than a theoretical difference from linear regression or an issue in the algorithm.

The output for that 3dinfo call is float 0.000000.