Hello AFNI experts,

I am trying to linearly detrend a preprocessed dataset.

I know that it is not recommended to detrend because 3dDeconvolve does a better job but I want to run a CompCor analysis to extract nuisance regressors for wm and csf and they recommend to motion correct and linearly detrend beforehand.

I ran the following: 3dTproject -input pb02.P1002.r01.volreg+orig. -prefix ldetrended -polort 1

I get the following output:

++ 3dTproject: AFNI version=AFNI_20.3.03 (Dec 7 2020) [64-bit]
++ Authored by: Cox the Algebraic (Linear)
++ Setting up regressors
++ 1 Blocks * 2 polynomials -- 2 polort regressors
++ 150 retained time points MINUS 2 regressors ==> 148 D.O.F. left
++ no -mask option ==> processing all 622592 voxels in dataset
++ Compute pseudo-inverse of fixed orts
++ Loading dataset
*+ WARNING: 3dTproject input data :: 18 vectors are constant
++ Starting project-orization
++ Convert results to output dataset
++ Output dataset ./ldetrended+orig.BRIK
++ ===== clock time = 8s 913ms

I have two question:
1. Does the Warning about the 18 constant vectors mean that something is not OK with the output dataset?

2. Why does it say 2 polort regressors when I wrote in the command -polort 1 ? Is it just B0 and B1?

Thank you very much in advance
Carolin