Each PVR adds a single column to the design matrix. In a two group design, this can make it tricky to set up. Let's say you have 10 of group1 and 20 of group2. You will need to create two PVR files, each with 30=10+20 frames. In the first one, the first 10 frames will be cortical thickness (or amyloid sampled on the surface) of group1; the next 20 frames will be all zeros. For the 2nd PVR, the first 10 frames will be 0s and the next 20 frames will be the cortical thickness (or amyloid) for group2. I would start by running mris_preproc for the two groups separate (so 2 files, one with 10 frames the other 20 frames). Then create the file of zeros using
fscalc group2.mgz mul 0 -o group2.zeros.mgz
Then
mri_concat group1.mgz group2.zeros.mgz --o pvr1.mgz
Then create the contrast based on the FSGD, but then add two more numbers, one for PVR1 (which tests for the within group correlation), and one for PVR2


On 8/1/2019 3:14 PM, miracle ozzoude wrote:

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Please, can anyone help me with this. 
Thank you

Paul

---------- Forwarded message ---------
From: miracle ozzoude <miracooloz@gmail.com>
Date: Wed, Jul 31, 2019 at 2:11 PM
Subject: multimodal analysis (pet and cortical thickness relationship) using --pvr
To: Douglas N Greve <freesurfer@nmr.mgh.harvard.edu>


Hello Experts, 

I am performing an analysis looking at the relationship between amyloid uptake and cortical thickness using --pvr flag in mri_glmfit. I've 2 groups and 2 variables (age and education). I want to run a within group analysis while regressing out age and education (i.e. Within group 1, is there a negative relationship between amyloid uptake and cortical thickness  regressing out the effects of age and education). 

However, i'm not sure how my pvr contrasts will look like. Below are my fsgd and an attempt at creating contrasts. Please, can you let me know if my contrasts are correct based on my questions. 

Thank you. 

Best, 
Paul

The fsgd file lists:
-------------------------------------------------------------
GroupDescriptorFile 1
Title Relationship Amy-thick reg out age and education
Class g1
Class g2
Variable Age Education
Input XX1 g1 60 16
Input XX2 g1 58 14
Input YY1 g2 62 20
Input YY1 g2 62 20
-------------------------------------------------------------
matrix for group1:
pvrgroup1= 1 0 0 0 0 0 0 
is there a relationship between amyloid-thickness in group1 regressing out age
and education? 
matrix for group2:
pvrgroup2= 0 1 0 0 0 0 0
is there a relationship between amyloid-thickness in group2 regressing out age
and education?




















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