QDEC is a single-binary application, named 'qdec', included in the Freesurfer distribution. QDEC is an acronym for Query, Design, Estimate, Contrast. It is intended to aid researchers performing inter-subject / group averaging and inference on the morphometry data (cortical surface and volume) produced by the Freesurfer processing stream. Prior to usage of qdec, all subcortical segmentation and surface reconstructions for all subjects in a study must be complete. The 'recon-all' script performs that work, and is not the subject of this document.

The qdec binary is a GUI front-end to a 'statistics engine' (the mri_glmfit binary, included in Freesurfer, currently fills this role) intended to:

  1. select the subjects meeting the criteria under study
  2. generate the necessary input to the stats engine, which, for mris_glm, includes:
    1. a Design matrix (called X in the GLM equation) containing the explanatory variables,

    2. a parameter Estimate matrix (called A in the GLM equation), and

    3. the Contrast vector(s)

  3. generate and optionally display the output data and/or images

Without the aid of the qdec tool, a researcher has to manually perform the stats analysis, which, when using the mri_glmfit utility, requires manual creation of an 'FSDG' file which specifies the X and A matrices for the group of subjects under study, and manually specifying the contrast vector(s) for the hypothesis to test. This can be quite tedious, and greatly slows down the "Test for an effect of factor X" research task. This is, in fact, the procedure described in the [wiki:FsTutorial/GroupAnalysis 'Group Analysis' tutorial].



Prior to using the qdec application, your group subject data must be processed by the standard Freesurfer processing stream, via the recon-all script. [ A freesurfer tutorial is available.] This processing stream supplies the surfaces and morphometry data on each subject.


The primary input to qdec is a text file, named qdec.table.dat, containing the subject IDs, and discrete and continuous factors, in table format. Here is an example file:

fsid            gender  age diagnosis   Left-Cerebral-White-Matter
011121_vc8048   Female  70  Demented    202291
021121_62313-2  Female  71  Demented    210188
010607_vc7017   Female  73  Nondemented 170653
021121_vc10557  Male    75  Demented    142029
020718_62545    Male    76  Demented    186087
020322_vc8817   Male    77  Nondemented 149810


First, set your SUBJECTS_DIR environment variable to point at your group subjects directory:

setenv SUBJECTS_DIR /space/boing/users/me/subjects/my_big_study

Or, there is a sample data set you can use to learn:

setenv SUBJECTS_DIR /autofs/space/sake_025/users/nicks/subjects/qdec3

Then, setup the 'dev' Freesurfer environment, which contains qdec:

source /usr/local/freesurfer/nmr-dev-env


For display purposes, you will need to have an average subject included in this subject group. Freesurfer's fsaverage will do fine:

ln -s $FREESURFER_HOME/subjects/fsaverage



Now start qdec:

qdec &

The first thing you will need to do is to load your qdec.table.dat file. Click File -> Load Data Table and traverse to your subjects directory and select your qdec.table.dat file that you created. When you click Open, it should load your file, the contents scrolling-by in the terminal window. If the data is loaded correctly, you should see in the terminal window a summary, like this example:

Number of subjects:   295
Number of factors:    201 (2 discrete, 199 continuous)
Number of classes:    4
Number of regressors: 800

Your discrete and continous factors should appear in the Design tab of the qdec application.


Select up to four factors in the Design tab to regress against. For the example data, you could select 'gender', 'diagnosis' and 'age'. Leaving the 'Smoothness', 'Measure' and 'Hemisphere' at their defaults (10mm, thickness, and lh), clicking the 'Analyze' button will begin the stats processing, executing the mri_glmfit executable. Upon clicking 'Analyze', the terminal will display the output of this processing. Also, progress information is shown in the bottom bar of the qdec application.


Once the analysis is complete (taking up to several minutes for a large subject set), the fsaverage inflated surface will appear in the display window. Now click on the 'Display' tab. You will see displayed in the form of questions a summary of the various analyses that were completed. Click on one of these questions to load those results. Here is an example display:


Notice the green cross-hairs. You can display a plot of the data for a particular vertex by right-clicking on a point while holding-down the Ctrl key. Here is an example plot that corresponds to the shown selected vertex:


Notice that thickness correlates with age at this vertex (thinning occurs).

To rotate the display, hold-down the right mouse button while moving. It takes quite a bit of practice to learn how to move the display. Holding-down the middle button while moving the mouse will move the display. Holding-down the left mouse button while moving will zoom the display.