How to make an atlas
How to make an atlas…
Making a new atlas is really pretty simple. There are a few steps involved:
1. Identify a structure with an intensity level (all voxels with a value of 10 represent the structure named the flasturgium, for instance). For probability maps a structure is identified by a particular volume, so sub-brick 3 might represent the probability of finding the flasturgium at any voxel.
2. Add the information to the dataset header in a particular way (a NIML table).
3. Add the atlas to a file to let AFNI know that it exists.
4. If you want AFNI to use it by default in the AFNI GUI, add the atlas to an AFNI environment variable, and the structures will show up in the whereami output.
If you are also using a new template, you might need to create a new template “space” too. Also pretty simple.
5. Add the new template and the new space to the same file from step 3 that contains the atlas information.
6. Add the new space to the default list of template spaces used by AFNI.
Each of the steps is detailed below.
1. The hardest part is figuring out what’s what – drawing or making a region. You might use AFNI’s Draw Dataset to assign a number to that region. Soon those will automatically be the equivalent of atlases, and you can skip step 2 if you use this method. Otherwise, programs like FreeSurfer can do automatic segmentation. You can also draw regions with other programs like MIPAV, Amira, Osirix and many others. It’s easiest to bring the data into AFNI as NIFTI and then convert that to AFNI format. Many of the traditional atlases have been drawn using expert anatomical knowledge and take many months or years to complete. A few minor details:
Non-probabilistic atlases can be multiple sub-bricks (see the TTatlas dataset for an example). This is useful for cases where regions might overlap, or different ways to draw regions (gyral/region). You can also use separate atlas datasets.
Left or right as part of the name is preferred, but if it isn’t there, the program will try to figure this out from the x>=0 is left, x<0 is right.
Probabilistic atlases can take up a fair amount of disk space and memory because a separate volume is loaded for each structure. It’s useful to compress the data to save on disk space. Use “setenv AFNI_COMPRESSOR gzip”.
Values for structures should be positive integers for non-probabilistic atlases and stored as byte or short (16-bit integer) datasets. For probabilistic atlases, the probability values can be floating point or integer data. If the data is floating point, the dataset can still be byte or short with a scaling factor, but it can also be a float dataset. The byte dataset will save memory and disk space. If the data is greater than one after any scale factors, then the probability is assumed to be the value divided by 250.
2. The remaining steps have been incorporated into a single script to make this all easier. First, put all the custom atlases in a specific directory. Then add them with @Atlasize. The script takes an atlas dataset and a file with two columns specifying intensity level and structure name. See the @Atlasize help for more details on the options.
@AfniEnv -set AFNI_SUPP_ATLAS_DIR ~/CustomAtlases/
@Atlasize -space MNI -dset hmat_spm_final.nii \
-lab_file keys.txt 1 0 -lab_file_delim ';' \
-atlas_type G -atlas_name HMAT -atlas_description 'Motor Meta'
The @Atlasize script takes care of the details, but you can do this by hand if needed. Otherwise, just skip the remaining steps. Simple!
Adding the NIML information for the structure means using a specific format that organizes the description of the atlas structures with a simple text description. Using a text editor, create a new file. For this example, name the file MyAtlasStructures.niml. Add text like the example below, with an entry for each structure in the atlas:
# ----------- Simple Atlas ---------------------
COG="0.0 0.0 0.0"
COG="0.0 0.0 0.0"
COG="0.0 0.0 0.0"
Notice each structure is associated with an “ATLAS_POINT” NIML element, and all the ATLAS_POINT’s are part of a group called “atlas_point_list”. The ATLAS_POINT’s begin with “<ATLAS_POINT” and end with “/>”. Similarly, the atlas_point_list is enclosed by ‘<atlas_point_list ni_form="ni_group">’. The two attributes that need to be completed are the “STRUCT=” line and the “VAL=” line. For the first line, put the name of the structure after “STRUCT=” and then put the intensity value that is associated with that structure in the “VAL=” line. The structure name can have spaces, but punctuation will make it difficult to work with later. Again, the value should be a positive integer. Other attributes are less important and not strictly required. If there had been a different value used previously for the atlas, you may set the original key value, OKEY, (not important for this example). GyoAR sets whether the structure should be identified as gyrus or area. If you don’t want to distinguish between the two, leave it as 0; otherwise, set it to 1 or 2 for gyrus or area, respectively. Finally, the “COG” attribute sets the center of gravity position in RAI coordinates. Use a position that you would like as a center (maybe a maximum probability or a center of mass). This position is used in the “Go to atlas location” function in the AFNI GUI. If you don’t need a central location for the structure, just put “0.0 0.0 0.0”.
For probabilistic atlases, you will need to add an additional attribute of “SB_LABEL=” to give the label of the sub-brick that is associated with the structure. This label can be the same as the structure and should be at least two characters. For probabilistic atlases, the values correspond to the sub-brick number, so the structure in the first sub-brick gets a value of 0 in the “VAL=0” line. The second structure gets “VAL=1” and so on.
If there are many structures, you can script the creation of this niml file. See the examples here for how to do this with Matlab or a tcsh script. For probabilistic atlases, you will need to make sure the sub-brick labels match the NIML table SB_LABELs for each sub-brick. You will need to assign sub-brick labels to the dataset if those have not already been set. Use commands like this to set the sub-brick labels:
3drefit -sublabel 0 "sub_brick_0_label" MyAtlas+tlrc
3drefit -sublabel 1 "sub_brick_1_label" MyAtlas+tlrc
Add this NIML table to the header of the dataset with this command:
3drefit -atrstring ATLAS_LABEL_TABLE file:MyAtlasStructures.niml \
Make the atlas show up in the Overlay panel with an integral colormap using this command:
3drefit -cmap INT_CMAP MyAtlas+tlrc
Use CONT_CMAP for a continuous colormap for probabilistic atlases. Probabilistic atlases also need the additional attribute, ATLAS_PROB_MAP.
3drefit -cmap CONT_CMAP
3drefit -atrint ATLAS_PROB_MAP 1 MyAtlas+tlrc
If the dataset is not
already associated with a template space, add that here; otherwise, AFNI won’t
know for which kinds of datasets this atlas is useful. For example, if the
dataset was made from data that was aligned to the TT_N27 dataset, you might use
a command like this:
3drefit -space TT_N27 MyAtlas+tlrc
3. Now you’re really almost done. Just add the atlas to a new text file (for this example, name the file myafniatlases.niml. The file that contains entries for all the atlases that come with AFNI, AFNI_atlas_spaces.niml, is overwritten with AFNI updates, but we’ll be using a similar format. Use this text as an example:
comment="Created by me for my site and my subjects…"
For AFNI to use this file to define atlases, just set an environment variable to point to this file. This is best done in the .afnirc file, but it can also be done on the command line with this:
setenv AFNI_SUPP_ATLAS myafniatlases.niml
You can also use the variable, AFNI_LOCAL_ATLAS, for a third atlas definition file.
4. Finally, for AFNI to use the atlas automatically, add the new atlas to AFNI environment list. If you only want to see the new atlas and no others when you use whereami or the AFNI GUI, add this to your .afnirc file or type on the command line:
setenv AFNI_ATLAS_LIST “MyAtlas”
or if you want to use any other atlases too at the same time, add those here:
setenv AFNI_ATLAS_LIST “MyAtlas,TT_Daemon,CA_EZ_ML,Desai_DD_MPM”
For the other places in the AFNI GUI where atlases are used besides the whereami menu, like "Show atlas colors" or "Go to atlas location", you can also set this variable to use your new atlas by default:
setenv AFNI_ATLAS_COLORS MyAtlas
That’s all that is needed for your own atlas, but if you need to create a new space, there are a couple more things you might want to do.
5. Add a template space to the AFNI atlas definition file you created earlier. This is a similar format.
comment="Aligned to my average group or specific subject"
The generic space is the rough equivalent for the space; this might be useful if you want to distinguish between a Talairached subject and generally the Talairach space, for example. In this case, we are assuming a completely new space.
In the same file, you may also add a definition for a new template and transformations from or to this space from any other defined template spaces. The template definition isn't strictly required yet, but will likely be used in future versions of programs like @auto_tlrc. The transformations are a little more complicated to describe, but these transformations provide a connection between a pair of spaces so that AFNI knows how to use atlases made in one space with a dataset that is in another space. If you want to use the TLRC or MNI_ANAT atlases that come with AFNI, and your data is not in either of these spaces, you can define that transformation in the same file. There will be another page describing the different ways to define these transformations, but look at the existing AFNI_atlas_spaces.niml file for reference.
6. Add the template space to the environment variable for the default list of spaces to include in the whereami and AFNI GUI whereami output:
setenv AFNI_ TEMPLATE_SPACE_LIST "MySpace,TLRC,MNI,MNI_ANAT"
You have now defined everything required for a new atlas and a new template space. AFNI will use the variables and definitions you have created just the same as the TLRC daemon or any other AFNI atlas.
Last modified 2012-01-06 14:42