Preparing a BIDS dataset by hand and from scratch

⚠️ Note that this is purely for learning purposes and it is NOT recommended to BIDSify real datasets by hand . ⚠️

Table of content

• Ingredients
• Recipe
  • Preheat the oven: folder structure
  • Starters: anatomical data
  • Main course: functional data
  • Dessert: defacing, QA...
• Useful links

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... to see what I hide !!!

Ingredients and tools

Get them fresh from your local market:

• MRI scanner 🧲
• EEG amplifier 🌩
• MEG squid 🦑
• ...

🧠 some source data to be converted into BIDS

We will work with the multi-modal face dataset from SPM .

Very often MRI source data will be in a DICOM format and will require to be converted. Here the MRI data is in "3D" Nifti format .hdr/.img and we will need to change that to a "4D" Nifti .nii format.

This dataset contains EEG, MEG and fMRI data on the same subject within the same paradigm. We also extracted some of the information about the data from the SPM manual and put it into the source/README.md.

Similarly when you have DICOM data, it is usually a good idea to keep the PDF of MRI acquisition parameters with your source data.

🖋 a text editor

Several common options top choose from:

• Visual Studio code
• Sublime
• Atom
• Notepad does not really count.

♻ some format conversion tools

For the MRI data we will be using some of the SPM built-in functions to convert Nifti files into the proper format.

📥 [OPTIONAL] BIDS validator

• Install Node.js (at least version 12.12.0).
• Update npm to be at least version 7 (npm install --global npm@^7)
• From a terminal run npm install -g bids-validator
• Run bids-validator to start validating datasets.

See the full instruction here.

[OPTIONAL] Datalad to version control your data

You can follow the installation instruction in the Datalad handbook.

🐳 [OPTIONAL] Docker

Check the install instruction for your system here.

Recipe

1. Preheat the oven: creating folders

• Create a raw folder to host your BIDS data and inside it create:

  • a sourcedata folder and put your source data in it
  • a code/conversion folder and put this README.md in it
  • a subject folder: sub-01
    • with session folder: ses-mri
      • with an anat folder for the structural MRI data
      • with an func folder for the functional MRI data

By now you should have this.

  ├── code
  │   └── conversion
  ├── sourcedata
  │   ├── multimodal_fmri
  │   │   └── fMRI
  │   │       ├── Session1
  │   │       └── Session2
  │   └── multimodal_smri
  │       └── sMRI
  └── sub-01
      └── ses-mri
          ├── anat
          └── func
  

2. Starters: converting the anatomical MRI file

• In Matlab launch SPM: spm fmri.

• In SPM:

  • use the SPM 3D to 4D module:

      Batch --> SPM --> Utils --> 3D to 4D File conversion
    

  • select the *.img file to convert
  • keep track of what you did by saving the batch in code/conversion
  • run the batch

a. Cooking is not just about the taste, it is also about how things look: naming files

• Move the .nii file you have just created into sub-01/ses-mri/anat.
• Give this file a valid BIDS filename.

✅ Valid BIDS filenames

• BIDS filenames are composed of:
  • extension
  • suffix preceded by a _
  • entity-label pairs separated by a _
• So a BIDS filename can look like: entity1-label1_entity2-label2_suffix.extension
• entities and labels can only contain letters and / or numbers.
• For a given suffix, some entities are required and some others are [optional].
• entity-label pairs pairs have a specific order in which they must appear in filename.

In case you do not remember which suffix to use and which entities are required or optional, the BIDS specification has:

• filename templates at the beginning of the section for each imaging modality,
• a summary entity table.

b. Taste your dish while you prepare it: using the BIDS validator

Try it directly in your browser.

c. Season to taste: adding missing files

• README
• dataset_description.json

You can get content for those files from:

• from the BIDS specification (use the search bar)
• the BIDS starter templates

Suggestion:

Add the "table" output of the BIDS validator to your README to give a quick overview of the content of your dataset.

🚨 About JSON files

JSON files are text files to store key-value pairs.

If your editor cannot help you format them properly, you can always use the online editor.

More information on how read and write JSON files is available on the BIDS stater kit.

JSON CONTENT EXAMPLE:

{
  "key": "value",
  "key2": "value2",
  "key3": {
    "subkey1": "subvalue1"
  },
  "array": [ 1, 2, 3 ],
  "boolean": true,
  "color": "gold",
  "null": null,
  "number": 123,
  "object": {
    "a": "b",
    "c": "d"
  },
  "string": "Hello World"
}

d. Icing on the cake: adding extra information

• Add T1w.json file. Use information from source/README.md to create it.
• Add a participants participants.tsv. You can use excel or google sheet to create them.

🚨 About TSV files

A Tab-Separate Values (TSV) file is a text file where tab characters (\t) separate fields that are in the file.

It is structured as a table, with each column representing a field of interest, and each row representing a single data point.

More information on how read and write TSV files is available on the BIDS stater kit

TSV CONTENT EXAMPLE:

participant_id\tage\tgender\n
sub-01\t34\tM

By now you should have this.

  ├── code
  ├── sourcedata
  ├── sub-01
  │   └── ses-mri
  │       ├── anat
│ │       │   ├── sub-01_ses-mri_T1w.json
│ │       │   └── sub-01_ses-mri_T1w.nii
  │       └── func
  ├── README
  ├── participants.tsv
  ├── participants.json
  └── dataset_description.json
  

e. BIDS is data jam: let's preserve some

[OPTIONAL]

• Create a Datalad dataset
• make a commit when you have a valid dataset to use as a checkpoint.

datalad create --force -c text2git .
datalad save -m 'initial commit'

3. Main course: converting the functional MRI files

• Convert the 2 runs of made of 3D series of *.img into 2 single 4D *.nii images by using the same SPM module used for the anatomical conversion.
• Make sure to use enter the repetition time in the interscan interval.
• Give the output files valid BIDS filenames. You will need to use task and the run entities.
• Use the BIDS validator and any eventual missing file (like *_bold.json file).
• Create events.tsv: you can run the function multimodal/code/convert_func_event_mat.m to help you convert the files multimodal/source/multimodal_fmri/fMRI/trials_ses*.mat
• Put the events.tsv files in the func folders and give them BIDS valid names.
• Remove duplicate json files to make use of the "inheritance principle"

By now you should have this.

  ├── code
  ├── sourcedata
  ├── sub-01
  │   └── ses-mri
  │       ├── anat
│ │       │   └── sub-01_ses-mri_T1w.nii
  │       └── func
  │           ├── sub-01_ses-mri_task-FaceSymmetry_run-1_bold.nii
  │           ├── sub-01_ses-mri_task-FaceSymmetry_run-1_events.tsv
  │           ├── sub-01_ses-mri_task-FaceSymmetry_run-2_bold.nii
  │           └── sub-01_ses-mri_task-FaceSymmetry_run-2_events.tsv
  ├── README
  ├── participants.tsv
  ├── participants.json
  ├── T1w.json
  ├── task-FaceSymmetry_bold.json
  └── dataset_description.json
  

4. Dessert: defacing, quality control, upload your data to GIN...

Defacing

• with SPM:

    Batch --> SPM --> Utils --> De-face images
  

MRIQC

# from within the `raw` folder

bids_dir=`pwd`

output_dir=${bids_dir}/../derivatives/mriqc

docker run -it --rm \
  -v ${bids_dir}:/data:ro \
  -v ${output_dir}:/out \
  poldracklab/mriqc:0.16.1 /data /out \
  --participant_label sub-01 \
  --verbose-reports \
  participant

uploading your data on GIN

• create an account on GIN
• upload your public SSH key to GIN for SSH access
  • you might need to create one first
• create an empty repository on GIN

  datalad siblings add --name gin --url git@gin.g-node.org:/your_username/your_repository.git
  datalad push --to gin
  

• More information on the datalad handbook

Things to improve ?

Useful links

• BIDS specification
• BIDS starter kit
  • templates
  • wiki
    • validator
• BIDS validator
• BIDS examples
• Neurostars forum
• Other conversion tutorial
  • reproducibility.stanford.edu
  • fieldtrip
• Conversion tools
• Datalad handbook
• GIN

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