How to create a configuration file

Configuration file example

{
  "descriptions": [
    {
      "datatype": "anat",
      "suffix": "T2w",
      "criteria": {
        "SeriesDescription": "*T2*",
        "EchoTime": 0.1
      },
      "sidecar_changes": {
        "ProtocolName": "T2"
      }
    },
    {
      "id": "task_rest",
      "datatype": "func",
      "suffix": "bold",
      "custom_entities": "task-rest",
      "criteria": {
        "ProtocolName": "func_task-*",
        "ImageType": ["ORIG*", "PRIMARY", "M", "MB", "ND", "MOSAIC"]
      }
    },
    {
      "datatype": "fmap",
      "suffix": "fmap",
      "criteria": {
        "ProtocolName": "*field_mapping*"
      },
      "sidecar_changes": {
        "IntendedFor": "task_rest"
      }
    },
    {
      "id": "id_task_learning",
      "datatype": "func",
      "suffix": "bold",
      "custom_entities": "task-learning",
      "criteria": {
        "SeriesDescription": "bold_task-learning"
      },
      "sidecar_changes": {
        "TaskName": "learning"
      }
    },
    {
      "datatype": "fmap",
      "suffix": "epi",
      "criteria": {
        "SeriesDescription": "fmap_task-learning"
      },
      "sidecar_changes": {
        "TaskName": "learning",
        "IntendedFor": "id_task_learning"
      }
    }
  ]
}

The descriptions field is a list of descriptions, each describing some acquisition. In this example, the configuration describes five acquisitions, a T2-weighted, a resting-state fMRI, a fieldmap, and an fMRI learning task with another fieldmap.

Each description tells dcm2bids how to group a set of acquisitions and how to label them. In this config file, Dcm2Bids is being told to collect files containing

{
  "SeriesDescription": "AXIAL_T2_SPACE",
  "EchoTime": 0.1
}

in their sidecars¹ and label them as anat, T2w type images.

criteria

dcm2bids will try to match the sidecars¹ of dcm2niix to the descriptions of the configuration file. The values you enter inside the criteria dictionary are patterns that will be compared to the corresponding key of the sidecar.

The pattern matching is shell-style. It's possible to use wildcard *, single character ? etc ... Please have a look at the GNU documentation to know more.

For example, in the second description, the pattern *T2* will be compared to the value of SeriesDescription of a sidecar. AXIAL_T2_SPACE will be a match, AXIAL_T1 won't.

dcm2bids has a SidecarFilename key, as in the first description, if you prefer to also match with the filename of the sidecar. Note that filename are subject to change depending on the dcm2niix version in use.

You can enter several criteria. All criteria must match for a description to be linked to a sidecar.

datatype

It is a mandatory field. Here is a definition from bids v1.2.0 :

Data type - a functional group of different types of data. In BIDS we define six data types: func (task based and resting state functional MRI), dwi (diffusion weighted imaging), fmap (field inhomogeneity mapping data such as field maps), anat (structural imaging such as T1, T2, etc.), meg (magnetoencephalography), beh (behavioral).

suffix

It is a mandatory field. It describes the modality of the acquisition like T1w, T2w or dwi, bold.

custom_entities

It is an optional field. For some acquisitions, you need to add information in the file name. For resting state fMRI, it is usually task-rest.

To know more on how to set these fields, read the BIDS specifications.

For a longer example of a Dcm2Bids config json, see here.

Note that the different bids labels must come in a very specific order to be bids valid filenames. If the custom_entities fields that are entered that are in the wrong order, then dcm2bids will reorder them for you.

For example if you entered:

"custom_entities": "run-01_task-rest"

when running dcm2bids, you will get the following warning:

WARNING:dcm2bids.structure:✅ Filename was reordered according to BIDS entity table order:
                from:   sub-ID01_run-01_task-rest_bold
                to:     sub-ID01_task-rest_run-01_bold

custom_entities could also be combined with extractors. See custom_entities combined with extractors

sidecar_changes, id and IntendedFor

Optional field to change or add information in a sidecar.

IntendedFor is now considered a sidecar_changes.

Example:

{
  "sidecar_changes": {
    "IntendedFor": "task_rest"
  }
}

If you want to add an IntendedFor entry or any extra sidecar linked to a specific file, you will need to set an id to the corresponding description and put the same id with IntendedFor.

For example, task_rest means it is intended for task-rest_bold and id_task_learning is intended for task-learning_bold.

You could also use this feature to feed sidecar such as `Source`` for example or anything that suits your needs.

Multiple config files

It is possible to create multiple config files and iterate the dcm2bids command over the different config files to structure data that have different parameters in their sidecar files.

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1. For each acquisition, dcm2niix creates an associated .json file, containing information from the dicom header. These are known as sidecars. These are the sidecars that dcm2bids uses to filter the groups of acquisitions.

   To define the filters you need, you will probably have to review these sidecars. You can generate all the sidecars for an individual participant using the dcm2bids_helper command. ↩↩

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Last update: 2023-07-27
Created: 2022-04-19