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Motion

For information on how to cite this extension when referencing it in the context of the academic literature, please read Citing BIDS.

!!! example "Example datasets"

Motion datasets formatted using this specification are available on the
[BIDS examples repository](https://bids.neuroimaging.io/datasets/examples.html#motion)
and can be used as helpful guidance when curating new datasets.

Motion recording data

{{ MACROS___make_filename_template( "raw", datatypes=["motion"], suffixes=["motion", "events", "physio", "stim"]) }}

A wide variety of motion capture systems are used in human research, resulting in different proprietary data formats.

This BIDS extension deals with common outputs from motion capture systems such as positions, orientations, or their time derivatives.

The extension is not limited to motion data in physical space but also encompasses simulated movement in virtual space, as far as these are comparable to movements in physical space. Other dynamic objects than human body parts whose motion is tracked may as well be included as tracked objects. This specification does not include raw camera footages (from camera-based or optical motion capture recordings), but includes the positions or orientations computed using such data.

In this specification, positions (and their time derivatives) are represented as Cartesian coordinates along up to three spatial axes, and orientations (and their time derivatives) are represented as Euler angles. However, to cover recordings from computer graphics applications (for example, virtual 3D motion or immersive virtual reality recording in physical space), orientations are also allowed to be represented as quaternions.

In this case, the quaternion channels can be distinguished from channels containing Euler angles based on the entries in columns component and units in the *_channels.tsv file. See subsection on Channels description for further details.

Motion data from one tracking system MUST be stored in a single *_motion.tsv file. A tracking system is defined as a group of motion channels that share hardware properties (the recording device) and software properties (the recording duration and number of samples). For example, if the position time series of multiple optical markers is processed via one recording unit, this MAY be defined as a single tracking system. Note that it is not uncommon to have multiple tracking systems to record at the same time.

Each tracking system MUST have its own *_tracksys-<label>_motion.tsv file, where <label> is a user-defined keyword to be used to identify each file belonging to a tracking system. This is especially helpful when more than one tracking system is used. Data from different tracking systems MUST be stored in different *_tracksys-<label>_motion.tsv files, each of which is accompanied by *_tracksys-<label>_motion.json and *_tracksys-<label>_channels.tsv files. Between tracksys-<label> entity and *_motion.tsv, *_motion.json, or *_channels.tsv suffixes, optional acq-<label> or run-<index> entity MAY be inserted.

One column in the *_tracksys-<label>_motion.tsv file represents one data channel. Motion files MUST NOT have a header row; the ordering of columns is given by the order of rows in the associated *_channels.tsv file. The number of columns in _motion.tsv files MUST equal the number of rows in the associated _channels.tsv file. All relevant metadata about a tracking systems is stored in accompanying sidecar *_tracksys-<label>_motion.json file.

When multiple tracking systems are used to record motion or motion capture is used alongside the recording of other BIDS modalities and recordings should be interpreted together, it is advised to provide a possibility to synchronize recordings. The preferred way to do so is to use the acquisition time of the first data point of recordings and to store this information in the acq_time column of the *_scans.tsv file. Note that the BIDS date time format allows optional fractional seconds, which SHOULD be used to maximize the precision of the synchronization. Only if the precision of the synchronization is not high enough, the *_events.tsv file SHOULD be used to synchronize recordings. In this file, the start- and stop time of the recording of a system are specified in relation to a system to synchronize with. If more than two systems are to be synchronized, it is up to the user to identify the "main" system.

In case a tracking system provides time information with every recorded sample, these time information MAY be stored in form of latencies to recording onset (first sample) in the *_motion.tsv file. If a system has uneven sampling rate behavior, the LATENCY channel can be used to share these information.

To store events alongside motion data when there are multiple tracking systems simultaneously in use, it is RECOMMENDED to designate a tracking system to the events file. Such an events filename SHOULD include the tracksys key and looks like sub-<label>[_ses-<label>]_task-<label>[_tracksys-<label>][_acq-<label>][_run-<index>]_events.tsv. Event latencies can then be related to motion samples of multiple tracking systems also by using acq_time column entries in the *_scans.tsv. The same principle applies when the events file is saved alongside a simultaneously recorded non-motion data (for example EEG).

Sidecar JSON (*_motion.json)

Task information

{{ MACROS___make_sidecar_table("motion.motionTaskInformation") }}

Hardware information

{{ MACROS___make_sidecar_table("motion.motionHardware") }}

Institution information

{{ MACROS___make_sidecar_table("motion.motionInstitutionInformation") }}

Motion specific fields

Motion specific fields MUST be present:

{{ MACROS___make_sidecar_table("motion.motionRequired") }}

Motion specific fields SHOULD be present:

{{ MACROS___make_sidecar_table("motion.motionRecommended") }}

Example *_tracksys-<label>_motion.json

{
 "SamplingFrequency": 60,
 "SamplingFrequencyEffective": 60.00197437,
 "TaskName": "BIDS Motion fictive example",
 "TrackingSystemName": "IMU Right Hand",
 "TaskDescription": "walking and talking",
 "InstitutionAddress": "Fictive address",
 "InstitutionName": "Fictive Institution",
 "MotionChannelCount": 18,
 "RecordingDuration": 4667.641106,
 "SubjectArtefactDescription": "n/a",
 "TrackedPointsCount" : 2,
 "ACCELChannelCount": 6,
 "GYROChannelCount": 6,
 "MAGNChannelCount": 6,
 "Manufacturer": "BWSensing",
 "ManufacturersModelName": "BW-IMU600"
}

In this example, the *_motion.json contains data from one tracking system consisting of two inertial measurement units (imu). If there are additional tracking systems (for example optical motion capture), data from these MUST be stored as separate files like *_tracksys-omcA_motion.tsv and *_tracksys-omcB_motion.tsv. All specified tracking systems MAY share tracked_point defined in *_channels.tsv, when tracking devices are placed on the same object or body part.

Note that the onsets of the recordings SHOULD be stored in the study key file (scans.tsv). Here, date-time information MUST be expressed as indicated in Units. The scans.tsv file contains the filename and the acquisition time of a recording, which MAY be used to synchronize multiple recordings.

Channels description (*_channels.tsv)

{{ MACROS___make_filename_template( "raw", datatypes=["motion"], suffixes=["channels"]) }}

This file is REQUIRED as it makes it easy to browse or query over larger collections of datasets. The REQUIRED columns are channel name, component, type, tracked_point and units. Any number of additional columns MAY be added to provide additional information about the channels. The *_tracksys-<label>_channels.tsv file SHOULD give additional information about individual recorded channel, some of which my not be found summarized in *_motion.json. To store information about reference frames for a channel, the reference_frame column SHOULD be used (see Reference frame description (*_channels.json)).

The columns of the channels description table stored in *_channels.tsv are:

{{ MACROS___make_columns_table("motion.motionChannels") }}

Restricted keyword list for channel component

Restricted keyword list for column component. When using quaternions to represent orientations, the axial components that corresponds to the three spatial axes MUST be specified as "quat_x", "quat_y", "quat_z", and the non-axial component as "quat_w".

Keyword Description
x Position along the X-axis, or rotation about the X-axis among the Euler angles that represent the orientation, or magnetic field strength along the X-axis.
y Position along the Y-axis or rotation about the Y-axis among the Euler angles that represent the orientation, or magnetic field strength along the Y-axis.
z Position along the Z-axis or rotation about the Z-axis among the Euler angles that represent the orientation, or magnetic field strength along the Z-axis.
quat_x Quaternion component associated with the X-axis.
quat_y Quaternion component associated with the Y-axis.
quat_z Quaternion component associated with the Z-axis.
quat_w Non-axial quaternion component.
n/a Channels that have no corresponding spatial axis.

Restricted keyword list for channel type

Restricted keyword list for column type in alphabetic order. Note that upper-case is REQUIRED:

Keyword Description
ACCEL Accelerometer channel, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y, or z).
ANGACCEL Angular acceleration channel, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y, or z).
GYRO Gyrometer channel, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y, or z).
JNTANG Joint angle channel between two fixed axis belonging to two bodyparts. Angle SHOULD be defined between proximal and distal bodypart in deg.
LATENCY Latency of samples in seconds from recording onset (see acq_time column of the respective *_scans.tsv file). MUST be in form of s[.000000], where s reflects whole seconds, and .000000 reflects OPTIONAL fractional seconds.
MAGN Magnetic field strength, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y or z).
MISC Miscellaneous channels.
ORNT Orientation channel, one channel for each spatial axis or quaternion component. Column component for the axis or quaternion label MUST be added to the *_channels.tsv file (x, y, z, quat_x, quat_y, quat_z, or quat_w).
POS Position in space, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y or z).
VEL Velocity, one channel for each spatial axis. Column component for the axis MUST be added to the *_channels.tsv file (x, y or z).

Example *_channels.tsv

name	component	type	tracked_point	units	reference_frame
t1_acc_x	x	ACCEL	LeftFoot	m/s^2	global
t1_acc_y	y	ACCEL	LeftFoot	m/s^2	global
t1_acc_z	z	ACCEL	LeftFoot	m/s^2	global
t1_gyro_x	x	GYRO	LeftFoot	rad/s	global
t1_gyro_y	y	GYRO	LeftFoot	rad/s	global
t1_gyro_z	z	GYRO	LeftFoot	rad/s	global
…
t2_acc_x	x	ACCEL	RightWrist	m/s^2	global
t2_acc_y	y	ACCEL	RightWrist	m/s^2	global
t2_acc_z	z	ACCEL	RightWrist	m/s^2	global
t2_gyro_x	x	GYRO	RightWrist	rad/s	global
t2_gyro_y	y	GYRO	RightWrist	rad/s	global
t2_gyro_z	z	GYRO	RightWrist	rad/s	global

Reference frame description (*_channels.json)

A reference frame specifies the origin and orientation of the spatial axes with respect to which motion data is to be interpreted. In case the information is available, sharing this can immensely boost the usability of shared data. The description of the reference_frame column SHOULD use the "Levels" field to describe the named field using objects with following fields.

Key name Requirement Level Data type Description
RotationOrder RECOMMENDED string The sequence in which the extrinsic rotations are applied around the three axes. One of "XYZ", "XZY", "YXZ", "YZX", "ZXY", or "ZYX".
RotationRule RECOMMENDED string The direction of rotation around each axis. One of "left-hand" or "right-hand".
SpatialAxes RECOMMENDED string The coordinate system in which the motion data are to be interpreted. A sequence of characters from the set {'A', 'P', 'L', 'R', 'S', 'I', '_'} indicating the direction of each axis. For example "ARS" indicates positive values in the X, Y, Z axes are respectively anterior, right, and superior of the origin, while "PLI" indicates positive values are posterior, left, and inferior of the origin. The "_" character may be used for unused axes.
Description OPTIONAL, but RECOMMENDED if no other keys are present string A description of the reference_frame

Example of *_channels.json

{
  "reference_frame": {
    "Levels": {
      "global": {
        "SpatialAxes": "ALS",
        "RotationOrder": "ZXY",
        "RotationRule": "right-hand"
      },
      "local": {
        "Description": "Joint angles are described following [...]"
      }
    }
  }
}