Multi-Pose Data¶

What is Multi-Pose Imaging?¶

In multi-pose fUSI, a probe is physically stepped to a series of positions along one spatial axis. At each position (a pose), one or more volumes are acquired. Stacking the poses together extends the field of view beyond what a single probe position can cover.

The probe at each pose can image a 2D plane or a 3D volume, depending on the probe type:

• Linear probes (e.g., standard linear probes): each pose yields a single 2D image (one elevation slice). Stepping across N poses and stacking gives a 3D volume of N elevation slices.
• 2D probes (e.g., matrix, RCA, or stacked linear probes): each pose already yields a 3D volume. Stepping across N poses concatenates these volumes into a larger 3D volume.

Multiple fUSI systems support this approach, including Iconeus, EchoFrame, and AUTC. ConfUSIus represents multi-pose data with a pose dimension and per-pose affine transformations that record the world position of each pose.

The Multi-Pose Data Model¶

A multi-pose VoxelData array follows the same dimension order as any other VoxelData array, (..., time, pose, k, j, i), with pose sitting between time and the native voxel dimensions k/j/i. This reflects the fact that probe poses are typically acquired sequentially during a single recording.

pose is not just an extra axis: it changes what the z/y/x world coordinates and the time coordinate mean, because both are per-pose rather than per-array.

• Geometry is per-pose. Each pose has its own voxel-to-world affine, so the world coordinates z/y/x are pose-dependent with shape (pose, k, j, i). Resolving a voxel to a single world position requires selecting a scalar pose first, e.g. data.isel(pose=0).sel(z=1.2). See The VoxelData Model for the full explanation of pose-dependent voxel-to-world geometry.

• Timing can be per-pose. When poses are acquired sequentially rather than simultaneously, each pose is captured at a slightly different time. The time coordinate then holds each pose's own acquisition timestamp directly, shaped (time, pose) instead of the usual 1D (time,)—mirroring how z/y/x require a scalar pose selection before they resolve to a single value:

  pwd.time  # (time, pose) in seconds.
  pwd.isel(pose=0).time  # (time,) in seconds, for pose 0 only.
  

  This matters for slice timing correction (see Slice Timing Correction below), which accounts for the fact that different poses were not acquired simultaneously.

  Per-volume quantities such as confounds and sample masks are timed by the whole volume, not by one pose: their 1D time coordinate is the whole-volume time, as a consolidated array would have.

These two rules—pose-dependent world coordinates, pose-dependent timing—drive everything else in this guide: how multi-pose data is loaded and built, what you can and cannot do with it directly, and what consolidation resolves.

Loading and Constructing Multi-Pose Data¶

Iconeus SCAN Files¶

When loading Iconeus SCAN files containing multi-pose data (3Dscan or 4Dscan acquisition modes), the resulting VoxelData array has a pose dimension. The example below illustrates a recording from a mouse acquired with an IcoPrime-4D MultiArray probe—four linear probes stacked along the elevation axis, giving 4 elevation slices per pose—translated across multiple regularly spaced positions:

>>> from confusius.io import load_scan
>>> pwd = load_scan("sub-01_task-awake_pwd.scan")
>>> pwd
<xarray.DataArray 'scan_data' (time: 409, pose: 4, k: 4, j: 92, i: 118)> Size: 568MB
dask.array<transpose, shape=(409, 4, 4, 92, 118), dtype=float64, chunksize=(106, 4, 4, 92, 106), chunktype=numpy.ndarray>
Coordinates:
    time       (time, pose) float64 13kB 0.4 2.05 0.95 1.5 ... 899.7 898.6 899.1
  * pose       (pose) int64 32B 0 1 2 3
  * k          (k) int64 32B 0 1 2 3
  * j          (j) int64 736B 0 1 2 3 ... 89 90 91
  * i          (i) int64 944B 0 1 2 3 ... 116 117
    z          (pose, k, j, i) float64 1MB ...
    y          (pose, k, j, i) float64 1MB ...
    x          (pose, k, j, i) float64 1MB ...
Attributes:
    affines:               {}
    device_serial_number:  ASAO0000
    software_version:      IcoScan v.1.9.0
    iconeus_scan_mode:     4Dscan
    iconeus_subject:       sub-01
    iconeus_session:       task-awake
    iconeus_scan:          sub-01_task-awake_pwd
    ...

The probe was stepped across 4 positions, each contributing 4 elevation slices—a total of 16 slices once consolidated.

Reloading from Zarr¶

Saving unconsolidated multi-pose data to Zarr preserves the pose dimension and the per-pose affine stack (see Saving below). cf.load round-trips this transparently: reloading a Zarr store written by cf.save rebuilds the pose-dependent VoxelToWorldIndex from attrs["voxel_to_world"], so a reloaded array is indistinguishable from the one that was saved.

import confusius as cf

pwd = cf.load("sub-01_task-awake_pwd_multipose.zarr")  # dims: (time, pose, k, j, i)

Other Systems¶

For fUSI systems without a dedicated loader, multi-pose data must be assembled manually, in one of two ways:

• From a raw array, using create_voxeldata with a pose dimension, an (npose, 4, 4) voxel_to_world affine stack, and, if poses were acquired sequentially, a 1D t0 array giving each pose's own time origin:

  import confusius as cf
  
  multipose = cf.create_voxeldata(
      raw_power,  # shape: (i, j, k, pose, time)
      dims=("i", "j", "k", "pose", "time"),
      dt=0.6,  # seconds, shared across poses.
      t0=[0.0, 0.15, 0.3, 0.45],  # per-pose time origin, in seconds.
      voxel_to_world=pose_affines,  # shape: (4, 4, 4)
  )
  

• From independently loaded single-pose arrays, using stack_poses to combine them into one pose-dependent array.

  from confusius.multipose import stack_poses
  
  poses = [cf.load(f"sub-01_pose-{i:02d}_pwd.nii.gz") for i in range(4)]
  multipose = stack_poses(poses)  # dims: (time, pose, k, j, i)
  

  If the individual poses were acquired sequentially rather than simultaneously and carry different time values, stack_poses automatically produces a pose-dependent (time, pose) coordinate instead of a shared 1D time.

Working with Multi-Pose Arrays¶

Most Xarray operations that don't touch world coordinates or timing work on multi-pose arrays exactly as on any other VoxelData array—reductions, arithmetic, .isel, mask extraction, and so on all treat pose as an ordinary dimension.

Operations that resolve positions or timestamps, however, need a scalar pose first, per The Multi-Pose Data Model above:

pose0 = pwd.sel(pose=0)
shallow = pose0.sel(z=slice(0, 2.5))       # world-coordinate selection now works.
subset = pose0.sel(time=slice(10, 60))     # time is now 1D, .sel(time=...) works too.

A number of ConfUSIus functions go further and require multi-pose data to be reduced to a single spatial grid entirely, since they operate on one voxel-to-world affine:

Slice Timing Correction¶

Because poses in a sequential acquisition are not captured simultaneously, comparing voxels across poses at the same nominal time index compares samples taken at slightly different real times. correct_slice_timings resamples each pose's time series onto a shared, per-volume reference time, removing this offset. It accepts either unconsolidated data with a pose-dependent (time, pose) time coordinate, or already-consolidated data with a slice_time coordinate:

from confusius.multipose import correct_slice_timings

corrected = correct_slice_timings(pwd)  # time coordinate becomes a plain 1D index.

Pose Consolidation¶

consolidate_poses merges the pose dimension and the sweep spatial dimension into a single axis with physically meaningful coordinates, producing a VoxelData array with a single, non-pose-dependent voxel-to-world affine. consolidate_poses performs the following steps:

1. Detect the swept voxel dimension (sweep_dim) from the per-pose affines: match the pose-translation direction against each voxel dimension's world-space direction, and take the best-aligned one.
2. Read the per-pose affines to compute the world position of every (pose, sweep_dim) voxel.
3. Find the precise sweep direction via SVD of all these voxel positions, and project each voxel onto that axis.
4. Check that the resulting positions form a regular grid.
5. Reindex the data in ascending position order, replacing pose and sweep_dim with a single consolidated coordinate in world space.

>>> import confusius as cf
>>> pwd = cf.load("sub-01_task-awake_pwd.scan")
>>> volume = cf.multipose.consolidate_poses(pwd)
>>> volume
<xarray.DataArray 'scan_data' (time: 409, k: 16, j: 92, i: 118)> Size: 568MB
array([...])
Coordinates:
  * time        (time) float64 3kB 2.05 4.25 6.45 8.65 ... 895.2 897.5 899.7
  * k           (k) int64 128B 0 1 2 3 ... 13 14 15
    slice_time  (time, k) float64 52kB 0.4 2.05 0.95 1.5 ... 899.7 898.6 899.1
  * j           (j) int64 736B 0 1 2 3 ... 89 90 91
  * i           (i) int64 944B 0 1 2 3 ... 116 117
    z           (k, j, i) float64 1MB ...
    y           (k, j, i) float64 1MB ...
    x           (k, j, i) float64 1MB ...
Attributes:
    affines:               {}
    device_serial_number:  ASAO0000
    software_version:      IcoScan v.1.9.0
    iconeus_scan_mode:     4Dscan
    iconeus_subject:       sub-01
    iconeus_session:       task-awake
    iconeus_scan:          sub-01_task-awake_pwd
    ...

4 poses × 4 slices = 16 consolidated z positions, spanning −4.95 to 2.93 mm in world space. The pose-dependent time coordinate is reduced to a single whole-volume time and a slice_time coordinate with dims (time, k): each slice retains the timestamp of the pose it came from.

After consolidation, the per-pose affine stack is reduced to a single (4, 4) matrix representing the consolidated volume's orientation in world space.

Saving¶

After Consolidation¶

Once consolidated, a multi-pose array is still a VoxelData array and can be saved to any format:

import confusius as cf

anat = cf.load("sub-01_acq-anat_pwd.scan")
volume = cf.multipose.consolidate_poses(anat)

# Save to NIfTI (creates .nii.gz and a matching fUSI-BIDS JSON sidecar).
volume.fusi.save("sub-01_acq-anat_pwd.nii.gz")

# Or to Zarr.
volume.to_zarr("sub-01_acq-anat_pwd.zarr")

Without Consolidation¶

Non-consolidated data can be saved to Zarr directly, preserving the pose dimension and all per-pose affines:

data.to_zarr("sub-01_acq-anat_pwd_multipose.zarr")

Saving non-consolidated data to NIfTI is not straightforward because NIfTI stores a single affine per file. If you need NIfTI output before consolidating (e.g., for per-pose slice timing correction), save each pose as a separate file:

for i, pose in enumerate(anat.pose.values):
    # The pose entity is defined in the fUSI-BIDS specification.
    anat.sel(pose=pose).fusi.save(f"sub-01_acq-anat_pose-{i:02d}_pwd.nii.gz")