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Co-authored-by: Chris Markiewicz <[email protected]>
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docs/methods.rst

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Methods and implementation
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==========================
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*SDCFlows* defines a clear :abbr:`API (application programming interface)` that divides
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"*the problem of susceptibility distortions (SD)*" into two stages:
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the problem of susceptibility distortions (SD) into two stages:
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#. **Estimation**:
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the MRI acquisitions in the protocol for :abbr:`SD (susceptibility distortions)` are
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:math:`T_\text{ro}` is the readout time of one slice of the EPI dataset
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we want to correct for distortions.
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Let :math:`V` represent the «*fieldmap in Hz (or equivalently,
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«*voxel-shift-velocity map*» as Hz are equivalent to voxels/s), with
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Let :math:`V` represent the *fieldmap* in Hz (or equivalently,
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*voxel-shift-velocity map*, as Hz are equivalent to voxels/s), with
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:math:`V(i,j,k) = \gamma \cdot \Delta B_0 (i, j, k)`, then, introducing
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the voxel zoom along the phase-encoding direction, :math:`s_\text{PE}`,
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we obtain the nonzero component of the associated displacements field
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. admonition:: BIDS Specification
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See `this section of the BIDS specification
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<https://bids-specification.readthedocs.io/en/latest/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#case-3-direct-field-mapping>`__.
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See the section `Types of fieldmaps - Case 3: Direct field mapping
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<https://bids-specification.readthedocs.io/en/latest/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#case-3-direct-field-mapping>`__
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in the BIDS specification.
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Some MR schemes such as :abbr:`SEI (spiral-echo imaging)` can directly
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reconstruct an estimate of *the fieldmap in Hz*, :math:`V(i,j,k)`.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. admonition:: BIDS Specification
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See `this section of the BIDS specification
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<https://bids-specification.readthedocs.io/en/latest/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#case-2-two-phase-maps-and-two-magnitude-images>`__.
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See the section `Types of fieldmaps - Case 2: Two phase maps and two magnitude images
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<https://bids-specification.readthedocs.io/en/latest/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#case-2-two-phase-maps-and-two-magnitude-images>`__
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in the BIDS specification.
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Some scanners produce one ``phasediff`` map, where the drift between the two echos has
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already been calculated, see `the corresponding section of BIDS
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already been calculated, see the section
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`Types of fieldmaps - Case 1: Phase-difference map and at least one magnitude image
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<https://bids-specification.readthedocs.io/en/latest/04-modality-specific-files/01-magnetic-resonance-imaging-data.html#case-1-phase-difference-map-and-at-least-one-magnitude-image>`__.
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The fieldmap variation in T, :math:`\Delta B_0 (i, j, k)`, that is necessary to obtain
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The process is divided in two steps.
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First, the two images to be aligned (anatomical and one or more EPI sources) are prepared for
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registration, including a linear pre-alignment of both, calculation of a 3D EPI *reference* map,
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intensity/histogram enhancement, and *deobliquing* (meaning, for images were the physical
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intensity/histogram enhancement, and *deobliquing* (meaning, for images where the physical
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coordinates axes and the data array axes are not aligned, the physical coordinates are
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rotated to align with the data array).
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Such a preprocessing is implemented in :py:func:`init_syn_preprocessing_wf`.
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Estimation tooling
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~~~~~~~~~~~~~~~~~~
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The workflows provided by :py:mod:`sdcflows.fit` make use of several utilities.
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Perhaps, the centerpiece of these tools is the fieldmap representation with B-Splines
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The cornerstone of these tools is the fieldmap representation with B-Splines
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(:py:mod:`sdcflows.interfaces.bspline`).
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B-Splines are very adequate to plausibly smooth the fieldmap and provide a compact
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B-Splines are well-suited to plausibly smooth the fieldmap and provide a compact
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representation of the field with fewer parameters.
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This representation is also more accurate in the case the images that were used for estimation
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are not aligned with the target images to be corrected because the fieldmap is not directly

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