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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
@@ -58,8 +58,9 @@ Direct B0 mapping sequences
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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)`.
@@ -72,11 +73,13 @@ Phase-difference B0 estimation
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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
@@ -148,7 +151,7 @@ The implementation is a variation on those developed in [Huntenburg2014]_ and
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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 `.
@@ -184,9 +187,9 @@ surfaces onto the distorted :abbr:`EPI (echo-planar imaging)` data [Esteban2016]
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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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