Merge pull request nipreps#115 from oesteban/rf/uniform-interface

ENH: New estimation API (featuring a TOPUP implementation!)

docs/api.rst

@@ -7,5 +7,6 @@ Information on specific functions, classes, and methods.
    :glob:
 
    api/sdcflows.interfaces
+   api/sdcflows.models
    api/sdcflows.viz
    api/sdcflows.workflows

sdcflows/conftest.py

@@ -2,6 +2,7 @@
 import os
 from pathlib import Path
 import numpy
+import nibabel
 import pytest
 from bids.layout import BIDSLayout
 
@@ -23,6 +24,7 @@ def pytest_report_header(config):
 @pytest.fixture(autouse=True)
 def add_np(doctest_namespace):
     doctest_namespace['np'] = numpy
+    doctest_namespace['nb'] = nibabel
     doctest_namespace['os'] = os
     doctest_namespace['Path'] = Path
     for key, val in list(layouts.items()):
@@ -35,10 +37,15 @@ def workdir():
 
 
 @pytest.fixture
-def output_path():
+def outdir():
     return None if test_output_dir is None else Path(test_output_dir)
 
 
 @pytest.fixture
 def bids_layouts():
     return layouts
+
+
+@pytest.fixture
+def datadir():
+    return Path(test_data_env)

sdcflows/interfaces/epi.py

@@ -0,0 +1,208 @@
+# emacs: -*- mode: python; py-indent-offset: 4; indent-tabs-mode: nil -*-
+# vi: set ft=python sts=4 ts=4 sw=4 et:
+"""
+Interfaces to deal with the various types of fieldmap sources.
+
+    .. testsetup::
+
+        >>> tmpdir = getfixture('tmpdir')
+        >>> tmp = tmpdir.chdir() # changing to a temporary directory
+        >>> nb.Nifti1Image(np.zeros((90, 90, 60)), None, None).to_filename(
+        ...     tmpdir.join('epi.nii.gz').strpath)
+
+
+"""
+
+from nipype.interfaces.base import (
+    BaseInterfaceInputSpec,
+    TraitedSpec,
+    File,
+    isdefined,
+    traits,
+    SimpleInterface,
+)
+
+
+class _GetReadoutTimeInputSpec(BaseInterfaceInputSpec):
+    in_file = File(exists=True, desc="EPI image corresponding to the metadata")
+    metadata = traits.Dict(mandatory=True, desc="metadata corresponding to the inputs")
+
+
+class _GetReadoutTimeOutputSpec(TraitedSpec):
+    readout_time = traits.Float
+
+
+class GetReadoutTime(SimpleInterface):
+    """Calculate the readout time from available metadata."""
+
+    input_spec = _GetReadoutTimeInputSpec
+    output_spec = _GetReadoutTimeOutputSpec
+
+    def _run_interface(self, runtime):
+        self._results["readout_time"] = get_trt(
+            self.inputs.metadata,
+            self.inputs.in_file if isdefined(self.inputs.in_file) else None,
+        )
+        return runtime
+
+
+def get_trt(in_meta, in_file=None):
+    r"""
+    Extract the *total readout time* :math:`t_\text{RO}` from BIDS.
+
+    Calculate the *total readout time* for an input
+    :abbr:`EPI (echo-planar imaging)` scan.
+
+    There are several procedures to calculate the total
+    readout time. The basic one is that a ``TotalReadoutTime``
+    field is set in the JSON sidecar. The following examples
+    use an ``'epi.nii.gz'`` file-stub which has 90 pixels in the
+    j-axis encoding direction.
+
+    >>> meta = {'TotalReadoutTime': 0.02596}
+    >>> get_trt(meta)
+    0.02596
+
+    If the *effective echo spacing* :math:`t_\text{ees}`
+    (``EffectiveEchoSpacing`` BIDS field) is provided, then the
+    total readout time can be calculated reading the number
+    of voxels along the readout direction :math:`T_\text{ro}`
+    and the parallel acceleration factor of the EPI :math:`f_\text{acc}`.
+
+      .. math ::
+
+          T_\text{ro} = t_\text{ees} \, (N_\text{PE} / f_\text{acc} - 1)
+
+    >>> meta = {'EffectiveEchoSpacing': 0.00059,
+    ...         'PhaseEncodingDirection': 'j-',
+    ...         'ParallelReductionFactorInPlane': 2}
+    >>> get_trt(meta, in_file='epi.nii.gz')
+    0.02596
+
+    Some vendors, like Philips, store different parameter names:
+
+    >>> meta = {'WaterFatShift': 8.129,
+    ...         'MagneticFieldStrength': 3,
+    ...         'PhaseEncodingDirection': 'j-',
+    ...         'ParallelReductionFactorInPlane': 2}
+    >>> get_trt(meta, in_file='epi.nii.gz')
+    0.018721183563864822
+
+    """
+    import nibabel as nb
+
+    # Use case 1: TRT is defined
+    trt = in_meta.get("TotalReadoutTime", None)
+    if trt is not None:
+        return trt
+
+    # All other cases require the parallel acc and npe (N vox in PE dir)
+    acc = float(in_meta.get("ParallelReductionFactorInPlane", 1.0))
+    npe = nb.load(in_file).shape[_get_pe_index(in_meta)]
+    etl = npe // acc
+
+    # Use case 2: TRT is defined
+    ees = in_meta.get("EffectiveEchoSpacing", None)
+    if ees is not None:
+        return ees * (etl - 1)
+
+    # Use case 3 (philips scans)
+    wfs = in_meta.get("WaterFatShift", None)
+    if wfs is not None:
+        fstrength = in_meta["MagneticFieldStrength"]
+        wfd_ppm = 3.4  # water-fat diff in ppm
+        g_ratio_mhz_t = 42.57  # gyromagnetic ratio for proton (1H) in MHz/T
+        wfs_hz = fstrength * wfd_ppm * g_ratio_mhz_t
+        return wfs / wfs_hz
+
+    raise ValueError("Unknown total-readout time specification")
+
+
+def get_ees(in_meta, in_file=None):
+    r"""
+    Extract the *effective echo spacing* :math:`t_\text{ees}` from BIDS.
+
+    Calculate the *effective echo spacing* :math:`t_\text{ees}`
+    for an input :abbr:`EPI (echo-planar imaging)` scan.
+
+
+    There are several procedures to calculate the effective
+    echo spacing. The basic one is that an ``EffectiveEchoSpacing``
+    field is set in the JSON sidecar. The following examples
+    use an ``'epi.nii.gz'`` file-stub which has 90 pixels in the
+    j-axis encoding direction.
+
+    >>> meta = {'EffectiveEchoSpacing': 0.00059,
+    ...         'PhaseEncodingDirection': 'j-'}
+    >>> get_ees(meta)
+    0.00059
+
+    If the *total readout time* :math:`T_\text{ro}` (``TotalReadoutTime``
+    BIDS field) is provided, then the effective echo spacing can be
+    calculated reading the number of voxels :math:`N_\text{PE}` along the
+    readout direction and the parallel acceleration
+    factor of the EPI
+
+      .. math ::
+
+           =  T_\text{ro} \,  (N_\text{PE} / f_\text{acc} - 1)^{-1}
+
+    where :math:`N_y` is the number of pixels along the phase-encoding direction
+    :math:`y`, and :math:`f_\text{acc}` is the parallel imaging acceleration factor
+    (:abbr:`GRAPPA (GeneRalized Autocalibrating Partial Parallel Acquisition)`,
+    :abbr:`ARC (Autocalibrating Reconstruction for Cartesian imaging)`, etc.).
+
+    >>> meta = {'TotalReadoutTime': 0.02596,
+    ...         'PhaseEncodingDirection': 'j-',
+    ...         'ParallelReductionFactorInPlane': 2}
+    >>> get_ees(meta, in_file='epi.nii.gz')
+    0.00059
+
+    Some vendors, like Philips, store different parameter names (see
+    http://dbic.dartmouth.edu/pipermail/mrusers/attachments/20141112/eb1d20e6/attachment.pdf
+    ):
+
+    >>> meta = {'WaterFatShift': 8.129,
+    ...         'MagneticFieldStrength': 3,
+    ...         'PhaseEncodingDirection': 'j-',
+    ...         'ParallelReductionFactorInPlane': 2}
+    >>> get_ees(meta, in_file='epi.nii.gz')
+    0.00041602630141921826
+
+    """
+    import nibabel as nb
+    from sdcflows.interfaces.epi import _get_pe_index
+
+    # Use case 1: EES is defined
+    ees = in_meta.get("EffectiveEchoSpacing", None)
+    if ees is not None:
+        return ees
+
+    # All other cases require the parallel acc and npe (N vox in PE dir)
+    acc = float(in_meta.get("ParallelReductionFactorInPlane", 1.0))
+    npe = nb.load(in_file).shape[_get_pe_index(in_meta)]
+    etl = npe // acc
+
+    # Use case 2: TRT is defined
+    trt = in_meta.get("TotalReadoutTime", None)
+    if trt is not None:
+        return trt / (etl - 1)
+
+    # Use case 3 (philips scans)
+    wfs = in_meta.get("WaterFatShift", None)
+    if wfs is not None:
+        fstrength = in_meta["MagneticFieldStrength"]
+        wfd_ppm = 3.4  # water-fat diff in ppm
+        g_ratio_mhz_t = 42.57  # gyromagnetic ratio for proton (1H) in MHz/T
+        wfs_hz = fstrength * wfd_ppm * g_ratio_mhz_t
+        return wfs / (wfs_hz * etl)
+
+    raise ValueError("Unknown effective echo-spacing specification")
+
+
+def _get_pe_index(meta):
+    pe = meta["PhaseEncodingDirection"]
+    try:
+        return {"i": 0, "j": 1, "k": 2}[pe[0]]
+    except KeyError:
+        raise RuntimeError('"%s" is an invalid PE string' % pe)