ENH: Allow gradient compensated data in maxwell_filter

doc/changes/latest.inc

@@ -41,6 +41,7 @@ Enhancements
 - Allow an image with intracranial electrode contacts (e.g. computed tomography) to be used without the freesurfer recon-all surfaces to locate contacts so that it doesn't have to be downsampled to freesurfer dimensions (for microelectrodes) and show an example :ref:`ex-ieeg-micro` with :func:`mne.transforms.apply_volume_registration_points` added to aid this transform (:gh:`11567` by `Alex Rockhill`_)
 - Use new :meth:`dipy.workflows.align.DiffeomorphicMap.transform_points` to transform a montage of intracranial contacts more efficiently (:gh:`11572` by `Alex Rockhill`_)
 - Improve performance of raw data browsing with many annotations (:gh:`11614` by `Eric Larson`_)
+- Add support for :func:`mne.preprocessing.maxwell_filter` with gradient-compensated CTF data, e.g., for tSSS-only mode (:gh:`10554` by `Eric Larson`_)
 - Add support for eyetracking data using :func:`mne.io.read_raw_eyelink` (:gh:`11152` by `Dominik Welke`_ and `Scott Huberty`_)
 
 Bugs

mne/epochs.py

@@ -3671,7 +3671,8 @@ def average_movements(epochs, head_pos=None, orig_sfreq=None, picks=None,
                                         _check_usable, _col_norm_pinv,
                                         _get_n_moments, _get_mf_picks_fix_mags,
                                         _prep_mf_coils, _check_destination,
-                                        _remove_meg_projs, _get_coil_scale)
+                                        _remove_meg_projs_comps,
+                                        _get_coil_scale, _get_sensor_operator)
     if head_pos is None:
         raise TypeError('head_pos must be provided and cannot be None')
     from .chpi import head_pos_to_trans_rot_t
@@ -3684,7 +3685,7 @@ def average_movements(epochs, head_pos=None, orig_sfreq=None, picks=None,
         head_pos = head_pos_to_trans_rot_t(head_pos)
     trn, rot, t = head_pos
     del head_pos
-    _check_usable(epochs)
+    _check_usable(epochs, ignore_ref)
     origin = _check_origin(origin, epochs.info, 'head')
     recon_trans = _check_destination(destination, epochs.info, True)
 
@@ -3697,6 +3698,7 @@ def average_movements(epochs, head_pos=None, orig_sfreq=None, picks=None,
         _get_mf_picks_fix_mags(info_to, int_order, ext_order, ignore_ref)
     coil_scale, mag_scale = _get_coil_scale(
         meg_picks, mag_picks, grad_picks, mag_scale, info_to)
+    mult = _get_sensor_operator(epochs, meg_picks)
     n_channels, n_times = len(epochs.ch_names), len(epochs.times)
     other_picks = np.setdiff1d(np.arange(n_channels), meg_picks)
     data = np.zeros((n_channels, n_times))
@@ -3761,6 +3763,9 @@ def average_movements(epochs, head_pos=None, orig_sfreq=None, picks=None,
         # (We would need to include external here for regularization to work)
         exp['ext_order'] = 0
         S_recon = _trans_sss_basis(exp, all_coils_recon, recon_trans)
+        if mult is not None:
+            S_decomp = mult @ S_decomp
+            S_recon = mult @ S_recon
         exp['ext_order'] = ext_order
         # We could determine regularization on basis of destination basis
         # matrix, restricted to good channels, as regularizing individual
@@ -3779,7 +3784,7 @@ def average_movements(epochs, head_pos=None, orig_sfreq=None, picks=None,
     evoked = epochs._evoked_from_epoch_data(data, info_to, picks,
                                             n_events=count, kind='average',
                                             comment=epochs._name)
-    _remove_meg_projs(evoked)  # remove MEG projectors, they won't apply now
+    _remove_meg_projs_comps(evoked, ignore_ref)
     logger.info('Created Evoked dataset from %s epochs' % (count,))
     return (evoked, mapping) if return_mapping else evoked
 

mne/io/base.py

@@ -281,8 +281,8 @@ def _dtype(self):
         return self._dtype_
 
     @verbose
-    def _read_segment(self, start=0, stop=None, sel=None, data_buffer=None,
-                      projector=None, verbose=None):
+    def _read_segment(self, start=0, stop=None, sel=None, data_buffer=None, *,
+                      verbose=None):
         """Read a chunk of raw data.
 
         Parameters
@@ -344,26 +344,22 @@ def _read_segment(self, start=0, stop=None, sel=None, data_buffer=None,
 
         # set up cals and mult (cals, compensation, and projector)
         n_out = len(np.arange(len(self.ch_names))[idx])
-        cals = self._cals.ravel()[np.newaxis, :]
-        if projector is not None:
-            assert projector.shape[0] == projector.shape[1] == cals.shape[1]
-        if self._comp is not None:
+        cals = self._cals.ravel()
+        projector, comp = self._projector, self._comp
+        if comp is not None:
+            mult = comp
             if projector is not None:
-                mult = self._comp * cals
-                mult = np.dot(projector[idx], mult)
-            else:
-                mult = self._comp[idx] * cals
-        elif projector is not None:
-            mult = projector[idx] * cals
+                mult = projector @ mult
         else:
-            mult = None
-        del projector
+            mult = projector
+        del projector, comp
 
         if mult is None:
-            cals = cals.T[idx]
+            cals = cals[idx, np.newaxis]
             assert cals.shape == (n_out, 1)
             need_idx = idx  # sufficient just to read the given channels
         else:
+            mult = mult[idx] * cals
             cals = None  # shouldn't be used
             assert mult.shape == (n_out, len(self.ch_names))
             # read all necessary for proj
@@ -504,8 +500,7 @@ def _preload_data(self, preload):
             data_buffer = None
         logger.info('Reading %d ... %d  =  %9.3f ... %9.3f secs...' %
                     (0, len(self.times) - 1, 0., self.times[-1]))
-        self._data = self._read_segment(
-            data_buffer=data_buffer, projector=self._projector)
+        self._data = self._read_segment(data_buffer=data_buffer)
         assert len(self._data) == self.info['nchan']
         self.preload = True
         self._comp = None  # no longer needed
@@ -752,8 +747,7 @@ def _getitem(self, item, return_times=True):
         if self.preload:
             data = self._data[sel, start:stop]
         else:
-            data = self._read_segment(start=start, stop=stop, sel=sel,
-                                      projector=self._projector)
+            data = self._read_segment(start=start, stop=stop, sel=sel)
 
         if return_times:
             # Rather than compute the entire thing just compute the subset
@@ -1669,7 +1663,7 @@ def append(self, raws, preload=None):
             nsamp = c_ns[-1]
 
             if not self.preload:
-                this_data = self._read_segment(projector=self._projector)
+                this_data = self._read_segment()
             else:
                 this_data = self._data
 
@@ -1681,8 +1675,7 @@ def append(self, raws, preload=None):
                 if not raws[ri].preload:
                     # read the data directly into the buffer
                     data_buffer = _data[:, c_ns[ri]:c_ns[ri + 1]]
-                    raws[ri]._read_segment(data_buffer=data_buffer,
-                                           projector=self._projector)
+                    raws[ri]._read_segment(data_buffer=data_buffer)
                 else:
                     _data[:, c_ns[ri]:c_ns[ri + 1]] = raws[ri]._data
             self._data = _data

mne/preprocessing/maxwell.py

@@ -23,6 +23,7 @@
                           quat_to_rot, rot_to_quat)
 from ..forward import _concatenate_coils, _prep_meg_channels, _create_meg_coils
 from ..surface import _normalize_vectors
+from ..io.compensator import make_compensator
 from ..io.constants import FIFF, FWD
 from ..io.meas_info import _simplify_info, Info
 from ..io.proc_history import _read_ctc
@@ -376,7 +377,7 @@ def _prep_maxwell_filter(
 
     # triage inputs ASAP to avoid late-thrown errors
     _validate_type(raw, BaseRaw, 'raw')
-    _check_usable(raw)
+    _check_usable(raw, ignore_ref)
     _check_regularize(regularize)
     st_correlation = float(st_correlation)
     if st_correlation <= 0. or st_correlation > 1.:
@@ -478,7 +479,6 @@ def _prep_maxwell_filter(
     exp['extended_proj'] = extended_proj
     del extended_proj
     # Reconstruct data from internal space only (Eq. 38), and rescale S_recon
-    S_recon /= coil_scale
     if recon_trans is not None:
         # warn if we have translated too far
         diff = 1000 * (info['dev_head_t']['trans'][:3, 3] -
@@ -520,13 +520,20 @@ def _prep_maxwell_filter(
             np.zeros(3)])
     else:
         this_pos_quat = None
+
+    # Figure out our linear operator
+    mult = _get_sensor_operator(raw, meg_picks)
+    if mult is not None:
+        S_recon = mult @ S_recon
+    S_recon /= coil_scale
+
     _get_this_decomp_trans = partial(
         _get_decomp, all_coils=all_coils,
         cal=calibration, regularize=regularize,
         exp=exp, ignore_ref=ignore_ref, coil_scale=coil_scale,
         grad_picks=grad_picks, mag_picks=mag_picks, good_mask=good_mask,
         mag_or_fine=mag_or_fine, bad_condition=bad_condition,
-        mag_scale=mag_scale)
+        mag_scale=mag_scale, mult=mult)
     update_kwargs.update(
         nchan=good_mask.sum(), st_only=st_only, recon_trans=recon_trans)
     params = dict(
@@ -536,15 +543,15 @@ def _prep_maxwell_filter(
         this_pos_quat=this_pos_quat, meg_picks=meg_picks,
         good_mask=good_mask, grad_picks=grad_picks, head_pos=head_pos,
         info=info, _get_this_decomp_trans=_get_this_decomp_trans,
-        S_recon=S_recon, update_kwargs=update_kwargs)
+        S_recon=S_recon, update_kwargs=update_kwargs, ignore_ref=ignore_ref)
     return params
 
 
 def _run_maxwell_filter(
         raw, skip_by_annotation, st_duration, st_correlation, st_only,
         st_when, ctc, coil_scale, this_pos_quat, meg_picks, good_mask,
         grad_picks, head_pos, info, _get_this_decomp_trans, S_recon,
-        update_kwargs,
+        update_kwargs, *, ignore_ref=False,
         reconstruct='in', copy=True):
     # Eventually find_bad_channels_maxwell could be sped up by moving this
     # outside the loop (e.g., in the prep function) but regularization depends
@@ -564,7 +571,7 @@ def _run_maxwell_filter(
     del raw
     if not st_only:
         # remove MEG projectors, they won't apply now
-        _remove_meg_projs(raw_sss)
+        _remove_meg_projs_comps(raw_sss, ignore_ref)
     # Figure out which segments of data we can use
     onsets, ends = _annotations_starts_stops(
         raw_sss, skip_by_annotation, invert=True)
@@ -745,7 +752,19 @@ def _get_coil_scale(meg_picks, mag_picks, grad_picks, mag_scale, info):
     return coil_scale, mag_scale
 
 
-def _remove_meg_projs(inst):
+def _get_sensor_operator(raw, meg_picks):
+    comp = raw.compensation_grade
+    if comp not in (0, None):
+        mult = make_compensator(raw.info, 0, comp)
+        logger.info(f'    Accounting for compensation grade {comp}')
+        assert mult.shape[0] == mult.shape[1] == len(raw.ch_names)
+        mult = mult[np.ix_(meg_picks, meg_picks)]
+    else:
+        mult = None
+    return mult
+
+
+def _remove_meg_projs_comps(inst, ignore_ref):
     """Remove inplace existing MEG projectors (assumes inactive)."""
     meg_picks = pick_types(inst.info, meg=True, exclude=[])
     meg_channels = [inst.ch_names[pi] for pi in meg_picks]
@@ -754,6 +773,10 @@ def _remove_meg_projs(inst):
         if not any(c in meg_channels for c in proj['data']['col_names']):
             non_meg_proj.append(proj)
     inst.add_proj(non_meg_proj, remove_existing=True, verbose=False)
+    if ignore_ref and inst.info['comps']:
+        assert inst.compensation_grade in (None, 0)
+        with inst.info._unlock():
+            inst.info['comps'] = []
 
 
 def _check_destination(destination, info, head_frame):
@@ -959,9 +982,9 @@ def _check_pos(pos, head_frame, raw, st_fixed, sfreq):
     return pos
 
 
-def _get_decomp(trans, all_coils, cal, regularize, exp, ignore_ref,
+def _get_decomp(trans, *, all_coils, cal, regularize, exp, ignore_ref,
                 coil_scale, grad_picks, mag_picks, good_mask, mag_or_fine,
-                bad_condition, t, mag_scale):
+                bad_condition, t, mag_scale, mult):
     """Get a decomposition matrix and pseudoinverse matrices."""
     from scipy import linalg
     #
@@ -970,6 +993,8 @@ def _get_decomp(trans, all_coils, cal, regularize, exp, ignore_ref,
     S_decomp_full = _get_s_decomp(
         exp, all_coils, trans, coil_scale, cal, ignore_ref, grad_picks,
         mag_picks, mag_scale)
+    if mult is not None:
+        S_decomp_full = mult @ S_decomp_full
     S_decomp = S_decomp_full[good_mask]
     #
     # Extended SSS basis (eSSS)
@@ -1143,16 +1168,16 @@ def _check_regularize(regularize):
         raise ValueError('regularize must be None or "in"')
 
 
-def _check_usable(inst):
+def _check_usable(inst, ignore_ref):
     """Ensure our data are clean."""
     if inst.proj:
         raise RuntimeError('Projectors cannot be applied to data during '
                            'Maxwell filtering.')
     current_comp = inst.compensation_grade
-    if current_comp not in (0, None):
+    if current_comp not in (0, None) and ignore_ref:
         raise RuntimeError('Maxwell filter cannot be done on compensated '
-                           'channels, but data have been compensated with '
-                           'grade %s.' % current_comp)
+                           'channels (data have been compensated with '
+                           'grade {current_comp}) when ignore_ref=True')
 
 
 def _col_norm_pinv(x):

mne/preprocessing/tests/test_maxwell.py

@@ -303,27 +303,49 @@ def test_other_systems():
     _assert_shielding(raw_sss_auto, power, 0.7)
 
     # CTF
-    raw_ctf = read_crop(fname_ctf_raw)
-    assert raw_ctf.compensation_grade == 3
-    with pytest.raises(RuntimeError, match='compensated'):
-        maxwell_filter(raw_ctf)
-    raw_ctf.apply_gradient_compensation(0)
+    raw_ctf_3 = read_crop(fname_ctf_raw)
+    assert raw_ctf_3.compensation_grade == 3
+    raw_ctf_0 = raw_ctf_3.copy().apply_gradient_compensation(0)
+    assert raw_ctf_0.compensation_grade == 0
+    # 3rd-order gradient compensation works really well (better than MF here)
+    _assert_shielding(raw_ctf_3, raw_ctf_0, 20, 21)
+    origin = (0., 0., 0.04)
+    raw_sss_3 = maxwell_filter(raw_ctf_3, origin=origin, verbose=True)
+    _assert_n_free(raw_sss_3, 70)
+    _assert_shielding(raw_sss_3, raw_ctf_3, 0.12, 0.14)
+    _assert_shielding(raw_sss_3, raw_ctf_0, 2.63, 2.66)
+    assert raw_sss_3.compensation_grade == 3
+    raw_sss_3.apply_gradient_compensation(0)
+    assert raw_sss_3.compensation_grade == 0
+    _assert_shielding(raw_sss_3, raw_ctf_3, 0.15, 0.17)
+    _assert_shielding(raw_sss_3, raw_ctf_0, 3.18, 3.20)
     with pytest.raises(ValueError, match='digitization points'):
-        maxwell_filter(raw_ctf)
-    raw_sss = maxwell_filter(raw_ctf, origin=(0., 0., 0.04))
-    _assert_n_free(raw_sss, 68)
-    _assert_shielding(raw_sss, raw_ctf, 1.8)
+        maxwell_filter(raw_ctf_0)
+    raw_sss_0 = maxwell_filter(raw_ctf_0, origin=origin, verbose=True)
+    _assert_n_free(raw_sss_0, 68)
+    _assert_shielding(raw_sss_0, raw_ctf_3, 0.07, 0.09)
+    _assert_shielding(raw_sss_0, raw_ctf_0, 1.8, 1.9)
+    raw_sss_0.apply_gradient_compensation(3)
+    _assert_shielding(raw_sss_0, raw_ctf_3, 0.07, 0.09)
+    _assert_shielding(raw_sss_0, raw_ctf_0, 1.63, 1.67)
+    with pytest.raises(RuntimeError, match='ignore_ref'):
+        maxwell_filter(raw_ctf_3, ignore_ref=True)
+    # ignoring ref outperforms including it in maxwell filtering
     with catch_logging() as log:
-        raw_sss = maxwell_filter(raw_ctf, origin=(0., 0., 0.04),
+        raw_sss = maxwell_filter(raw_ctf_0, origin=origin,
                                  ignore_ref=True, verbose=True)
     assert ', 12/15 out' in log.getvalue()  # homogeneous fields removed
     _assert_n_free(raw_sss, 70)
-    _assert_shielding(raw_sss, raw_ctf, 12)
-    raw_sss_auto = maxwell_filter(raw_ctf, origin=(0., 0., 0.04),
+    _assert_shielding(raw_sss, raw_ctf_0, 12, 13)
+    # if ignore_ref=True, we remove compensators because they will not
+    # work the way people expect (it puts noise back in the data!)
+    with pytest.raises(ValueError, match='Desired compensation.*not found'):
+        raw_sss.copy().apply_gradient_compensation(3)
+    raw_sss_auto = maxwell_filter(raw_ctf_0, origin=origin,
                                   ignore_ref=True, mag_scale='auto')
     assert_allclose(raw_sss._data, raw_sss_auto._data)
     with catch_logging() as log:
-        maxwell_filter(raw_ctf, origin=(0., 0., 0.04), regularize=None,
+        maxwell_filter(raw_ctf_0, origin=origin, regularize=None,
                        ignore_ref=True, verbose=True)
     assert '80/80 in, 12/15 out' in log.getvalue()  # homogeneous fields