enh(BSplineApprox): integrate downsampling when the input is high-res

Adds a new input ``zooms_min`` that will trigger the downsampling of the
input field map when it comes with a very high resolution (typically
this is the case for SDC SyN).

Although the input is resampled, the output field is interpolated at the
original resolution for debugging purposes.

Downsampling dramatically speeds up approximation.
Alternatively, a solution (for the SDC SyN) would be to calculate a
heavily dilated mask of the brain, and use it to limit the number of
voxels that are fit in regression.

sdcflows/interfaces/bspline.py

@@ -47,7 +47,6 @@
 DEFAULT_ZOOMS_MM = (40.0, 40.0, 20.0)  # For human adults (mid-frequency), in mm
 DEFAULT_LF_ZOOMS_MM = (100.0, 100.0, 40.0)  # For human adults (low-frequency), in mm
 DEFAULT_HF_ZOOMS_MM = (16.0, 16.0, 10.0)  # For human adults (high-frequency), in mm
-BSPLINE_SUPPORT = 2 - 1.82e-3  # Disallows weights < 1e-9
 LOGGER = logging.getLogger("nipype.interface")
 
 
@@ -76,6 +75,11 @@ class _BSplineApproxInputSpec(BaseInterfaceInputSpec):
         usedefault=True,
         desc="generate a field, extrapolated outside the brain mask",
     )
+    zooms_min = traits.Float(
+        1.4,
+        usedefault=True,
+        desc="limit minimum image zooms, set 0.0 to use the original image",
+    )
 
 
 class _BSplineApproxOutputSpec(TraitedSpec):
@@ -123,16 +127,45 @@ class BSplineApprox(SimpleInterface):
 
     def _run_interface(self, runtime):
         from sklearn import linear_model as lm
-        from scipy.sparse import vstack as sparse_vstack
+
+        # Output name baseline
+        out_name = fname_presuffix(
+            self.inputs.in_data, suffix="_field", newpath=runtime.cwd
+        )
 
         # Load in the fieldmap
         fmapnii = nb.load(self.inputs.in_data)
+        zooms = fmapnii.header.get_zooms()
+
+        # Get a mask (or define on the spot to cover the full extent)
+        masknii = (
+            nb.load(self.inputs.in_mask)
+            if isdefined(self.inputs.in_mask)
+            else None
+        )
+
+        need_resize = np.any(np.array(zooms) < self.inputs.zooms_min)
+        if need_resize:
+            from niworkflows.utils.images import resample_by_spacing
+
+            LOGGER.info(
+                "Resampling image with resolution exceeding 'zooms_min' "
+                f"({'x'.join(str(s) for s in zooms)})."
+            )
+            fmapnii = resample_by_spacing(fmapnii, [self.inputs.zooms_min] * 3)
+
+            if masknii is not None:
+                masknii = resample_by_spacing(masknii, [self.inputs.zooms_min] * 3)
+
         data = fmapnii.get_fdata(dtype="float32")
+
+        # Generate a numpy array with the mask
         mask = (
-            nb.load(self.inputs.in_mask).get_fdata() > 0
-            if isdefined(self.inputs.in_mask)
-            else np.ones_like(data, dtype=bool)
+            np.ones_like(fmapnii.dataobj, dtype=bool) if masknii is None
+            else np.asanyarray(masknii.dataobj) > 1e-4
         )
+
+        # Convert spacings to numpy arrays
         bs_spacing = [np.array(sp, dtype="float32") for sp in self.inputs.bs_spacing]
 
         # Recenter the fieldmap
@@ -145,45 +178,29 @@ def _run_interface(self, runtime):
         elif self.inputs.recenter == "mean":
             data -= np.mean(data[mask])
 
-        # Calculate the spatial location of control points
-        bs_levels = []
-        ncoeff = []
-        weights = None
-        for sp in bs_spacing:
-            level = bspline_grid(fmapnii, control_zooms_mm=sp)
-            bs_levels.append(level)
-            ncoeff.append(level.dataobj.size)
-
-            weights = (
-                gbsw(fmapnii, level)
-                if weights is None
-                else sparse_vstack((weights, gbsw(fmapnii, level)))
-            )
+        # Calculate collocation matrix & the spatial location of control points
+        colmat, bs_levels = _collocation_matrix(fmapnii, bs_spacing)
 
-        regressors = weights.T.tocsr()[mask.reshape(-1), :]
+        bs_levels_str = ['x'.join(str(s) for s in level.shape) for level in bs_levels]
+        bs_levels_str[-1] = f"and {bs_levels_str[-1]}"
+        LOGGER.info(
+            f"Approximating B-Splines grids ({', '.join(bs_levels_str)} [knots]) on a grid of "
+            f"{'x'.join(str(s) for s in fmapnii.shape)} ({np.prod(fmapnii.shape)}) voxels,"
+            f" of which {mask.sum()} fall within the mask."
+        )
 
         # Fit the model
         model = lm.Ridge(alpha=self.inputs.ridge_alpha, fit_intercept=False)
-        model.fit(regressors, data[mask])
-
-        interp_data = np.zeros_like(data)
-        interp_data[mask] = np.array(model.coef_) @ regressors.T  # Interpolation
-
-        # Store outputs
-        out_name = fname_presuffix(
-            self.inputs.in_data, suffix="_field", newpath=runtime.cwd
-        )
-        hdr = fmapnii.header.copy()
-        hdr.set_data_dtype("float32")
-        fmapnii.__class__(interp_data, fmapnii.affine, hdr).to_filename(out_name)
-        self._results["out_field"] = out_name
+        model.fit(colmat[mask.reshape(-1), :], data[mask])
 
+        # Store coefficients
         index = 0
         self._results["out_coeff"] = []
-        for i, (n, bsl) in enumerate(zip(ncoeff, bs_levels)):
+        for i, bsl in enumerate(bs_levels):
+            n = bsl.dataobj.size
             out_level = out_name.replace("_field.", f"_coeff{i:03}.")
             bsl.__class__(
-                np.array(model.coef_, dtype="float32")[index : index + n].reshape(
+                np.array(model.coef_, dtype="float32")[index:index + n].reshape(
                     bsl.shape
                 ),
                 bsl.affine,
@@ -192,6 +209,27 @@ def _run_interface(self, runtime):
             index += n
             self._results["out_coeff"].append(out_level)
 
+        # Interpolating in the original grid will require a new collocation matrix
+        if need_resize:
+            fmapnii = nb.load(self.inputs.in_data)
+            data = fmapnii.get_fdata(dtype="float32")
+            mask = (
+                np.ones_like(fmapnii.dataobj, dtype=bool) if masknii is None
+                else np.asanyarray(nb.load(self.inputs.in_mask).dataobj) > 1e-4
+            )
+            colmat, _ = _collocation_matrix(fmapnii, bs_spacing)
+
+        regressors = colmat[mask.reshape(-1), :]
+        interp_data = np.zeros_like(data)
+        # Interpolate the field from the coefficients just calculated
+        interp_data[mask] = regressors @ model.coef_
+
+        # Store interpolated field
+        hdr = fmapnii.header.copy()
+        hdr.set_data_dtype("float32")
+        fmapnii.__class__(interp_data, fmapnii.affine, hdr).to_filename(out_name)
+        self._results["out_field"] = out_name
+
         # Write out fitting-error map
         self._results["out_error"] = out_name.replace("_field.", "_error.")
         fmapnii.__class__(
@@ -205,8 +243,8 @@ def _run_interface(self, runtime):
             self._results["out_extrapolated"] = self._results["out_field"]
             return runtime
 
-        extrapolators = weights.tocsc()[:, ~mask.reshape(-1)]
-        interp_data[~mask] = np.array(model.coef_) @ extrapolators  # Extrapolation
+        extrapolators = colmat[~mask.reshape(-1), :]
+        interp_data[~mask] = extrapolators @ model.coef_  # Extrapolation
         self._results["out_extrapolated"] = out_name.replace("_field.", "_extra.")
         fmapnii.__class__(interp_data, fmapnii.affine, hdr).to_filename(
             self._results["out_extrapolated"]
@@ -457,6 +495,24 @@ def bspline_grid(img, control_zooms_mm=DEFAULT_ZOOMS_MM):
     return img.__class__(np.zeros(bs_shape, dtype="float32"), bs_affine)
 
 
+def _collocation_matrix(image, knot_spacing):
+    from scipy.sparse import vstack as sparse_vstack
+
+    bs_levels = []
+    weights = None
+    for sp in knot_spacing:
+        level = bspline_grid(image, control_zooms_mm=sp)
+        bs_levels.append(level)
+
+        weights = (
+            gbsw(image, level)
+            if weights is None
+            else sparse_vstack((weights, gbsw(image, level)))
+        )
+
+    return weights.T.tocsr(), bs_levels
+
+
 def _fix_topup_fieldcoeff(in_coeff, fmap_ref, pe_dir, out_file=None):
     """Read in a coefficients file generated by TOPUP and fix x-form headers."""
     from pathlib import Path