ENH: Add number of volumes per *b*-value to QC report

dmriprep/interfaces/vectors.py

@@ -29,6 +29,7 @@ class _CheckGradientTableOutputSpec(TraitedSpec):
     out_bvec = File(exists=True)
     full_sphere = traits.Bool()
     pole = traits.Tuple(traits.Float, traits.Float, traits.Float)
+    num_shells = traits.Dict()
     b0_ixs = traits.List(traits.Int)
 
 
@@ -47,6 +48,8 @@ class CheckGradientTable(SimpleInterface):
     (0.0, 0.0, 0.0)
     >>> check.outputs.full_sphere
     True
+    >>> check.outputs.num_shells
+    {0.0: 12, 1200.0: 32, 2500.0: 61}
 
     >>> check = CheckGradientTable(
     ...     dwi_file=str(data_dir / 'dwi.nii.gz'),
@@ -56,6 +59,8 @@ class CheckGradientTable(SimpleInterface):
     (0.0, 0.0, 0.0)
     >>> check.outputs.full_sphere
     True
+    >>> check.outputs.num_shells
+    {0: 12, 1200: 32, 2500: 61}
     >>> newrasb = np.loadtxt(check.outputs.out_rasb, skiprows=1)
     >>> oldrasb = np.loadtxt(str(data_dir / 'dwi.tsv'), skiprows=1)
     >>> np.allclose(newrasb, oldrasb, rtol=1.e-3)
@@ -81,6 +86,7 @@ def _run_interface(self, runtime):
         pole = table.pole
         self._results["pole"] = tuple(pole)
         self._results["full_sphere"] = np.all(pole == 0.0)
+        self._results["num_shells"] = table.count_shells
         self._results["b0_ixs"] = np.where(table.b0mask)[0].tolist()
 
         cwd = Path(runtime.cwd).absolute()

dmriprep/utils/vectors.py

@@ -1,5 +1,6 @@
 """Utilities to operate on diffusion gradients."""
 from .. import config
+from collections import Counter
 from pathlib import Path
 from itertools import permutations
 import nibabel as nb
@@ -174,6 +175,11 @@ def bvals(self, value):
             raise ValueError("The number of b-vectors and b-values do not match")
         self._bvals = np.array(value)
 
+    @property
+    def count_shells(self):
+        """Count the number of volumes per b-value."""
+        return Counter(sorted(self._bvals))
+
     @property
     def b0mask(self):
         """Get a mask of low-b frames."""