BUG: Fix bug with volume (#12544)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: larsoner

doc/changes/devel/12544.bugfix.rst

@@ -0,0 +1 @@
+Fix bug with :func:`mne.SourceSpaces.export_volume` where the ``img.affine`` was not set properly, by `Eric Larson`_.

mne/source_space/_source_space.py

@@ -668,10 +668,11 @@ def export_volume(
 
         # Figure out how to get from our input source space to output voxels
         fro_dst_t = invert_transform(transform)
-        dest = transform["to"]
         if coords == "head":
             head_mri_t = _get_trans(trans, "head", "mri")[0]
-            fro_dst_t = combine_transforms(head_mri_t, fro_dst_t, "head", dest)
+            fro_dst_t = combine_transforms(
+                head_mri_t, fro_dst_t, "head", transform["to"]
+            )
         else:
             fro_dst_t = fro_dst_t
 

mne/source_space/tests/test_source_space.py

@@ -874,6 +874,27 @@ def test_combine_source_spaces(tmp_path):
     with pytest.warns(RuntimeWarning, match="2 surf vertices lay outside"):
         src.export_volume(image_fname, mri_resolution="sparse", overwrite=True)
 
+    # gh-12495
+    image_fname = tmp_path / "temp-image.nii"
+    lh_cereb = mne.setup_volume_source_space(
+        "sample",
+        mri=aseg_fname,
+        volume_label="Left-Cerebellum-Cortex",
+        add_interpolator=False,
+        subjects_dir=subjects_dir,
+    )
+    lh_cereb.export_volume(image_fname, mri_resolution=True)
+    aseg = nib.load(str(aseg_fname))
+    out = nib.load(str(image_fname))
+    assert_allclose(out.affine, aseg.affine)
+    src_data = _get_img_fdata(out).astype(bool)
+    aseg_data = _get_img_fdata(aseg) == 8
+    n_src = src_data.sum()
+    n_aseg = aseg_data.sum()
+    assert n_aseg == n_src
+    n_overlap = (src_data & aseg_data).sum()
+    assert n_src == n_overlap
+
 
 @testing.requires_testing_data
 def test_morph_source_spaces():