diff --git a/src/depiction/image/image_normalization.py b/src/depiction/image/image_normalization.py
index 77e2f8f..0d66664 100644
--- a/src/depiction/image/image_normalization.py
+++ b/src/depiction/image/image_normalization.py
@@ -28,7 +28,7 @@ def normalize_xarray(self, image: xarray.DataArray, variant: ImageNormalizationV
         else:
             raise NotImplementedError("Multiple index columns are not supported yet.")
 
-    def _normalize_single_xarray(self, image: xarray.DataArray, variant: ImageNormalizationVariant):
+    def _normalize_single_xarray(self, image: xarray.DataArray, variant: ImageNormalizationVariant) -> xarray.DataArray:
         if variant == ImageNormalizationVariant.VEC_NORM:
             return image / (((image**2).sum(["c"])) ** 0.5)
         elif variant == ImageNormalizationVariant.STD:
@@ -36,7 +36,9 @@ def _normalize_single_xarray(self, image: xarray.DataArray, variant: ImageNormal
         else:
             raise NotImplementedError(f"Unknown variant: {variant}")
 
-    def _normalize_multiple_xarray(self, image: xarray.DataArray, index_dim: str, variant: ImageNormalizationVariant):
+    def _normalize_multiple_xarray(
+        self, image: xarray.DataArray, index_dim: str, variant: ImageNormalizationVariant
+    ) -> xarray.DataArray:
         dataset = image.to_dataset(dim=index_dim)
         normalized = dataset.map(
             lambda x: self._normalize_single_xarray(x, variant=variant),
diff --git a/src/depiction/image/multi_channel_image.py b/src/depiction/image/multi_channel_image.py
index 1e7084f..5299dd2 100644
--- a/src/depiction/image/multi_channel_image.py
+++ b/src/depiction/image/multi_channel_image.py
@@ -1,14 +1,17 @@
 from __future__ import annotations
 
 from functools import cached_property
-from pathlib import Path
-from collections.abc import Sequence
 
 import numpy as np
 import xarray
 from xarray import DataArray
 
 from depiction.image.sparse_representation import SparseRepresentation
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from collections.abc import Sequence
+    from pathlib import Path
 
 
 class MultiChannelImage:
diff --git a/src/depiction/image/sparse_representation.py b/src/depiction/image/sparse_representation.py
index 6f894eb..3e55c5e 100644
--- a/src/depiction/image/sparse_representation.py
+++ b/src/depiction/image/sparse_representation.py
@@ -40,6 +40,7 @@ def sparse_to_dense(cls, sparse_values: DataArray, coordinates: DataArray, bg_va
         for i_channel in range(n_channels):
             values_grid[tuple(coordinates_shifted.T) + (i_channel,)] = sparse_values[:, i_channel]
 
+        # TODO coordinates might come in the wrong order FIXME
         return DataArray(values_grid, dims=("y", "x", "c"))
 
     @classmethod
diff --git a/src/depiction/image/xarray_helper.py b/src/depiction/image/xarray_helper.py
index 319870a..96c37c1 100644
--- a/src/depiction/image/xarray_helper.py
+++ b/src/depiction/image/xarray_helper.py
@@ -60,8 +60,8 @@ def apply_on_spatial_view(array: DataArray, fn: Callable[[DataArray], DataArray]
             # remove nan
             result = result_flat.dropna("i", how="all").drop_isel(c=-1)
 
-            # TODO assigning the coords will be broken in the future, when "i" is a multi-index, however since in general
-            #      it is not, this will require a case distinction
+            # TODO assigning the coords will be broken in the future, when "i" is a multi-index,
+            #      however since in general it is not, this will require a case distinction
             return result.assign_coords(i=original_coords)
         else:
             raise ValueError(f"Unsupported dims={set(array.dims)}")