Tolerate missing ReferencedInstanceSequence (#380)

* Allow missing ReferencedInstanceSequence

* Add test

Author: CPBridge

src/highdicom/image.py

@@ -3980,10 +3980,14 @@ def _include_sequence(seq):
                             instance_sequence = (
                                 ref_series.ReferencedSOPSequence
                             )
-                        else:
+                        elif 'ReferencedInstanceSequence' in ref_series:
                             instance_sequence = (
                                 ref_series.ReferencedInstanceSequence
                             )
+                        else:
+                            # This is invalid according to the standard, but
+                            # has been observed in some data
+                            instance_sequence = []
 
                         for ref_ins in instance_sequence:
                             instance_data.append(

tests/test_image.py

@@ -1439,3 +1439,17 @@ def test_imread_from_dicom_bytes_io_lazy():
         # Two reads to ensure opening/closing is handled
         im.get_frame(1)
         im.get_frame(2)
+
+
+def test_imread_missing_referenced_instances():
+    # Test for the case where a ReferencedSeriesSequence is present but does
+    # not list instances. This is not valid by the standard but is observed and
+    # we should be tolerant to it
+    dcm = get_testdata_file('eCT_Supplemental.dcm', read=True)
+
+    # Add ReferencedSeriesSequence without any referenced instances
+    ref_series = pydicom.Dataset()
+    ref_series.SeriesInstanceUID = UID()
+    dcm.ReferencedSeriesSequence = [ref_series]
+
+    Image.from_dataset(dcm, copy=False)