FIX: Conditionally drop isotropic frames

Author: effigies

nibabel/nicom/dicomwrappers.py

@@ -509,11 +509,14 @@ def image_shape(self):
         if hasattr(first_frame, 'get') and first_frame.get([0x18, 0x9117]):
             # DWI image may include derived isotropic, ADC or trace volume
             try:
-                self.frames = pydicom.Sequence(
+                anisotropic = pydicom.Sequence(
                     frame
                     for frame in self.frames
                     if frame.MRDiffusionSequence[0].DiffusionDirectionality != 'ISOTROPIC'
                 )
+                # Image contains DWI volumes followed by derived images; remove derived images
+                if len(anisotropic) != 0:
+                    self.frames = anisotropic
             except IndexError:
                 # Sequence tag is found but missing items!
                 raise WrapperError('Diffusion file missing information')