get_average_of_timeseries
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.. py:function:: petpal.utils.useful_functions.get_average_of_timeseries(input_image: ants.ANTsImage) -> ants.ANTsImage

   Get average of a 4D ANTsImage and return as a 3D ANTsImage.

   :param input_image: 4D PET image over which to compute timeseries average.
   :type input_image: ants.ANTsImage

   :returns: *mean_image (ants.ANTsImage)* -- 3D mean over time in the PET image.