suvr
=========================================

.. py:function:: petpal.preproc.standard_uptake_value.suvr(input_image_path: str, output_image_path: str | None, segmentation_image_path: str, ref_region: int | list[int], start_time: float, end_time: float) -> ants.ANTsImage

   Computes an ``SUVR`` (Standard Uptake Value Ratio) by taking the average of
   an input image within a reference region, and dividing the input image by
   said average value.

   :param input_image_path: Path to 4D PET image.
   :type input_image_path: str
   :param output_image_path: Path to output image file which is written to. If None, no output
                             is written.
   :type output_image_path: str
   :param segmentation_image_path: Path to segmentation image, which we use
                                   to compute average uptake value in the reference region.
   :type segmentation_image_path: str
   :param ref_region: Region or list of region mappings over which to compute average SUV. If a
                      list is provided, combines all regions in the list as one reference region.
   :type ref_region: int
   :param start_time: Time in seconds from the start of the scan from which to begin sum calculation.
                      Only frames after selected time will be included in the sum. Default 0.
   :param end_time: Time in seconds from the start of the scan from which to end sum calculation.
                    Only frames before selected time will be included in the sum. If -1, use all frames
                    after `start_time` in the calculation. Default -1.

   :returns: *ants.ANTsImage* -- SUVR parametric image