def size
      volume = 0.0
      last_index = @slices.length - 1
      # Iterate each slice:
      @slices.each_index do |i|
        # Get the contoured area in this slice, convert it to volume and add to our total.
        # If the slice is the first or last, only multiply by half of the slice thickness:
        if i == 0 or i == last_index
          volume += @slices[i].area * image_series.slice_spacing * 0.5
        else
          volume += @slices[i].area * image_series.slice_spacing
        end
      end
      # Convert from mm^3 to cm^3:
      return volume / 1000.0
    end
def size
      volume = 0.0
      last_index = @slices.length - 1
      # Iterate each slice:
      @slices.each_index do |i|
        # Get the contoured area in this slice, convert it to volume and add to our total.
        # If the slice is the first or last, only multiply by half of the slice thickness:
        if i == 0 or i == last_index
          volume += @slices[i].area * image_series.slice_spacing * 0.5
        else
          volume += @slices[i].area * image_series.slice_spacing
        end
      end
      # Convert from mm^3 to cm^3:
      return volume / 1000.0
    end
Calculates the size (volume) of the ROI by evaluating the ROI's
 delination in the referenced image series.

 @return [Float] the ROI volume (in units of cubic centimeters)