def add_couch_position(item, tag, value, current)
      if !self.send(current) || value != self.send(current)
        self.send("#{current}=", value)
        DICOM::Element.new(tag, (value.empty? ? '' : value.to_f * 10), :parent => item)
      end
    end
def add_couch_position(item, tag, value, current)
      if !self.send(current) || value != self.send(current)
        self.send("#{current}=", value)
        DICOM::Element.new(tag, (value.empty? ? '' : value.to_f * 10), :parent => item)
      end
    end
Adds a Table Top Position element to a Control Point Item.
 Note that the element is only added if there is no 'current' attribute
 defined, or the given value is different form the current attribute.

 @param [DICOM::Item] item the DICOM control point item in which to create the element
 @param [String] tag the DICOM tag of the couch position element
 @param [String, NilClass] value the couch position
 @param [Symbol] current the instance variable that keeps track of the current value of this attribute