1. A method for providing respiration-gated CT image data of a patient, comprising the steps of: performing an emission scan of said patient together with measurement of patient respiration level data; selecting a lower respiration level and upper respiration level pair from said measured level data, which has a narrowest range and encompasses a predetermined portion of the entire emission scan time; and using said selected lower and upper respiration level pair to develop a trigger signal to gate a CT image scan using a respiration signal from said patient to acquire said gated CT image data.