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; determining strain levels from the respiration data level data; compiling a histogram of the strain levels; selecting a fixed fraction parameter, wherein the fixed fraction parameter comprises a predetermined portion of the histogram; selecting a lower strain threshold value from measured lower strain levels; selecting upper strain threshold values for each measured lower strain level, wherein an interval defined by the upper strain threshold values and the lower strain threshold value includes the predetermined portion of the histogram; automatically selecting a final upper strain threshold value and a final lower strain threshold value pair that has a narrowest interval and encompasses the predetermined portion of the histogram; and using said automatically selected final lower and upper strain threshold value 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.