A CT imaging system and a method for determining a CT collimator slit profile. The method includes determining a profile of two opposite edges of the collimator slit in a longitudinal direction thereof based on the following: a vertical distance between a focus of a radiation source to the collimator slit, a vertical distance between the focus and the radiation detector, an inclination angle between adjacent detector elements, a length of each detector element, a desired width of projection on the radiation detector by the radiation rays passing through the slit whose longitudinal edge profile is to be determined, and an offset angle of a connecting line from a point on a longitudinal center line of the slit to the focus relative to a plane passing said focus and perpendicular to the slit.
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