A device may include a collimator positioned between a radiation source of a scanner and a bore of the scanner. The bore may include a detecting region configured to accommodate a subject. The collimator may be configured to prevent at least one portion of radiation rays emitted from the radiation source from being incident on the subject. The device may further include a first filter and a second filter. The first filter may be positioned between the radiation source and the collimator. The second filter may be positioned between the collimator and the bore. The first filter and the second filter may be configured to adjust a distribution of radiation impinging upon the subject.