A method for assessing the time of flight (TOF) performance of a positron emission tomography (PET) scanner is provided. The method may include obtaining raw data relating to radiation originating from an object from a PET scan by a PET scanner, the raw data including TOF information. The method may also include generating, based on a back projection algorithm, a first back projection image by reconstructing the raw data including the TOF information. The method may further include generating, based on the back projection algorithm, a second back projection image by reconstructing the raw data excluding the TOF information. The method may further include comparing the first back projection image with the second back projection image. The method may also include assessing, based on the comparison, the TOF performance of the PET scanner.