The disclosure relates to a system and method for correcting PET image data. PET image data of a first part of a subject may be obtained. CT image data of a second part of the subject may be obtained. The first part may include the second part. PET voxel data of the first part may be obtained based on the PET image data of the first part. A relationship between the CT image data and PET voxel data of the second part may be determined. CT image data of a third part of the subject may be determined based on the relationship and PET voxel data of the third part. The first part may include the third part. An attenuation map may be determined based on the CT image data of the second part and the third part. The PET image data of the first part may be corrected based on the attenuation map.