A method for imaging cancer using a combined PET-MRI system takes advantage of the performance characteristics for both PET and MRI in the context of cancer imaging. MRI is used to assess a large area of the body with high sensitivity for cancer and PET is then used in localized areas of concern to provide physiological information. Optionally, MRI may also then be used to re-scan the localized areas of concern with high spatial resolution and additional tissue contrasts to provide anatomical information and soft tissue contrast to supplement the PET information. The use of a combined PET-MRI system ensures that the imaging data from both modalities is accurately referenced to the same locations in the body.