In simultaneous PET/MRI, the reconstruction of PET image uses an MRI-based attenuation coefficient (AC) map for accurate quantization of radiotracer activity. The AC map is typically derived from SPGR images, but these are severely distorted near metallic implants due to strong off-resonance artifacts. We investigated the influence of the metal artifacts in the MR-based AC map upon the PET image accuracy. To correct the metal artifacts in PET, we present the use of multi-spectral imaging with field-map-based determination of meta boundary and type. Our correction result mitigated the signal underestimation near metal in PET images from 50% to 8%.