The purpose of this study is to test two imaging techniques, one called whole body (WB) diffusion weighted (DWI) magnetic resonance imaging (MRI) (WB-DWI MRI), and another called Fluorine-18 3'-deoxy-3'-fluorothymidine positron emission tomography (PET) (F-18-FLT PET). The goal is to see whether these imaging techniques would allow the study doctors to see changes in the size of a tumor earlier for patients with metastatic melanoma receiving Pembrolizumab (MK-3475).