 This pilot clinical trial studies how well positron emission tomography PETmagnetic resonance imaging MRI works compared to PETcomputed tomography CT in diagnosing younger patients with cancer. PETMRI and PETCT are procedures that combine the pictures from a PET scan and an MRI scan or a CT scan. The PET and MRI scans or PET and CT scans are done at the same time with the same machine. The combined scans give more detailed pictures of areas inside the body than either scan gives by itself. It is not yet known whether PETMRI works better than PETCT in diagnosing younger patients with cancer. 