Diffusion Weighted Magnetic Resonance in Imaging Younger Patients With Newly Diagnosed Bone or Soft Tissue Sarcomas

Whole body (WB) and primary tumor diffusion weighted imaging (DWI) will be performed at baseline in all subjects. Additional DW-MRIs will be done up to 3 times during treatment at the same time as routine MRI examinations are scheduled. Follow-up primary tumor DWI MRI examinations will be performed at time points determined by the participant's therapeutic treatment protocol. Follow-up primary tumor DWI examinations will be performed until completion of local control (surgical resection or completion of radiation therapy). All examinations will be performed on 1.5T Siemens magnetic resonance (MR) scanners unless there is a clinical indication for 3T imaging. Because investigators will correlate imaging parameters with patient outcome, participants will be followed until they are discharged to the After Completion of Therapy Clinic or until they have tumor progression or recurrence or develop a second malignancy or death, whichever comes first. PRIMARY OBJECTIVES: * To estimate the proportion of pediatric sarcoma patients whose bone/bone marrow and soft-tissue metastasis status are correctly staged by whole body diffusion weighted MRI (WB DWI) at the time of diagnosis by comparing it to clinical stage. * To determine the correlation between changes in primary pediatric sarcoma 18F-fluorodeoxyglucose positron emission tomography (FDG PET) maximum standardized uptake values (SUVmax) and average DWI apparent diffusion coefficient (ADC) values from baseline (pre-treatment) to just prior to local control. OTHER PRESPECIFIED OBJECTIVES: * To estimate the sensitivity, specificity, accuracy, and negative and positive predictive values of WB DWI, PET-CT and PET-CT + bone scan for detecting sites of metastatic tumor in pediatric sarcomas using biopsy or clinical follow-up (including imaging studies and clinical information) as the reference standards. * To compare the ability of WB MRI, PET-CT and PET-CT + bone scan to detect all sites of metastases in pediatric sarcoma patients using biopsy and clinical judgment as the reference standards. * To examine the associations of DWI ADC values of primary tumors at diagnosis and treatment protocol driven time points during therapy with tumor histology, tumor grade, RECIST response, % tumor necrosis, FDG SUVmax and patient outcome. * To compare whole body DWI to conventional T1W and STIR whole body MRI for the detection of nodal, bone/bone marrow, soft-tissue or lung metastases among pediatric bone and soft-tissue sarcoma patients using biopsy and clinical judgment as the reference standards. * To investigate the value of a variety of quantitative parameters obtained from software analysis of the primary tumor, such as; histogram kurtosis, range, peak, shifts in peak, or tumor heterogeneity.