18F-F-AraG PET Imaging to Evaluate Immunological Response to CAR T Cell Therapy in Lymphoma

Inclusion Criteria: * Age ≥ 18 years old * Histologically confirmed aggressive B cell NHL including the following types defined by WHO 2008: * DLBCL not otherwise specified; T cell/histiocyte rich large B cell lymphoma; DLBCL associated with chronic inflammation; Epstein Barr virus (EBV)+ DLBCL of the elderly; OR * primary mediastinal (thymic) large B cell lymphoma * transformation of follicular lymphoma, marginal zone lymphoma or chronic lymphocytic leukemia to DLBCL will also be included * Measurable disease by PET imaging (as defined by Cheson (2014)), that meets all the following criteria: * At least one measureable lesion away from head \& neck, liver, kidneys, GI tract and bladder * At least one biopsy-accessible lesion or lymph node. * Express willingness to undergo low risk FNA or core biopsy of subcutaneous accessible lesion or lymph node. * Scheduled to receive commercial or research CAR T cell therapy with axicabtagene ciloleucel (Yescarta ®) as part of anticancer therapy. * Adequate renal and hepatic function, defined as: 1. Creatinine clearance (as estimated by Cockcroft Gault) ≥ 60 mL/min or Cr \< 1.6 mg/dL 2. Serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤ 2.5x upper limit of normal (ULN) 3. Total bilirubin ≤ 1.5 mg/dL, except in cases of Gilbert's syndrome * Able to give informed consent. Subjects unable to give informed consent will not be eligible for this study Exclusion Criteria: * Women who are pregnant or breastfeeding. * Subjects with significant GI disease involvement by PET imaging * In the investigator's judgment, have any medical condition likely to interfere with assessment of safety or efficacy, be unable to tolerate additional radiation, or be unlikely to complete all protocol-required visits and procedures.