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:  Creatinine clearance (as estimated by Cockcroft Gault) ≥ 60 mL/min or Cr < 1.6 mg/dL Serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤ 2.5x upper limit of normal (ULN) 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.