Allogeneic hematopoietic stem cell transplant (HSCT) remains the only curative modality for patients with aggressive hematologic malignancies and many nonmalignant hematologic disorders (genetic disorders and immunodeficiency disorders). Graft vs host disease (GVHD) is the major complication and cause of non-relapse mortality in allogeneic HSCT. GVHD is attributable to donor T cell recognition of recipient alloantigen, presenting commonly in skin, liver and gastrointestinal (GI-GVHD). Initial clinical suspicion of GI-GVHD is made by symptoms of severe weight loss and increased stool output >500cc (500->1500)per day, without other cause identified. Clinical diagnosis of GI-GVHD separately from chemotherapy/infectious colitis (CI- Colitis) requires colonoscopy/endoscopy followed by biopsy, but has < 60% intra-expert variability of pathological diagnosis. To overcome these limitations in diagnosis of GI-GVHD, we propose to: 1) test the safety of Clinical MSOT detecting oxy-/deoxy-hemoglobin, total hemoglobin, and collagen contrast in patients with GI-GVHD and to 2) evaluate the potential of clinical MSOT to differentiate GI-GVHD from chemotherapy/infectious colitis (CI-Colitis) to ultimately increase diagnostic accuracy, decrease patient risk, and decrease time to therapy. Building upon our experience in HSCT and autoimmune responses in patients, as well as experience with multispectral optoacoustic tomography, these proposed studies aim to assess the potential of MSOT imaging to differentiate GVHD from CI-Colitis with the ultimate goal to provide patients a transabdominal non-invasive, accurate and objective method to identify disease for individual patients in support of personalized medicine. We hypothesize that clinical MSOT imaging is safe for patients with compromised immune systems and that clinical MSOT can distinguish GVHD from CI-Colitis based upon a combination of oxy- and deoxy-hemoglobin and collagen. We will test this hypothesis by the following Aims: 1) Assess safety of clinical features of MSOT in patients with GI-GVHD or CI-Colitis; and 2) Evaluate potential of clinical MSOT to identify and stratify GI-GVHD in HSCT patients. Our study will be the first to test clinical MSOT in HSCT patients and the first proposed study to use MSOT differentiate diseases of similar clinical presentation but radically different therapies.