Inclusion Criteria:  Male or female subject aged ≥ 18 years. Esophageal cancer patient eligible to undergo either neoadjuvant or definitive CRT.  Note: Stage IV patients with limited metastatic disease burden may be eligible if CRT is recommended by the multidisciplinary team.  ECOG Performance Status ≤ 2.  For female subjects: Negative pregnancy test or evidence of post-menopausal status. The post-menopausal status will be defined as having been amenorrheic for 12 months without an alternative medical cause. The following age-specific requirements apply:  Women < 50 years of age:  Amenorrheic for ≥ 12 months following cessation of exogenous hormonal treatments; and Luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution; or Underwent surgical sterilization (bilateral oophorectomy or hysterectomy).  Women ≥ 50 years of age:  Amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments; or Had radiation-induced menopause with last menses >1 year ago; or Had chemotherapy-induced menopause with last menses >1 year ago; or Underwent surgical sterilization (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy). Recovery to baseline or ≤ Grade 2 CTCAE v5 from toxicities related to any prior treatments, unless AE(s) are clinically non-significant and/or stable on supportive therapy. Able to provide informed consent and willing to sign an approved consent form that conforms to federal and institutional guidelines.  Exclusion Criteria:  Unable to undergo an MRI for any reason, including:  Severe claustrophobia not amenable to pre-medication Presence of metallic objects or implanted medical devices in the body that are not MRI-compatible (e.g., non-MRI-compatible cardiac pacemaker, deep brain stimulator, neurostimulator, aneurysm clips, surgical clips, prostheses, artificial hearts, valves with steel parts, metal fragments, shrapnel, tattoos near the eye, or steel implants) Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the radiation therapy are eligible for this trial.  The subject has severe, uncontrolled, significant intercurrent or recent illness that would exclude them from being a candidate for chemoradiation therapy.  --Any other condition that would, in the Investigator's judgment, contraindicate the subject's participation in the clinical study due to safety concerns or compliance with clinical study procedures.  Known HIV infection with a detectable viral load at the time of screening. Note: Patients on effective antiretroviral therapy or who will plan on going on antiretroviral therapy at the time of screening are eligible for this trial. HIV testing is not required for eligibility. Known prior severe hypersensitivity to gadolinium, FMISO or any component in its formulations (NCI CTCAE v5.0 Grade ≥ 3).