A new MRI technique may prove useful for identifying occult brain injury in critically ill patients. Specifically, highresolution, three-dimensional MRI can be used to assess noninvasively differences in brain tissue sodium concentration, which is a highly sensitive marker of tissue viability that highlights areas that traditional MRI can miss [82] [83] [84] [85] [86] . The method is based on sodium ion homeostasis, which is tightly regulated in the body and is a major energy consuming process. Any event that perturbs the energy level of the cell enough to disrupt the sodium ion gradient, such as ischemia, has an important impact on cell viability. Although tissue sodium concentration MRI has been successfully used to evaluate the CNS, including nonhuman primate studies and clinical studies of stroke and reversible focal brain ischemia [87] [88] [89] , it has not been used to assess patients with either acute or chronic critical illnessassociated CD.