Participants will undergo the imaging protocol with CTP of head at the time of initial diagnostic imaging upon hospital arrival. Non-contrast CT (NCCT) of the head will be performed as part of current imaging protocol for severe TBI patients. At the time of the NCCT, participants will undergo whole-brain CTP protocol, ensuring coverage of the entire brainstem. A total of 40 mL of nonionic iodinated contrast media will be injected. Images will be acquired according to the investigators previously published imaging protocol. Cerebral blood flow (CBF) and cerebral blood volume (CBV) images from the CTP will be qualitatively assessed for the presence or absence of a matched decrease of CBF and CBV in the brainstem. CTP will be assessed independently by two radiologists blinded from each other. If disagreements arise, they will be resolved by discussion.