 While there are studies that have invasively monitored cerebral saturation and brain tissue oxygen in severe traumatic brain injury sTBI patients, there are none using non-invasive cerebral saturation monitoring in TBI patients undergoing packed red blood cell pRBC transfusion. To date, all published studies have involved invasive monitoring with their concomitant potential side effects. Insertion of invasive probes and monitors has several risks and side effects including bleeding, local trauma and brain damage, and infection. Furthermore, they have limited utility as information is restricted to the region of the brain surrounding the probe, as opposed to a more global picture. We therefore propose an observational study using non-invasive near infrared spectroscopy to monitor brain tissue oxygen during the transfusion of packed red blood cells. 