## 1 Introduction
### Near-Infrared Spectroscopy (NIRS)  
More than 30 years of extensive research has been invested into methods of measuring brain tissue oxygenation using noninvasive spectroscopy. The optical characteristics of oxygenated and deoxygenated hemoglobin that make pulse oximetry possible were first used to monitor cerebral tissue perfusion and oxygenation in 1977 with the development of near-infrared spectroscopy (NIRS) [18]. There has since been increasing interest in applying the technique to noninvasively detect brain tissue at risk for secondary injury. NIRS emits and detects reflected wavelengths between 700 and 1000 nm, a range in which the absorption by water molecules is low enough to allow signal transmission through tissues. Moreover, since melanin, which is responsible for skin color, has minimal absorption above 700 nm, NIRS signals should also be substantially independent of skin color (although there is recent controversy over the influence of skin color on oximetry measurements [19]). The oxidation state (or redox state) of optically active biological molecules can then be detected and their relative concentrations measured. In neuro-critical care, the concentrations of chromophores such as HbO and HbR within the target brain tissue are the most commonly measured biological molecules. This technique is used to detect cerebral hypoxia and malperfusion and has been able to identify patients with increasing intracranial pressure. The measurements [20] obtained trend with invasively monitored brain tissue oxygen tension, sparing the need for probes placed directly into the brain tissue. HbO and HbR spectral signals have even been coupled with blood pressure signals to provide a continuously updating measure of cerebrovascular reactivity in traumatic brain injury. Spectroscopically derived cerebrovascular reactivity indices demonstrated reasonable agreement with other surrogates of patient autoregulatory status and can identify patients with an increased risk of mortality or a poor functional outcome. While NIRS is a promising technique, multiple challenges limit its clinical adoption. Signal contamination by injured extracranial tissue and intracranial hematomas limit tissue penetration and the reliability of the signal obtained. Patient temperature, movement, and ambient light also interfere with reliable readings, lending to concerns about its accuracy. Finally, tissue oxygenation is still only a surrogate measure of neuronal function. Interpretation of this data is unclear, as decreased oxygen utilization may either reflect cellular dysfunction or decreased oxygen demand (as occurs in deep sedation). To determine if changes in brain tissue oxygen tension are indicative of cellular dysfunction, cellular metabolism should also be measured. Shock-induced derangements in cellular metabolism may appear earlier than systemic changes in the laboratory or hemodynamic parameters and may be evident prior to changes in diagnostic imaging. While structural damage to neurons may be visible in imaging, functional damage is not. Early detection of brain cell metabolic dysfunction could alert providers to the progression of secondary brain injury and detect a time-sensitive window for treatment. As an example of other applications, NIRS has been used in clinical settings to detect cerebral and distal limb ischemia, and different clinical interventions can be established based on the NIRS data [21]. In addition, vascular occlusion tests have demonstrated that NIRS can be used for noninvasive detection of tissue ischemia [22; 23]. Moreover, NIRS has been applied to other organs such as the kidney [24], and the kidney was shown to have a different response from the cerebral measurements. However, most of the NIRS studies used a few wavelengths of light and only measured the hemodynamic response. The broadband SCISCCO with the simultaneous measurement of HbO and HbR as well as CCO could provide much more information on tissue viability, as illustrated in this paper.