Near-infrared spectroscopy (NIRS) is a light-based method for assessing tissue function and viability. Due to the ability of light in the near-infrared range (700 to 1000 nm) to penetrate tissue, it is possible to interrogate deep tissues non-invasively by NIRS probes placed on the skin. The advantage of NIRS compared to conventional medical imaging approaches is the ability to perform measurements right at the bedside. Functional measures related to tissue viability that can be assessed by NIRS include tissue oxygenation, perfusion and energy metabolism.
One of our research teams has developed a unique NIRS method of measuring cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO2) using a light-absorbing dye as a vascular contrast agent. This method has been validated in a number studies (Brown Pediatr Res 2002, Tichauer J Cereb Blood Flow Metab 2006, Diop Rev Sc Inst 2009, Elliott J Biomed Opt 2010) and was recently used to monitor CBF and CMRO2 in preterm infants in the neonatal intensive care unit (Arora Pediatr Res 2013).
(A) CT perfusion image of a pig’s head showing an ischemic region in the cortex caused by injecting the vasoconstrictor endothelin-1. (B) Outline of the same pig’s head with the sensitivity volume of the optical measurements overlaid.
For continuous monitoring of brain function, the group has combined their quantitative bolus-tracking method with another optical technique, diffuse correlation spectroscopy (DCS), which is capable of real-time monitoring of relative CBF (Diop Biomed Opt Express 2011, Verdecchia J Biomed Opt 2013). This combination provides a unique means of assessing if CBF drops to ischemic levels in critical care patients.
Through a CFI infrastructure grant, these methods were adapted to time-resolved NIRS (Diop J Biomed Opt 2010). This state-of-the-art equipment detects photon arrival times, which can be used to discriminate between signals from different tissue layers (Diop Biomed Opt Express 2013). Using this approach, the team has shown that NIRS has the ability to monitor CBF even if there is sizable signal contamination from scalp and skull.
Through funding support from the CIHR and the HSFO, the next step is to develop a portable NIRS/DCS system that can be used in the intensive care unit to monitor brain function in patients with neurological emergencies, including stroke, cerebral hemorrhage and traumatic brain injury. They also plan to combine NIRS with MRI/PET to validate the optical measurements of CBF and CMRO2 in adults and to evaluate the ability of time-resolved NIRS to map brain function.
Correlation of CBF measurements from CT and NIRS. In these experiments the depth from skin surface to brain was 10-15 mm
Mel Boulton, Neurosurgery
Sandrine de Ribaupierre, Neurosurgery
Jeffery Carson, Medical Biophysics
Mamadou Diop, Medical Biophysics
David Lee, Neonatology
Ting-Yim Lee, Medical Imaging
Keith St. Lawrence, Medical Biophysics
Bryan Young, Neurology
A Liebert, Polish Academy of Sciences
A Kofke and A. Yodh, U of Pennsylvania