New optical probe designs for absolute (self-calibrating) NIR tissue hemoglobin measurements.

Most of the instruments reported for near infrared absorption measurements in highly scattering media, suitable for noninvasive hemoglobin concentration measurements in living tissues, require same type of instrumental calibration, or knowledge of the initial concentrations of the two species. A new type of probe, which does not require calibration, has been developed. These new probes can provide a rapid and "absolute" measurement of absorption and scattering in tissue when combined with the "multi-distance" method developed, in part, at the Laboratory for Fluorescence Dynamics at the University of Illinois and ISS Incorporated. An initial evaluation of the performance and efficacy of these new probes has been performed using a frequency-domain ISS tissue oximeter. The new probes offer the convenience of not requiring calibration, and also provide more reliable measurements and greater long-term stability compared with the standard multi-distance design. The new probes rely on a symmetrical arrangement of light sources and detectors. These geometries allow for measurements that are, in theory, entirely independent of the intensity differences between light sources, phase differences between the multiple light sources, sensitivity differences between the detectors, and any optical coupling losses, including those at the probe sample interface.