Contribution of the flow effect caused by shear-dependent RBC aggregation to NIR spectroscopic signals

Near-infrared spectroscopy (NIRS) is widely used to record activation-related blood oxygenation changes in human brain tissue. However, the changes in the NIRS signal upon increased flow are influenced not only by the hemoglobin and oxyhemoglobin concentrations but also by changes in light scattering by various brain constituents. This paper points out the large contribution of flow-dependent red blood cell (RBC) aggregation as a cause of this altered light scattering, a phenomenon which has not previously been considered in the theoretical analysis of NIRS signals. Here, we show that RBCs, which constitute a major chromophore in the tissue, not only absorb light at hemoglobin molecules but also scatter it strongly at the cell membranes of aggregated RBCs, and that the blood optical density per se changes greatly with the size of the plasma gap, which varies according to flow. When local blood flow increases by 50%, the amount of the optical attenuation due to RBC dispersion/disaggregation (the flow effect) can reach 90% of the NIRS signal change for venous blood. The reasons why the optical signal due to blood oxygenation alone can be amount to less than 10% of the total are because the near-infrared lies in the most unfavorable range in the hemoglobin absorption spectrum for determining blood oxygenation, while the flow effect in the NIR range is large. We conclude that reported activation-related changes in brain blood oxygenation, at least in the peripheral region around the activation focus, based on NIRS can be mainly ascribed to the flow effect arising from RBC dispersion/disaggregation with increased flow in the venous system.

Flow effect impacts NIRS, jeopardizing quantification of tissue hemoglobin

It has been generally agreed that changes in light transmission detected by NIRS are not exclusively attributable to hemoglobin changes, but also include a contribution from scattering by constituents of the brain tissue, which is not transparent, but turbid. If the light scattering is constant, light transmission can be quantified by an equation similar to Lambert-Beer's law. However, upon brain activation, it is known that the blood flow invariably increases. We have shown that light scattering by blood per se increases flow-dependently, interfering with the measurement of the intrinsic signals from hemoglobin. I call this the flow effect. Here, in responses to Pourtian's comments, I present additional evidence of the flow effect, which arises from RBC aggregation/dispersion, and further point out the inappropriateness of using NIR range wavelengths for the quantification of blood hemoglobin, even though their use is unavoidable in practice, because this wavelength range offers the best transmission through the tissue. In summary, I believe that the NIRS signal is a function of both flow (flow effect, which becomes the predominant contributor when blood flow increases) and metabolism (blood oxygenation change, which becomes accordingly a minor contributor). If the contribution of the flow effect is unknown, any mathematical equation describing the hemoglobin oxygenation is a Pfaffian equation which has no solution. This does not mean that I deny the clinical usefulness of NIRS, which definitely provides an indication of some biological changes in the brain in response to stimuli. However, great care is needed in the interpretation of the signals.