Determination of the transport scattering coefficient of red blood cells

There are no reliable values for the transport scattering coefficient (mu(s)') of whole blood, particularly at the high hematocrits found in-vivo. Existing values are largely based on single scattering measurements of scattering cross-section and angular scattering. The accuracy of the single scattering method is limited by the accuracy with which the g-value can be measured, as it is near 0.99. Diffuse measurements, coupled to diffusion theory models, are normally hampered by the relatively high absorption coefficient of oxy-hemoglobin even in the near infrared. Blood was drawn from 9 healthy volunteers, heparinised, and the cells washed twice with phosphate buffered saline. Oxyhemoglobin was converted to carbonmonoxide-hemoglobin, which has an absorption coefficient that is approximately 5% of that of oxy-hemoglobin in the near infrared region. Picosecond pulses of light were transmitted through an 80x60x20 mm cuvette and the resulting temporal point spread functions recorded with a streak camera. The data were fitted to a time-dependent diffusion model to derive values for mu(s)' and mu(a) which is now valid because of the low mu(a) Of carbonmonoxide-hemoglobin. The effect of changing the hematocrit was determined by stepwise dilution of the washed red cells with further phosphate buffered saline. Mean values for mu(s)' and mu(a) at 802 nm were 3.15+/-0.12 mm(-1) and 0.0098+/-0.0004 mm(-1) respectively for the hematocrit in the normal range. The undiluted red cells showed a small negative wavelength dependence for mu(s)' over the range 740 to 860 nm with a slope of -0.0045+/-0.0011 mm(-1) nm(-1) which was highly significant (p=0.0001). The negative wavelength dependence of mu(s)' is compatible with Mie theory predictions of scattering by large particles. A near linear relationship between mu(s)' versus hematocrit (H) was found in this study (up to H=0.45) as opposed to the mu(s)' proportional to H(1 - H) or mu(s)' proportional to H(1 - H)(1.4 - H) macroscopic scattering coefficient relationship of others.