Measurement of blood hematocrit inside the magnetically suspended centrifugal pump using an optical technique: Application to assessment of pump flow

To measure blood hematocrit inside the magnetically suspended centrifugal p ump, we have performed both forward and backward light scattering measureme nts using a specially designed optical cell. In the forward scattering meas urement, an optical fiber was used to guide the near infrared light at 780 nm into a 250 mu m gap region, and the light that forward scattered toward a detector fiber was measured using a phototransistor. The light intensity decreased exponentially with an increase in the hematocrit to around 20%. T he forward scattering method suffered from sensitivity at the hematocrit le vels around 25-45% due to the diffusion effect. By making the optical path length larger than several millimeters, the sensitivity of the forward scat tering method in terms of hematocrit change can be improved. In the back sc attering method, however, better sensitivity in terms of hematocrit change from 0-50% was obtained. By making the optical fiber separation distance le ss than 1 mm, the system will measure the first order back scattering from the shallow layer while, by making the fiber separation distance larger tha n several millimeters, the system will primarily measure the diffuse reflec tance from the deeper layer. Both approaches will yield sensitive optical i ntensity change in terms of the physiological hematocrit range.