STUDY OF BLOOD SEDIMENTATION BY PHOTO-THE RMO RADIOMETRY WITH RANDOM-EXCITATION

The erythrocyte sedimentation rate is a complex phenomena involving a large number of parameters. The rate of sedimentation is highly depend ent on the haematocrit, the internal viscosity of the red cells and th e viscosity of the suspending medium and its composition. The experime ntal conditions also have a non-negligible effect (geometry and nature of the test tube, temperature, foreign substances in the medium...). In order to respond to the need for more precise and more rapid method s of analyzing the erythrocyte sedimentation rate, we developed new ph ysical methods allowing a real time evaluation of the phenomena involv ed. Several of these new photothermal methods have already been applie d for non-destructive evaluation of thin or layered material (such as composite material or glued structures) both in laboratory situations and in the industry. When a material is placed in a modulated laser be am, the incident rays absorbed heat the sample. The heat then diffuses throughout the material and the surface temperature of the sample inc reases locally with a periodicity. The surface thus emits a modulated flow of infrared radiation. The amplitude and phase shift of the photo thermal signal generated is characteristically dependent of the optic and thermal properties of the material for a given modulation frequenc y. The early photothermal modelization based on a two-layer model and a physico-mathematical theory of red cell sedimentation proposed by S. Oka made it possible to simulate the phenomena as they occur over tim e. We hypothesize that the temperature gradients created within the sa mple are to small to create a convection current and that the all heat transfer occurs by conduction. The experimental set-up is presented i n figure 1. The blood is irradiated with a green laser beam (488 nm). The light is absorbed at the plasma/blood interface creating a heat so urce within the sample which moves away from the free surface. The pho tothermic signal diminishes and at the phase term of the signal is mod ified as the thickness of the plasma increases. The experimental measu rements, including those made on samples of different concentrations, have generally been reproducible. Based on earlier work, blood sedimen tation and coagulation can be studied under modulated single frequency excitation. For the specific problem of whole blood sedimentation, mu ltifrequency measurements must be made to provide tomographic informat ion. These measurements must be made in a low frequency range in order to examine significant layer thicknesses and to observe the phenomena for a sufficient length of time. These advantages appear to be combin ed in photothermal radiometry under random excitation. The impulse and harmonic responses obtained on whole blood have given results coheren t with previous theories. Thus it has been possible to follow changes in plasma thickness in real time (fig. 2) and to produce a simultaneou s characterization of the changes in the optical absorption coefficien ts of blood and plasma during the first minutes of sedimentation (fig. 3a-b). Nevertheless, although this photothermal method appears to be useful for real-time analysis and evaluation of changes in blood compo nents without contact, the energy input required to obtain a photother mal signal is too great. Since the samples cannot be submitted to exce ssively strong excitation, the sensibility of the experimental set-up must be improved before further work on modelization and experimentati on can be performed.