Assessment of red blood cell aggregation using normalized power spectrum of high frequency ultrasound

Red blood cell (RBC) aggregation, which is one of the induces showing blood viscosity, plays an important role in blood rheology. RBC aggregation is formed by adhension of RBCs because electrostatic repulsion between RBCs weakens as protein and saturated fatty acid in blood increase. Excessive RBC aggregation promotes various circulatory diseases in the clinical situation. The purpose of this study is to establish a noninvasive and quantitative method for assessment of RBC aggregation. The spectrum of nonaggregating RBCs presents Rayleigh behavior, which means that the power of scattered wave is in proportion to the fourth power of frequency. By dividing the measured power spectrum by that of echo from a silicone plate, the frequency responses of transmitting and receiving transducer are removed from the measured power spectrum. The normalized power spectrum changes linearly with respect to logarithmic frequency. In non-Rayleigh scattering, on the other hand, the spectral slope decreases. This is derived that a larger scatterer also behaves as a reflector and on echo from a reflector does not show frequency dependence. As a result, the influence of Rayleigh scattering is getting weak. Therefore, it is possible to assess the RBC aggregation from the spectral slope value. The spectral slope of the normalized power spectrum of echoes from the lumen of the vein in dorsum manus of 24-year-old healthy male was close to that from microspheres with diameter of 11 mum, and the standard RBC diameter is similar to this value. These results show the possibility of the proposed method for the noninvasive assessment of RBC aggregation.