Estimation of Scatterer Diameter Using Ultrasonic Backscattering Property for Assessment of Red Blood Cell Aggregation

Abstract

Red blood cell (RBC) aggregation, a determinant of blood viscosity, plays an important role in blood flow rheology. RBC aggregation is induced by the adhesion of RBCs when the electrostatic repulsion between RBCs weakens owing to increases in protein and saturated fatty acid levels in blood, and excessive RBC aggregation may lead to various circulatory diseases. This study was conducted to establish a noninvasive quantitative method for the assessment of RBC aggregation. The spectrum of nonaggregating RBCs presents Rayleigh behavior, indicating that the power of a scattered wave is proportional to the fourth power of frequency. By dividing the measured power spectrum of echoes from scatterers by that from a silicone plate reflector, the frequency responses of transmitting and receiving transducers are removed from the former spectrum. This normalized power spectrum changes linearly with respect to logarithmic frequency. In non-Rayleigh scattering, on the other hand, the spectral slope decreases because a larger scatterer behaves as a reflector and echoes from a reflector do not show frequency dependence. Therefore, it is possible to assess RBC aggregation using the spectral slope value. In this study, spherical scatterers with diameters of 5, 11, 15, and 30 µm were measured in basic experiments. The spectral slope of the normalized power spectrum of echoes from the lumen of the vein in the dorsum manus of a 24-year-old healthy male was close to that from microspheres with a diameter of 15 µm, and the typical RBC diameter was smaller than this value. The frequency-dependent attenuation of ultrasound during propagation in a biomedical tissue was considered to be one reason for this. Furthermore, during avascularization, the slope gradually decreased owing to the aggregation of RBCs. These results show the possibility of using the proposed method for the noninvasive assessment of RBC aggregation.