High-Frequency Acoustic Backscattering from the Rayleigh Scatterers in Fluid Media

Abstract

The acoustic backscattering from Cochlodinium polykrikoides and red blood cell (RBC) was investigated using high-frequency acoustic measurement systems in the Rayleigh regime. C. polykrikoides is a phytoplankton which results in harmful algal blooms in the ocean. RBC is the most abundant cell in blood. The acoustic impedances of two objects are similar with those of corresponding media such as seawater and plasma so that C. polykrikoides and RBC are weak scatterers. Hence, high-frequency ultrasound was required for the measurement of backscattering. The acoustic integrated backscattered power (IBP) from C. polykrikoides was measured for 100 s every 15 min during a cultivation of 5 days. The duration of illumination was adjusted daily at 14 h of light and 10 h of darkness. As a result, the IBP was increased overall during the 5-day cultivation, but it was varied daily depending on the duration of illumination. The IBP was increased during light on and was decreased during light off. C. polykrikoides reproduces asexually by binary fission and forms a chain so that the IBP is increased overall during cultivation. However, the daily variation cannot be explained by the numerical abundance. It is hypothesized that photosynthesis affects the size of C. polykrikoides, because it was reported that the cell volume was increased due to the increase of chlorophyll a as the phytoplankton was photosynthesized. However, chlorophyll a was no longer produced when the photosynthesis was finished, but was released into the medium instead, resulting in the decrease of cell volume. This result expected to be utilized for the physiological research of plankton using acoustics. Blood echogenicity from whole blood (WB) and RBC suspension was measured at different flow speeds in a mock vessel system with a high-frequency ultrasound biomicroscopy (UBM). For WB, the blood echogenicity was increased as flow speed was decreased because the RBCs aggregated. These phenomena were clarified by the same measurement for RBC suspension. Blood echogenicity was not varied with flow speed for the RBC suspension because the RBC suspension prevents aggregation. Based on these results, blood echogenicity was measured in the radial artery of wrist. The cyclic variation of blood echogenicity was observed in the radial artery. This is because the blood echogenicity was increased and then decreased, resulting in RBC aggregation and disaggregation, respectively, under pulsatile flow, where the flow speed was changed due to the heartbeat. The cyclic variation of blood echogenicity in the radial artery was first observed by ultrasound up to our knowledge. This may contribute to the development of a system for measuring and monitoring blood properties noninvasively in vivo using acoustics. This paper describes the variation of acoustic backscattering from Rayleigh scatterers such as C. polykrikoides and RBC. The origins of acoustic backscattering variation were similar, but the mechanisms of biophysical phenomena of the scatterers were different from each other. Consequently, this study can be applied and help advance biological research using acoustics.