Ultrasonic energy backscattered from blood

Abstract

To determine how the ultrasonic energy scattered from flowing blood depends on the hematocrit, the rms amplitude of the audio Doppler signal from a range gated, pulsed Doppler ultrasonic flowmeter was measured for hematocrits ranging from 0.01 to 80%. For all investigated frequencies (5–27 MHz), the signal power, the square of the rms amplitude, was proportional to the hematocrit until 2%, reached a plateau at 10%, and was independent of the hematocrit over the rest of the investigated range. To determine how the scattered energy depends on the volume of the erythrocyte, we measured the variation of audio signal power with cell concentration for goat blood, and then without changing any of the instrument settings, repeated this measurement for dog blood. For cell concentrations ranging from 10⁴ to 10⁶ cells/mm³, the larger dog erythrocytes scattered 17 times more energy than the goat erythrocytes. This observation is consistent with the hypothesis that the energy scattered by a single erythrocyte is proportional to the square of the volume of the cell.