Cell concentration is a crucial quantity for both clinical diagnostic examinations and cell culture studies. However, typical modalities for cell concentration measurements are either time-consuming or not cost-effective. In the present study, cell concentration is estimated using high-frequency ultrasonic backscattering. Validation tests indicate that the proposed method can differentiate red blood cells (RBCs) of various hematocrits. A 50-MHz ultrasound system with appropriate sensitivity is utilized to estimate cell concentrations from a small volume of RBCs suspended in saline, with hematocrits ranging from 1.66 × 10-4 to 10%, and fibroblasts, with concentrations ranging from 2 × 104 to 128 × 104
cells/mL. The backscatter strength and statistical distribution, characterized by the Nakagami parameter, are calculated
from gated signals for quantitatively assessing the samples. Results show that the backscatter strength of RBCs linearly increases with increasing hematocrit level in the hematocrit range of 3 to 10%, which agrees well with results of previous studies. The backscatter strength of RBCS has an exponential relationship with the hematocrit level in the hematocrit range of 1.66 × 10-4 to 3%. The corresponding Nakagami parameter is sensitive to electronic noise as long as the signal-to-noise ratio decreasing follows with the decrease of RBC hematocrits at the concentration lower than 0.85%. The backscatter strength of fibroblasts exponentially increases with increasing fibroblasts concentration, which is consistent with results obtained from typical optical density measurements. A linear relationship, with correlation coefficient of 0.99, between the results of ultrasonic backscattering and those of the optical density measurements is established. High-frequency ultrasonic backscattering can be applied to sensitively estimate the concentrations of small volumes of cells.