Estimating concentration of ultrasound contrast agents with backscatter coefficients.

Abstract

Ultrasound contrast agents (UCAs) are used clinically to enhance the contrast of ultrasound images. Recently, microbubbles have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to acquire real-time estimates non-invasively. A method for measuring concentration, based upon backscatter coefficients (BSCs) at frequencies above resonance, was developed. Calculation of the BSC was accomplished using plane reference measurements from the back wall of a Plexiglas chamber. For each trial, an average of 500 snapshots of ultrasonic backscatter from Definity microbubbles flowing through the chamber was acquired. Immediately following this procedure, a sample of the UCAs was extracted from the flow path in order to optically verify the concentration estimates. Using measurements of attenuation coefficient through the bubble cloud, BSC was calculated for the 15–25 MHz range. UCA concentration was estimated by using a Levenberg–Marquardt fitting algorithm to match the experimental BSC to a linear scattering model. All BSC-based estimates were within one standard deviation of optically derived estimates. These results indicate that the BSC can be used to measure UCA concentration. [This work was supported by NIH Grant No. R37EB002641.]