Signal-to-noise ratio and attenuation of Optison® microbubbles in blood as a function of imaging frequency

Abstract

Using an active cavitation detector (ACD), the power backscattered by various concentrations of Optison® microbubbles (signal) was compared to the power backscattered by a 50% hematocrit suspension of red blood cells in saline (noise), as a function of imaging frequency (5–30 MHz). A theoretical model, based on direct experimental measurements of the size distribution of Optison® microbubbles, was developed to predict the signal-to-noise ratio (SNR) of microbubbles in blood, assuming no interactions between the populations of scatterers. The SNR was shown experimentally to decrease with increasing imaging frequency up to a point where Optison® no longer provided image enhancement. Measurements of the SNR were repeated in a suspension of 0.8% hematocrit, which has the same backscattering coefficient as a 50% hematocrit suspension. The SNR for Optison® in 50% hematocrit was found to be lower than for the 0.8% hematocrit suspension at all frequencies, suggesting that the large number density and close proximity of the red blood cells inhibits the acoustic response of the microbubbles. Measurements of sound attenuation through suspensions of red blood cells with or without Optison® were also obtained, indicating that the microbubbles barely contribute to the overall attenuation. [Work supported by the NSF and the ASA.]