Absolute measurement of ultrasonic backscatter from single microbubbles

Good quality acoustical experiments are needed to measure microbubble behavior. An absolute calibration of the transmitted ultrasound field is possible using a calibrated hydrophone, but characterization of the received ultrasound beam is a more elaborate process and is not described in the literature. A new system based on a hydrodynamically focused flow has been used to measure echoes from single microbubbles at well specified positions in the ultrasonic field. An experimental set-up was built around a commercial scanner (Sonos5500, Philips Medical Systems) to measure the scatter from solid spheres with radii between 30 to 60 mu m. The behavior of these linear scatterers is accurately predicted by theory and software was produced to incorporate a simulation of the experimental conditions. The calibration of a phased array transducer was achieved by quantifying the receiver's spectral sensitivity for the range of receive frequencies (1.2 to 4.5 MHz). Examples of echoes from the microbubble agent Definity are used to illustrate the implementation of the calibration technique.