Study on an evaluation method of shell characteristics of ultrasound contrast agent using a high-speed camera

New methods for ultrasound contrast imaging that use amplitude-encoded ultrasound pulses have recently been developed. An understanding of various behaviors of bubbles exposed to ultrasound pulses with different pressures is important for using these methods. In order to study the dependence of bubble behavior pressure, a high-speed observation system was improved to generate pulses that have various pressures at pulse intervals of several tens of microseconds. A microbubble was exposed to five ultrasound pulses with increasing pressures (normalized pressure of 1.0, 1.2, 1.4, 1.6 and 1.8 at pulse intervals of 20 μs) The total frame number of the high-speed camera is 24, and 5 frames were assigned to the first 4 pulses and 4 frames were assigned to the first to the last pulse. The ultrasound contrast agents Definity and Levovost were used in the experiments, and the center frequency of ultrasound pulses was set at 1 MHz. It was found that the amplitude of bubble oscillation generally increased with increase in pulse pressure, though discontinuous increase in oscillation amplitude was frequently observed in the results obtained using Levovist. This strong nonlinear relationship between pressure of ultrasound pulse and amplitude of radial oscillation should be considered in amplitude-encoded contrast imaging methods.