Radial modulation imaging of microbubble contrast agents at high frequency.

In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3-MPa 20-MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals related to the generation of harmonics by bubbles strongly resonating at the modulation frequency were observed. The bubble response to simultaneous modulation and imaging excitations was simulated for different combinations of bubble sizes and modulation amplitudes. Simulation results confirm the hypothesis that the generation of harmonics of the modulation frequency can be detected by the imaging transducer. Simulations indicate that the modulation frequency should be chosen lower than the resonant frequency of the biggest bubbles present in the population. The simulation also suggests that a 10% variation of bubble diameter induced by the modulation excitation is sufficient for radial modulation imaging. In conclusion, the effects of radial modulation are detectable at a high frequency. Therefore, radial modulation imaging has potential for high-resolution imaging of microbubbles in the microvasculature.