The effects of bubble size on nonlinear scattering from microbubbles

The influence of bubble size on nonlinear scattering at high ultrasound frequencies is investigated for a lipid encapsulated microbubble contrast agent. Simulations indicate that bubbles on the order of 1-2 microns in diameter and below can potentially participate in nonlinear scattering at transmit frequencies in the 20-30 MHz range. Experimental measurements of nonlinear scattering were made when the agent size distribution was manipulated by means of mechanical filtering and decantation techniques. By preferentially excluding larger bubbles (> 1-3 microns in diameter) it was observed that the ratios of harmonic (sub harmonic, ultra harmonic, and second harmonic) to fundamental energy were improved for transmit frequencies of 20 and 30 MHz. These results suggests that subpopulations of micron and sub-micron sized bubbles, which comprise a very small volume fraction of current agents, are giving rise to nonlinear signals in the ultrasound biomicroscopy (UBM) frequency range.