Wideband sub-forcing harmonic phase inversion imaging

A new imaging technique is proposed which presents a significant new opportunity to differentiate bubble echoes from tissue echoes while maintaining maximum transmitted bandwidth. When insonified with a single-cycle pulse near twice the bubble´s linear resonance frequency, the echo spectrum is dependent on the phase of transmission. When rarefaction precedes compression in the transmission, the echo is centered near the frequency of transmission. When compression precedes rarefaction, the echo has a major peak near the bubble´s resonance frequency (one-half the driving frequency). We call this portion of the echo at one-half of the driving frequency the “sub-forcing harmonic.” The mechanism responsible for generating the lower center frequency involves exciting a resonant frequency oscillation of the bubble. The bubble echo can then be recognized by the shift in mean frequency between the two phases of transmission. With this technique, tissue echoes are rejected by both the lack of sub-forcing harmonic frequency content and the response to the phase inversion sequence. Thus, the discrimination of bubble and tissue echoes is significantly improved. To study the effect of transmission and bubble parameters, the modified Herring equation with shell terms is solved for the time dependent bubble radius and wall velocity and these outputs are used to formulate the predicted echo from a single encapsulated bubble. Experimental echoes are also recorded from a single bubble in a phantom vessel. The transmission of a pair of wideband (1.5 cycle) pulses with opposite phases yields echoes with considerable differences in frequency content. For a 1.1 μm radius bubble and a 6.5 MHz transmission with a peak negative pressure of 850 kPa, the mean frequency of the echo received from an 180° pulse is more than 1 MHz higher than that from the 0° pulse. The frequency shift due to sub-forcing harmonic generation is high for a range of transmitted center frequencies (5-9 MHz) and bubble diameters (1-3 μm) and the experimental echoes from a 5 MHz transmission confirm a shift of 1 MHz for a bubble in this size range