Comparison of pulse inversion and second harmonic for ultrasound contrast imaging

Real-time modalities for ultrasound contrast imaging based on the pulse inversion (PI) principle are proposed to be more sensitive than second harmonic (SH) imaging modalities. This is explained as a better separation of nonlinear bubble response. By using two pulses, PI will also be sensitive to changes in the contrast agent due to bubble destruction and motion. By analysis of RF data from in vitro experiments, we are able to compare PI and SH under equal conditions. The RF data were acquired with output amplitude varying over a large scale, and allowed separation of nonlinear bubble responses from decorrelation effects caused by bubble destruction and motion. Our findings are that PI give an increase both in contrast-to-noise ratio and contrast-to-tissue ratio. The increased sensitivity observed at low mechanical index is partly a result of bubble motion and destruction, but also get a significant contribution from asymmetric response of bubbles. Frequency analysis shows that the observed nonlinear response from bubbles is not restricted to the harmonic band, but also occur in the fundamental band. Because of this, special care must be taken to optimize the band-pass filter to use for PI