Comparison of pulse subtraction Doppler and pulse inversion Doppler

A challenge in molecular imaging with targeted contrast agents is the development of methods able to detect and distinguish microbubbles attached to regions of disease from the high background signal from freely circulating microbubbles and tissue. Combining a nonlinear contrast agent detection technique with Doppler has potential to perform this discrimination. So far, pulse inversion Doppler (PID), is able to distinguish linear from nonlinear moving scatterers by making use of the even harmonics generated by the microbubbles. However it has been shown that adherent microbubbles emit a stronger fundamental signal than freely flowing ones. In this paper, a new Doppler technique based on pulse subtraction imaging (PSD) is described and compared with PID. The results show that PSD is able to differentiate bubble motion from tissue motion and could find applications in targeted microbubble imaging. For Doppler processing conducted at the fundamental frequency, the contrast to tissue ratio (CTR) in PSD was on average 3.3 (± 0.4) times higher than PID at a mechanical index (MI) of 0.1. At the harmonic frequency, PID has shown to have a 3.1 (± 0.4) times higher CTR than PSD. The CTR has also been shown to increase with increasing MI.