Counter-propagation interaction for contrast-enhanced ultrasound imaging

Most techniques for contrast-enhanced ultrasound imaging such as phase inversion and amplitude modulation require linear propagation to detect nonlinear scattering of contrast agent microbubbles. Waveform distortion due to nonlinear propagation impairs the ability to distinguish contrast agent microbubbles from tissue. This leads to erroneous detection of contrast agent behind any vessel or blood pool containing contrast agent. These artifacts can significantly impair the accuracy of medical diagnoses. Unlike biological tissue, lipid-coated gas microbubbles used as contrast agent allow the interaction of two ultrasound waves propagating in opposite directions (counter-propagation). We describe a strategy to detect microbubbles based on counter-propagation interaction that is free from nonlinear propagation artifacts. The new method was implemented in an ultrasound scanner (Philips iU22 and L9-3 probe) and tested in vitro in a tissue-mimicking phantom containing a cavity filled with a 1: 1000 dilution of BR14 contrast agent (Bracco). It is compared to the default contrast mode of the scanner employing amplitude modulation as a technique to detect contrast agent microbubbles. A low acoustic pressure (110 kPa peak pressure) as used in vivo for carotid imaging is employed in our in vitro study. Whereas amplitude modulation produces a pseudo-enhancement behind the cavity in the contrast image, the pulse sequence exploiting counterpropagating wave interaction is free from artifacts due nonlinear propagation in contrast agent.