Preliminary study in differential contrast echography

Abstract

Microbubble-based contrast agents have backscattering properties that differ greatly from those of soft tissues. These agents exhibit nonlinear scattering properties in response to incident acoustic energy, causing harmonic components in reflected energy. Even in linear scattering conditions, an important difference is observed in the frequency dependence of their backscatter coefficient, when compared to that of tissues. In this situation, any such differential behaviour can be exploited by imaging instrumentation to enhance the detection of agent-containing vessels against background tissue. The resulting B-mode image brightness (or pixel level) can thus be the representation of a local parameter computed from radio frequency (rf) signal processing of the raw echoes. The object of this article is to report on parametric imaging studies performed with Sonovue ™ (formerly code named BR1, Bracco Research SA), aimed at assessing the contrast-enhancing potential of spectral rf signal processing. The technique used, termed here “differential contrast echography” (DCE), is applicable in real-time B-mode imaging. It is based on an on-line subtractive approach, using dual-channel signal processing to implement differential filtering and demodulation. In this preliminary work, DCE was implemented off-line, on rf echo signals digitised from commercial B-mode scanners. Data acquisition, DCE processing and complete B-mode image reconstruction were programmed on a personal computer. The images presented were produced from test phantoms as well as animal phased array scanning. The phantom included a flow channel, background scattering material, fixed echogenic targets and echo-free regions. Animal scanning was performed on rabbit liver. The results obtained from DCE show promising contrast-enhancing properties. The regions containing no contrast agent are significantly suppressed from the image, preferentially leaving the regions perfused by the contrast agent. This property was favourable to experimenting with image-overlay presentations, superimposing colour-coded DCE imaging with standard log-compressed grey-scale B-mode, in a way analogous to duplex imaging combining colour Doppler and B-mode.