A nonlinear processing technique for removing coherent interference artefacts

Ultrasound pulses propagating through human tissue appear to retain most of their initial coherence, and are coherently scattered from the many inhomogeneities within a tissue. A complex echo field is generated which exhibits many interference effects, the most familiar of these manifests itself as the ubiquitous speckle artefact. Speckle pervades almost all medical ultrasound pulse-echo signals and imposes a fundamental limit on signal and image quality. It is commonly assumed that the removal of speckle will produce a great advantage in a large number of practical applications. The novel approach developed here provides a general descriptive framework for interference effects, and is based on a description of interference by the presence of what we have termed structure zeros in the analytic continuation of the real data into the complex frequency, and complex time domains. The structure zeros may be uniquely identified if the form of the interrogating ultrasound pulse is precisely known. In practice, the latter requirement cannot be satisfied, and the presence of noise introduces a further element of uncertainty, but the structure zeros which make the dominant contribution to signal corruption may be unambiguously identified (via a sensitivity index) when short data segments are considered. Appropriate manipulation of the structure zero locations results in a specific, desired correction to the signal.