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Contrast echo processing for very wideband systems using higher order statistics

For many clinical applications, it is desirable to use a single transmitted pulse per line of sight for contrast imaging in order to reduce the effects of tissue motion. Transmission of a low intensity is also preferred in order to reduce tissue clutter and avoid bioeffects. We present a new application of higher order statistical analysis for processing wideband contrast echoes. The imaging technique involves transmission at a frequency near resonance (2.25 MHz) with wideband reception centered at 4 times the transmitted frequency (10 MHz). It is well known that higher order statistics including the third and fourth order cumulant statistics and corresponding Fourier domain bispectra and trispectra are useful for identifying harmonics due to nonlinearities. The application of such signal processing to ultrasound contrast imaging remains relatively unexplored. Here, echoes obtained from a lipid-shelled ultrasound contrast agent are acquired using a 200-micron tube embedded in a tissue- mimicking phantom and characterized in terms of the fourth order cumulant. Salient differentiating features are shown when comparing statistics for tissue only and bubble plus tissue echoes. We take advantage of these features to detect bubble echoes with a suitable choice of cumulant lags and simple thresholding to produce a parametric image of UCA location.