Barker-coded ultrasound color flow imaging: theoretical and practical design considerations

Barker-coded excitation is applied to improve the sensitivity versus resolution tradeoff for ultrasound color flow imaging (CFI). Direct sequence encoding and complex baseband decoding methods that enable flexible combination of demodulation frequency and Barker-chip duration are proposed. Based on a general Wiener decoding filter formulation, three different conditions that pertain to the relationship between the Barker-chip duration and demodulation frequency are found, such that they result in real, complex symmetric and complex asymmetric decoding filter sequences, respectively. It is also shown that the matched filter and the inverse filter represent two extreme cases of the Wiener filter, and the latter is proposed for decoding Barker-coded CFI signals with maximum range sidelobe suppression. Some practical considerations such as coding gain, integrated sidelobe level (ISL) and peak sidelobe level (PSL) for various decoding filter lengths, and the influence of flow rate also are analyzed. Linear array imaging of steady flow in straight tubes is simulated based on a 4-cycle base pulse at 6.25 MHz, a 5-chip Barker code, a 32-tap decoding filter, and standard color flow data processing. The resultant color flow images demonstrate the expected improvements in penetration and axial resolution.