Direct and Post-Compressed Sound Fields for Different Coded Excitations

Abstract

Coded ultrasonography is intensively studied in many laboratories due to its remarkable properties; increased penetration depth, signal to noise (SNR) gain and improved axial resolution. However, no data concerning the spatial behavior of the pressure field generated by coded bursts transmissions were yet reported. This work reports the results of investigations using five different excitations schemes: two and sixteen periods sine bursts, 8 μs chirp (1 MHz bandwidth), and sinusoidal sequences phase modulated with 13 bits Barker code and 16 bits Golay complementary codes. Flat, circular transducer with 15 mm diameter, 2 MHz center frequency and 50% bandwidth was used in all experiments. The data were recorded using PVDF membrane hydrophone and collected using computerized scanning system developed in our laboratory. The results of measured pressure field distributions before and after compression were compared to those recorded using standard ultrasonographic short pulse excitation. The comparison clearly indicated increase in the SNR of the decoded pressure fields. Also, the modification of the spatial pressure field distribution, especially in the intensity and shape of the sidelobes, was apparent. As coded sequences are relatively long, intuitively, the beam shape or directivity pattern would be expected to be very similar to those produced by a long sine burst. Whereas the outcome of the experiments confirmed that this is correct for non-compressed distributions of the examined signals, in the case of the compressed sound fields, especially those generated for binary Golay sequences, the field patterns exhibited characteristics corresponding to those obtained using short, wideband bursts