Barker coded excitation with linear frequency modulated carrier for ultrasonic imaging

A new Barker coded excitation using linear frequency modulated (LFM) carrier (called LFM-Barker) is proposed for improving ultrasound imaging quality in terms of axial resolution and signal-to-noise ratio (SNR). The LFM-Barker coded excitation has two independent parameters: one is the bandwidth of LFM carrier, and the other is the chip duration of Barker code. To improve the axial resolution, increase the bandwidth of LFM carrier; and to improve the SNR, increase the chip duration of Barker code. In this study, a LFM pulse with proper (<5.5) time-bandwidth product is considered as the carrier in order to avoid sidelobes inside the mainlobe of matched filtered output. A pulse compression scheme for the LFM-Barker coded excitation is developed, and it consists of the LFM matched filter and Barker code mismatched filter. In the simulations, the impulse response of transducer can be approximated by a Gaussian shaped sinusoid with 5 MHz central frequency of 60% -6 dB fractional bandwidth. The pulse compression filter is performed to suppress sidelobes below -40 dB roughly, which is acceptable in medical imaging. Simulation results show that in comparison with conventional Barker coded excitation using sinusoid carrier (called Sinusoid-Barker), the axial resolution of the LFM-Barker coded excitation system can be doubled, and the SNR can be improved by about 3 dB. Simulation of B-mode images with the Field II program demonstrates that the axial resolution is improved from 0.7 mm to 0.4 mm. In addition, the LFM-Barker coded excitation is robust for frequency dependent attenuation of tissues.