Coded tissue harmonic imaging with nonlinear chirp signals

Coded tissue harmonic imaging with pulse inversion (CTHI-PI) based on a linear chirp signal can improve the signal-to-noise ratio with minimizing the peak range sidelobe level (PRSL), which is the main advantage over CTHI with bandpass filtering (CTHI-BF). However, the CTHI-PI technique could suffer from motion artifacts due to decreasing frame rate caused by two firings of opposite phase signals for each scanline. In this paper, a new CTHI method based on a nonlinear chirp signal (CTHI-NC) is presented, which can improve the separation of fundamental and harmonic components without sacrificing frame rate. The nonlinear chirp signal is designed to minimize the PRSL value by optimizing its frequency sweep rate and time duration. The performance of the CTHI-NC method was evaluated by measuring the PRSL and mainlobe width after compression. From the in vitro experiments, the CTHI-NC provided the PRSL of −40.6 dB and the mainlobe width of 2.1 μs for the transmit quadratic nonlinear chirp signal with the center frequency of 2.1 MHz, the fractional bandwidth at −6 dB of 0.6 and the time duration of 15 μs. These results indicate that the proposed method could be used for improving frame rates in CTHI while providing comparable image quality to CTHI-PI.

Research highlights
► Pulse inversion(PI) is preferred for coded tissue harmonic imaging with linear chirp.
► It could suffer from slow frame rates and motion artifacts.
► The quadratic nonlinear chirp significantly lowers these artifacts without PI.