Third harmonic transmit phasing for SNR improvement in tissue harmonic imaging with Golay-encoded excitation

BackgroundUltrasound tissue harmonic signal generally provides superior image quality as compared to the linear signal. However, since the generation of the tissue harmonic signal is based on finite amplitude distortion of the propagating waveform, the penetration and the sensitivity in tissue harmonic imaging are markedly limited because of the low signal-to-noise ratio (SNR).MethodsThe method of third harmonic (3f0) transmit phasing can improve the tissue harmonic SNR by transmitting at both the fundamental (2.25 MHz) and the 3f0 (6.75 MHz) frequencies to achieve mutual enhancement between the frequency-sum and the frequency-difference components of the second harmonic signal. To further increase the SNR without excessive transmit pressure, coded excitation can be incorporated in 3f0 transmit phasing to boost the tissue harmonic generation.ResultsOur analyses indicate that the phase-encoded Golay excitation is suitable in 3f0 transmit phasing due to its superior transmit bandwidth efficiency. The resultant frequency-sum and frequency-difference components of tissue harmonic signal can be simultaneously Golay-encoded for SNR improvement. The increase of the main-lobe signal with the Golay excitation in 3f0 transmit phasing are consistent between the tissue harmonic measurements and the simulations. B-mode images of the speckle generating phantom also demonstrate the increases of tissue harmonic SNR for about 11 dB without noticeable compression artifacts.ConclusionFor tissue harmonic imaging in combination with the 3f0 transmit phasing method, the Golay excitation can provide further SNR improvement. Meanwhile, the axial resolution can be effectively restored by pulse compression while the lateral resolution remains unchanged.

Research highlights
► Golay-encoded excitation with dual-frequency transmit phasing improves tissue harmonic SNR.
► The harmonic components are phased to be constructively combined to enhance the harmonic magnitude.
► Experimental B-mode images show the SNR increase with negligible compression artifacts.
► The overall tissue harmonic image quality improves without degradation of image resolution.