Investigation of chirp coded excitation with dualfrequency transmit for ultrasonic harmonic contrast detection

Tissue background suppression is essential for harmonic detection of ultrasonic contrast microbubbles. To reduce the tissue harmonic amplitude for improvement of contrast-to-tissue ratio (CTR), the method of third harmonic (3f₀) transmit phasing utilizes dual-frequency transmit at both fundamental (f₀) and 3f₀ frequencies to provide mutual cancellation of the frequency-sum component (i.e. f₀+f₀) and the frequency-difference component (i.e. 3f₀-f₀) in tissue harmonic generation. To further increase the SNR in harmonic detection, however, conventional chirp transmit leads to the frequency-difference component of a single-frequency sinusoid while the frequency-sum component remains a chirp signal. The spectral mismatch between the cancellation pair produces marked harmonic residue in the tissue background and compromise the CTR improvement. Instead, we proposed a chirp waveform suitable for the 3f₀ transmit phasing, the different-bandwidth chirp signal (DBCS), that will generate the chirp signal for the frequency-difference component. Results indicate that the DB chirp not only markedly suppress the tissue harmonic amplitude over the entire second harmonic band for CTR improvement but also effectively increase the SNR. The DB chirp waveform can provide 6-dB improvement of SNR together with a CTR increase of 3 dB. The design of the DBCS waveform is also applicable to other dual-frequency imaging techniques which rely on the harmonic generation at the difference frequency.