The effect of amplitude modulation on subharmonic imaging with chirp excitation
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control
Subharmonic generation from ultrasound contrast agents depends on the spectral and temporal properties of the excitation signal. The subharmonic response can be improved by using wideband and long duration signals. However, for sinusoidal tone-burst excitation the effective bandwidth of the signal is inversely proportional with the signal duration. Linear frequency modulated (LFM) and nonlinear frequency modulated (NLFM) chirp excitations allow independent control over the signal bandwidth and
... uration, therefore in this study LFM and NLFM signals were used for the insonation of microbubble populations. The amplitude modulation of the excitation waveform was achieved by applying different window functions. A customized window was designed for the NLFM chirp excitation by focusing on reducing the spectral leakage at the subharmonic frequency and increasing the subharmonic generation from microbubbles. Subharmonic scattering from a microbubble population was measured for various excitation signals and window functions. At a peak-negative pressure of 600 kPa, the generated subharmonic energy by ultrasound contrast agents was 15.4 dB more for NLFM chirp excitation with 40% fractional bandwidth when compared to tone-burst excitation. For this reason, the NLFM chirp with a customized window was used as an excitation signal to perform subharmonic imaging in an ultrasound flow phantom. Results showed that the NLFM waveform with a customized window improved the subharmonic contrast by 4.35 ± 0.42 dB on average over a Hann windowed LFM excitation.