Contrast-Enhanced Ultrasound Imaging with Chirps: Signal Processing and Pulse Compression

Contrast-enhanced ultrasound imaging creates one of the worst case scenarios for pulse compression due to depth and frequency dependent attenuation, high level of harmonic generation, phase variations due to resonance behavior of microbubbles, and increased broadband noise by microbubble destruction. This study investigates the feasibility of pulse compression with a matched filter in the existence of microbubbles with resonant behavior. Simulations and experimental measurements showed that the scattered pressure from a microbubble population excited by a chirp waveform preserves its chirp rate even for harmonic frequencies. Although, pulse compression by a matched filter was possible due to the conservation of the chirp rate, an increase on sidelobe levels were observed at fundamental and second harmonic frequencies. Therefore, using chirp excitation and a matched filter pair will increase the contrast-to-tissue ratio with a trade-off of decreased image quality

The Use of Fractional Fourier Transform for the Extraction of Overlapped Harmonic Chirp Signals

ABSTRACT In ultrasound harmonic imaging, high bandwidth is essential to provide good axial resolution at the receiver. However, increasing the bandwidth will cause overlapping amongst the nonlinear harmoni

Superharmonic imaging with chirp coded excitation: Filtering spectrally overlapped harmonics

Superharmonic imaging improves the spatial resolution by using the higher order harmonics generated in tissue. The superharmonic component is formed by combining the third, fourth, and fifth harmonics, which have low energy content and therefore poor SNR. This study uses coded excitation to increase the excitation energy. The SNR improvement is achieved on the receiver side by performing pulse compression with harmonic matched filters. The use of coded signals also introduces new filtering capabilities that are not possible with pulsed excitation. This is especially important when using wideband signals. For narrowband signals, the spectral boundaries of the harmonics are clearly separated and thus easy to filter; however, the available imaging bandwidth is underused. Wideband excitation is preferable for harmonic imaging applications to preserve axial resolution, but it generates spectrally overlapping harmonics that are not possible to filter in time and frequency domains. After pulse compression, this overlap increases the range side lobes, which appear as imaging artifacts and reduce the Bmode image quality. In this study, the isolation of higher order harmonics was achieved in another domain by using the fan chirp transform (FChT). To show the effect of excitation bandwidth in superharmonic imaging, measurements were performed by using linear frequency modulated chirp excitation with varying bandwidths of 10% to 50%. Superharmonic imaging was performed on a wire phantom using a wideband chirp excitation. Results were presented with and without applying the FChT filtering technique by comparing the spatial resolution and side lobe levels. Wideband excitation signals achieved a better resolution as expected, however range side lobes as high as -23 dB were observed for the superharmonic component of chirp excitation with 50% fractional bandwidth. The proposed filtering technique achieved >50 dB range side lobe suppression and improved the image quality without affecting the axial resolution