Abstract
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.
Highlights
I N medical ultrasound imaging, the spatial resolution is defined by the minimum resolvable distance between two point-scatterers
Since this study focuses on finite duration signals, for the real signal s(t), centered at the origin with duration T, the limits of the integral in Eq (3) reduce to −T /2 and T /2 as [41]: T /2
The pressure field was measured between the depths of 20 − 100 mm, where the maximum energy transfer to the superharmonic component was achieved at 86 mm
Summary
I N medical ultrasound imaging, the spatial resolution is defined by the minimum resolvable distance between two point-scatterers. To improve the lateral resolution, the aperture size of the ultrasound probe or the excitation frequency should be increased. Tissue harmonic imaging can improve both the lateral and axial resolution of an image without changing the excitation frequency or bandwidth. A harmonic image is formed by exploiting the second harmonic generated in tissue through nonlinear propagation, which effectively has twice the center frequency and the bandwidth of the excitation waveform [1]. Another advantage of harmonic imaging is the reduced near-field artifacts, since harmonics are generated in tissue through nonlinear propagation
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