Improvement of Performance Degradation in Synthetic Aperture Extension of Enhanced Axial Resolution Ultrasound Imaging Based on Frequency Sweep.

Jing Zhu,Norio Tagawa

doi:10.3390/s19102414

Abstract

We are studying a method based on the carrier frequency sweep for axial high resolution ultrasonic imaging to provide the range resolution that corresponds to the carrier wavelength. The first proposal for this type of method was based on the focused pulse transmission. Then, to improve the frame rate, the method was extended to a synthetic aperture-type method that transmits divergent pulses. While the method is effective in terms of the frame rate, degradation of the enhanced axial resolution performance is a concern. Therefore, using finite element method simulations and simple experiments, the performance of the synthetic aperture method with high axial resolution is evaluated via comparison with the original method using focused pulses. The evaluation confirmed that the performance degradation of the synthetic aperture method is caused by weakness in the transmitted wave intensity and deterioration of the phase coherence in the reception beamforming. Based on this result, we propose a method that is less affected by the latter cause and show its effectiveness.

Highlights

In high-quality ultrasound imaging, the signal-to-noise ratio (SNR) is of primary importance, and a pulse compression technique (PCT) [1] that uses a frequency modulation (FM) chirp signal as the transmission pulse is commonly applied in this type of imaging
The four results are plotted together and are numbered as follows: (i) represents SA-pulse compression technique (SA-PCT), in which pulse compression was applied to the echoes that were received by each transducer and the synthetic aperture imaging (SAI) process was performed on these compressed echoes for all transmissions; (ii) represents the SA-super-resolution FM-chirp correlation method (SCM); (iii) represents the normal
For the SCM and PCT, the calculations were performed along the horizontal center line only, and the transmitted FM chirp pulse was focused on the object position

Summary

Introduction

In high-quality ultrasound imaging, the signal-to-noise ratio (SNR) is of primary importance, and a pulse compression technique (PCT) [1] that uses a frequency modulation (FM) chirp signal as the transmission pulse is commonly applied in this type of imaging. The improvement of the resolution in the range direction is mostly due to the broadening of the frequency band, especially harmonic imaging [4,5]. We focus on the resolution in the range direction, which is usually determined by the pulse width that is to be transmitted. By using a pulse composed of one cycle of a sine wave, the pulse width is narrowed, and the axial resolution is improved, but the SNR reduced. A pulse composed of a plurality of sine waves is often used. From this standpoint, we are studying a method to realize the resolution corresponding to the wavelength of the carrier wave

Methods

Results

Conclusion