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Optimal transmit phasing for harmonic-background suppression with bipolar square-wave pulser.

Che-Chou Shen¹, Yun-Chian Yang, Wen-Shiang Chen

¹Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. choushen@mail.ntust.edu.tw

Ultrasonic Imaging

|August 10, 2010

Summary

This study introduces a method using sigma-delta modulation for optimal transmit phasing in harmonic imaging. Bipolar waveforms effectively improve contrast-to-tissue ratio (CTR) in cardiac imaging, though phase selection can be challenging.

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Area of Science:

Ultrasound Physics
Medical Imaging Technology
Signal Processing

Background:

Optimal transmit phasing aims to enhance contrast-to-tissue ratio (CTR) in harmonic imaging by canceling harmonic components.
Clinical systems often use bipolar square-wave pulsers, necessitating binary conversion of continuous signals for arbitrary phasing.
Effective binary conversion is crucial for implementing optimal transmit phasing with bipolar waveforms.

Purpose of the Study:

To develop and evaluate a method for generating arbitrarily-phased bipolar waveforms for optimal transmit phasing.
To assess the efficacy of bipolar waveforms in achieving harmonic suppression and improving CTR in ultrasound imaging.
To investigate the clinical applicability of optimal transmit phasing using bipolar waveforms.

Main Methods:

Combined sigma-delta modulation with code tuning to generate bipolar waveforms.
Investigated harmonic suppression using a wire phantom comparing original and bipolar waveforms.
Performed in-vivo cardiac imaging in a rabbit model on a clinical ultrasound system.

Main Results:

Bipolar waveforms, when used in optimal transmit phasing, enable effective harmonic suppression in the tissue background.
In-vivo cardiac imaging demonstrated an average CTR improvement of 4.9 dB (p < 10(-8)) with bipolar waveforms.
Abrupt harmonic magnitude changes with transmit phasing were observed, potentially complicating optimal phase selection.

Conclusions:

Sigma-delta modulation provides a viable method for generating bipolar waveforms for optimal transmit phasing.
Bipolar transmit waveforms can significantly improve CTR in clinical harmonic imaging applications.
Challenges in optimal phase selection due to abrupt harmonic magnitude may limit the widespread clinical adoption of this technique.