Article

Phase-dependent dual-frequency contrast imaging at sub-harmonic frequency.

Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control (Impact Factor: 1.8). 02/2011; 58(2):379-88. DOI: 10.1109/TUFFC.2011.1815
Source: PubMed

ABSTRACT Sub-harmonic imaging techniques have been shown to provide a higher contrast-to-tissue ratio (CTR) at the cost of relatively low signal intensity from ultrasound contrast agents (UCAs). In this study, we propose a method of dual-frequency excitation to further enhance the CTR of subharmonic imaging. A dual-frequency excitation pulse is an amplitude-modulated waveform which consists of two sinusoids with frequencies of f₁ (e.g., 9 MHz) and f₂ (e.g., 6 MHz) and the resulting envelope component at (f₁ - f₂) (e.g., 3 MHz) can serve as a driving force to excite the nonlinear response of UCAs. In this study, the f₂, at twice of the resonance frequency of UCAs, is adopted to efficiently generate a sub-harmonic component at half of the f₂ frequency, and f₁ is included to enhance the high-order nonlinear response of UCAs at the sub-harmonic frequency. The second- and third-order nonlinear components resulting from the envelope component would spectrally overlap at the sub-harmonic frequency when f₁ and f₂ are properly selected. We further optimize the generation of the sub-harmonic component by tuning the phase terms between second- and third-order nonlinear components. The results show that, with dual-frequency excitation, the CTR at sub-harmonic frequency improves compared with the conventional tone-burst method. Moreover, the CTR changes periodically with the relative phase of the separate frequency component in the dual-frequency excitation, leading to a difference of as much as 9.1 dB between the maximal and minimal CTR at 300 kPa acoustic pressure. The echo produced from the envelope component appears to be specific for UCAs, and thus the proposed method has the potential to improve both SNR and CTR in sub-harmonic imaging. Nevertheless, the dual-frequency waveform may suffer from frequency-dependent attenuation that degrades the generation of the envelope component. The deviation of the microbubble's resonance characteristics from the selection of dual-frequency transmission may also decrease the CTR improvement.

0 Bookmarks
 · 
106 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new ultrasound contrast imaging technique was proposed for eliminating the harmonic components from the emission signal transmitted by the broadband ultrasonic system. Reversal phase-inversion pulse was used for the first time to separate the contrast harmonics from the harmonics in the emission signal to improve the detection of contrast micro-bubbles. Based on the nonlinear acoustic theory of finite-amplitude effects and the associated distortion of the propagating wave, the Bessel-Fubini series model was applied to describe the nonlinear propagation effects of the reversal phase-inversion pulse, and the Church’s equation for zero-thickness encapsulation model was used to produce the scattering-pulse of the bubble. For harmonic imaging, the experiment was performed using a 64-element linear array, which was simulated by Field II. The results show that the harmonic components from the emission signal can be completely cancelled, and the harmonics generated by the nonlinear propagation of the wave through the tissue, can be reduced by 15–30 dB. Compared with the short pulse, the reversal phase-inversion pulse can improve the contrast and definition of the harmonic image significantly.
    Journal of Central South University. 20(3).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 to the signal duration. Linear frequency-modulated (LFM) and nonlinear frequencymodulated (NLFM) chirp excitations allow independent control over the signal bandwidth and duration; 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 with 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.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 12/2013; 60(12):2532-44. · 1.80 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nonlinear imaging was implemented in commercial ultrasound systems over the last 15 years offering major advantages in many clinical applications. In this work, pulsing schemes coupled with a dual frequency pulse are presented. The pulsing schemes considered were pulse inversion, power modulation, and power modulated pulse inversion. The pulse contains a fundamental frequency f and a specified amount of its second harmonic 2f. The advantages and limitations of this method were evaluated with both acoustic measurements of harmonic generation and theoretical simulations based on the KZK equation. The use of two frequencies in a pulse results in the generation of the sum and difference frequency components in addition to the other harmonic components. While with single frequency pulses, only power modulation and power modulated pulse inversion contained odd harmonic components, with the dual frequency pulse, pulse inversion now also contains odd harmonic components.
    The Journal of the Acoustical Society of America 05/2014; 135(5):2545. · 1.65 Impact Factor