Optimal Transmit Phasing on Tissue Background Suppression in Contrast Harmonic Imaging

Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
Ultrasound in medicine & biology (Impact Factor: 2.21). 07/2008; 34(11):1820-31. DOI: 10.1016/j.ultrasmedbio.2008.04.010
Source: PubMed


Ultrasonic harmonic imaging provides superior image quality than linear imaging and has become an important diagnostic tool in many clinical applications. Nevertheless, the contrast-to-tissue ratio (CTR) in harmonic imaging is generally limited by tissue background signal comprising both the leakage harmonic signal and the tissue harmonic signal. Harmonic leakage generally occurs when a wideband transmit pulse is used for better axial resolution. In addition, generation of tissue harmonic signal during acoustic propagation also decreases the CTR. In this paper, suppression of tissue background signal in harmonic imaging is studied by selecting an optimal phase of the transmit signal to achieve destructive cancellation between the tissue harmonic signal and the leakage harmonic signal. With the optimal suppression phase, our results indicate that the tissue signal can be significantly reduced at second harmonic band, whereas the harmonic amplitude from contrast agents shows negligible change with the selection of transmit phase. Consequently, about 5-dB CTR improvement can be achieved from effective reduction of tissue background amplitude in optimal transmit phasing. (E-mail: [email protected]
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    • "shen et al. proposed a technique transmitting a fundamental and a phase-shifted third-harmonic wave simultaneously to cancel the second harmonic [12], [13]. The same group also used the harmonic leakage signal to produce the secondharmonic reduction signal [14], [15]. all of the aforementioned methods are single-pulse techniques, and many multi-pulse techniques have also been proposed to increase the cTr. "
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    ABSTRACT: In ultrasound contrast imaging, many techniques based on multiple transmissions have been proposed to increase the contrast-to-tissue ratio (CTR). They are generally based on the response of static scatterers inside the imaged region. However, scatterer motion, for example in blood vessels, has an inevitable influence on multi-pulse techniques, which can either enhance or degrade the technique involved. This paper investigates the response of static nonlinear media insonated by multi-pulses with various phase shifts, and the influence of scatterer motion on multi-pulse techniques. Simulations and experimental results from a single bubble and clouds of bubbles show that the phase shift of the echoes backscattered from bubbles is dependent on the transmissions' phase shift, and that the bubble motion influences the efficiency of multi-pulse techniques: fundamental and second-harmonic amplitudes of the processed signal change periodically, exhibiting maximum or minimum values, according to scatterer motion. Furthermore, experimental results based on the second-harmonic inversion (SHI) technique reveal that bubble motion can be taken into account to regulate the pulse repetition frequency (PRF). With the optimal PRF, the CTR of SHI images can be improved by about 12 dB compared with second-harmonic images.
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    • "Currently, only the experimental transducers with interleaved high-and low-frequency elements are available (Bouakaz et al 2004, van Neer et al 2010). An alternative method to improve the ATR 2 by reducing P tis 2 was introduced by Shen and Hsieh (2008), who proposed to use the harmonic leakage signal to generate the second harmonic reduction signal. The imaging protocol was based on a constant phase shift between the emitted fundamental pulse and the leakage signal. "
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    ABSTRACT: Ultrasound contrast agents (UCAs) are small micro-bubbles that behave nonlinearly when exposed to an ultrasound wave. This nonlinear behavior can be observed through the generated higher harmonics in a back-scattered echo. In past years several techniques have been proposed to detect or image harmonics produced by UCAs. In these proposed works, the harmonics generated in the medium during the propagation of the ultrasound wave played an important role, since these harmonics compete with the harmonics generated by the micro-bubbles. We present a method for the reduction of the second harmonic generated during nonlinear-propagation-dubbed second harmonic inversion (SHI). A general expression for the suppression signals is also derived. The SHI technique uses two pulses, p' and p″, of the same frequency f(0) and the same amplitude P(0) to cancel out the second harmonic generated by nonlinearities of the medium. Simulations show that the second harmonic is reduced by 40 dB on a large axial range. Experimental SHI B-mode images, from a tissue-mimicking phantom and UCAs, show an improvement in the agent-to-tissue ratio (ATR) of 20 dB compared to standard second harmonic imaging and 13 dB of improvement in harmonic power Doppler.
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