Optimal transmit phasing on tissue background suppression in contrast harmonic imaging.
ABSTRACT 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.
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ABSTRACT: The impulse response of a transducer can be rep- resented in the frequency domain by its complex analog, the transfer function. The amplitude transfer function is measured regularly in contrast to the phase transfer function (PTF). Applications for the PTF range from adjusting the emitted pulse shape for coding based imaging to the optimization of ultrasound contrast imaging methods based on destructive interference. A number of acoustic methods to measure a transducer's PTF ex- ists, but they usually require accurate distance and acoustic wave speed measurements. Small discrepancies in these cause large phase errors. We present a pulse-echo method to measure a trans- ducer's PTF without needing a measurement of the wave travel distance and speed. We generalize it to rectangular transducers. In our method the transducer is excited by a monofrequency sine burst with a rectangular envelope. The transducer initially vibrates at resonance (transient regime) prior to the forcing frequency (steady state regime). The PTF value of the system is the difference between the phases deduced from the transient and the steady state regimes at different forcing frequencies. As the PTF is calculated from a relative difference measuring the wave travel distance or speed is unnecessary. The approach assumes linear wave propagation and uses a pulse-echo setup. The method was tested on a custom built single element transducer (square: 13 x 13 mm, center frequency 4 MHz, no backing or matching layers). The results were compared with KLM model simulations. Also, we phase calibrated a hydrophone, which was then used to measure the PTF of the square transducer. The simulated and measured resonance frequencies differed by 0.17 MHz. The mean PTF difference between simulation and measurements was 7 - 14 . The method's reproducibility was 15 . The PTF of the transducer was measured with good reproducibility, without measuring the wave travel distance or speed of sound in the medium. Our simple setup requires basic laboratory ultrasound equipment. Index Terms—phase; pulse-echo; hydrophone; rectangular transducers; transducer calibration.01/2010;
Conference Paper: Influences of bubble motion to second-harmonic inversion imaging[Show abstract] [Hide abstract]
ABSTRACT: In ultrasound contrast harmonic imaging, the generated harmonics during wave propagation in tissues lead to a limited contrast-to-tissue ratio (CTR). Second-harmonic inversion (SHI), based on two 90° phase-shifted transmissions with the same frequencies and amplitudes, can effectively reduce the tissue-harmonic signals and improve CTR. However, the bubbles motion strongly influences the SHI effectiveness. Therefore, in-vitro experiments on circulating UCA were conducted to quantify the influence of moving bubbles to SHI technique, by regulating the pulse repetition frequency (PRF), in order to guarantee and to optimize the SHI technique. Experimental results show that when bubbles move in axial direction, the second-harmonic amplitudes exhibits an oscillating evolution with increasing bubbles motions. The maximum improvement reaches 12 dB when compared to SHI without bubbles motion. The influence of lateral bubbles motion is much less than the influence of axial bubbles motion.Ultrasonics Symposium (IUS), 2012 IEEE International; 10/2012
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ABSTRACT: Ultrasound harmonic imaging is limited by the harmonic components which are produced during wave propagation because of the native nonlinearities of the tissue. A newly proposed method, named Second Harmonic Inversion (SHI) suggests transmitting successively two pulses with the same frequency, the same amplitude and a 90° phase difference to reduce the second harmonic generated by tissue. This newly proposed SHI method is carried out on an open system equipped with a bipolar square-wave pulser and a linear probe. Measurements performed in water, on a general purpose ultrasound phantom and with a tissue mimicking phantom with circulating contrast agents are investigated. Experimental results show that SHI method can be easily implemented on an open system. Both radio frequency signals analysis and B-mode ultrasound images show that SHI method decreases significantly the native second-harmonic tissue components existing in standard harmonic images. Contrast-to-tissue ratio (CTR) of SHI image is improved by 4.6dB when compared to standard harmonic image and improved by 3.6dB when compared to PI image. SHI method enhances CTR through effective tissue generated second harmonic reduction. Moreover, the easy implementation procedure and the better specificity make SHI an interesting alternative to PI method.Ultrasonics Symposium (IUS), 2011 IEEE International; 10/2011