Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.     

Coded tissue harmonic imaging with nonlinear chirp signals

Coded tissue harmonic imaging with pulse inversion (CTHI-PI) based on a linear chirp signal can improve the signal-to-noise ratio with minimizing the peak range sidelobe level (PRSL), which is the main advantage over CTHI with bandpass filtering (CTHI-BF). However, the CTHI-PI technique could suffer from motion artifacts due to decreasing frame rate caused by two firings of opposite phase signals for each scanline. In this paper, a new CTHI method based on a nonlinear chirp signal (CTHI-NC) is presented, which can improve the separation of fundamental and harmonic components without sacrificing frame rate. The nonlinear chirp signal is designed to minimize the PRSL value by optimizing its frequency sweep rate and time duration. The performance of the CTHI-NC method was evaluated by measuring the PRSL and mainlobe width after compression. From the in vitro experiments, the CTHI-NC provided the PRSL of −40.6 dB and the mainlobe width of 2.1 μs for the transmit quadratic nonlinear chirp signal with the center frequency of 2.1 MHz, the fractional bandwidth at −6 dB of 0.6 and the time duration of 15 μs. These results indicate that the proposed method could be used for improving frame rates in CTHI while providing comparable image quality to CTHI-PI.     

Nakagami-m parametric imaging for characterization of thermal coagulation and cavitation erosion induced by HIFU

Nowadays, both thermal and mechanical ablation techniques of HIFU associated with cavitation have been developed for noninvasive treatment. A specific challenge for the successful clinical implementation of HIFU is to achieve real-time imaging for the evaluation and determination of therapy outcomes such as necrosis or homogenization. Ultrasound Nakagami-m parametric imaging highlights the degrading shadowing effects of bubbles and can be used for tissue characterization. The aim of this study is to investigate the performance of Nakagami-m parametric imaging for evaluating and differentiating thermal coagulation and cavitation erosion induced by HIFU. Lesions were induced in basic bovine serum albumin (BSA) phantoms and ex vivo porcine livers using a 1.6 MHz single-element transducer. Thermal and mechanical lesions induced by two types of HIFU sequences respectively were evaluated using Nakagami-m parametric imaging and ultrasound B-mode imaging. The lesion sizes estimated using Nakagami-m parametric imaging technique were all closer to the actual sizes than those of B-mode imaging. The p-value obtained from the t-test between the mean m values of thermal coagulation and cavitation erosion was smaller than 0.05, demonstrating that the m values of thermal lesions were significantly different from that of mechanical lesions, which was confirmed by ex vivo experiments and histologic examination showed that different changes result from HIFU exposure, one of tissue dehydration resulting from the thermal effect, and the other of tissue homogenate resulting from mechanical effect. This study demonstrated that Nakagami-m parametric imaging is a potential real-time imaging technique for evaluating and differentiating thermal coagulation and cavitation erosion.     

A preliminary in vitro assessment of polymer-shelled microbubbles in contrast-enhanced ultrasound imaging

This paper focuses on the use of poly (vinyl alcohol)-shelled microbubbles as a contrast agent in ultrasound medical imaging. The objective was an in vitro assessment of the different working conditions and signal processing methods for the visual detection (especially in small vessels) of such microbubbles, while avoiding their destruction. Polymer-shelled microbubbles have recently been proposed as ultrasound contrast agents with some important advantages. The major drawback is a shell that is less elastic than that of the traditional lipidic microbubbles. Weaker echoes are expected, and their detection at low concentrations may be critical. In vitro experiments were performed with a commercial ultrasound scanner equipped with a dedicated acquisition board. A concentration of 100 bubbles/mm³, excitation pressure amplitudes from 120 kPa to 320 kPa, and a central frequency of 3 MHz or 4.5 MHz were used. Three multi-pulse techniques (i.e., pulse inversion, contrast pulse sequence based on three transmitted signals, and contrast pulse sequence in combination with the chirp pulse) were compared. The results confirmed that these microbubbles produce a weaker ultrasound response than lipidic bubbles with a reduced second-order nonlinear component. Nevertheless, these microbubbles can be detected by the contrast pulse sequence technique, especially when the chirp pulse is adopted. The best value of the contrast-to-tissue ratio was obtained at an excitation pressure amplitude of 230 kPa: although this pressure amplitude is higher than what is typically used for lipidic microbubbles, it does not cause the rupture of the polymeric contrast agent.     

Dual-high-frequency ultrasound excitation on microbubble destruction volume

Objective and motivation

The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects.

Methods

Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 μs 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue^TM contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the - 6 dB level.

Results

The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2 mm.

Conclusion

These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles.     

Nonlinear chirp compensation by scan filtering in SAIL

The nonlinear chirp of a tunable laser generates the phase errors and damages the range resolution in synthetic aperture imaging ladar (SAIL). In the compensation algorithms establishing matched and nonmatched reference paths, the phase errors were compensated in a whole echo pulse. In this paper a compensation algorithm of nonlinear chirp by scan filtering is proposed.The heterodyne signals of different echoes from all target points in a footprint are scan filtered from one whole heterodyne signal of one whole echo pulse in the spectrum. The phase errors of these heterodyne signals are measured by phase-shifting algorithm in reference path and compensated separately. Then all the compensated signals are combined back into a whole heterodyne pulse and compressed in range. After all heterodyne pulses are compressed in range, the azimuth compensation is followed.The mathematical flow of this compensation algorithm is established. The simulation of the airborne SAIL model validates the feasibility, and the bandwidth of range compression decreases obviously. The effects of nonlinear chirp and the pass bandwidth of the scan filter are analyzed and discussed finally.     

Compression and amplification of an ultrasonic pulse reflected from a one-dimensional layered structure

Compression of ultrasonic pulses reflected from layered structures is studied. A short pulse is emitted into water towards a structure consisting of solid plates backed with an air layer. Due to multiple reflections in the structure, the signal is elongated. The reflected signal is received by the same transducer and digitized. After that, the wave is reversed in time and emitted towards the layered structure for the second time; then, the reflected signal is received. Due to the invariance of the processes under the time reversal, the pulse is compressed by the structure: the reflected signal becomes shorter and acquires the waveform of the initial pulse. The possibility of an efficient compression of signals is demonstrated experimentally. Numerical simulations show that the use of more complex structures can considerably increase the compression ratio and produce short signals of a much higher amplitude than that emitted by the transducer. An efficient compression algorithm is proposed.     

Third harmonic transmit phasing for SNR improvement in tissue harmonic imaging with Golay-encoded excitation

Background

Ultrasound tissue harmonic signal generally provides superior image quality as compared to the linear signal. However, since the generation of the tissue harmonic signal is based on finite amplitude distortion of the propagating waveform, the penetration and the sensitivity in tissue harmonic imaging are markedly limited because of the low signal-to-noise ratio (SNR).

Methods

The method of third harmonic (3f₀) transmit phasing can improve the tissue harmonic SNR by transmitting at both the fundamental (2.25 MHz) and the 3f₀ (6.75 MHz) frequencies to achieve mutual enhancement between the frequency-sum and the frequency-difference components of the second harmonic signal. To further increase the SNR without excessive transmit pressure, coded excitation can be incorporated in 3f₀ transmit phasing to boost the tissue harmonic generation.

Results

Our analyses indicate that the phase-encoded Golay excitation is suitable in 3f₀ transmit phasing due to its superior transmit bandwidth efficiency. The resultant frequency-sum and frequency-difference components of tissue harmonic signal can be simultaneously Golay-encoded for SNR improvement. The increase of the main-lobe signal with the Golay excitation in 3f₀ transmit phasing are consistent between the tissue harmonic measurements and the simulations. B-mode images of the speckle generating phantom also demonstrate the increases of tissue harmonic SNR for about 11 dB without noticeable compression artifacts.

Conclusion

For tissue harmonic imaging in combination with the 3f₀ transmit phasing method, the Golay excitation can provide further SNR improvement. Meanwhile, the axial resolution can be effectively restored by pulse compression while the lateral resolution remains unchanged.     

Study for imaging of inside bone using FM-chirp pulse compression system

We measured the attenuation of ultrasound in animal bone in vitro, and determined successfully the correct dynamic range for the detection of echo signals from inside the bone. It was possible to display a B-mode image of a 6-mm thick porcine rib bone using a high frequency (5 MHz) transducer. However, it was difficult to obtain the image of a large bone (20-mm thick femur). This paper describes a possibility of the improvement for the detection of echoes from inside the bone using the FM-chirp pulse. An ultrasonic wave generator that generates the FM-chirp pulse for a frequency range of 100 KHz to 5 MHz was fabricated. And the pulse was transmitted from a transducer and received by a hydrophone in the water. The received waveform and the frequency spectrum were displayed on an oscilloscope. We measured the amplitude spectrum of the received waves before and after inserting a sample bone between the transducer and the hydrophone. In the experiment, three types of focused transducers (0.5, 1 and 2.25 MHz) and three kinds of sample bones (spine, femur and rib of a pig) were used. From the results of the measurement, we could find the useful information for imaging of inside the bone.     

Coded excitation for ultrasound tissue harmonic imaging

Coded excitation can improve the signal-to-noise ratio (SNR) in ultrasound tissue harmonic imaging (THI). However, it could suffer from the increased sidelobe artifact caused by incomplete pulse compression due to the spectral overlap between the fundamental and harmonic components of ultrasound signal after nonlinear propagation in tissues. In this paper, three coded tissue harmonic imaging (CTHI) techniques based on bandpass filtering, power modulation and pulse inversion (i.e., CTHI-BF, CTHI-PM, and CTHI-PI) were evaluated by measuring the peak range sidelobe level (PRSL) with varying frequency bandwidths. From simulation and in vitro studies, the CTHI-PI outperforms the CTHI-BF and CTHI-PM methods in terms of the PRSL, e.g., −43.5 dB vs. −24.8 dB and −23.0 dB, respectively.     

Study of ultrasound stiffness imaging methods using tissue mimicking phantoms

A pilot study was carried out to investigate the performance of ultrasound stiffness imaging methods namely Ultrasound Elastography Imaging (UEI) and Acoustic Radiation Force Impulse (ARFI) Imaging. Specifically their potential for characterizing different classes of solid mass lesions was analyzed using agar based tissue mimicking phantoms. Composite tissue mimicking phantom was prepared with embedded inclusions of varying stiffness from 50 kPa to 450 kPa to represent different stages of cancer. Acoustic properties such as sound speed, attenuation coefficient and acoustic impedance were characterized by pulse echo ultrasound test at 5 MHz frequency and they are ranged from (1564 ± 88 to 1671 ± 124 m/s), (0.6915 ± 0.123 to 0.8268 ± 0.755 db cm^-1 MHz^-1) and (1.61×10⁶ ± 0.127 to 1.76 × 10⁶ ± 0.045 kg m^-2 s^-1) respectively. The elastic property Young’s Modulus of the prepared samples was measured by conducting quasi static uni axial compression test under a strain rate of 0.5 mm/min upto 10 % strain, and the values are from 50 kPa to 450 kPa for a variation of agar concentration from 1.7% to 6.6% by weight. The composite phantoms were imaged by Siemens Acuson S2000 (Siemens, Erlangen, Germany) machine using linear array transducer 9L4 at 8 MHz frequency; strain and displacement images were collected by UEI and ARFI. Shear wave velocity 4.43 ± 0.35 m/s was also measured for high modulus contrast (18 dB) inclusion and X.XX m/s was found for all other inclusions. The images were pre processed and parameters such as Contrast Transfer Efficiency and lateral image profile were computed and reported. The results indicate that both ARFI and UEI represent the abnormalities better than conventional US B mode imaging whereas UEI enhances the underlying modulus contrast into improved strain contrast. The results are corroborated with literature and also with clinical patient images.     

Continuous-wave ultrasound reflectometry for surface roughness imaging applications

Background

Measurement of surface roughness irregularities that result from various sources such as manufacturing processes, surface damage, and corrosion, is an important indicator of product quality for many nondestructive testing (NDT) industries. Many techniques exist, however because of their qualitative, time-consuming and direct-contact modes, it is of some importance to work out new experimental methods and efficient tools for quantitative estimation of surface roughness.

Objective and method

Here we present continuous-wave ultrasound reflectometry (CWUR) as a novel nondestructive modality for imaging and measuring surface roughness in a non-contact mode. In CWUR, voltage variations due to phase shifts in the reflected ultrasound waves are recorded and processed to form an image of surface roughness.

Results

An acrylic test block with surface irregularities ranging from 4.22 μm to 19.05 μm as measured by a coordinate measuring machine (CMM), is scanned by an ultrasound transducer having a diameter of 45 mm, a focal distance of 70 mm, and a central frequency of 3 MHz. It is shown that CWUR technique gives very good agreement with the results obtained through CMM inasmuch as the maximum average percent error is around 11.5%.

Conclusion

Images obtained here demonstrate that CWUR may be used as a powerful non-contact and quantitative tool for nondestructive inspection and imaging of surface irregularities at the micron-size level with an average error of less than 11.5%.     

A novel method for estimating the focal size of two confocal high-intensity focused ultrasound transducers

Estimating the focal size and position of a high-intensity focused ultrasound (HIFU) transducer remains a challenge since traditional methods, such as hydrophone scanning or schlieren imaging, cannot tolerate high pressures, are directional, or provide low resolution. The difficulties increase when dealing with the complex beam pattern of a multielement HIFU transducer array, e.g., two transducers facing each other. In the present study we show a novel approach to the visualization of the HIFU focus by using shockwave-generated bubbles and a diagnostic B-mode scanner. Bubbles were generated and pushed by shock waves toward the HIFU beam, and were trapped in its pressure valleys. These trapped bubbles moved along the pressure valleys and thereby delineated the shape and size of the HIFU beam. The main and sidelobes of 1.1- and 3.5 MHz HIFU beams were clearly visible, and could be measured with a millimeter resolution. The combined foci could also be visualized by observing the generation of sustained inertial cavitation and enhanced scattering. The results of this study further demonstrate the possibility of reducing the inertial cavitation threshold by the local introduction of shock wave-generated bubbles, which might be useful when bubble generation and cavitation-related bioeffects are intended within a small region in vivo.     

超声换能器对线性调频信号脉冲压缩性能的影响分析*

该文研究了不同编码带宽下超声换能器对编码信号脉冲压缩后信噪比与轴向分辨率的影响关系。将脉冲压缩后的时域峰值转化为频域积分的形式,得到考虑换能器影响的脉冲压缩信噪比公式。以线性调频信号为例,仿真与实验结果表明,编码激励相对于方波激励的信噪比增益随编码带宽的减小而增大,因为受换能器带宽限制,编码激励的轴向分辨率随编码带宽的增大先减小后趋于稳定。该研究为编码激励方法更有效地应用于超声检测的背景中提供了参考。     

Reflector-based phase calibration of ultrasound transducers

Recently, the measurement of phase transfer functions (PTFs) of piezoelectric transducers has received more attention. These PTFs are useful for e.g. coding and interference based imaging methods, and ultrasound contrast microbubble research. Several optical and acoustic methods to measure a transducer’s PTF have been reported in literature. The optical methods require a setup to which not all ultrasound laboratories have access to. The acoustic methods require accurate distance and acoustic wave speed measurements. A small error in these leads to a large error in phase, e.g. an accuracy of 0.1% on an axial distance of 10 cm leads to an uncertainty in the PTF measurement of ±97° at 4 MHz. In this paper we present an acoustic pulse-echo method to measure the PTF of a transducer, which is based on linear wave propagation and only requires an estimate of the wave travel distance and the acoustic wave speed. 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 response (steady state regime). The PTF value of the system is the difference between the phases deduced from the transient and the steady state regimes. Good agreement, to within 7°, was obtained between KLM simulations and measurements on two transducers in a 1-8 MHz frequency range. The reproducibility of the method was ±10°, with a systematic error of 2° at 1 MHz increasing to 16° at 8 MHz. This work demonstrates that the PTF of a transducer can be measured in a simple laboratory setting.     

Simultaneous wavelength conversion and pulse compression exploiting cascaded second-order nonlinear processes in LiNbO3 waveguides

Simultaneous wavelength conversion and pulse compression are proposed and demonstrated exploiting cascaded second-order nonlinear processes in periodically domain-inverted LiNbO₃ waveguides. The influences of initial pulse widths and waveguide length on the conversion efficiency and converted pulse compression are theoretically analyzed. Tunable wavelength conversion is performed for the signal pulse with the temporal width of 7.5 ps and repetition rate of 40 GHz. Conversion efficiency of more than −24 dB is obtained for 35-nm conversion span under average signal power of 10 dBm when a CW control wave is adopted.     

The effect of high intensity focused ultrasound on luciferase activity on two tumor cell lines in vitro, under the control of a CMV promoter

In this study, we compared the effect of high intensity focused ultrasound (HIFU) and thermal stress on the luciferase activity, controlled by a cytomegaly virus (CMV) promoter in an in vitro model using two tumor cell lines (M21, SCCVII). HIFU was applied in a pulsed-wave mode with increasing voltage at constant pulse duration, or thermal stress was delivered over a range of temperatures (36-52 °C) for 5 min. The resulting luciferase activity was measured in live cells using a cooled CCD camera. Luciferase activity was measured at set time intervals over a total of 48 h post-stress. Compared to baseline, the luciferase activity of the M21 tumor cell line when exposed to HIFU was approximately 54.2 ± 67.5% (p < 0.01) higher at a temperature of 42 °C, and approximately 52.9±128.5% (p < 0.01) higher at 44 °C. In the SCCVII tumor cell line, the luciferase activity after HIFU application was 55.4 ± 66.6% (p < 0.01) higher compared to baseline at a temperature of 42 °C. The M21 and SCCVII tumor cell line when exposed to thermal stress alone did not increase the luciferase activity. M21 and SCCVII tumor cells exposed to HIFU showed a maximum decrease in cell viability to 45.3 ± 7.5% and 10.3 ± 7.5%, respectively, and when exposed to thermal stress to 85.3 ± 3.5% and 20.4 ± 6.5%, respectively, compared to the untreated control. In M21 and SCCVII cells exposed to HIFU, free radicals could be detected using the dichlorofluorescein dye. Our findings demonstrate that HIFU can enhance the luciferase activity controlled by a CMV promoter. However it also has a higher damaging effect on the cells.     

Comparison of continuous-wave (CW) and tone-burst (TB) excitation modes in vibro-acoustography: Application for the non-destructive imaging of flaws

This paper describes the application of continuous-wave (CW) and tone-burst (TB) vibro-acoustography (VA) experiments for imaging a flawed composite plate. For both modes, the ultrasound frequency is set at f₁ = 3 MHz and f₂ = 3 MHz + ∣Δf∣. The plate was placed at the focus of the transducer and scanned point-by-point over an area of 60 mm by 50 mm on its frontal face with an increment step equal to 0.25 mm/pixel. The resulting acoustic emission amplitude at ∣Δ f∣ is recorded. For the CW mode the difference frequency was set at ∣Δf∣ = 12.9 kHz. For the TB mode, the burst-emitted signal was 100 μs long at a pulse repetition frequency (PRF) of 100 Hz corresponding to bursts of 300 cycles at 3 MHz, and the difference frequency was set at ∣Δf∣ = 44 kHz. The resulting VA images readily show the shape of the flaws. The images also reveal considerable detail of internal substructures such as the fibers used to reinforce the plate. However, the CW VA image shows an artifact caused by the effect of ultrasound standing waves established between the plate and the concave surface of the transducer, resulting in masking some of the flaws. On the other hand, the TB-VA image is free from such artifact. Despite some advantages of using TB-VA, there are some limitations related to this mode. Advantages and limitations of using the two modes are discussed.     

Noninvasive electromechanical wave imaging and conduction-relevant velocity estimation in vivo

Electromechanical wave imaging is a novel technique for the noninvasive mapping of conduction waves in the left ventricle through the combination of ECG gating, high frame rate ultrasound imaging and radio-frequency (RF)-based displacement estimation techniques. In this paper, we describe this new technique and characterize the origin and velocity of the wave under distinct pacing schemes. First, in vivo imaging (30 MHz) was performed on anesthetized, wild-type mice (n = 12) at high frame rates in order to take advantage of the transient electromechanical coupling occurring in the myocardium. The RF signal acquisition in a long-axis echocardiographic view was gated between consecutive R-wave peaks of the mouse electrocardiogram (ECG) and yielded an ultra-high RF frame rate of 8000 frames/s (fps). The ultrasound RF signals in each frame were digitized at 160 MHz. Axial, frame-to-frame displacements were estimated using 1D cross-correlation (window size of 240 μm, overlap of 90%). Three pacing protocols were sequentially applied in each mouse: (1) sinus rhythm (SR), (2) right-atrial (RA) pacing and (3) right-ventricular (RV) pacing. Pacing was performed using an eight-electrode catheter placed into the right side of the heart with the capability of pacing from any adjacent bipole. During a cardiac cycle, several waves were depicted on the electromechanical wave images that propagated transmurally and/or from base to apex, or apex to base, depending on the type of pacing and the cardiac phase. Through comparison between the ciné-loops and their corresponding ECG obtained at different pacing protocols, we were able to identify and separate the electrically induced, or contraction, waves from the hemodynamic (or, blood-wall coupling) waves. In all cases, the contraction wave was best observed along the posterior wall starting at the S-wave of the ECG, which occurs after Purkinje fiber, and during myocardial, activation. The contraction wave was identified based on the fact that it changed direction only when the pacing origin changed, i.e., it propagated from the apex to the base at SR and RA pacing and from base to apex at RV pacing. This reversal in the wave propagation direction was found to be consistent in all mice scanned and the wave velocity values fell within the previously reported conduction wave range with statistically significant differences between SR/RA pacing (0.85 ± 0.22 m/s and 0.84 ± 0.20 m/s, respectively) and RV pacing (−0.52 ± 0.31 m/s; p < 0.0001). This study thus shows that imaging the electromechanical function of the heart noninvasively is feasible. It may therefore constitute a unique noninvasive method for conduction wave mapping of the entire left ventricle. Such a technology can be extended to 3D mapping and/or used for early detection of dyssynchrony, arrhythmias, left-bundle branch block, or other conduction abnormalities as well as diagnosis and treatment thereof.