Third harmonic transmit phasing for tissue harmonic generation

Generation of tissue harmonic signals during acoustic propagation is based on the combined effect of two different spectral interactions of the transmit signal. One produces harmonic whose frequency is the sum of transmit frequencies. The other results in harmonic at difference frequency of the transmit signals. Both the frequency-sum component and the frequency-difference component are sensitive to the phase of their constitutive spectral signals. In this study, a novel approach for modifying the amplitude of tissue harmonic signal is proposed based on phasing these two components to achieve either harmonic enhancement or suppression. Both experiments and simulations were performed to investigate the effects of 3f0 transmit phasing on tissue harmonic generation. Results indicate that the relative phasing between the frequency-sum component and the frequency-difference component markedly changes the amplitude of the second harmonic signal. For harmonic enhancement, approximate 6 dB increase of second harmonic amplitude can be achieved while the lateral harmonic beam pattern also is improved as compared to conventional situations in which only the frequency-sum component is considered. For harmonic suppression, the second harmonic signal also could be significantly reduced by about 11 dB when the frequency-difference component is out of phase with the frequency-sum component. Hence, the method of 3f0 transmit phasing has potentials for both improving signal-to-noise ratio in tissue harmonic imaging and enhancing image contrast in contrast-agent imaging by suppression of tissue harmonic background.     

Golay-encoded excitation for dual-frequency harmonic detection of ultrasonic contrast agents

Golay-encoded excitation in combination with the third harmonic (3f?) transmit phasing is examined for both signal-to-noise ratio (SNR) and contrast-to-tissue ratio (CTR) improvements in harmonic imaging of contrast microbubbles. To produce the cancellation pair of tissue harmonic signal in 3f? transmit phasing, the phase of the bit waveform is properly designed for both the fundamental and the 3f? transmit signals to provide the Golay encoding of the received harmonic responses. Results indicate that the proposed Golay excitation can effectively suppress the tissue harmonic amplitude to increase CTR. Meanwhile, the SNR of the contrast harmonic signal also improves because of the elongated waveform of Golay excitation. Nevertheless, the generation of marked range side-lobes of the bubble region would degrade the achievable SNR improvement and the image contrast, especially when the bit of Golay excitation increases. The range side-lobes could result from the nonlinear resonance of the microbubbles that interferes with the phase modulation of the Golay encoding.     

A thin film phantom for blood flow simulation and Doppler test

The thin film phantom is a new type of ultrasound resolution test object. It consists of a thin planar substrate that is acoustically matched to the surrounding media. Precisely located scatterers on the surface of the substrate generate echo signals. The patterning of scatterers on the substrate allows echogenicity to be controlled as a function of position, which enables the production of a test object with highly reproducible and controllable scattering characteristics. We show that by vibrating the substrate in a suitable manner, an echo signal may be generated that simulates bidirectional flow. We demonstrate that a vibration of low amplitude at frequency f₀ produces a Doppler spectral signal at f₀ and -f₀, within the limits of aliasing. Furthermore, by driving the film with a bandlimited noise signal, we illustrate how a velocity distribution may be simulated. A time-varying flow velocity may be simulated by varying the noise bandwidth with time. Finally, using this technique, we demonstrate a system that simulates an arterial flow pattern, including its characteristic velocity distribution in forward and reverse directions simultaneously     

Harmonic chirp imaging method for ultrasound contrast agent

Coded excitation is currently used in medical ultrasound to increase signal-to-noise ratio (SNR) and penetration depth. We propose a chirp excitation method for contrast agents using the second harmonic component of the response. This method is based on a compression filter that selectively compresses and extracts the second harmonic component from the received echo signal. Simulations have shown a clear increase in response for chirp excitation over pulse excitation with the same peak amplitude. This was confirmed by two-dimensional (2-D) optical observations of bubble response with a fast framing camera. To evaluate the harmonic compression method, we applied it to simulated bubble echoes, to measured propagation harmonics, and to B-mode scans of a flow phantom and compared it to regular pulse excitation imaging. An increase of approximately 10 dB in SNR was found for chirp excitation. The compression method was found to perform well in terms of resolution. Axial resolution was in all cases within 10% of the axial resolution from pulse excitation. Range side-lobe levels were 30 dB below the main lobe for the simulated bubble echoes and measured propagation harmonics. However, side-lobes were visible in the B-mode contrast images.     

Contrast imaging with chirped excitation

Coded excitation has been successfully used in imaging to increase the signal-to-noise ratio (SNR) and penetration depth. With a contrast agent, wideband signals have been hypothesized to increase the contrast-to-tissue ratio (CTR). However, nonlinear properties of contrast agents make decoding difficult when applying coded excitation to contrast imaging. We propose two chirped excitation methods to image contrast agents, with a mechanical index (MI) ranging from 0.05 to 0.34. In the single chirp method, one chirp is transmitted, followed by a clutter filter to reject tissue echoes, then a matched filter is used to recover range resolution. In the chirp sequence method, an increasing and decreasing chirp sequence is transmitted followed by subtraction of the compressed echoes to reject tissue echoes (assuming tissue is a linear scatterer at low MI). Ten independent acoustic experiments were performed to evaluate the CTR for chirp and tone burst insonation, with the same spatial peak temporal averaged intensity (I(SPTA)). A significant increase in CTR, ranging from 4 dB to 8 dB, is observed for chirped excitation as compared with tone burst insonation, at an I(SPTA) of 0.1 and 0.3 mW/cm2 (P < or = 5e-3). To achieve the same CTR of 15 dB, the spatial peak pulse averaged intensity (I(SPPA)) can be decreased by 6 dB for chirp insonation as compared with tone burst insonation (P < 1e-5). Additionally, an increase of more than 10 dB in tissue rejection ratio (TRR) is observed for a chirp sequence insonation compared to tone burst phase inversion for this set of parameters (P < or = 1e-9). Deconvolution of the linear microbubble response from the received echoes is proposed as a method to recover spatial resolution. The difference in the axial resolution resulting from chirp and three-cycle tone burst insonation is approximately 220 microm. The difference in the mainlobe width between experimental and predicted compressed echoes is less than 20%. The side-lobe amplitude is 9 dB to 16 dB below the mainlobe with a transmitted I(SPTA) from 0.1 to 6.6 mW/cm2.     

Synthesizing a Field Source for Magnetic Stimulation of Peripheral Nerves

We have developed an automated method for optimizing the design of a coil for magnetic stimulation of peripheral nerves, acting on the current distribution of the coil. The method consists of these steps: 1 generating a 3-D finite-element model of a coil laid on a forearm; 2 solving the model for two parameters,f₁ and f₂ of the activating function, which is responsible for the peripheral nerve stimulation; and 3 starting from an existing prototype, applying an evolutionary algorithm to maximize f₁ and minimize f₂ . The final aim is to obtain a focused and strong stimulation. In order to determine robust solutions, the method considers a sensitivity constraint and obtains two solutions, one without the sensitivity constraint and the other taking it into account. In both cases, the objective functions are improved with respect to the existing prototype. In conclusion, we discuss the structure of the objective space.     

Adaptive ultrasonic measurement of blood vessel diameter and wall thickness: theory and experimental results

An adaptive ultrasonic technique for measuring blood vessel diameter and wall thickness is presented. This technique allows one to use a target-specific transmitted waveform/receiver filter to obtain a larger signal-to-noise ratio (SNR) in the received signal than conventional techniques. Generally, SNR of a received wave increases as the intensity of the transmit wave increases; however, because of the FDA limitations placed on the amount of transmit energy, it is important to be able to make the most efficient use of the energy that is available to obtain the best possible SNR in the received signal. Adaptive ultrasonic measurement makes the most efficient use of the energy that is available by placing the maximum amount of energy in the largest target scattering mode. This results in more energy backscatter from a given target, which leads to a higher SNR in the received waveform. Computer simulations of adaptive ultrasonic measurement of blood vessel diameter show that for a SNR of 0 dB in the transmitted waveform, the standard deviation of the diameter measurements for a custom-designed transmitted waveform is about two orders of magnitude less than the standard deviation of the diameter measurements using more conventional waveforms. Diameter and wall thickness measurement experiments were performed on a latex tube and a bovine blood vessel using both custom-made and conventionally used transmitted waveforms. Results show that the adaptively designed waveform gives a smaller uncertainty in the measurements. The adaptive ultrasonic blood vessel diameter and wall thickness measuring technique has potential applications in examining vessels which are either too deep inside the body or too small for conventional techniques to be used, because of the low SNR in the received signal.     

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

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 f? (e.g., 9 MHz) and f? (e.g., 6 MHz) and the resulting envelope component at (f? - f?) (e.g., 3 MHz) can serve as a driving force to excite the nonlinear response of UCAs. In this study, the f?, at twice of the resonance frequency of UCAs, is adopted to efficiently generate a sub-harmonic component at half of the f? frequency, and f? 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 f? and f? 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.     

Measurement of static and vibration-induced phase noise in UHF thin-film resonator (TFR) filters

Measurements of the static phase noise and vibration sensitivity of thin-film resonator (TFR) filters operating at 640 and 2110 MHz have been made. They show that the short-term frequency instability of the filters is small compared with that induced in the oscillator signal by the sustaining stage amplifier PM (phase modulation) noise. In-oscillator measurement of filter performance under vibration indicates that fractional frequency vibration sensitivities (δf ₀/f₀) are on the order of several parts in 10^-9/g. Because the percentage bandwidth and order (number of poles) of the filters was fairly constant, so was the product of the center frequency and group delay. Thus, the fractional frequency vibration sensitivity of the filters can be expressed alternatively as carrier signal phase sensitivity to vibration. The τ-ω₀ product for the filters that were tested was on the order of 300 rad, so that the equivalent phase sensitivity to vibration was approximately 1 grad/g     

Harmonic leakage and image quality degradation in tissue harmonic imaging

Image quality degradation caused by harmonic leakage was studied for finite amplitude distortion-based harmonic imaging. Various sources of harmonic leakage, including transmit waveform, signal bandwidth, and system nonlinearity, were investigated using both simulations and hydrophone measurements. Effects of harmonic leakage in the presence of sound velocity inhomogeneities were also considered. Results indicated that sidelobe levels of the harmonic beam pattern were directly affected by harmonic leakage when the harmonic signal was obtained by filtering out the fundamental signal. Because sidelobe levels also increase with the bandwidth of the transmitted signal, a trade-off exists between axial resolution and contrast resolution. It is concluded that accurate control of the frequency content of the waveform prior to propagation is necessary to optimize imaging performance of tissue harmonic imaging. The filtering technique was also compared with the pulse inversion technique. It was shown that the pulse inversion technique effectively suppresses harmonic leakage at the cost of imaging frame rate and potential motion artifacts     

Low current density, inverted polarity, high-speed, top-emitting 850 nm vertical-cavity surface-emitting lasers

High-speed, oxide-confined, inverted polarity (n-up), polyimide-planarised 850 nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated and characterised. The lasers exhibit a -3 dB frequency modulation bandwidth (f_3dB) up to 15.2 GHz with a 10 mum oxide aperture diameter, at the lowest current density (/bias) ever reported of 6.4 kA/cm². The ratio f_3dB ²/J_bias = 36.1 (GHz²/kA/cm²) represents a 21% increase when compared with the highest previously reported ratio. The threshold voltage and current were as low as 1.45 V and 0.9 mA, respectively, with a series resistance of 65 Omega. A rate-equation-based thermal VCSEL model was used to predict the device performance at different temperatures. Good agreements between measured and simulated DC characteristics were obtained.     

A harmonic cancellation technique for an ultrasound transducer excited by a switched-mode power converter

The aim of this study is to evaluate the feasibility of using harmonic cancellation for a therapeutic ultrasound transducer excited by a switched-mode power converter without an additional output filter. A switching waveform without the third harmonic was created by cascading two switched-mode power inverter modules at which their output waveforms were pi/3 phase shifted from each other. A PSPICE simulation model for the power converter output stage was developed. The simulated results were in good agreement with the measurement. The waveform and harmonic contents of the acoustic pressure generated by a 1-MHz, self-focused piezoelectric transducer with and without harmonic cancellation have been evaluated. Measured results indicated that the acoustic third harmonicto- fundamental ratio at the focus was small (-48 dB) with harmonic cancellation, compared to that without harmonic cancellation (-20 dB). The measured acoustic levels of the fifth harmonic for both cases with and without harmonic cancellation also were small (-46 dB) compared to the fundamental. This study shows that it is viable to drive a piezoelectric ultrasound transducer using a switched-mode power converter without the requirement of an additional output filter in many high-intensity focused ultrasound (HIFU) applications.     

Impact of propagation through an aberrating medium on the linear effective apodization of a nonlinearly generated second harmonic field

Techniques based on the nonlinearly generated second harmonic signal (tissue harmonic imaging) have rapidly supplanted linear (fundamental) imaging methods as the standard in two-dimensional echocardiography. Enhancements to the compactness of the nonlinearly generated second harmonic (2f) field component with respect to the fundamental (1f) field component are widely considered to be among the factors contributing to the observed image quality improvements. The objective of this study was to measure the impact of phase and amplitude aberrations resulting from propagation through an inhomogeneous tissue, on the beamwidths associated with: the fundamental (1f); the nonlinearly generated second harmonic (2f); and the linearly propagated, effective apodization signal at the same (21) frequency. Modifications to the transmit characteristics of a phased-array imaging system were validated with hydrophone measurements. Results demonstrate that the characteristics of the diffraction pattern associated with the linear-propagation effective apodization transmit case were found to be in good agreement with the detailed spatial characteristics of the nonlinearly generated second harmonic field. The effects of the abdominal wall tissue aberrators are apparent for all three of the beam profiles studied. Consistent with the improved image quality associated with harmonic imaging, the aberrated nonlinearly generated second harmonic beam was shown to remain more compact than the corresponding aberrated fundamental beam patterns in the presence of the interposed aberrator.     

Factorial Modeling for Transient Solid Particle Velocity in a Fluidized Bed Combustor Cold Model

Transient hydrodynamics phenomena in the fluidized bed combustor (FBC) freeboard have been critical in the past two decades. Within a 152 mm ID FBC cold model, solid particle transient velocities were measured and analyzed with the assistance of advanced laser-based particle image velocimetry (PIV) instrumentation. Two layers of swirling secondary air were injected into the cold model. The PIV system was applied to the FBC cold model to visualize transient solid particle velocity. A series of transient solid particle velocity profiles were generated for the factorial analysis. In each profile, the solid particle velocity vectors (Vx and Vy) for 10 × 10 grids were generated. Analysis of variance (ANOVA) was used to determine the significant factors that affect transient solid particle velocities, time, and position coordinates. Then, the 10¹⁰ factorial design method was used to develop a specific empirical model of transient solid particle velocity in the FBC freeboard, which was in the shape of V_x = f₁(t, x, y) and V_y = f₂(t, x, y).

This unique factorial analysis method proved to be a very effective and practical method to evaluate experimental conditions and analyze experimental results in the FBC systems.     

Suppression of propagating second harmonic in ultrasound contrast imaging

Contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear methods such as harmonic imaging can be used. Contrast is limited, however, by tissue second harmonic signals. We show that a major source of this signal is nonlinear propagation through tissue. In addition, we present methods to suppress this second harmonic generation. One simple approach is to decrease the f/number of the imaging system. Simulations show that doubling the size of the array, while keeping total power output constant, decreases propagating second harmonic generation. A second approach uses active noise cancellation to suppress second harmonic generation. A specific method, the harmonic cancellation system (HCS), is developed and presented as an example. In simulations, HCS decreased second harmonic generation by over 30 dB. Using such methods, contrast can be improved between tissue and bubbles in harmonic imaging.     

Evaluation of a pulse coding technique for spacial structurecharacterization

Pulse coding techniques have been used in the past primarily to improve signal to (electronic) noise ratio. However, the flexibility inherent in pulse coding can be exploited to solve several problems in medical imaging and nondestructive testing. We have experimentally examined its potential for spacial structure characterization of a scattering medium on a scale below the resolution of the imaging system. The ability to change the point spread function and the spectral content of the interrogating pulse with frequency modulated (FM) pulse coding has been utilized. Water filled sponge with pore size much smaller than the resolution cell volume was used as the scattering medium. A nonfocused transducer was driven with FM coded pulses. The pulse compression processing was carried out digitally on a computer. FM pulses with 143 different combinations of center frequency f₀ and 6 dB bandwidth Δf were used. Amplitude signal to noise ratio (SNR_A) was calculated on the envelope detected signal for spacial structure characterization. SNR_A showed significant increase from its Rayleigh limit value of 1.91 at certain specific frequencies. Both simulation and theoretical considerations are used to show that this resonance effect is a signature of the underlying semiperiodic scattering structure of the medium     

Effect of Dual-Magnetic-Layer on Performance of CoCrPt-SiO2 Perpendicular Media

CoCrPt-SiO₂ perpendicular recording media have been investigated in order to increase media signal-to-noise ratio (SNR) by introducing a dual-magnetic-layer structure into the CoCrPt-SiO₂ recording layer. The dual-magnetic-layer structure consists of low- and high-M_s granular layers. The magnetic properties and recording performances of two kinds of dual-magnetic-layer media, one with a low-M_s bottom layer and another with a low-M_s top layer, were measured. The coercivity of the low-M_s bottom medium is higher than that of the low-M_s top medium, although they have the same thermal stability (K_uV/k_B T). We showed that using the low-M_s bottom layer gives a higher SNR. That is to say, SNR was improved by 3 dB compared with that of the high-M_s single-magnetic-layer medium, and it was even higher than that of the low-M_s single-magnetic-layer medium. It is thus concluded that the dual-magnetic-layer structure with the low-M_s bottom layer is effective in increasing SNR of perpendicular media     

Coded excitation for diagnostic ultrasound: a system developer's perspective

Resolution and penetration are primary criteria for clinical image quality. Conventionally, high bandwidth for resolution was achieved with a short pulse, which results in a tradeoff between resolution and penetration. Coded excitation extends the bounds of this tradeoff by increasing signal-to-noise ratio (SNR) through appropriate coding on transmit and decoding on receive. Although used for about 50 years in radar, coded excitation was successfully introduced into commercial ultrasound scanners only within the last 5 years. This delay is at least partly due to practical implementation issues particular to diagnostic ultrasound, which are the focus of this paper. After reviewing the basics of biphase and chirp coding, we present simulation results to quantify tradeoffs between penetration and resolution under frequency-dependent attenuation, dynamic focusing, and nonlinear propagation. Next we compare chirp and Golay code performance with respect to image quality and system requirements, then we show clinical images that illustrate the current applications of coded excitation in B-mode, harmonic, and flow imaging.     

DAC阶梯信号频谱的新简化分析及其基波的非滤波测量

提出一个新的简化方法分析了DAC生成的阶梯波频谱,包括幅值和相角。以正弦数字信号的离散值为输入,DAC将生成一个阶梯信号,其中的基波分量与输入的正弦信号之间幅值差为-1. 644 9/n²,相位差为-p/n,n为一个周期内离散点的个数;而阶梯信号中的谐波位于(tn±1)阶次上,其幅值是基波分量的1/(tn±1)。设计了一个不需滤波器而仅对阶梯波中基波分量作测量的实验,实验结果证实了上述关于基波的分析结论。讨论了各种可能的应用。     

High electro-optic response in polymers doped with novel chromophores with good thermal and orientational stability

Novel chromophores designed towards the elaboration of highly electro-optic (EO) polymers have been synthesized and characterized. EO doped polymers were prepared and characterised using second harmonic generation and the Teng and Man techniques at 1.3 μm for measurements of d₃₃ and r₃₃, respectively. The measured values of r₃₃ (18 pm/V) in a polycarbonate matrix doped with 5.5% of chromophore qualify these materials for further elaboration in EO devices for optical signal processing.