Second harmonic imaging: Good reverberations

Nonlinear acoustics allows for applications like tissue harmonic imaging in medicine and parametric arrays in underwater acoustics. Mainstream sonars transmit and receive signals at the same frequency and up to now energy transferred to higher harmonic frequencies has been mainly seen as a disturbance for target strength estimation, e.g., in fishery research. This paper investigates the feasibility of utilizing the part of the signal generated around the second harmonic frequency band by nonlinear propagation of sound in water. It presents the potential enhancements the second harmonic signal may provide for target imaging as well as multifrequency target recognition. It compares measurements of the pressure field radiated by commercial transducers in water at 121 and 200 kHz up to a range of 12 m with numerical simulations. The detected levels of higher harmonic signals agree with simulations of nonlinear wave propagation. This verifies the implementation of the simulator and allows a comparison of the beam characteristics at longer ranges when filtered around the fundamental or second harmonic frequencies. An example of pulse-echo imaging with spherical targets is also shown using signals at the fundamental and second harmonic frequencies where the second harmonic signal can detect one of the targets that the fundamental signal cannot. Using the active sonar equation to estimate the maximum range, simulations based on a simple model including ambient noise and volume reverberation confirm that with a source level of 228 dB and a detection threshold of 12 dB the fundamental signal at 200 kHz can detect a fish of target strength -36 dB to approximately 343 m while the detection range of the second harmonic signal is approximately 243 m. The combined use of the signal components in the second harmonic and fundamental frequency bands provides a high-resolution image at short range and a long-range imaging capability at a lower resolution as well as a multifrequency characterization of targets.

Recently, the technique of harmonic contrast imaging has been developed which exploits the nonlinear properties of ultrasound contrast agents by processing the second harmonic echo signal of the transmitted ultrasound pulse. The contrast agent employed is Levovist (Schering), which was released in Japan in 1999. The advantage of harmonic imaging is its selective imaging of differences between tissue and blood perfusion areas.The purpose of the present study was to investigate a method of detecting human brain perfusion by ultrasound with second harmonic signals and contrast agents in eight patients with brain tumors. We used pulse inversion harmonic imaging, which is the latest technique for visualizing ultrasound contrast agents, thereby allowing us to image brain perfusion, and well enhanced perfusion images were obtained in all cases.We performed analysis resulting in wash-in wash-out curves, and concluded that this method could be used for diagnosis of brain hemodynamics.

The performance of tissue imaging is improved by tissue harmonic imaging (THI) to expand and enhance the range and level of clinical diagnosis for many diseases. The separation based on High-pass filtering (S_HPF) is a commonly used method for extracting harmonic components from ultrasonic echo signals. However, the cut-off frequency, order and algorithm to realize a high-pass filter have a great influence on the separation accuracy of harmonic signals. In present study, a novel separation method (S_CEEMDAN) based on the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) algorithm is proposed for adaptively separating harmonic components from ultrasonic echo signals. First, we calculate the ensemble size of CEEMDAN adaptively based on the added noise level, and draw a sum of intrinsic mode functions (IMFs) from the ultrasonic echo signals by CEEMDAN. Then, the spectrum of each IMF is computed and evaluated, and the IMFs containing both fundamental and harmonic components are further decomposed by using the CEEMDAN algorithm. This separation process is end until all of IMFs have been divided into either fundamental or harmonic categories. Finally, the corresponding fundamental and harmonic echo signals are yielded by accumulating separately these two categories. In experiments, simulated ultrasonic echo signals with a center frequency of 3.5MHz are separated by the proposed S_CEEMDAN method, and the results are compared with those processed by S_HPF. The edge of the harmonic image by S_CEEMDAN marginally better defined than S_HPF. The indices for the harmonic signals separated by S_CEEMDAN and S_HPF, respectively, are as follows: the center frequencies of 6.66 MHz and 6.65 MHz, 3 dB bandwidths of 1.04 MHz and 1.05 MHz, 6 dB bandwidths of 1.59 MHz and 1.55 MHz, signal-to-noise ratios (SNRs) of 14.00 and 13.63, and image contrasts of 7.01 and 7.06. In conclusion, due to good adaptive characteristics, and lower reconstruction errors, the proposed S_CEEMD method is superior to S_HPF in the performance of spectral accuracy and harmonic imaging. This method could be potentially alternative to the current method for the ultrasonic harmonic separation.

Is there a role for intravenous transpulmonary contrast imaging in pediatric stress echocardiography?

901–49 Second Harmonic Imaging of Echocardiographic Contrast Agents: Unique Features and Promising Characteristics

Harmonic waves are generated from nonlinear distortion of an acoustic signal as an ultrasound wave insonates tissues in the body. These beams are integer multiples of a fundamental transmitted frequency. Potential advantages of harmonic imaging include improved axial resolution due to higher frequencies and better lateral resolution due to narrower beams. Decreased noise from side lobes improves signal-to-noise ratios and reduces artifacts. Deleterious effects of the body wall are also reduced. The authors prospectively studied ultrasonographic (US) findings in 100 adult patients with 202 abdominal lesions by comparing harmonic US images with conventional US images. The results were subjected to statistical analysis. Harmonic imaging was superior to conventional US in regard to lesion visibility and diagnostic confidence. Harmonic imaging was particularly useful for depicting cystic lesions and those containing echogenic tissues such as fat, calcium, or air. In patients with a body mass index of 30 or more, harmonic imaging was clearly better for lesion visibility and confidence of diagnosis. The authors recommend routine use of harmonic imaging for abdominal US studies in all adult patients.

Harmonic downmixing is an important effect that must be taken into account when performing sensitive spectrum sensing using direct-conversion receivers. When the local oscillator waveform contains harmonics of the fundamental frequency, the quadrature mixer in the receiver will downconvert RF signals found at these harmonics, termed harmonic images, in addition to the RF signals around the fundamental frequency. The harmonic images will be detected by power spectral density based sensing algorithms and will cause certain parts of the desired spectrum to be mistakenly flagged as occupied. Although harmonic downmixing is important to consider, it is an often negelected effect. This paper presents a harmonic rejection spectral sensing technique, that exploits two quadrature mixers. The mixers work with different LO frequencies, which decorrelate the harmonic images so that cross-correlation of their outputs renders an improved spectral estimate. In addition to rejecting the harmonic images, spurious signals entering the receiver through the analog baseband inputs was also be rejected. The frequency resolution of the spectral estimate is scalable and the rejection of the harmonic images increases 15 dB per 1000-fold increase of the correlation time. The complexity of the algorithm is analyzed and its performance is shown by means of simulations. The effect of I/Q imbalance is also taken into account.

Feasibility and accuracy of left ventricular volumes and ejection fraction determination by fundamental, tissue harmonic, and intravenous contrast imaging in difficult-to-image patients

Dual-frequency tissue harmonic suppression using phase-coded pulse sequence: Proof of concept using a phantom

Multi-frequency harmonic arrays: initial experience with a novel transducer concept for nonlinear contrast imaging

Ultrasonic interaction with fatigue damage in a material can lead to nonlinear ultrasonic harmonic generation. Unfortunately, nonlinear generation is often very weak and measurement sensors need to have a large dynamic range to pick weak second harmonic signals. Furthermore, couplant dependence of the ultrasonic measurements can mask any variations in second harmonic generation due to fatigue damage. To overcome this, a couplant‐independent method for ultrasonic nonlinearity measurements was recently proposed by us. This method requires two sets of transducers that can each generate/detect both the fundamental and second harmonic signals. In this paper, we describe a dynamically reconfigurable interdigitated Surface Acoustic Wave transducer mat can effectively be used to generate/receive bom the fundamental and the second harmonic signals. This is achieved using simple external electronic reconfiguration of the signals from a pair of interdigitated transducer arrays.

It is of great significance to study the weak harmonic signal detection from strong chaotic background. Current detection methods mainly use the chaotic phase space reconstruction method based on Takens theory, among which the neural network method has attracted the most attention. However, these methods require high signal-to-interference-plus-noise ratio (SINR) and are sensitive to Gaussian white noise, etc. Noticing the fact that the second-order statistical properties of chaotic signals are stationary, we propose a harmonic signal detection method from strong chaotic background based on optimal filter. We first construct a data matrix, whose rows are the detection signal and reference signals. The reference signals only contain chaotic interference. Then we calculate the one-dimensional fast Fourier transformation of the data matrix to make each column of the matrix form a frequency channel. The harmonic signal can be detected by searching each frequency channel in the frequency domain, thus the signal detection problem is converted into an optimization problem. Further, we use the optimization theory to design a filter such that it can maintain the gain of the signal from the current frequency channel and suppress signals from other frequency channels as far as possible. Finally, the harmonic signal can be obtained by calculating the output SINR of each frequency channel. In order to reduce the calculation, we can further design a local region optimal filter. We choose part of frequency channels to constitute a local area, thus the dimension of the chaotic interference covariance matrix is greatly reduced. Theoretically speaking, the more the number of auxiliary frequency channels, the better the detection results are. However, in the practical application, choosing two channels on the left and right side of current channel each can obtain a very good detection effect. After obtaining the chaotic interference covariance matrix, we can further achieve the output SINR of each frequency channel. Compared with the traditional methods, the proposed method has the following advantages: 1) it can detect a weak harmonic signal under lower SINR; 2) it can detect a greater range of signal amplitude; 3) it is robust against white Gaussian noise. The simulation results with taking Lorenz system as the strong chaotic background show that the proposed method still has a very good detection effect when SINR =-81.03 dB, and the stronger the harmonic signal, the better the detection effect is, while the neural network method can work under the condition of SINR higher than -67.03 dB; the proposed method still can correctly detect the target signal in the case that the SNR is as low as -20 dB, but the neural network method has a poor detection effect under the same condition.

In this study, a C~K band frequency synthesizer used in broadband electronic warfare equipment was designed, developed, and the measurement results were presented. The structure of the designed frequency synthesizer generates a signal up to 6 GHz using only direct digital synthesizer(DDS) without a mixer or multiplier. For signals above 6 GHz frequency, the structure uses a multiplier. In addition, a variable sampling frequency structure and a DDS circuit were designed to improve signal quality by reducing spurious signals, harmonic signals, and phase noise. The developed frequency synthesizer has different signal quality performance depending on the use of the multiplier, and the C~G band has output power flatness ±1.7 dB, spurious signal less than −55 dBc, and phase noise less than −95 dBc/Hz (@1 kHz). The H~K band using multiplier has characteristics of output power flatness ±2.0 dB, spurious signal less than −35 dBc, and phase noise less than −80 dBc/Hz (@1 kHz). The results of this study confirmed the miniaturization of the signal generation module in the C~K band.

Gaseous ultrasound contrast agents, such as Albunex, have been shown to be valuable clinically for the better identification of certain anatomic structures and for quantitating organ perfusion. However, the application of a contrast agent sometimes also reduces the image contrast between the blood and the surrounding tissues. This paper presents experimental results to show that this problem may be alleviated by using harmonic signals generated by nonlinear backscattering of these microbubbles. The results show that the second harmonic signal produced by Albunex is significant and that harmonic images in which backscattered echoes from solid tissues were filtered out, but not those from Albunex, can be obtained. In addition, a continuous wave Doppler arrangement has been implemented. It permits quantitative harmonic Doppler measurements at different frequencies, concentrations, and transmitted acoustic pressures. When Albunex was compared to polystyrene microsphere suspensions which possess exclusively linear behavior, Doppler power at second and subharmonic frequencies could only be detected from these microbubbles. A comparison of results on Doppler powers measured at the first harmonic from polystyrene spheres and Albunex indicates that at a transmitting frequency of 2.5 MHz, the Doppler power from Albunex is 12 dB stronger than the polystyrene spheres, whereas at 5 MHz, the Doppler power from Albunex is 3 dB weaker than that from the polystyrene spheres.

High harmonic generation (HHG) in pure nitrogen (N2) and its mixture with neon (Ne) gas was produced. The external laser field producing 6mJ optical pulse energy with 50fs pulse duration at 10Hz repetition rate was focused into a gas jet producing high harmonics. The N2 and Ne gas have different ionization potentials. The strong harmonic signal in pure N2 was produced, and a weak harmonic signal in a mixture of N2-Ne was generated compared to that in pure N2. The increase of Ne contribution in the N2-Ne mixture resulted in a decrease in harmonic signal. Harmonic signal increase and decrease were observed for different N2:Ne ratios. The harmonic generation mechanism was discussed in that the ionization of Ne is difficult compared to N2, and the neutral Ne atom leads to neutral atomic dispersion (phase mismatch), so the harmonic yield decreases.