Frequency Shift Method Research Articles

The Revisited Frequency-Shift Method for Shear Wave Attenuation Computation and Imaging.

Ultrasound (US) shear wave (SW) elastography has been widely studied and implemented on clinical systems to assess the elasticity of living organs. Imaging of SW attenuation reflecting viscous properties of tissues has received less attention. A revisited frequency shift (R-FS) method is proposed to improve the robustness of SW attenuation imaging. Performances are compared with the FS method that we originally proposed and with the two-point frequency shift (2P-FS) and attenuation measuring US SW elastography (AMUSE) methods. In the proposed R-FS method, the shape parameter of the gamma distribution fitting SW spectra is assumed to vary with distance, in contrast to FS. Second, an adaptive random sample consensus (A-RANSAC) line fitting method is used to prevent outlier attenuation values in the presence of noise. Validation was made on ten simulated phantoms with two viscosities (0.5 and 2 Pa [Formula: see text]) and different noise levels (15 to -5 dB), two experimental homogeneous gel phantoms, and six in vivo liver acquisitions on awake ducks (including three normal and three fatty duck livers). According to the conducted experiments, R-FS revealed mean reductions in coefficients of variation (CV) of 62.6% on simulations, 62.5% with phantoms, and 62.3% in vivo compared with FS. Corresponding reductions compared with 2P-FS were 45.4%, 77.1%, and 62.0%, respectively. Reductions in normalized root-mean-square errors for simulations were 63.9% and 48.7% with respect to FS and 2P-FS, respectively.

In Vivo Magnetic Resonance Thermometry for Brain and Body Temperature Variations in Canines under General Anesthesia

The core body temperature tends to decrease under general anesthesia. Consequently, monitoring the core body temperature during procedures involving general anesthesia is essential to ensure patient safety. In veterinary medicine, rectal temperature is used as an indicator of the core body temperature, owing to the accuracy and convenience of this approach. Some previous studies involving craniotomy reported differences between the brain and core temperatures under general anesthesia. However, noninvasive imaging techniques are required to ascertain this because invasive brain temperature measurements can cause unintended temperature changes by inserting the temperature sensors into the brain or by performing the surgical operations. In this study, we employed in vivo magnetic resonance thermometry to observe the brain temperatures of patients under general anesthesia using the proton resonance frequency shift method. The rectal temperature was also recorded using a fiber optic thermometer during the MR thermometry to compare with the brain temperature changes. When the rectal temperature decreased by 1.4 ± 0.5 °C (mean ± standard deviation), the brain temperature (white matter) decreased by 4.8 ± 0.5 °C. Furthermore, a difference in the temperature reduction of the different types of brain tissue was observed; the reduction in the temperature of white matter exceeded that of gray matter mainly due to the distribution of blood vessels in the gray matter. We also analyzed and interpreted the core temperature changes with the body conditioning scores of subjects to see how the body weight affected the temperature changes.

Comparing of Frequency Shift and Impedance Analysis Method Based on QCM Sensor for Measuring the Blood Viscosity.

Blood viscosity measurements are crucial for the diagnosis of cardiovascular and hematological diseases. Traditional blood viscosity measurements have obvious limitations because of their expensive equipment usage and large sample consumption. In this study, blood viscosity was measured by the oscillating circuit method and impedance analysis method based on single QCM. In addition, the effectiveness of two methods with high precision and less sample is proved by the experiments. Moreover, compared to the result from a standard rotational viscometer, the maximum relative errors of the proposed oscillating circuit method and impedance analysis method are ±5.2% and ±1.8%, respectively. A reliability test is performed by repeated measurement (N = 5), and the result shows that the standard deviation about 0.9% of impedance analysis is smaller than that of oscillating circuit method. Therefore, the impedance analysis method is superior. Further, the repeatability of impedance analysis method was evaluated by regression analysis method, and the correlation coefficient R2 > 0.965 demonstrated that it had excellent reproducibility.

Parabolic fitting method for quality factor estimation

The quality factor [Formula: see text] has a wide application range in seismic data processing and interpretation. Compared with the commonly used methods of Q estimation (e.g., the spectral ratio method and the centroid frequency shift method), the logarithmic spectral area difference (LSAD) method exhibits better noise immunity. However, the LSAD method uses the area difference in one frequency band, and the [Formula: see text] values estimated using this method are still unstable under the influence of noise. We have developed a new parabolic fitting (PF) method to estimate Q. This method is an extension of the LSAD method. We derive an analytical relationship between [Formula: see text] and area difference curve of the logarithmic amplitude spectra. Using the logarithmic area difference of multiple frequency bands to fit the parabola, we improve the accuracy and stability of [Formula: see text] estimation. In addition, the PF method does not require special assumptions regarding the source wavelet. We test the PF method in the presence of noise and at varying bandwidths and compare the results with those obtained using the LSAD method. The results of the theoretical examples indicate that the PF method is noise resistant and stable. Applying the PF method to real zero-offset vertical seismic profiling records also indicates that the proposed method can reasonably and stably estimate the [Formula: see text] value of the formation.

System for delivering microwave ablation to subcutaneous tumors in small-animals under high-field MRI thermometry guidance

Purpose Bio-effects following thermal treatments are a function of the achieved temperature profile in tissue, which can be estimated across tumor volumes with real-time MRI thermometry (MRIT). Here, we report on expansion of a previously developed small-animal microwave hyperthermia system integrated with MRIT for delivering thermal ablation to subcutaneously implanted tumors in mice. Methods Computational models were employed to assess suitability of the 2.45 GHz microwave applicators for delivering ablation to subcutaneous tumor targets in mice. Phantoms and ex-vivo tissues were heated to temperatures in the range 47–67 °C with custom-made microwave applicators for validating MRIT with the proton resonance frequency shift method against fiberoptic thermometry. HAC15 tumors implanted in nude mice (n = 6) were ablated in vivo and monitored with MRIT in multiple planes. One day post ablation, animals were euthanized, and excised tumors were processed for viability assessment. Results Average absolute error between temperatures from fiberoptic sensors and MRIT was 0.6 °C across all ex-vivo ablations. During in-vivo experiments, tumors with volumes ranging between 5.4–35.9 mm3 (mean 14.2 mm3) were ablated (duration: 103–150 s) to achieve 55 °C at the tumor boundary. Thermal doses ≥240 CEM43 were achieved across 90.7–98.0% of tumor volumes for four cases. Ablations were incomplete for remaining cases, attributed to motion-affected thermometry. Thermal dose-based ablative tumor coverage agreed with viability assessment of excised tumors. Conclusions We have developed a system for delivering microwave ablation to subcutaneous tumors in small animals under MRIT guidance and demonstrated its performance in-vivo.

High-Q Toroidal Dipole Metasurfaces Driven By Bound States in the Continuum for Ultrasensitive Terahertz Sensing

A novel metallic toroidal dipole (TD) metasurface driven by Friedrich-Wintgen bound states in the continuum (FW-BIC) is theoretically proposed for terahertz (THz) sensing. By tuning the middle gap distance without breaking the symmetry of unit cell, the FW-BIC and quasi-BIC mode can be excited via resonance coupling between dipole modes. Based on the cyclic distribution of anti-aligned magnetic dipoles and the calculated scattering powers, TD resonance is demonstrated qualitatively and quantitatively; also FW-BIC is verified by far-field transmission spectrum and near-field enhancement spectrum. More importantly, it is the first time to exploit this quasi-BIC TD resonance for THz sensing to the best of our knowledge. For micron film sensing with frequency shift ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FS</i> ) method, numerical results show the sensitivity, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor and the corresponding figure of merit (FoM) can simultaneously reach 775.7 GHz/RIU, 1016, and 284, respectively. Moreover, for nano film sensing where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FS</i> method is inapplicable, the amplitude difference method is utilized and the simulated results show it has superior sensing capability. Our proposed structure opens up an avenue to develop multifunctional and ultrasensitive THz sensors.

Improved two-point frequency shift power method for measurement of shear wave attenuation

Quantitative assessment of mechanical properties of biological soft tissues is frequently evaluated using a noninvasive modality, called ultrasound shear wave elastography (SWE). SWE typically exerts an acoustic radiation force (ARF) to produce shear waves propagating in the lateral direction for which velocities and attenuations are measured. The tissue viscoelasticity is commonly studied by investigating the shear wave phase velocity curves. Viscoelastic tissue properties can also be characterized through utilizing various shear wave attenuation techniques. In this study, we propose an improved method for measuring the shear wave attenuation, called two-point frequency shift power (2P-FSP), which is an improved version of the two-point frequency shift (2P-FS) method. The technique is fully data driven and does not use a rheological model for mathematical modeling. The 2P-FSP method utilizes the power spectra frequency shift of shear waves measured at two spatial positions, which provides robustness to noise. The conceptual basis for the 2P-FSP is provided and tested with numerical and experimental data. We investigated how the location of the first signal and the distance interval between the two locations influence the shear wave attenuation measurement in the 2P-FSP technique. We utilized the 2P-FSP method on numerical phantom data generated using a finite-difference-based method in tissue-mimicking viscoelastic media. Moreover, we tested the 2P-FSP method with data from custom-made tissue-mimicking viscoelastic phantom experiments, and ex vivo porcine liver. We compared results from the proposed technique with results from 2P-FS and analytical values in the case of simulations. The results showed that the 2P-FSP method provides improved results over the 2P-FS technique for lower signal-to-noise ratio (SNR) and locations farther from the push location considered, and can be used to measure attenuation of viscoelastic soft tissues.

Heat Modulation of Intrinsic MR Contrasts for Tumor Characterization.

Simple SummaryMulti-parametric magnetic resonance imaging (MRI) is a paradigm that combines several MR imaging contrast types to provide added layers of information for the characterization of tissue types, including benign and malignant tumors. The approach detailed in this manuscript evaluates heat-induced changes in intrinsic MRI contrast types in vivo for tumor characterization. Specifically, the quantitative longitudinal relaxation time (T1), transverse relaxation time (T2), water proton chemical shift (CS), and apparent diffusion coefficient (ADC) were measured at various temperatures for benign and malignant tumors in rats. Results indicate that heat-induced changes in these intrinsic contrast types can potentially improve MR imaging visualization and characterization of tumor tissue. The approach detailed here may have a strong impact on real-time interventional procedures where tumor boundaries need to be accurately delineated to maximize positive therapeutic response for MRI-guided focal therapy.(1) Background: The longitudinal relaxation time (T1), transverse relaxation time (T2), water proton chemical shift (CS), and apparent diffusion coefficient (ADC) are MR quantities that change with temperature. In this work, we investigate heat-induced intrinsic MR contrast types to add salient information to conventional MR imaging to improve tumor characterization. (2) Methods: Imaging tests were performed in vivo using different rat tumor models. The rats were cooled/heated to steady-state temperatures from 26–36 °C and quantitative measurements of T1, T2, and ADC were obtained. Temperature maps were measured using the proton resonance frequency shift (PRFS) method during the heating and cooling cycles. (3) Results: All tissue samples show repeatable relaxation parameter measurement over a range of 26–36 °C. Most notably, we observed a more than 3.3% change in T1/°C in breast adenocarcinoma tumors compared to a 1% change in benign breast fibroadenoma lesions. In addition, we note distinct values of T2/°C change for rat prostate carcinoma cells compared to benign tissue. (4) Conclusion: These findings suggest the possibility of improving MR imaging visualization and characterization of tissue with heat-induced contrast types. Specifically, these results suggest that the temporal thermal responses of heat-sensitive MR imaging contrast mechanisms in different tissue types contain information for improved (i) characterization of tumor/tissue boundaries for diagnostic and therapy purposes, and (ii) characterization of salient behavior of tissues, e.g., malignant versus benign tumors.

Estimation of Seismic Wave Attenuation from 3D Seismic Data: A Case Study of OBC Data Acquired in an Offshore Oilfield

Previous studies performed in Abu Dhabi oilfields, United Arab Emirates, revealed the direct link of seismic wave attenuation to petrophysical properties of rocks. However, all those studies were based on zero offset VSP data, which limits the attenuation estimation at one location only. This is due to the difficulty of estimating attenuation from 3D seismic data, especially in carbonate rocks. To overcome this difficulty, we developed a workflow based on the centroid frequency shift method and Gabor transform which is optimized by using VSP data. The workflow was applied on 3D Ocean Bottom Cable seismic data. Distinct attenuation anomalies were observed in highly heterogeneous and saturated zones, such as the reservoirs and aquifers. Scattering shows significant contribution in attenuation anomalies, which is unusual in sandstones. This is due to the complex texture and heterogeneous nature of carbonate rocks. Furthermore, attenuation mechanisms such as frictional relative movement between fluids and solid grains, are most likely other important causes of attenuation anomalies. The slight lateral variation of attenuation reflects the lateral homogeneous stratigraphy of the oilfield. The results demonstrate the potential of seismic wave attenuation for delineating heterogeneous zones with high fluid content, which can substantially help for enhancing oil recovery.

Bandwidth-insensitive extended centroid frequency-shift method for near-surfaceQestimation

Seismic waves often are strongly attenuated in the near surface. The measurement and compensation for attenuation are crucial for high-resolution seismic imaging. The quality factor ([Formula: see text]), as a measure of attenuation, is usually estimated by frequency-based methods. We have extended the centroid frequency-shift (CFS) method for [Formula: see text] estimation to high loss media without the usual assumption of [Formula: see text]. The adaptability of this approach for an unknown source wavelet also is demonstrated. Skewness and kurtosis have been used for analyzing the spectral shape change during attenuated wave propagation. Synthetic data tests show a strong relationship between skewness and [Formula: see text] factor estimation accuracy for the conventional CFS method. Surprisingly, our extended CFS approach shows frequency band insensitivity and accuracy for strongly attenuating media. The relative insensitivity of the method to the selected or available frequency bandwidth is shown to be the theoretical basis for its good noise immunity. Finally, a layered medium [Formula: see text] inversion method is derived, which is appropriate to vertical seismic profile (VSP) surveying and is applied to a multioffset field VSP data set to obtain a reliable and stable internal [Formula: see text] value depth distribution.

Frequency Shift Method: A Technique for 3-D Shape Acquisition in the Presence of Strong Interreflections

Frequency Shift Method: A Technique for 3-D Shape Acquisition in the Presence of Strong Interreflections

A fuzzy rule based approach for islanding detection in grid connected inverter systems

&lt;span lang="EN-US"&gt;Islanding is when an area of the electrical distribution system is isolated from the electrical system while being powered by distributed generators. An important condition for the interconnection of power plants and distribution systems is the ability of the power plant to detect islands. The presented and proposed method is a combination of best active sandia frequency shift (SFS) method with the intelligent fuzzy logic controller, which has been tested in distributed production using the island detection function. And the choice to improve the method by fuzzy logic control (FLC) is retained, as this process is more effective in decreasing the non-&lt;/span&gt;&lt;span lang="EN-US"&gt;detection zone (NDZ) and in further improving the efficiency of the islanding detection system. This paper proposes a new active islanding detection technique controlled by a fuzzy logic controller, for grid connected photovoltaic (PV) inverters. In addition, the efficiency and performance of the proposed method strategy for islanding detection has been analyzed and tested in the various situations of the network. In addition, the results of the simulations with the &lt;span lang="EN-US"&gt;power &lt;/span&gt;&lt;span lang="EN-US"&gt;simulation (&lt;/span&gt;PSIM) software will be provided to illustrate the main conclusions and the development of the control. Thus, will be used to show the feasibility and validity of the proposed new algorithm.&lt;/span&gt;

Effect of pore fluid on ultrasonic S-wave attenuation in partially saturated tight rocks

S-wave attenuation in porous media is affected by the presence and properties of saturating fluids. We measured the ultrasonic S-wave velocities of three types of tight rocks (siltstone, dolomite, and sandstone) under partial fluid saturation conditions using the pulse-transmission technique. The S-wave attenuation is estimated by the centroid frequency shift and spectral-ratio methods. The fluid-saturated test results show that the attenuation of most water-saturated dolomites is stronger than that of gas- or oil-saturated dolomites, whereas no apparent relationship exists between attenuation and fluid type in siltstone. The attenuation of siltstone and dolomite increases with increasing porosity and permeability, as well as with clay content for siltstone with clay contents less than 6%. The maximum attenuation of rocks partially saturated with gas and water occurs at high water saturation (55%–100%) in most siltstones. The attenuation of most dolomites shows an increasing trend with water saturation, but remains nearly unchanged in partially saturated sandstone. The attenuation of rocks partially saturated with oil and water is strongest at the partial saturation state in siltstone, but remains essentially constant with water saturation in dolomite. A squirt flow model is used to predict the S-wave attenuation of the fluid-saturated sample. Based on rock property and microstructure analyses, wave attenuation in the fluid-saturated rocks measured in this study can be explained by squirt flow mechanisms and the scattering effect owing to heterogeneities of water patches and grain clusters.

Seismic estimation of fluid saturation based on rock physics: A case study of the tight-gas sandstone reservoirs in the Ordos Basin

Tight-gas sandstone reservoirs of the Ordos Basin in China are characterized by high rock-fragment content, dissimilar pore types, and a random distribution of fluids, leading to strong local heterogeneity. We model the seismic properties of these sandstones with the double-double porosity theory, which considers water saturation, porosity, and the frame characteristics. A generalized seismic wavelet is used to fit the real wavelet, and the peak frequency-shift method is combined with the generalized S-transform to estimate attenuation. Then, we establish rock-physics templates (RPTs) based on P-wave attenuation and impedance. We use the log data and related seismic traces to calibrate the RPTs and generate a 3D volume of rock-physics attributes for the quantitative prediction of saturation and porosity. The predicted values are in good agreement with the actual gas production reports, indicating that the method can be effectively applied to heterogeneous tight-gas sandstone reservoirs.

Improved Carrier Frequency-Shifting Algorithm Based on 2-FFT for Phase Wrap Reduction

The number of phase wraps that result from the carrier component can be completely eliminated or reduced by first applying a fast Fourier transform (FFT) to the image and then shifting the spectrum to the origin. However, because the spectrum can only be shifted by an integer number, the phase wraps of the carrier component cannot be completely reduced. In this paper, an improved carrier frequency-shifting algorithm based on 2-FFT for phase wrap reduction is proposed which allows the spectrum to be shifted by a rational number. Firstly, the phase wraps are reduced by the conventional FFT frequency shift method. Secondly, the wrapped phase with residual carrier components is filtered and magnified sequentially; the amplified phase is transformed into the frequency domain using an FFT, and then, the wrapped phase with the residual carrier components can be further reduced by shifting the spectrum by a rational number. Simulations and experiments were conducted to validate the efficiency of the proposed method.

An Efficient Algorithm for Frequency Estimation of Sinusoid Signal Based on Improved Quinn

To balance the performance and complexity for frequency estimation of sinusoid wave using DFT-based methods, an efficient estimation algorithm based on improved Quinn is proposed. The amplitude and phase information are used to interpolate which ensures the accuracy of the frequency shift direction. A method of piecewise frequency shift is performed, and it has the characteristics of higher precision and lower computational costs. Simulation results show that the judgment of direction factor is more correct using Quinn, the Root Mean Square Error (RMSE) of this method is closer to Cramer-Rao Lower Bound (CRLB) under the conditions of different frequencies and SNRs comparing with Quinn and other improved algorithms, and it can attain remarkable decrease of computational cost which facilitates the algorithm implementation.

Characterization of Gas Saturation in Tight-Sandstone Reservoirs with Rock-Physics Templates Based on Seismic Q

Tight-sandstone gas reservoirs have low porosity and permeability, dissimilar pore types, and generally high clay content. Partial saturation leads to local fluid flow induced by seismic waves, resulting in velocity dispersion and attenuation, and this is the reason why the dissipation factor (Q−1) (inverse quality factor) is highly sensitive to fluid saturation. A relation between Q and saturation can be based on the self-consistent approximation and poroelasticity theory, to build, in principle, a two-dimensional (2D) rock-physics template. However, there is an ambiguity in the process of estimation because one value of Q may correspond to two saturations. Thus, this paper addresses this limitation. Moreover, a well-log in the Sichuan Basin and reported experimental data show that these reservoirs may have a high clay content, which affects the estimation. To take into account this factor, the hydration effect of clay is considered in the framework of the double double-porosity theory of wave propagation. Ultrasonic measurements were performed on a tight sandstone and the spectral-ratio method was used to estimate Q. Then, three-dimensional (3D) rock-physics templates are built by introducing the phase velocity ratio (VP/VS), clay content, and seismic Q estimated with an improved frequency-shift method. The template is calibrated and tested with ultrasonic, well-log, and seismic data and applied to estimate reservoir porosity, clay content, and gas saturation on 2D and 3D seismic data.

Fluid Discrimination in Ultra-Deep Reservoirs Based on a Double Double-Porosity Theory

We develop a methodology, based on rock-physics templates, to effectively identify reservoir fluids in ultra-deep reservoirs, where the poroelasticity model is based on the double double-porosity theory. P-wave attenuation, the ratio of the first Lamé constant to mass density (λ/ρ) and Poisson ratio are used to build the templates at the ultrasonic and seismic frequency bands to quantitatively predict the total and crack (soft) porosities and oil saturation. Attenuation on these frequency bands is estimated with the spectral-ratio and frequency-shift methods. We apply the methodology to fault-controlled karst reservoirs in the Tarim Basin (China), which contain ultra-deep hydrocarbon resources with a diverse pore-crack system, low porosity/permeability and complex oil-water spatial distributions. The results are consistent with well-log data and actual oil recovery. Crack porosity can be used as an indicator to find regions with high oil saturation, since high values implies a good pore connectivity.

How gas-hydrate saturation and morphology control seismic attenuation: A case study from the south Hydrate Ridge

Prior studies have shown an ambiguous relationship between gas-hydrate saturation and seismic attenuation in different regions, but the effect of gas-hydrate morphology on seismic attenuation of hydrate-bearing sediments was often overlooked. We have combined seismic data with rock-physics modeling to elucidate how gas-hydrate saturation and morphology may control seismic attenuation. To extract P-wave attenuation, we process the vertical seismic profile data within a frequency range of 30–150 Hz and the sonic logging data within 10–15 kHz from three wells in the south Hydrate Ridge, offshore Oregon (USA), collected during Ocean Drilling Program Leg 204 in 2000. We calculate the P-wave attenuation using spectral matching and centroid frequency shift methods, and we use Archie’s relationship to derive gas-hydrate saturation from the resistivity data above the bottom-simulating reflection at the same wells. To interpret the observed seismic attenuation in terms of the effects of the gas-hydrate saturation and the morphology, we use the hydrate-bearing effective sediment rock-physics model. By comparing the observed and model-predicted attenuation values, we infer that (1) seismic attenuation appears to not be dominated by any single factor — instead, its variation is likely governed by the gas-hydrate saturation and the morphology; (2) the relationship between seismic attenuation and gas-hydrate saturation varies with different hydrate morphologies; (3) the squirt flow, occurring at different compliances of adjacent pores driven by pressure gradients, may be responsible for the significantly large or small attenuation over a broad frequency range.

Attenuation methods for quantifying gas saturation in organic-rich shale and tight gas formations

Quantitative interpretation of waveform attenuation for determining petrophysical properties remains one of the most challenging problems associated with rock physics. In this study, two effective methods are proposed to compute gas saturation in organic-rich shale and tight gas formations from full-waveform sonic attenuations. We first extract compressional (P)- and shear (S)-wave attenuations from monopole and dipole waveforms by median frequency shift and multi-frequency inversion methods, respectively. Crossplots of the P- to S-wave attenuation ratio ([Formula: see text]) and core gas saturation show a positive linear correlation. The [Formula: see text] value and the neutron-density porosity difference exhibit an identical log trend across different formations. The coincidence of these two different hydrocarbon indicators implies that [Formula: see text] is most sensitive to pore-fluid saturation and is less affected by variations in lithology. In the first method, the core-calibrated [Formula: see text] yields an accurate estimate of gas saturation. The second method is suited for the absence of core saturation data, which use the probability distribution of [Formula: see text] for the evaluation of gas saturation. Compared to conventional resistivity methods, the proposed attenuation method, as a nonelectric approach, provides a more accurate gas saturation prediction for low-resistivity reservoir rocks. Finally, we analyze the characteristics of attenuation-saturation relations in low-porosity rocks and discover the possible attenuation mechanisms.