| 1. |
- Aitomäki, Yvonne, et al.
(författare)
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Estimating material properties of solid and hollow fibers in suspension using ultrasonic attenuation
- 2013
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 60:7, s. 1424-1434
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Tidskriftsartikel (refereegranskat)abstract
- Estimates of the material properties of hollow fibers suspended in a fluid using ultrasound measurements and a simple, computationally efficient analytical model are made. The industrial application is to evaluate the properties of wood fibers in paper pulp. The necessity of using a layered cylindrical model (LCM) as opposed to a solid cylindrical model (SCM) for modeling ultrasound attenuation in a suspension of hollow fibers is evaluated. The two models are described and used to solve the inverse problem of estimating material properties from attenuation in suspensions of solid and hollow polyester fibers. The results show that the measured attenuation of hollow fibers differs from that of solid fibers. Elastic properties estimates using LCM with hollow-fiber suspension measurements are similar to those using SCM with solid-fiber suspension measurements and compare well to block polyester values for elastic moduli. However, using the SCM with the hollow-fiber suspension did not produce realistic estimations. In conclusion, the LCM gives reasonable estimations of hollow fiber properties and the SCM is not sufficiently complex to model hollow fibers. The results also indicate that the use of a distributed radius in the model is important in estimating material properties from fiber suspensions.
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| 2. |
- Berge, John, et al.
(författare)
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Tunable bulk acoustic wave resonators based on Ba0.25Sr0.75TiO3 thin films and a HfO2/SiO2 Bragg reflector
- 2011
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 58:12, s. 2768-2771
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Tidskriftsartikel (refereegranskat)abstract
- A switchable and tunable bulk acoustic wave resonator based on a paraelectric phase Ba0.25Sr0.75TiO3 thin film and an all-dielectric HfO2/SiO2 Bragg reflector is presented. The achieved tuning range (3.8%) and effective electromechanical coupling coefficient (7.1%) are the highest reported for solidly mounted tunable bulk acoustic wave resonators. The non-conductive Bragg reflector stack provides excellent integration possibilities.
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| 3. |
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| 4. |
- Bjurström, Johan, et al.
(författare)
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Synthesis of textured thin piezoelectric AlN films with a nonzero C-axis mean tilt for the fabrication of shear mode resonators
- 2006
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 53:11, s. 2095-2100
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Tidskriftsartikel (refereegranskat)abstract
- A method for the deposition of thin piezoelectric aluminum nitride (AlN) films with a nonzero c-axis mean tilt has been developed. The deposition is done in a standard reactive magnetron sputter deposition system without any hardware modifications. In essence, the method consists of a two-stage deposition process. The resulting film has a distinct tilted texture with the mean tilt of the c-axis varying roughly in the interval 28 to 32 degrees over the radius of the wafer excluding a small exclusion zone at the center of the latter. The mean tilt angle distribution over the wafer has a circular symmetry. A membrane-type shear mode thickness-excited thin film bulk acoustic resonator together with a micro-fluidic transport system has been subsequently fabricated using the two stage AIN deposition as well as standard bulk micro machining of Si. The resonator consisted of a 2-mu m-thick AlN film with 200-nm-thick A1 top and bottom electrodes. The resonator was characterized with a network analyzer when operating in both air and water. The shear mode resonance frequency was about 1.6 GHz, the extracted device Q around 350, and the electromechanical coupling k(t)(2) 2% when the resonator was operated in air, whereas the latter two dropped down to 150 and 1.8%, respectively, when the resonator was operated in pure water.
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| 5. |
- Carlson, Johan, et al.
(författare)
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Prediction of molar fractions in two-component gas mixtures using pulse-echo ultrasound and PLS regression
- 2006
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 53:3, s. 606-613
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Tidskriftsartikel (refereegranskat)abstract
- The composition, and thus the energy content and monetary value of natural gas and biogas, vary considerably depending on the source. Present energy measurement techniques are riot suitable for online use. We show with experiments on mixtures of ethane (C/sub 2/H/sub 6/) and oxygen (O/sub 2/) how partial least squares regression (PLSR) can be used to predict the molar fraction of ethane in the mixtures, given spectral data from ultrasonic pulse-echo measurements. The PLSR technique is compared with the standard principal component regression (PCR), and we show that PLSR yields better predictive performance.
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| 6. |
- Chen, Hongjian, et al.
(författare)
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On the Development of a Novel Contrast Pulse Sequence for Polymer-Shelled Microbubbles
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955.
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Tidskriftsartikel (refereegranskat)abstract
- Contrast agents are routinely used in ultrasound examinations. Nonlinear ultrasound imaging techniques have been developed over decades to enhance the contrast between the tissue and the blood pool after the injection of ultrasound contrast agents. In this study, we introduce a new contrast pulse sequence, CPS4. The CPS4 combines pulse inversion, sub-harmonic, and ultra-harmonic techniques to remove propagation distortion while capturing the unique sub-harmonic, and ultra-harmonic responses from ultrasound contrast agents. The novel CPS4 and conventional pulse inversion, sub-harmonic, and ultra-harmonic techniques were used to detect the presence of a research-grade, thick shell, polymer microbubble in a tissue-mimicking flow phantom. The contrast-to-tissue ratio (CTR) obtained from the applications of all techniques were compared. The results show that the highest CTR of approximately 16 dB was obtained using CPS4, which was superior to the individual reference techniques: pulse inversion, sub-harmonic, and ultra-harmonic techniques, at all scenarios considered in this study.
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| 7. |
- Chen, Hongjian, et al.
(författare)
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On the Development of a Novel Contrast Pulse Sequence for Polymer-Shelled Microbubbles
- 2021
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 68:5, s. 1569-1579
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Tidskriftsartikel (refereegranskat)abstract
- Contrast agents are routinely used in ultrasound examinations. Nonlinear ultrasound imaging techniques have been developed over decades to enhance the contrast between the tissue and the blood pool after the injection of ultrasound contrast agents (UCAs). In this study, we introduce a new contrast pulse sequence, CPS4. The CPS4 combines pulse inversion (PI), subharmonic (SH), and ultraharmonic (UH) techniques to remove propagation distortion while capturing the unique SH and UH responses from UCAs. The novel CPS4 and conventional PI, SH, and UH techniques were used to detect the presence of a research-grade, thick-shell, polymer microbubble in a tissue-mimicking flow phantom. The contrast-to-tissue ratios (CTRs) obtained from the applications of all techniques were compared. The results show that the highest CTR of approximately 16 dB was obtained using CPS4, which was superior to the individual reference techniques: PI, SH, and UH techniques, in all scenarios considered in this study.
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| 8. |
- Cinthio, M., et al.
(författare)
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Evaluation of an ultrasonic echo-tracking method for measurements of arterial wall movements in two dimensions
- 2005
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 52:8, s. 1300-1311
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Tidskriftsartikel (refereegranskat)abstract
- The longitudinal movement of blood vessel walls has so far gained little or no attention, as it has been presumed that these movements are of a negligible magnitude. However, modern high-resolution ultrasound scanners can demonstrate that the inner layers of the arterial wall exhibit considerable movements in the longitudinal direction. This paper evaluates a new, noninvasive, echo-tracking technique, which simultaneously can track both the radial and the longitudinal movements of the arterial wall with high resolution in vivo. Initially, the method is evaluated in vitro using a specially designed ultrasound phantom, which is attached to and moved by an X-Y system, the movement of which was compared with two high-resolution triangulation lasers. The results show an inaccuracy of 2.5% full scale deflection (fsd), reproducibility of 12 µm and a resolution of 5 µm, which should be more than sufficient for in vivo studies. The ability of the method is also demonstrated in a limited in vivo study in which a preselected part of the inner vessel wall of the right common carotid artery of a healthy volunteer is tracked in two dimensions over many cardiac cycles. The results show well reproducible x-y movement loops in which the recorded radial and longitudinal movements both are of the magnitude millimetre. © 2005 IEEE.
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| 9. |
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| 10. |
- Engholm, Marcus, et al.
(författare)
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Adaptive Beamforming for Array Imaging of Plate Structures Using Lamb Waves
- 2010
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 57:12, s. 2712-2724
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Tidskriftsartikel (refereegranskat)abstract
- Lamb waves are considered a promising tool for the monitoring of plate structures. Active arrays employing beamforming techniques enables monitoring large areas of plate structures. Dispersion and multiple propagating modes are issues that need to be addressed when working with Lamb waves. Previous work has mainly focused on standard beamforming while reducing the effects of dispersion and multiple modes through frequency selectivity and transducer design. This paper presents a minimum variance distortionless response (MVDR) approach for Lamb waves using a uniform rectangular array (URA) and a single transmitter. Theoretically calculated dispersion curves are used to compensate for dispersion. The combination of the MVDR approach and the two dimensional array improves the suppression of interfering Lamb modes. The proposed approach is evaluated on simulated and experimental data and compared to the standard delay-and-sum (DAS) beamformer. It is shown that the MVDR algorithm performs better in terms of higher resolution and better sidelobe and mode suppression capabilities. Known issues of the MVDR approach, such as, signal cancellation in highly correlated environments, and poor robustness, are addressed using methods that have proven effective for the purpose in other fields of active imaging.
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| 11. |
- Eriksson, Håkan, et al.
(författare)
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A robust correlation receiver for distance estimation
- 1994
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - New York : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 41:5, s. 596-603
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Tidskriftsartikel (refereegranskat)abstract
- Many methods for distance estimation, such as the ultrasonic pulse-echo method, involve the estimation of a time-of-flight (TOF). In this paper, a signal model is developed that, apart from the TOF, accounts for an unknown, linear frequency dependent distortion as well as for additive noise. We derive a TOF estimator for this model based on the criteria of maximum likelihood. The resulting receiver can be seen as an extension or generalization of the well known cross-correlation, or "matched filter", estimator described, e.g., by Nilsson. The novel receiver is found to be more robust against unknown pulse shape distortion than the cross-correlation estimator, giving less biased TOF estimates. Also, bias versus noise sensitivity can be controlled by proper model order selection
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| 12. |
- Grennberg, Anders, et al.
(författare)
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Estimation of subsample time delay differences in narrowband ultrasonic echoes using the Hilbert transform correlation
- 1994
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 41:5, s. 588-595
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Tidskriftsartikel (refereegranskat)abstract
- In many areas the time delay of arrival (TDOA) is desired. In the case of narrowband signals we propose a fast and simple method to estimate small time delays. This method is shown to have the same or better accuracy as the cross correlation methods for small delays in the order of fractions of the sample interval. It is based on using the Hilbert transform in correlation between two signals and consists of only one scalar product, which makes it fast. It may also be used in applications with narrowband signals where the measurements are repeatable, such as ultrasonic imaging and nondestructive testing. In ultrasonic applications, due to fluctuations in the insonified media, a small random time shift may be present causing the signals to be misaligned in time. Averaging signals under these conditions will result in a distortion of the signal shape. We propose an averaging method to avoid this and to accomplish a higher SNR without the distortion. Simulations and experiments from ultrasonic applications are presented.
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| 13. |
- Hägglund, Fredrik, et al.
(författare)
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Model-based estimation of thin multi-layered media using ultrasonic measurements
- 2009
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 56:8, s. 1689-1702
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Tidskriftsartikel (refereegranskat)abstract
- In ultrasonic measurement situations, when dealing with media of multi-layered structures consisting of 1 or more thin layers, analysis of the measured ultrasonic waveform can be difficult because of overlapping and reverberant echoes. Information from the individual layers is then difficult to extract because the individual echoes cannot be detected. In this study, we use a parametric layer model to analyze the multi-layered material in a system identification approach. The parameters of the model are connected to physical properties of the investigated material, e.g., the reflection coefficients, the time-of-flight, and the attenuation. The main advantage using this model is that the complexity of the model is connected to the number of layers rather than the number of observable echoes in the received ultrasonic waveform. A system of linear equations is presented, giving the opportunity to find the model for both pulse-echo and through-transmission measurements. A thorough effort is made on the parameter estimation and optimization algorithm. The model is validated with practical measurements on a 3-layered structure using both pulse-echo and through-transmission techniques. The 3-layered material consists of a thin embedded middle layer with the time-of-flight in that layer shorter than the emitted signal¿s time support, giving rise to overlapping echoes. Finally the relation between the model parameters and physical properties of the material is established.
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| 14. |
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| 15. |
- Johansson, Jonny, et al.
(författare)
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Simulation of absolute amplitudes of ultrasound signals using equivalent circuits
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:10, s. 1977-1983
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Tidskriftsartikel (refereegranskat)abstract
- Equivalent circuits for piezoelectric devices and ultrasonic transmission media can be used to cosimulate electronics and ultrasound parts in simulators originally intended for electronics. To achieve efficient systemlevel optimization, it is important to simulate correct, absolute amplitude of the ultrasound signal in the system, as this determines the requirements on the electronics regarding dynamic range, circuit noise, and power consumption.This paper presents methods to achieve correct, absolute amplitude of an ultrasound signal in a simulation of a pulse-echo system using equivalent circuits. This is achieved by taking into consideration loss due to diffraction and the effect of the cable that connects the electronics and the piezoelectric transducer. The conductive loss in the transmission line that models the propagation media of the ultrasound pulse is used to model the loss due to diffraction.Results show that the simulated amplitude of the echo follows measured values well in both near and far fields, with an offset of about 10%. The use of a coaxial cable introduces inductance and capacitance that affect the amplitude of a received echo. Amplitude variations of 60% were observed when the cable length was varied between 0.07 m and 2.3 m, with simulations predicting similar variations. The high precision in the achieved results show that electronic design and system optimization can rely on system simulations alone. This will simplify the development of integrated electronics aimed at ultrasound systems.
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| 16. |
- Jonsson, Jonas, 1979-, et al.
(författare)
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Simulation, manufacturing, and evaluation of a sonar for a miniaturized submersible explorer
- 2010
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 57:2, s. 490-495
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Tidskriftsartikel (refereegranskat)abstract
- Single-beam side-scan sonar elements, to be fitted on a miniaturized submersible, are here simulated, manufactured, and evaluated. Finite element analysis simulations are compared with measurements, and an overall observation is that the agreement between simulations and measurements deviates from the measured values of 1.5 to 2°, for the narrow lobe angle, by less than 10% for most models. An overall finding is that the lobe width along the track direction can be accurately simulated and, hence, the resolution of the sonars can be predicted. This paper presents, to the authors’ knowledge, the world’s smallest side-scan sonars.
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| 17. |
- Jonsson, Ulf G., 1950-, et al.
(författare)
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A FEM-Based Method Using Harmonic Overtones to Determine the Effective Elastic, Dielectric, and Piezoelectric Parameters of Freely Vibrating Thick Piezoelectric Disks
- 2013
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - New York : IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society. - 0885-3010 .- 1525-8955. ; 60:1, s. 243-255
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Tidskriftsartikel (refereegranskat)abstract
- To gain an understanding of the electroelasticproperties of tactile piezoelectric sensors used in the characterizationof soft tissue, the frequency-dependent electric impedanceresponse of thick piezoelectric disks has been calculatedusing finite element modeling. To fit the calculated to the measuredresponse, a new method was developed using harmonicovertones for tuning of the calculated effective elastic, piezoelectric,and dielectric parameters. To validate the results, theimpedance responses of 10 piezoelectric disks with diameterto-thickness ratios of 20, 6, and 2 have been measured from10 kHz to 5 MHz. A two-dimensional, general purpose finiteelement partial differential equation solver with adaptive meshingcapability run in the frequency-stepped mode, was used.The equations and boundary conditions used by the solver arepresented. Calculated and measured impedance responses arepresented, and resonance frequencies have been compared indetail. The comparison shows excellent agreement, with averagerelative differences in frequency of 0.27%, 0.19%, and0.54% for the samples with diameter-to-thickness ratios of 20,6, and 2, respectively. The method of tuning the effective elastic,piezoelectric, and dielectric parameters is an importantstep toward a finite element model that describes the propertiesof tactile sensors in detail.
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| 18. |
- Jonsson, Ulf G, et al.
(författare)
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Modeling the high-frequency complex modulus of a silicone rubber using standing lamb waves and an inverse finite element method
- 2014
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : IEEE Press. - 0885-3010 .- 1525-8955. ; 61:12, s. 2106-2120
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Tidskriftsartikel (refereegranskat)abstract
- To gain an understanding of the high-frequency elastic properties of silicone rubber, a finite element model of a cylindrical piezoelectric element, in contact with a silicone rubber disk, was constructed. The frequency dependent elastic modulus of the silicone rubber was modeled by a four parameter fractional derivative viscoelastic model in the 100 kHz to 250 kHz frequency range. The calculations were carried out in the range of the first radial resonance frequency of the sensor. At the resonance, the hyperelastic effect of the silicone rubber was modeled by a hyperelastic compensating function. The calculated response was matched to the measured response by using the transitional peaks in the impedance spectrum that originates from the switching of standing Lamb wave modes in the silicone rubber. To validate the results, the impedance responses of three 5 mm thick silicone rubber disks, with different radial lengths, were measured. The calculated and measured transitional frequencies have been compared in detail. The comparison showed very good agreement, with average relative differences of 0.7 %, 0.6 %, and 0.7 % for the silicone rubber samples with radial lengths of 38.0 mm, 21.4 mm, and 11.0 mm, respectively. The average, complex, elastic modulus of the samples were: (0.97 + 0.009i) GPa at 100 kHz and (0.97 + 0.005i) GPa at 250 kHz.
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| 19. |
- Kothapalli, Veera Venkata Satya Naray, 1985-, et al.
(författare)
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Unique pumping-out fracturing mechanism of a polymer-shelled contrast agent : An acoustic characterization and optical visualization
- 2014
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 62:3, s. 451-462
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Tidskriftsartikel (refereegranskat)abstract
- This work describes the fracturing mechanism of air-filled microbubbles (MBs) encapsulated by a cross-linked poly(vinyl alcohol) (PVA) shell. The radial oscillation and fracturing events following the ultrasound exposure were visualized with an ultrahigh-speed camera, and backscattered timedomain signals were acquired with the acoustic setup specific for harmonic detection. No evidence of gas emerging from defects in the shell with the arrival of the first insonation burst was found. In optical recordings, more than one shell defect was noted, and the gas core was drained without any sign of air extrusion when several consecutive bursts of 1 MPa amplitude were applied. In acoustic tests, the backscattered peak-to-peak voltage gradually reached its maximum and exponentially decreased when the PVA-based MB suspension was exposed to approximately 20 consecutive bursts arriving at pulse repetition frequencies of 100 and 500 Hz. Taking into account that the PVA shell is porous and possibly contains large air pockets between the cross-linked PVA chains, the aforementioned acoustic behavior might be attributed to pumping gas from these pockets in combination with gas release from the core through shell defects. We refer to this fracturing mechanism as pumping-out behavior, and this behavior could have potential use for the local delivery of therapeutic gases, such as nitric oxide.
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| 20. |
- Larsson, Matilda, et al.
(författare)
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Ultrasound-Based Radial and Longitudinal Strain Estimation of the Carotid Artery : A Feasibility Study
- 2011
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 58:10, s. 2244-2251
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Tidskriftsartikel (refereegranskat)abstract
- Ultrasound-based estimation of arterial wall elasticity is commonly used to assess arterial stiffness. However, previous elastography studies have mostly addressed radial strain measurements, and the longitudinal strain has been more or less ignored. This study shows the feasibility of a speckle-tracking-based algorithm for simultaneous estimation of radial and longitudinal strain in the carotid artery in silico. Additionally, these results were preliminarily confirmed in vivo.
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| 21. |
- Lingvall, Fredrik, et al.
(författare)
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On time-domain model-based ultrasonic array imaging
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:8, s. 1623-1633
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Tidskriftsartikel (refereegranskat)abstract
- This paper treats time-domain model-based Bayesian image reconstruction for ultrasonic array imaging and, in particular, two reconstruction methods are presented. These two methods are based on a linear model of the array imaging system and they perform compensation in both the spatial and temporal domains using a minimum mean squared error (MMSE) criterion and a maximum a posteriori MAP) estimation approach, respectively. The presented estimators perform compensation for both the electrical and acoustical wave propagation effects for the ultrasonic array system at hand. The estimators also take uncertainties into account, and, by the incorporation of proper prior knowledge, high-contrast superresolution reconstruction results are obtained. The novel nonlinear MAP estimator constrains the scattering amplitudes to be positive, which applies in applications where the scatterers have higher acoustic impedance than the surrounding medium. The linear MMSE and nonlinear MAP estimators are compared to the traditional delay-and-sum (DAS) beamformer with respect to both resolution and signal-to-noise ratio. The algorithms are compared using both simulated and measured data. The results show that the model-based methods can successfully compensate for both sidelobes and grating lobes, and they have a superior temporal and lateral resolution compared to DAS beamforming. The ability of the nonlinear MAP estimator to suppress noise is also superior compared to both the linear MMSE estimator and the DAS beamformer.
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| 22. |
- Martinsson, Jesper, et al.
(författare)
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Model-based phase velocity and attenuation estimation in wideband ultrasonic measurement systems
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:1, s. 138-146
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Tidskriftsartikel (refereegranskat)abstract
- A parametric method to estimate frequencydependent phase velocity and attenuation is presented in this paper. The parametric method is compared with standard nonparametric Fourier analysis techniques using numerical simulations as well as real pulse-echo experiments. Approximate standard deviations are derived for bothmethods and validated with numerical simulations. Compared to standard Fourier analysis, the parametric model gives considerably lower variance when estimating attenuation and phase velocity. In contrast to nonparametric techniques, the proposed estimator avoids the phase unwrapping problem because analytical expressions for thecontinuous phase velocity and attenuation can be derived.
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| 23. |
- Olofsson, Tomas
(författare)
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Phase Shift Migration for Imaging Layered Objects and Objects Immersed in Water
- 2010
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 57:11, s. 2522-2530
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Tidskriftsartikel (refereegranskat)abstract
- This paper proposes the use of phase shift migration for ultrasonic imaging of layered objects and objects immersed in water. The method, which was developed in reflection seismology, is a frequency domain technique that in a computationally efficient way restores images of objects that are isotropic and homogeneous in the lateral direction but inhomogeneous in depth. The performance of the proposed method was evaluated using immersion test data from a block with side-drilled holes with an additional scatterer residing in water. In this way, the method's capability of simultaneously imaging scatterers in different media and at different depths was investigated. The method was also applied to a copper block with flat bottom holes. The results verify that the proposed method is capable of producing high-resolution and low-noise images for layered or immersed objects.
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| 24. |
- Olofsson, Tomas, et al.
(författare)
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Sparse deconvolution of B-scan images
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:8, s. 1634-1641
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a new computationally efficient sparse deconvolution algorithm for the use on B-scan images from objects with relatively few scattering targets is presented. It is based on a linear image formation model that has been used earlier in connection with linear minimum mean squared error (MMSE) two-dimensional (2-D) deconvolution. The MMSE deconvolution results have shown improved resolution compared to synthetic aperture focusing technique (SAFT), but at the cost of increased computation time. The proposed algorithm uses the sparsity of the image, reducing the degrees of freedom in the reconstruction problem, to reduce the computation time and to improve the resolution. The dominating task in the algorithm consists in detecting the set of active scattering targets, which is done by iterating between one up-dating pass that detects new points to include in the set, and a down-dating pass that removes redundant points. In the up-date, a spatio-temporal matched filter is used to isolate potential candidates. A subset of those are chosen using a detection criterion. The amplitudes of the detected scatterers are found by MMSE. The algorithm properties are illustrated using synthetic and real B-scan. The results show excellent resolution enhancement- and noise-suppression capabilities. The involved computation times are analyzed.
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| 25. |
- Plessky, Victor, et al.
(författare)
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Two Resonances of Different Nature in STW Resonators With Aperture-Weighted Metallization
- 2015
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 62:4, s. 766-770
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Tidskriftsartikel (refereegranskat)abstract
- In a narrowband 2-port resonator on 37 degrees-cut quartz with Al electrodes (h(Alu) = 100 nm) exploiting surface transverse waves (STW), we have observed parasitic ripples which have been attributed to transverse modes unusually situated on the left side of the main resonance. To suppress these modes, we have used metallization coefficient weighting across the aperture, with more metal in the middle and reduced metallization close to the busbars. The parasitic modes indeed disappeared, but at significantly higher frequency, we have found an additional strong response which does not exist in a resonator with uniform electrode metallization. 3-D simulations showed that the structure has another very different mode, with the wave propagating mainly along the edge of the busbars, but excited with the interdigital electrode system.
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| 26. |
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| 27. |
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| 28. |
- Skjelvareid, Martin H., et al.
(författare)
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Synthetic Aperture Focusing of Ultrasonic Data From Multilayered Media Using an Omega-K Algorithm
- 2011
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 58:5, s. 1037-1048
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Tidskriftsartikel (refereegranskat)abstract
- The synthetic aperture focusing technique (SAFT) is used to create focused images from ultrasound scans. SAFT has traditionally been applied only for imaging in a single medium, but the recently introduced phase shift migration (PSM) algorithm has expanded the use of SAFT to multilayer structures. In this article we present a similar focusing algorithm called multi-layer omega-k (MULOK), which combines PSM and the omega-k algorithm to perform multilayer imaging more efficiently. The asymptotic complexity is shown to be lower for MULOK than for PSM, and this is confirmed by comparing execution times for implementations of both algorithms. To facilitate the complexity analysis, a detailed description of algorithm implementation is included, which also serves as a guide for readers interested in practical implementation. Using data from an experiment with a multilayered structure, we show that there is essentially no difference in image quality between the two algorithms.
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| 29. |
- Stepinski, Tadeusz
(författare)
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An Implementation of Synthetic Aperture Focusing Technique in Frequency Domain
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:7, s. 1399-1408
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Tidskriftsartikel (refereegranskat)abstract
- A new implementation of a synthetic aperture focusing technique (SAFT) based on concepts used in synthetic aperture radar and sonar is presented in the paper. The algorithm, based on the convolution model of the imaging system developed in frequency domain, accounts for the beam pattern of the finite-sized transducer used in the synthetic aperture. The 2D fast Fourier transform (FFT) is used for the calculation of a 2D spectrum of the ultrasonic data. The spectrum is then interpolated to convert the polar coordinate system used for the acquisition of ultrasonic signals to the rectangular coordinates used for the presentation of imaging results. After compensating the transducer lobe amplitude profile using a Wiener filter, the transformed spectrum is subjected to the 2D inverse Fourier transform to get the time-domain image again. The algorithm is computationally attractive due to the use of 2D FFT. The performance of the proposed frequency-domain algorithm and the classical time-domain SAFT are compared in the paper using simulated and real ultrasonic data.
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| 30. |
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| 31. |
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| 32. |
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| 33. |
- Thornell, Greger, et al.
(författare)
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Residual stress in sputtered gold films on quartz measured by the cantilever beam deflection technique
- 1999
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 46:4, s. 981-992
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Tidskriftsartikel (refereegranskat)abstract
- With resonator applications in mind, the residual stress in sputtered gold electrodes on quartz has been investigated with respect to varioiis deposition rates (2, 10, and 50 A/s), pressures (1.0 and 3 0- mbar), deposition temperatures (80OC and room temperature (RT)), film thicknesses (approx. 400 to 800 A), and substrate smoothnesses (lapped and polished), u:;ing the cantilever beam deflection method. Samples were monitored for 4 weeks at room temperature followed by 13 weeks of annealing at 85OC. The initial stress (ranging from -180 to -60 MPa) was compressive for all samples but turned tensile (a few megapascals) in some of the samples after annealing. A significant decrease in initial compressive stress appeared with samples coated at an elevated temperature. From samples preparecd at lower pressure and differing only in film thickness and substrate roughness, an increased compressive stress was found in thicker films and on rougher surfaces. The stress relaxation has been fitted to an exponential expression, and an attempt to relate the stress to a frequency shift (typically a few parts per million for ordinary, 100-pm thick AT blanks) has been made. With the help of transmission electron microscopy (TEM) the film morphology was investigated and related to the deposition parameters and aging. Judging from the increase in compressive stress and grain refinement with increased deposition rate and decreased pressure the atomic peening mechanism is the most likely reascm for the induced stress. Rutherford backscattering spectrometry (RBS) was employed to rule our the inclusion oj- argon (below or around 0.5%) as an explanation. From the vague, but clearly discernible, trend toward faster RT stress relaxation with higher initial stress, together with the liner film morphology, the relief mechaniism is believed to he stress-promoted grain boundary diffusion.
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| 34. |
- Vallin, Örjan, et al.
(författare)
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Polishing of quartz by rapid etching in ammonium bifluoride
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:7, s. 1454-1462
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Tidskriftsartikel (refereegranskat)abstract
- The etch rate and surface roughness of polished and lapped AT-cut quartz subjected to hot (90, 110, and 130 degrees C), concentrated (50, 65, 80 wt %) ammonium bi-fluoride have been investigated. Having used principal component analysis to verify experimental solidity and analyze data, we claim with confidence that this parameter space does not, as elsewhere stated, allow for a polishing effect or even a preserving setting. Etch rates were found to correlate well, and possibly logarithmically, with temperature except for the hottest etching applied to lapped material. Roughness as a function of temperature and concentration behaved well for the lapped material, but lacked systematic variation in the case of the polished material. At the lowest temperature, concentration had no effect on etch rate or roughness. Future efforts are targeted at temperatures and concentrations closer to the solubility limit.
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| 35. |
- van Deventer, Jan, et al.
(författare)
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PSpice simulation of ultrasonic systems
- 2000
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 47:4, s. 1014-1024
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Tidskriftsartikel (refereegranskat)abstract
- The usage of electrical analogies for the simulation of wave generation and propagation in ultrasound transducers is well established. In this paper a PSpice approach that includes the temperature and frequency dependency of the transducer performance is proposed. The analogy between acoustic wave propagation and wave propagation in an electric transmission line is given. Further ways to deduce temperature and frequency dependencies are discussed. The simulation approach is applied to a pulse-echo setup for the determination of speed of sound and attenuation in liquids and solids. Experiments and simulations are made for three temperatures and in the frequency range 1-12 MHz using water, glycerine, and polymers (PMMA and PEEK) as test samples. Comparison shows a good agreement between simulation and experiments. Results for glycerine indicates that the available attenuation models for high viscosity liquids is inappropriate
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| 36. |
- van Deventer, Jan, et al.
(författare)
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Thermostatic and dynamic performance of an ultrasonic density probe
- 2001
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 48:3, s. 675-682
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Tidskriftsartikel (refereegranskat)abstract
- The thermally static and dynamic performance of an ultrasonic density probe for liquids is investigated in the density range of 750 to 1300 kg/m3 at temperature ranging from 0 to 40°C. The single transducer probe uses a pulse echo technique to obtain the characteristic acoustic impedance of the liquid and, subsequently, the speed of sound through the liquid to obtain the density of the liquid. Variations in the initial sound amplitude are addressed by the design of a layered two material probe. It is shown that it is possible to obtain an accuracy of 0.4% in the experiments carried out. For changing temperature, the probe exhibits large errors because of problems in estimating the temperatures in certain regions of the probe
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| 37. |
- Wennerström, Erik, et al.
(författare)
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Model-based correction of diffraction effects of the virtual source element
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:8, s. 1614-1622
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Tidskriftsartikel (refereegranskat)abstract
- A method for ultrasonic synthetic aperture imaging using finite-sized transducers is introduced that is based on a virtual source (VS) concept. In this setup, a focused transducer creates a VS element at its focal point that facilitates the use of synthetic aperture focusing technique (SAFT). It is shown that the performance of the VS method may be unsatisfactory due to the distortion introduced by the diffraction effects of the aperture used for creating the VS element. A solution to this problem is proposed that consists of replacing the classical SAFT by the extended synthetic aperature focusing technique (ESAFT) algorithm presented in our earlier works. In ESAFT, the full geometry of the VS is modeled, instead of applying the simplified point source approximation used when VS is combined with classical SAFT. The proposed method yields a substantial improvement in spatial resolution compared to that obtained using SAFT. Performance of the proposed algorithm is first demonstrated on simulated data, then verified on real data acquired with an array system.
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| 38. |
- Yantchev, Ventsislav
(författare)
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Coupling-of-modes analysis of thin film plate acoustic wave resonators utilizing the S0 Lamb mode
- 2010
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 57:4, s. 801-807
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Tidskriftsartikel (refereegranskat)abstract
- In this work the applicability of the coupling-ofmodes (COM) approach to the analysis of thin AlN film plate acoustic resonators (FPAR), utilizing the S0 Lamb wave, is discussed. Analysis based on the Floquet-Bloch theorem as well as COM parameter extraction from a micromachined FPAR test structure are simultaneously used to verify the applicability of the COM approach. Finite element model simulation is used to further study the contribution of the higher order mass loading effects over the Lamb wave propagation under a periodical grating. A possibility to achieve zero sensitivity of the FPAR resonance with respect to the grating strip thickness is identified and physically interpreted for the first time.
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| 39. |
- Yantchev, Ventsislav, et al.
(författare)
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Micromachined Thin Film Plate Acoustic Resonators Utilizing the Lowest Order Symmetric Lamb Wave Mode
- 2007
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 54:1, s. 87-95
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Tidskriftsartikel (refereegranskat)abstract
- Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) filmmembranes have been studied. The 2-μm thick, highly coriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rockingcurve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-μm wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateralfield excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technologyproved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology.
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| 40. |
- Yantchev, Ventsislav, et al.
(författare)
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Micromachined Thin Film Plate Acoustic Wave Resonators (FPAR) Part II
- 2009
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010 .- 1525-8955. ; 56:12, s. 2701-2710
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Tidskriftsartikel (refereegranskat)abstract
- Improved performance thin-film plate acousticwave resonators (FPAR) using the lowest order symmetricLamb wave (S0) propagating in highly textured AlN membraneshave been previously demonstrated for the first time.In this work, an experimental study of the resonators’ performancevs. a variety of design parameters is performed. Devicesoperating in the vicinity of the stopband center exhibiting aQ-value of up to 3000 at a frequency of around 875 MHz aredemonstrated. Further, low-loss high-Q micromachined 2-portlongitudinally coupled thin-film resonators using the S0 modeare demonstrated for the first time. For the analysis of theproposed structures, the coupling-of-modes (COM) approachis successfully employed. Initially, the COM model is used forthe extraction of physical parameters from one-port FPARmeasurements. Subsequently, using the COM model, a satisfactoryagreement with the proposed experimental frequencycharacteristics of S0 2-port FPARs has been achieved, andpossibilities for further improvements in the performance discussed.Finally, the frequency spectrum of the one-port deviceshas been studied and the excited plate modes at differentfrequencies identified and presented with their Q-factors andtemperature coefficients of frequency (TCF).
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| 41. |
- Zhou, Guang-Quan, et al.
(författare)
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A Single-Shot Region-Adaptive Network for Myotendinous Junction Segmentation in Muscular Ultrasound Images
- 2020
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-3010 .- 1525-8955. ; 67:12, s. 2531-2542
-
Tidskriftsartikel (refereegranskat)abstract
- Tracking the myotendinous junction (MTJ) in consecutive ultrasound images is crucial for understanding the mechanics and pathological conditions of the muscletendon unit. However, the lack of reliable and efficient identification of MTJ due to poor image quality and boundary ambiguity restricts its application in motion analysis. In recent years, with the rapid development of deep learning, the region-based convolution neural network (RCNN) has shown great potential in the field of simultaneous objection detection and instance segmentation in medical images. This article proposes a region-adaptive network (RAN) to localize MTJ region and to segment it in a single shot. Our model learns about the salient information of MTJ with the help of a composite architecture. Herein, a region-based multitask learning network explores the region containing MTJ, while a parallel end-to-end U-shaped path extracts the MTJ structure from the adaptively selected region for combating data imbalance and boundary ambiguity. By demonstrating the ultrasound images of the gastrocnemius, we showed that the RAN achieves superior segmentation performance when compared with the state-of-the-art Mask RCNN method with an average Dice score of 80.1. Our proposed method is robust and reliable for advanced muscle and tendon function examinations obtained by ultrasound imaging.
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| 42. |
- Almqvist, Monica, et al.
(författare)
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Characterization of micromachined ultrasonic transducers using light diffraction tomography
- 2005
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010. ; 52:12, s. 2298-2302
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Tidskriftsartikel (refereegranskat)abstract
- This paper demonstrates that light diffraction tomography can be used to measure the acoustic field of micromachined ultrasonic transducers (MUT) in cases in which standard methods like hydrophone and microphone measurements fail. Two types of MUTs have been characterized with the method, one air-coupled capacitive MUT (cMUT) and one waterloaded continuous wave (CW) miniature multilayer lead zirconate titanate (PZT) transducer. Light diffraction tomography is an ultrasound measurement method with some special characteristics. Based on the interaction of light and ultrasound, it combines light intensity measurements with tomography algorithms to produce a measurement system. The method offers nonperturbing pressure measurements with high spatial resolution. It has been shown that, under certain circumstances, light diffraction tomography can be used as an absolute pressure measurement method with accuracy in the order of 10% in water and 13% in air. The results show that air-coupled cMUTs in the frequency range of about 1 MHz as well as the extreme near field of a miniaturized CW 10 MHz waterloaded transducer were successfully characterized with light diffraction tomography.
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| 43. |
- Boström, Anders E, 1951, et al.
(författare)
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In-plane P-SV waves from a piezoelectric strip actuator: exact versus effective boundary condition solutions
- 2005
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. - 0885-3010. ; 52:9, s. 1594-1600
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Tidskriftsartikel (refereegranskat)abstract
- A piezoelectric strip of finite width and thickness is placed on top of anisotropic elastic half-space. It operates in actuator mode and a timeharmonic voltage is thus applied across it. The piezoelectric material isof type $6mm$ oriented so that a 2D inplane (P-SV) problem results.By Fourier series expansions the problem is solved exactly and thisresult is compared to the case when the piezoelectric strip is replaced byan effective boundary condition, which is derived by series expansionsin the thickness coordinate in the piezoelectric strip. At low frequenciesthe results agree very well and this corresponds to the situation oftenmet in practice. In general, the effective boundary condition should bemuch easier to apply, for example when a FEM program is used.
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| 44. |
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| 45. |
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| 46. |
- Evertsson, Maria, et al.
(författare)
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Frequency- and Phase-Sensitive Magnetomotive Ultrasound Imaging of Superparamagnetic Iron Oxide Nanoparticles
- 2013
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010. ; 60:3, s. 481-491
-
Tidskriftsartikel (refereegranskat)abstract
- It has recently been demonstrated that superparamagnetic iron oxide nanoparticles can be used as magnetomotive ultrasound contrast agents. A time-varying external magnetic field acts to move the particles and, thus, the nanoparticle-laden tissue. However, the difficulty of distinguishing this magnetomotive motion from undesired movement induced in regions without nanoparticles or other motion artifacts has not been well reported. Using a high-frequency linear-array system, we found that displacements outside nanoparticle-laden regions can be similar in magnitude to those in regions containing nanoparticles. We also found that the displacement outside the nanoparticle regions had a phase shift of approximately p radians relative to that in the nanoparticle regions. To suppress signals arising from undesirable movements, we developed an algorithm based on quadrature detection and phase gating at the precise frequency of nanoparticle displacement. Thus, clutter at other frequencies can be filtered out, and the processed signal can be color-coded and superimposed on the B-mode image. The median signal-to-clutter ratio improvement using the proposed algorithm was 36 dB compared with simply summing the movement energy at all frequencies. This clutter rejection is a crucial step to move magnetomotive ultrasound imaging of nanoparticles toward in vivo investigations.
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| 47. |
- Evertsson, Maria, et al.
(författare)
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Multimodal Detection of Iron Oxide Nanoparticles in Rat Lymph Nodes Using Magnetomotive Ultrasound Imaging and Magnetic Resonance Imaging
- 2014
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010. ; 61:8, s. 1276-1283
-
Tidskriftsartikel (refereegranskat)abstract
- Detection and removal of sentinel lymph nodes (SLN) is important in the diagnosis and treatment of cancer. The SLN is the first regional lymph node draining the primary tumor, and if the cancer has spread, it is most likely to find metastases in the SLN. In this study, we have for the first time been able to image the very same contrast agent, superparamagnetic iron oxide nanoparticles (SPIO-NPs), in rat SLNs by using both our frequency-and phase-gated magnetomotive ultrasound (MMUS) algorithm and conventional magnetic resonance imaging (MRI); MMUS post mortem, MRI in vivo. For both higher NP-concentration and smaller NPs, we found that the MMUS data showed a larger magnetomotive displacement (1.56 +/- 0.43 and 1.94 +/- 0.54 times larger, respectively) and that the MR-images were affected to a higher degree. The MMUS displacement also increased with lower excitation frequency (1.95 +/- 0.64 times larger for 5 Hz compared with 15 Hz) and higher excitation voltage (2.95 +/- 1.44 times larger for 30 V compared with 10 V). The results show that MMUS has potential to be used as bedside guidance during SLN surgery, imaging the same particles that were used in prior staging with other imaging techniques.
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| 48. |
- Gevorgian, Spartak, 1948, et al.
(författare)
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Impedance of DC-Bias-Controlled Composite FBARs
- 2013
-
Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. - 0885-3010. ; 60:4, s. 795-804
-
Tidskriftsartikel (refereegranskat)abstract
- A closed-form analytic formula is derived for composite resonators consisting of two layers of paraelectric-phase ferroelectrics. It is used to analyze the properties of these resonators. Under an applied dc electric field, the layers exhibit an induced piezoelectric effect. The signs of the induced piezoelectric coefficients may be changed by changing the polarity of the dc voltages applied to the layers. In the case of a composite in which the layers have similar thicknesses, changing the sign of the piezoelectric coefficient causes a switching between the resonant frequencies of the first and second harmonics. In the case in which one of the layers is substantially thinner, changing the sign of the piezoelectric coefficient causes switching of the resonant frequency of the first harmonic between two values. In both cases, a continuous change in the dc bias voltage causes continuous changes in the resonant frequencies.
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| 49. |
- Gran, Fredrik, et al.
(författare)
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Adaptive spectral Doppler estimation
- 2009
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Ingår i: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. - 0885-3010. ; 56:4, s. 700-714
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, 2 adaptive spectral estimation techniques are analyzed for spectral Doppler ultrasound. The purpose is to minimize the observation window needed to estimate the spectrogram to provide a better temporal resolution and gain more flexibility when designing the data acquisition sequence. The methods can also provide better quality of the estimated power spectral density (PSD) of the blood signal. Adaptive spectral estimation techniques are known to provide good spectral resolution and contrast even when the observation window is very short. The 2 adaptive techniques are tested and compared with the averaged periodogram (Welch's method). The blood power spectral capon (BPC) method is based on a standard minimum variance technique adapted to account for both averaging over slow-time and depth. The blood amplitude and phase estimation technique (BAPES) is based on finding a set of matched filters (one for each velocity component of interest) and filtering the blood process over slow-time and averaging over depth to find the PSD. The methods are tested using various experiments and simulations. First, controlled flow-rig experiments with steady laminar flow are carried out. Simulations in Field II for pulsating flow resembling the femoral artery are also analyzed. The simulations are followed by in vivo measurement on the common carotid artery. In all simulations and experiments it was concluded that the adaptive methods display superior performance for short observation windows compared with the averaged periodogram. Computational costs and implementation details are also discussed.
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| 50. |
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