Echo Amplitude Research Articles

Axis-matching excitation pulses for CPMG-like sequences in inhomogeneous fields

The performance of the standard CPMG sequence in inhomogeneous fields can be improved with the use of broadband excitation and refocusing pulses. Here we introduce a new class of excitation pulses, so-called axis-matching excitation pulses, that optimize the response for a given refocusing pulse. These new excitation pulses are tailored to the refocusing pulses and take their imperfections into account. Rather than generating purely transverse magnetization, these pulses are designed to generate magnetization pointing along the axis of the effective rotation of the refocusing cycle. This approach maximizes the CPMG component and minimizes the CP component of the signal. Replacing a standard 90° pulse with a new excitation pulse matched to the 180° refocusing pulse increases the signal bandwidth and improves the echo amplitudes by 30% in inhomogeneous fields in comparison to the standard CPMG sequence. Larger gains are obtained with more advanced refocusing pulses. Recent work demonstrated that it is possible to increase the signal to noise ratio (SNR) of individual echoes by more than a factor of 1.5 (in power units) without increasing the duration or amplitude of the refocusing pulses. This was achieved by replacing the standard 180° refocusing pulse by a short phase alternating pulse and the standard 90° excitation pulse by a broadband excitation pulse. We show here that with suitable axis-matching excitation pulses, the SNR further increases by over a factor of 2. We discuss the underlying theory and present several practical implementations of purely phase modulated axis-matching excitation pulses for a number of different refocusing pulses that were derived using methods of optimal control. To gain the full benefit of these new excitation pulses, it is essential to replace the standard phase cycling scheme based on 180° phase shifts by a new scheme involving phase inversion. We tested the new pulses experimentally and observe excellent agreement with the theoretical expectations. We also demonstrate that an additional benefit of axis-matching excitation pulses is the decrease of the transient that appears in the amplitudes of the first few echoes, thus enabling better measurements of short relaxation times.

Complete elastic characterization of lithium phosphorous oxynitride films using picosecond ultrasonics

We report on a complete elastic characterization by picosecond ultrasonics technique of lithium phosphorous oxynitride (LiPON) thin films, which are nowadays, considered as promising solid-state electrolytes for microbatteries technology. Longitudinal sound velocity is deduced from Brillouin oscillation detection at variable incidence angles. The density is estimated through an interfacial energy analysis related to the echo amplitude decay. Transverse wave velocity is obtained across the dispersion curve of Rayleigh surface waves. Combining these different approaches, Young's modulus and Poisson's ratio of LiPON are finally found to be 79GPa and 0.27, respectively.

High latitude artificial periodic irregularity observations with the upgraded EISCAT heating facility

We present a recently developed ionospheric modification experiment that produces artificial periodic irregularities in the ionosphere and uses them to make observations of the spatiotemporal behaviour of the irregularities. In addition, the method can be used to measure Faraday rotation and vertical velocities. We also introduce a novel experiment that allows monitoring the formation of the irregularities during heating, in addition to observing their decay after heating. The first measurements indicate, contrary to existing theory, that the amplitude of the radar echoes from the periodic irregularities grows faster than they decay. We focus on the API effects in the D- and E-region of the ionosphere.

Modification of Attenuation Rate in Range Normalization of Echo Levels for Obtaining Frequency-Dependent Intensity Data from 0.6 MHz and 1.0 MHz Devices

This investigation aims to propose an attenuation rate for range normalization of echo amplitudes recorded by an acoustic backscattering instrument working at a frequency of 1.0 MHz. The intention of the use of such an attenuation rate is to obtain equal echo levels when using a device from the same family of products with a different working frequency, i.e. a 0.6 MHz instrument, at an identical site. This work is based on a field experiment with a 1.0 MHz Acoustic Wave and Current (AWAC) profiler and a 0.6 MHz Aquadopp profiler. Both profilers were deployed upward, side-by-side in the Semak Daun reef lagoon, Seribu Islands, Java Sea, Indonesia. It was found that the proposed attenuation rate for the 1.0 MHz instrument was one-order magnitude higher with respect to the one used for the 0.6 MHz instrument, and logarithmically depth dependent. The proposed attenuation rate for the 1.0 MHz AWAC is –7.925log(R) + 8.551, with R is the slant range from the transducers to the measured layer. Accordingly, the overall agreement between the 1.0 MHz AWAC echo amplitude and the one recorded by the 0.6 MHz Aquadopp was improved by 18dB, which is quite significant considering that the average echo amplitude discrepancy recorded by each transducer was 2.4dB.

Quantitative Evaluation of Liver Fibrosis Using Multi-Rayleigh Model with Hypoechoic Component

To realize a quantitative diagnosis method of liver fibrosis, we have been developing a modeling method for the probability density function of the echo amplitude. In our previous model, the approximation accuracy is insufficient in regions with hypoechoic tissue such as a nodule or a blood vessel. In this study, we examined a multi-Rayleigh model with three Rayleigh distributions, corresponding to the distribution of the echo amplitude from hypoechoic, normal, and fibrous tissue. We showed quantitatively that the proposed model can model the amplitude distribution of liver fibrosis echo data with hypoechoic tissue adequately using Kullback–Leibler (KL) divergence, which is an index of the difference between two probability distributions. We also found that fibrous indices can be estimated stably using the proposed model even if hypoechoic tissue is included in the region of interest. We conclude that the multi-Rayleigh model with three components can be used to evaluate the progress of liver fibrosis quantitatively.

Use of UVP to investigate the evolution of bed configuration

Ultrasonic Velocity Profiler (UVP) is a widely used instrument which allows the determination of the complete velocity profile. UVP can also measure the echo amplitude in the mean time. In this study, it is intended to use the UVP transducers in an indirect way to detect the bottom surface by orienting them vertically downward. Preliminary experiments were conducted with immobile bed by using uniform, non-uniform sediment as well as wooden and plastic surface. It was appeared that the first remarkable peak corresponds to the bottom surface. The time variations of local bed elevations were registered by applying this technique in order to determine the erosion and deposition around a bridge pier and local scour around a horizontal submerged pipe. It was revealed that the so registered scour depths were in good agreement with those observed during the experiments. Consequently, it is demonstrated that time varied bed elevations can be measured via the first peak value of echo amplitudes by placing UVP transducers vertically.

Effect of Tensile Load for Detecting Broken Wires of Parallel Wire Cable Based on Guided Waves

Condition assessment of cables has gained more attention for the bridge safety. The cable is under tensile load in service. The effect of tensile load for detecting broken wires on parallel wire cable based on guided waves is investigated. The sample cable with broken wires under several levels of tensile load is detected using guided waves based on the magnetostrictive effect. The amplitude of flaw echo increases monotonically with the increasing of tension. The results indicate that the tensile load brings benefit for detecting broken wires in the parallel wire cable.

Ringing effects eliminated spin echo in solids

Two types of ringing effects eliminated spin echo sequences have been introduced. To achieve the task, two additional 90° pulses with proper phase cycles are placed at the beginning of the pulse sequences. The spin echo time is calculated with the perturbation method to the first order, i.e. taking into account only the dipolar secular term. The non-secular term causes an imaginary part of the FID, leading to an unsymmetrical NMR spectrum. This effect, according to a symmetry of NMR sequences under phase inversion, can be compensated by inverting all the x and −x or y and −y phases. The properties of the symmetry are derived based on the theory of density matrix. In addition, the non-secular term also results in a small drop (several per cent) of the echo amplitude, but it nearly does not affect the echo time. With these pulse sequences we are able to get a spectrum with an echo delay only 1.1μs without distortion using a Bruker AVANCE III NMR instrument.

High-precision measurement of satellite range and velocity using the EISCAT radar

Abstract. This paper is a continuation of an earlier work by Nygrén et al. (2012), where the velocity of a hard target was determined from a set of echo pulses reflected by the target flying through the radar beam. Here the method is extended to include the determination of range at a high accuracy. The method is as follows. First, the flight time of the pulse from the transmitter to the target is determined at an accuracy essentially better than the accuracy given by the sampling interval. This method makes use of the fact that the receiver filtering creates slopes at the phase flips of the phase modulated echo pulse. A precise flight time is found by investigating the echo amplitude within this slope. A value of velocity is calculated from each echo pulse as explained in the earlier paper. Next, the ranges together with velocities from a single beam pass are combined to a measurement vector for a linear inversion problem. The solution of the inversion problem gives the time-dependent range and velocity from the time interval of satellite flight through the radar beam. The method is demonstrated using the EISCAT (European Incoherent Scatter) UHF radar and radio pulses reflected by a satellite. The achieved standard deviations of range are about 5–50 cm and those of velocity are about 3–25 mm s−1.

Nuclear quadrupole moments as a microscopic probe to study the motion of atomic tunneling systems in amorphous solids.

The properties of amorphous solids below 1 K are dominated by atomic tunneling systems. A basic description is given by the standard tunneling model. Despite its success, the standard tunneling model still remains phenomenological and little is known about the microscopic nature of tunneling systems in amorphous solids. We present dielectric polarization echo experiments on partially deuterated amorphous glycerol. Nuclear quadrupoles, introduced by the deuteration, influence the echo amplitude in a characteristic way and allow us to draw for the first time detailed conclusions about the microscopic nature of the tunneling processes in amorphous glycerol.

Case studies of casing inspection with multi-functional ultrasonic imaging logging tool

The multi-functional ultrasonic imaging logging tool can provide casing inspection and cement bonding evaluation by using ultrasonic pulse echo technique. The casing’s internal conditions can be inspected from echo amplitude and transit time curves. The casing thickness and the cement impedance can be calculated from resonance frequency and resonance decay. The paper describes three well cases histories of casing inspection in detail with this instrument. The first case reports the evaluation of perforation interval in the cased hole; hole enlargement of perforation interval and perforations can be seen from echo amplitude and casing internal radius imaging curves. In the second case, we cannot distinguish corrosion or deposits in the casing surface from the echo amplitude and transit time curves, but casing thickness imaging curve characterize some slight corrosion. Finally, the last case history describes evaluation of casing mechanical wear in a deviated well; casing thickness imaging curve shows mechanical wear is caused by logging instrument in low side well. All cases shows that the multi-functional ultrasonic imaging logging tool can provide both quantitative and qualitative evaluation and diagnosis of casing problems.

Exploring the capacity of neural networks to recognize objects from dolphin echoes across multiple orientations

Dolphins naturally recognize objects from multiple angles using echolocation. With training, humans can also learn to accurately classify objects based on their echoic features. In this study, we used neural networks to identify acoustic cues that enable objects to be recognized from varying aspects. In simulation 1, a self-organizing map was able to differentiate a subset of objects using only amplitude and frequency cues, but it classified some echoes from different objects as being from the same object. In simulation 2, a multilayer perceptron was trained through error correction to identify objects based on echoes from a single aspect, and then tested on its ability to recognize those objects using echoes from different orientations. Overall, perceptrons performed similarly to trained undergraduates. Analysis of network connection weights revealed that both the amplitude and frequency of echoes, as well as the temporal dynamics of these features over the course of an echo train, enabled perceptrons to accurately identify objects when presented with novel orientations. These findings suggest that learning may strongly impact an organism’s ability to echoically recognize an object from any viewpoint.

The New Non-Invasive Quantification of Hepatic Steatosis With Morbid Obesity by Acoustic Structure Quantification (ASQ) From Ultrasound Echo Amplitude

The New Non-Invasive Quantification of Hepatic Steatosis With Morbid Obesity by Acoustic Structure Quantification (ASQ) From Ultrasound Echo Amplitude

Dolphin “packet” use during long-range echolocation tasks

When echolocating, dolphins typically emit a single broadband "click," then wait to receive the echo before emitting another click. However, previous studies have shown that during long-range echolocation tasks, they may instead emit a burst, or "packet," of several clicks, then wait for the packet of echoes to return before emitting another packet of clicks. The reasons for the use of packets are unknown. In this study, packet use was examined by having trained bottlenose dolphins perform long-range echolocation tasks. The tasks featured "phantom" echoes produced by capturing the dolphin's outgoing echolocation clicks, convolving the clicks with an impulse response to create an echo waveform, and then broadcasting the delayed, scaled echo to the dolphin. Dolphins were trained to report the presence of phantom echoes or a change in phantom echoes. Target range varied from 25 to 800 m. At ranges below 75 m, the dolphins rarely used packets. As the range increased beyond 75 m, two of the three dolphins increasingly produced packets, while the third dolphin instead utilized very high click repetition rates. The use of click packets appeared to be governed more by echo delay (target range) than echo amplitude.

Determination of Density Variation and Microstructure in Reaction‐sintered SiC Ceramics Using Ultrasonic Time‐of‐Flight

Ultrasonic time‐of‐flight (TOF) is investigated as a predictor of density variation across reaction‐sintered silicon carbide (SiC) ceramics. The importance of this research is heightened by the fact that the reaction‐sintered SiC ceramic tiles being investigated are manufactured using the reaction sintering process that involves the infusion of liquid silicon into a porous ceramic preform. This can potentially lead to the formation of islands of free silicon, small closed areas of un‐sintered silicon material as well as conventional porosity. All these defects can result in local variations of density which cannot be detected by conventional bulk density measurement techniques. To study the microstructural differences, the porosity dependence of ultrasonic TOF of the reflected signals was investigated to establish a correlation between the velocity and density across the ceramic tile that aids in characterizing the material. The data suggest that for these ceramic tiles, TOF C‐scan mapping surfs as a much better indicator of sample homogeneity than the amplitude of the back wall echo. At the current time, ceramic tiles are inspected offline and this takes time and effort and is very expensive. In particular, this study contains the procedure followed to establish a quantitative method to quantify density variation across selected ceramic tile.

Multiple echoes due to distant dipolar fields in NMR of hyperpolarized noble gas solutions

We report on multiple echo measurements in hyperpolarized liquids using optically pumped spin-1/2 noble gas atoms: either 129Xe dissolved in cyclohexane or 3He dissolved in superfluid 4He. An NMR pulse sequence 90° − τ − 90° (with slice-selective flipping pulses for 129Xe experiments) was used and long echo trains have been observed in the presence of applied gradients due to average dipolar fields typically one order of magnitude larger than those of bulk water in high magnetic fields. We show that a mean field description is valid for explaining the multiple echoes observed in these liquids, even for spin temperatures as low as 10mK for 129Xe or 10μK for 3He, and the echoes originate from the distant dipolar fields within the samples. Numerical lattice simulations have been used to assess the effects of slice selection and of finite sample size in addition to those of atomic diffusion. They account for the observed echo widths and amplitudes much better than previously published models, which disregard finite size effects that appear to be of key importance. This opens the way to using multiple echoes resulting from distant dipolar fields for the determination of the absolute magnetization in hyperpolarized liquids without signal calibration.

HIGH-FREQUENCY GRAVITATIONAL WAVES FROM SUPERMASSIVE BLACK HOLES: PROSPECTS FOR LIGO-VIRGO DETECTIONS

It is commonly assumed that ground-based gravitational wave (GW) instruments will not be sensitive to supermassive black holes (SMBHs) because the characteristic GW frequencies are far below the ~ 10 - 1000 Hz sensitivity bands of terrestrial detectors. Here, however, we explore the possibility of SMBH gravitational waves to leak to higher frequencies. In particular, if the high frequency spectral tail asymptotes to h(f) ~ f^{-alpha}, where alpha<=2, then the spectral amplitude is a constant or increasing function of the mass M at a fixed frequency f>>c^3/GM. This will happen if the time domain waveform or its derivative exhibits a discontinuity. Ground based instruments could search for these universal spectral tails to detect or rule out such features irrespective of their origin. We identify the following processes which may generate high frequency signals: (i) gravitational bremsstrahlung of ultrarelativistic objects in the vicinity of a SMBH, (ii) ringdown modes excited by an external process that has a high frequency component or terminates abruptly, (iii) gravitational lensing echos and diffraction. In particular for (iii), SMBHs produce GW echos of inspiraling stellar mass binaries in galactic nuclei with a delay of a few minutes to hours. The lensed primary signal and GW echo are amplified if the binary is within a ~10 deg (r/100M)^{-1/2} cone behind the SMBH relative to the line of sight at distance r from the SMBH. For the rest of the binaries near SMBHs, the amplitude of the GW echo is ~0.1 (r/100M)^{-1} of the primary signal on average.

Effect of Ultrasound Radiation Force on the Choroid

While visualization of the retina and choroid has made great progress, functional imaging techniques have been lacking. Our aim was to utilize acoustic radiation force impulse (ARFI) response to probe functional properties of these tissues. A single element 18-MHz ultrasound transducer was focused upon the retina of the rabbit eye. The procedure was performed with the eye proptosed and with the eye seated normally in the orbit. The transducer was excited to emit ARFI over a 10-ms period with a 25% duty cycle. Phase resolved pulse/echo data were acquired before, during, and following ARFI. In the proptosed eye, ARFI exposure produced tissue displacements ranging from 0 to 10 μm, and an immediate increase in choroidal echo amplitude to over 6 dB, decaying to baseline after about 1 second. In the normally seated eye, ultrasound phase shifts consistent with flow were observed in the choroid, but enhanced backscatter following ARFI rarely occurred. ARFI-induced displacements of about 10 μm were observed at the choroidal margins. Larger displacements occurred within the choroid and in orbital tissues. We hypothesize that elevated intraocular pressure occurring during proptosis induced choroidal ischemia and that acoustic radiation force produced a transient local decompression and reperfusion. With the eye normally seated, choroidal flow was observed and little alteration in backscatter resulted from exposure. Clinical application of this technique may provide new insights into diseases characterized by altered choroidal hemodynamics, including maculopathies, diabetic retinopathy, and glaucoma.

Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B0 field

It is an efficient protocol to use the refocusing flip angle pulse optimization technique to solve special engineering technical problems in nuclear magnetic resonance (NMR) measurements. By reducing RF pulse duration, the low refocusing flip angle pulses can consume lower power, satisfy specific absorption rate of samples, and improve signal-to-noise ratio as well. To further analyze the function mechanism of pulse angles, the dependence of signal intensity on RF pulse is studied in homogenous magnetic field and constant gradient magnetic field respectively. Afterwards, echo amplitudes with various tip angles and flip angles ranging from 0° to 180° are compared with conventional sequence of 90° pulse followed by 180° pulses theoretically and experimentally. For the constant gradient field, the refocusing pulse of flip angle can be as low as 140°, defined as the optimum herein, to obtain the strongest signal intensity, enhanced by 13% compared with that of 180°. Moreover, T1 distributions measured by the conventional and optimal sequences for distilled water at room temperature are compared, and good conformances of T1 between the two pulse sequences are obtained, which demonstrates the optimal refocusing pulse can be directly applied to T1 measurement. The results provide constructive suggestion for designing pulse sequences for signal intensity enhancement in NMR logging while drilling and NMR online quick analysis.

Magnetic-Field Control of Photon Echo from the Electron-Trion System in a CdTe Quantum Well: Shuffling Coherence between Optically Accessible and Inaccessible States

We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions.