A coastal acoustic tomography experiment in the Tokyo Bay

Eight sets of coastal acoustic tomography (CAT) systems were deployed during November 29 to December 10, 2002 at the coasts on both sides of Tokyo Bay to measure tidal current structures at 15-min interval.Sound transmission across the Tokyo Bay (between Yokohama and Chiba)was successfully traced,even under severe interference from ship generated wakes and bubbles.Tidal current fields changing from northward to southward flow are well reconstructed by the inverse analysis of travel-time difference data for a period with the best sound transmission condition. It is suggested that the CAT is the most powerful tool to continuously map tidal current fields in the coastal seas with heavy shipping traffic and fisheries activity.

Observation of Internal Tides in the Qiongzhou Strait by Coastal Acoustic Tomography

In this study,power spectral density and inverse analyses were performed to obtain the frequency characteristics and spatial distribution of temperature in the Qiongzhou Strait using reciprocal sound transmission data obtained in a coastal acoustic tomo-graphy experiment conducted in 2013.The results reveal three dominant types of internal tides(diurnal,semidiurnal,and terdiurnal).Spectral analysis of the range-average temperature deviation along the northern and southern transmission paths shows that along the northern path,the energy of the diurnal internal tides was significantly larger than that of the semidiurnal tides.The semidiurnal internal tides,in contrast,were more pronounced along the southern path.A terdiurnal spectrum with an energy level equivalent to that of the semidiurnal internal tide was discernable for both the northern and southern paths.These three types of internal tides can also be recognized in the time variation of the zonal-average temperature deviation.The diurnal internal tides were strengthened along the northern coast,implying their westward propagation and the existence of coastally trapped effects.The other two types of internal tides,which have smaller wavelengths than the diurnal internal tides,were less resolved over the entire tomographic domain due to the insufficient resolution of the inversion.The data quality was verified to be satisfactory by error estimation.

A graphical user interface(GUI)for model-based radiation-induced acoustic computed tomography

Radiation-induced acoustic computed tomography(RACT)is an evolving biomedical imaging modality that aims to reconstruct the radiation energy deposition in tissues.Traditional backprojection(BP)reconstructions carry noisy and limited-view artifacts.Model-based algorithms have been demonstrated to overcome the drawbacks of BPs.However,model-based algorithms are relatively more complex to develop and computationally demanding.Furthermore,while a plethora of novel algorithms has been developed over the past decade,most of these algorithms are either not accessible,readily available,or hard to implement for researchers who are not well versed in programming.We developed a user-friendly MATLAB-based graphical user interface(GUI;RACT2D)that facilitates back-projection and model-based image reconstructions for twodimensional RACT problems.We included numerical and experimental X-ray-induced acoustic datasets to demonstrate the capabilities of the GUI.The developed algorithms support parallel computing for evaluating reconstructions using the cores of the computer,thus further accelerating the reconstruction speed.We also share the MATLAB-based codes for evaluating RACT reconstructions,which users with MATLAB programming expertise can further modify to suit their needs.The shared GUI and codes can be of interest to researchers across the globe and assist them in e±cient evaluation of improved RACT reconstructions.

Ring artifacts removal in X-ray-induced acoustic computed tomography

X-ray-induced acoustic computed tomography(XACT)is a hybrid imaging modality for detecting X-ray absorption distribution via ultrasound emission.It facilitates imaging from a single projection X-ray illumination,thus reducing the radiation exposure and improving imaging speed.Nonuniform detector response caused by the interference between multichannel data acquisition for ring array transducers and amplifier systems yields ring artifacts in the reconstructed XACT images,which compromises the image quality.We propose model-based algorithms for ring artifacts corrected XACT imaging and demonstrate their effcacy on numerical and experimental measurements.The corrected reconstructions indicate significantly reduced ring artifacts as compared to their conventional counterparts.

New acoustic system for continuous measurement of river discharge and water temperature

In many cases, river discharge is indirectly estimated from water level or streamflow velocity near the water surface. However, these methods have limited applicability. In this study, an innovative system, the fluvial acoustic tomography system （FATS）, was used for continuous discharge measurement. Transducers with a central frequency of 30 kHz were installed diagonally across the river. The system＇s significant functions include accurate measurement of the travel time of the transmission signal using a GPS clock and the attainment of a high signal-to-noise ratio as a result of modulation of the signal by the 10th order M-sequence. In addition, FATS is small and lightweight, and its power consumption is low. Operating in unsteady streamflow, FATS successfully measured the cross-sectional average velocity. The agreement between FATS and acoustic Doppler current profilers （ADCPs） on water discharge was satisfactory. Moreover, the temporal variation of the cross-sectional average temperature deduced from the sound speed of FATS was similar to that measured by a temperature sensor near the bank.

Magnetoacoustic position imaging for liquid metal in animal interstitial structure

Magnetoacoustic tomography with magnetic induction(MAT-MI),as a new kind of in-vivo imaging method,has potential application value in interstitial fluid research.In this paper,we propose the application of MAT-MI with liquid metal serving as a tracer of the interstitial structure to study its fluid behavior,and use it to implement the positional imaging of the spatial distribution of liquid metal.Owing to the particularity of liquid metal magnetoacoustic pressure(MAP)signals,we propose an envelope analysis method to extract the rising edge of the amplitude envelope of the detected waveform as effective position data.And for the first time,we propose the method of superpositing pixel matrix to achieve the position imaging of liquid metal.Finally,the positional imaging of the liquid metal sample embedded in the gel is achieved to have relatively accurate results.This study provides a method of effectively extracting data and implementing the position imaging for liquid metal in the interstitial structure in the frame of MAT-MI.

Variability in salt flux and water circulation in Ota River Estuary, Japan

In this study the sub-tidal and intra-tidal variations of salt fluxes in the upstream section of a shallow estuary （with a water depth of less than 3 m） were investigated. The salt fluxes were estimated based on the cross-sectional average salinity and velocity measured by the fluvial acoustic tomography system （FATS）. The results indicate that the magnitude of seaward fluxes is approximately two times greater than that of landward fluxes under normal conditions. The results of short-term observation in the study area indicate that there is a phase lag of the bottom and surface salinities between the regions with the largest and smallest depths. The vertical shear flux with a peak value of-0.7 m2/s during the ebb tide indicated an important contribution to the total salt flux compared with the advective flux. A phase lag occurred between the vertical shear terms in the regions with the largest and smallest depths, which resulted from the correlation between the vertical variations of the salinity and velocity and the existence of transversal velocity circulations.

The Fourier slice theorem for ultrasonic reflectivity tomography and the correction to the effect caused by curvature of integral lines

For ultrasonic reflective tomographic imaging of different transmitter-receiver mode, we demonstrate that the Fourier slice theorem can be used when the distance between the transducer and origin becomes much larger than the object to be reconstructed. Iterative reconstruction formula based on the Fourier slice theorem is proposed for the case in which the paraxial approximation holds. The effect caused by the curvature of integral lines may be eliminated iteratively and better reconstructed images can be expected.