Phase-shift Map Research Articles

Robust Single-Probe Quantitative Ultrasonic Imaging System With a Target-Aware Deep Neural Network.

The speed of sound (SoS) has great potential as a quantitative imaging biomarker since it is sensitive to pathological changes in tissues. In this paper, a target-aware deep neural (TAD) network reconstructing an SoS image quantitatively from pulse-echo phase-shift maps gathered from a single conventional ultrasound probe is presented. In the proposed TAD network, the reconstruction process is guided by feature maps created from segmented target images for accuracy and contrast. In addition, the feature extraction process utilizes phase difference information instead of direct pulse-echo radio frequency (RF) data for robust image reconstruction against noise in the pulse-echo data. The TAD network outperforms the fully convolutional network in root mean square error (RMSE), contrast-to-noise ratio (CNR), and structural similarity index (SSIM) in the presence of nearby reflectors. The measured RMSE and CNR are 5.4 m/s and 22 dB, respectively with the tissue attenuation coefficient of 2 dB/cm/MHz, which are 72% and 13 dB improvement over the state of the art design in RMSE and CNR, respectively. In the in-vivo test, the proposed method classifies the tissues in the neck area using SoS with a p-value below 0.025. The proposed TAD network is the most accurate and robust single-probe SoS image reconstruction method reported to date. The accuracy and robustness demonstrated by the proposed SoS imaging method open up the possibilities of wide-spread clinical application of the single-probe SoS imaging system.

Studies of terahertz discharge in noble gases using a Michelson interferometer

The measurements of the spatial-time evolution of the discharge in argon and krypton at a wide range of pressures were performed. The gyrotron was used for plasma ignition. The radiation was 40 kW with frequency 0.67 THz. The pulse duration was 20 μs. A Michelson interferometer has been exploited for investigation of the discharge in the noble gases in a focused beam of terahertz waves. The diameter of the laser beam near the plasma cloud was 25 mm at 1/e 2 level, λ = 532 nm. The rectangle area (about 20 × 15 mm) near the focus of THz beam was examined synchronously by the interferometer and a CCD camera. The spatial-time evolution of laser beam phase-shift of THz discharge in Kr and Ar from atmospheric pressure down to a few torrs was obtained. The intensity of the discharge glow in optical range correlates with the phase shift map. Previous measurements of plasma density made in Ar under a background gas pressure at a level of a few torrs, based on the Stark effect, are in good agreement with the gained results.

Improved detection of far-side solar active regions using deep learning

Context. The analysis of waves on the visible side of the Sun allows the detection of active regions on the far side through local helioseismology techniques. Knowing the magnetism in the whole Sun, including the non-visible hemisphere, is fundamental for several space weather forecasting applications. Aims. Seismic identification of far-side active regions is challenged by the reduced signal-to-noise ratio, and only large and strong active regions can be reliable detected. Here we develop a new method to improve the identification of active region signatures in far-side seismic maps. Methods. We constructed a deep neural network that associates the far-side seismic maps obtained from helioseismic holography with the probability that active regions lie on the far side. The network was trained with pairs of helioseismic phase-shift maps and Helioseismic and Magnetic Imager (HMI) magnetograms acquired half a solar rotation later, which were used as a proxy for the presence of active regions on the far side. The method was validated using a set of artificial data, and it was also applied to actual solar observations during the period of minimum activity of solar cycle 24. Results. Our approach shows a higher sensitivity to the presence of far-side active regions than standard methods that have been applied up to date. The neural network can significantly increase the number of detected far-side active regions, and will potentially improve the application of far-side seismology to space weather forecasting.

Investigation of the refractive index decrement of 3D printing materials for manufacturing breast phantoms for phase contrast imaging

3D breast modelling for 2D and 3D breast x-ray imaging would benefit from the availability of digital and physical phantoms that reproduce accurately the complexity of the breast anatomy. While a number of groups have produced digital phantoms with increasing level of complexity, physical phantoms reproducing that software approach have been scarcely developed. One possibility is offered by 3D printing technology. This implies the assessment of the energy dependent absorption index β of 3D printing materials for absorption based imaging, as well as the assessment of the refractive index decrement, δ, of the printing material, for phase contrast imaging studies, at the energies of interest for breast imaging. In this work we set-up a procedure and performed a series of measurements (at 30, 45 and 60 keV, at the European Synchrotron Radiation Facility) for assessing the relative value of δ with respect to that of breast tissues, for twelve 3D printing materials. The method included propagation based phase contrast 2D imaging and retrieval of the estimated phase shift map, using the Paganin’s algorithm. Breast glandular, adipose and skin tissues were used as reference materials of known ratio δ/β. A percentage difference Δδ was introduced to assess the suitability of the printing materials as tissue substitutes. The accuracy of the method (about 4%) was assessed based on the properties of PMMA and Nylon, acting as gold standard. Results show that, for the above photon energies, ABS is a good substitute for adipose tissue, Hybrid as a substitute of the glandular tissue and PET-G for simulating the skin. We plan to realize a breast phantom manufactured by fused deposition modelling (FDM) technology using ABS, Hybrid and PET-G as substitutes of the glandular and skin tissue and a second phantom by stereolithography (SLA) technology with the resins Flex, Tough and Black.

Comparison of image properties in full-field phase X-ray microscopes based on grating interferometry and Zernike's phase contrast optics

A grating interferometer (GI) system has been installed in an X-ray microscope equipped with a Zernike phase contrast (ZPC) system and a Cu rotating anode X-ray source. The GI and ZPC systems are switchable, and their performances of phase information extraction have been compared. The GI system is based on a Lau interferometer consisting of an absorption grating and a π/2 phase grating, which extracts a magnified phase shift map of a sample via a phase-stepping measurement. The ZPC system generates a phase contrast image by using a phase plate and a corresponding condenser device. The ZPC system and the GI system are compared in terms of detectability of phase objects. By the Fourier analysis of images of a logarithmic ruler pattern, the spatial resolution was found to be identical between the two systems. Although the sensitivity depends on the sample size, the signal-to-noise ratio of polystyrene spheres with a few microns in diameter was used for sensitivity comparison, showing the superior sensitivity of the GI system to that of the ZPC system. The quantitativeness of the GI system with the phase-stepping measurement was also demonstrated over the ZPC system, which generates halo and shade-off artifacts. The GI system exhibits twin image artifacts that need to be resolved for practical applications of the technique.

Interferometric analysis of sub-nanosecond laser-induced optical breakdown dynamics in the bulk of fused-silica glass.

Dynamics of laser-induced optical breakdown in the bulk of fused-silica glass irradiated by a sub-nanosecond laser pulse at a wavelength of 790 nm with a fluence of 522 J/cm2 was studied by the femtosecond time-resolved complex interferometry in Nomarski arrangement utilising a Fresnel bi-prism. Evolution of the plasma channel and the development of the free electron density were in focus of the investigation. The measured ultimate length of the plasma channel was equal to 30 μm and almost doubled the length estimated within the moving breakdown model. The history of the transient electron density distribution in the plasma was reconstructed from the phase shift maps using the inverse Abel transform and it revealed further deviation from this model. The core of the plasma channel exhibited at the last stages of the development a considerable level of the electron density up to 2.4×1020 cm-3. The signature of the pre-breakdown phase has been identified as radiation caused by ionization-released electrons interacting with ions and has been demonstrated in solids for the first time in this way. Origin of the discrepancy between the theoretical prediction of the moving breakdown model and the measured values of the channel length is discussed as well.

Cyclic Continuous Max-Flow: A Third Paradigm in Generating Local Phase Shift Maps in MRI.

Sensitivity to phase deviations in MRI forms the basis of a variety of techniques, including magnetic susceptibility weighted imaging and chemical shift imaging. Current phase processing techniques fall into two families: those which process the complex image data with magnitude and phase coupled, and phase unwrapping-based techniques that first linearize the phase topology across the image. However, issues, such as low signal and the existence of phase poles, can lead both methods to experience error. Cyclic continuous max-flow (CCMF) phase processing uses primal-dual-variational optimization over a cylindrical manifold, which represent the inherent topology of phase images, increasing its robustness to these issues. CCMF represents a third distinct paradigm in phase processing, being the only technique equipped with the inherent topology of phase. CCMF is robust and efficient with at least comparable accuracy as the prior paradigms.

Susceptibility‐weighted imaging using inter‐echo‐variance channel combination for improved contrast at 7 tesla

To implement and optimize a new approach for susceptibility-weighted image (SWI) generation from multi-echo multi-channel image data and compare its performance against optimized traditional SWI pipelines. Five healthy volunteers were imaged at 7 Tesla. The inter-echo-variance (IEV) channel combination, which uses the variance of the local frequency shift at multiple echo times as a weighting factor during channel combination, was used to calculate multi-echo local phase shift maps. Linear phase masks were combined with the magnitude to generate IEV-SWI. The performance of the IEV-SWI pipeline was compared with that of two accepted SWI pipelines-channel combination followed by (i) Homodyne filtering (HPH-SWI) and (ii) unwrapping and high-pass filtering (SVD-SWI). The filtering steps of each pipeline were optimized. Contrast-to-noise ratio was used as the comparison metric. Qualitative assessment of artifact and vessel conspicuity was performed and processing time of pipelines was evaluated. The optimized IEV-SWI pipeline (σ = 7 mm) resulted in continuous vessel visibility throughout the brain. IEV-SWI had significantly higher contrast compared with HPH-SWI and SVD-SWI (P < 0.001, Friedman nonparametric test). Residual background fields and phase wraps in HPH-SWI and SVD-SWI corrupted the vessel signal and/or generated vessel-mimicking artifact. Optimized implementation of the IEV-SWI pipeline processed a six-echo 16-channel dataset in under 10 min. IEV-SWI benefits from channel-by-channel processing of phase data and results in high contrast images with an optimal balance between contrast and background noise removal, thereby presenting evidence of importance of the order in which postprocessing techniques are applied for multi-channel SWI generation. 2 J. Magn. Reson. Imaging 2017;45:1113-1124.

Experimental Study of the Laser-Triggered Discharge for the Application in a Gap Switch

Laser triggering is interesting for gap switches used in power systems and pulsed power because of its well-controllable delay and jitter time, as well as its electrically uncoupled characteristics. Despite the large attention that was paid to various parameters affect triggering characteristics, the underlying physics of laser triggering remain unclear. In this paper, the nanosecond-laser-triggered pulsed discharge was investigated in atmospheric air. Using a picosecond laser, a high-time-resolution interferometer was constructed and the laser- and discharge-produced plasmas were detected. The shockwaves formed by instantaneous release of the laser and electrical energy into the ambient air were well distinguished. Using the interferograms, phase-shift maps were extracted by a 2-D fast Fourier transform method. Finally, preliminary experiments were performed on laser-triggered discharge from a prototype gap switch, and delay and jitter results were given.

Spatial light interference tomography (SLIT)

We present spatial light interference tomography (SLIT), a label-free method for 3D imaging of transparent structures such as live cells. SLIT uses the principle of interferometric imaging with broadband fields and combines the optical gating due to the micron-scale coherence length with that of the high numerical aperture objective lens. Measuring the phase shift map associated with the object as it is translated through focus provides full information about the 3D distribution associated with the refractive index. Using a reconstruction algorithm based on the Born approximation, we show that the sample structure may be recovered via a 3D, complex field deconvolution. We illustrate the method with reconstructed tomographic refractive index distributions of microspheres, photonic crystals, and unstained living cells.

Combined mixed approach algorithm for in‐line phase‐contrast x‐ray imaging

In the past decade, phase-contrast imaging (PCI) has been applied to study different kinds of tissues and human body parts, with an increased improvement of the image quality with respect to simple absorption radiography. A technique closely related to PCI is phase-retrieval imaging (PRI). Indeed, PCI is an imaging modality thought to enhance the total contrast of the images through the phase shift introduced by the object (human body part); PRI is a mathematical technique to extract the quantitative phase-shift map from PCI. A new phase-retrieval algorithm for the in-line phase-contrast x-ray imaging is here proposed. The proposed algorithm is based on a mixed transfer-function and transport-of-intensity approach (MA) and it requires, at most, an initial approximate estimate of the average phase shift introduced by the object as prior knowledge. The accuracy in the initial estimate determines the convergence speed of the algorithm. The proposed algorithm retrieves both the object phase and its complex conjugate in a combined MA (CMA). Although slightly less computationally effective with respect to other mixed-approach algorithms, as two phases have to be retrieved, the results obtained by the CMA on simulated data have shown that the obtained reconstructed phase maps are characterized by particularly low normalized mean square errors. The authors have also tested the CMA on noisy experimental phase-contrast data obtained by a suitable weakly absorbing sample consisting of a grid of submillimetric nylon fibers as well as on a strongly absorbing object made of a 0.03 mm thick lead x-ray resolution star pattern. The CMA has shown a good efficiency in recovering phase information, also in presence of noisy data, characterized by peak-to-peak signal-to-noise ratios down to a few dBs, showing the possibility to enhance with phase radiography the signal-to-noise ratio for features in the submillimetric scale with respect to the attenuation-based imaging. It has been shown that phase-retrieved radiographies can be used both to have quantitative phase information about soft tissues, complementary to the attenuation information, and to enhance the visibility of details inside soft tissues, with higher efficiency with respect to phase radiography.

The Progress of the Taiwan Oscillation Network Project

We describe the present status of the project of the Taiwan Oscillation Network (TON) and discuss a scientific result using the TON data. The TON is a ground-based network to measure solar intensity oscillations for the study of the solar interior. Four telescopes have been installed in appropriate longitudes around the world. The TON telescopes take K-line full-disk solar images of diameter 1000 pixels at a rate of one image per minute. The data has been collected since October of 1993. The TON high-spatial-resolution data are specially suitable for the study of local properties of the Sun. In 1997 we developed a new method, acoustic imaging, to construct the acoustic signals inside the Sun with the acoustic signals measured at the solar surface. From the constructed signals, we can form intensity map and phase-shift map of an active region at various depths. The direct link between these maps and the subsurface wave-speed perturbation suffers from the poor vertical resolution of acoustic imaging. Recently an inversion method has been developed to invert the measured phase travel time perturbation to estimate the distribution of wave-speed perturbation based on the ray approximation. This technique of acoustic imaging has been used to image the far-side of the Sun that could provides information on space weather prediction.

Analyzing laser plasma interferograms with a continuous wavelet transform ridge extraction technique: the method

Laser plasma interferograms are currently analyzed by extraction of the phase-shift map with fast Fourier transform (FFT) techniques [Appl. Opt. 18, 3101 (1985)]. This methodology works well when interferograms are only marginally affected by noise and reduction of fringe visibility, but it can fail to produce accurate phase-shift maps when low-quality images are dealt with. We present a novel procedure for a phase-shift map computation that makes extensive use of the ridge extraction in the continuous wavelet transform (CWT) framework. The CWT tool is flexible because of the wide adaptability of the analyzing basis, and it can be accurate because of the intrinsic noise reduction in the ridge extraction. A comparative analysis of the accuracy performances of the new tool and the FFT-based one shows that the CWT-based tool produces phase maps considerably less noisy and that it can better resolve local inhomogeneties.

A generalization of Abel inversion to non-axisymmetric density distribution

Abel inversion is currently used in laser-plasma studies in order to estimate the electronic density n e from the phase-shift map δ φ obtained via interferometry. The main limitation of the Abel method is due to the assumption of axial symmetry of the electronic density, which is often hardly fulfilled. In this paper we present an improvement to the Abel inversion technique in which the axial symmetry condition is relaxed by means of a truncated Legendre polynomial expansion in the azimuthal angle. With the help of simulated interferograms, we will show that the generalized Abel inversion generates accurate densities maps when applied to non-axisymmetric density sources.

A Study of Sunspots with Phase Time and Travel Time ofp‐Mode Waves in Acoustic Imaging

Solar acoustic signals constructed with the technique of acoustic imaging contain both intensity and phase information. The phase of constructed signals can be studied by computing the cross-correlation function between time series constructed with ingoing and outgoing waves. The envelope peak of the cross-correlation function provides information about wave travel time, associated with the group velocity along the wave path. The phase time of the cross-correlation function provides information about phase changes along the wave path, including the phase change at boundaries of the mode cavity and boundaries of flux tubes. We study the active region NOAA 7978 using the data taken with the Michelson Doppler Imager (MDI) instrument. We construct two-dimensional phase-shift maps and envelope-shift maps of the active region at the surface by determining the phase time and envelope time of the cross-correlation function at each point. Both phase time and envelope time in magnetic regions are smaller than those in the quiet Sun. The phase shift and envelope shift at each point are computed by subtracting the average values in the quiet Sun. The envelope shift is greater than the phase shift by about 2.5 minutes inside sunspots. Both the envelope shift and the phase shift in sunspots increase approximately linearly with frequency. The difference between the envelope shift and phase shift is the same at different frequencies. There is evidence for an additional phase shift in sunspots relative to plages. We discuss inferences about the structure of sunspots from these phenomena.

Acoustic Imaging of Solar Active Regions (Invited Review)

Acoustic imaging is a new method to construct the acoustic signal at a point on the solar surface or in the solar interior with the signals measured at the solar surface. The constructed signals contain both intensity information and phase information. The intensity is computed by summing the squared amplitude of the constructed signal over time. The phase of constructed signals can be studied by the cross-correlation function between the time series constructed with ingoing waves and outgoing waves. The location of the envelope peak of the cross-correlation function and the phase of the cross-correlation function contain different information on the physical conditions of the plasma along the wave path. From the constructed signals, one can form the two-dimensional outgoing intensity map, absorption map, phase-shift map, and envelope-shift map of a target region at different focal depths. The perturbed physical conditions caused by the magnetic fields of active regions manifest in these maps. The outgoing intensity is lower in magnetic regions than the quiet Sun. The group travel time and phase travel time are smaller in magnetic regions than in the quiet Sun. In this paper, we review the studies of active regions, including emerging flux regions, with acoustic imaging.

In Search of Emerging Magnetic Flux underneath the Solar Surface with Acoustic Imaging.

With the method of acoustic imaging in helioseismology, we study an emerging flux region, NOAA 7978, using data taken with instruments of the Taiwan Oscillation Network (TON) and the Michelson Doppler Imager (MDI). Phase-shift maps of NOAA 7978 focusing at different depths are constructed, which show the evolution of emerging magnetic flux. Although the emerging flux below the solar surface is not easy to be directly recognized in the phase-shift maps, average phase shifts over the active region reveal the signature of upward-moving magnetic flux in the solar interior. The average phase shifts in the interior normalized to their surface values are larger at earlier times in both the TON and the MDI data. This suggests that the flux below the surface is moving upward during the development of the active region.

Probing the Subsurface Structure of Active Regions with the Phase Information in Acoustic Imaging

We present the phase information of solar p-mode waves constructed with an acoustic imaging technique in the solar interior. There exists a phase shift between the time series constructed with ingoing waves and outgoing waves. We find that this phase shift is different in an active region and the quiet Sun. The p-mode travel time is shorter in the magnetic regions than in the quiet Sun. We construct a three-dimensional phase shift map of the solar interior. As with the acoustic absorption images, the phase shift features of the active region in maps at the surface correlate with magnetic fields. The vertical extension of phase shift features in the active region is smaller in the phase maps constructed with shorter wavelengths. This indicates the vertical spatial resolution of these three-dimensional phase maps is sensitive to the range of modes used in constructing the signal. The actual depths of the phase shift features in the active region may be smaller than those shown in the three-dimensional phase maps.