Yae-Lin Sheu

Subband photoacoustic imaging for contrast improvement

Contrast in photoacoustic imaging is primarily determined by optical absorption. This paper proposes a subband imaging method to further enhance the image contrast. The method is based on media with different absorptions generating acoustic waves with different frequency contents. Generally, assuming all other conditions remain the same, a high-absorption medium generates acoustic waves with higher frequency components, and hence the imaging contrast can be enhanced by appropriate selection of the spectral subbands. This study employed both finite-difference, time-domain-based simulations and phantom imaging. The numerical results show that the peak frequencies of the signals for objects with absorption coefficients of 1 and 100 cmM(-1) were 2.4 and 7.8 MHz, respectively. Imaging an agar-based phantom further demonstrated that the contrast between two objects with absorption coefficients of 5.01 and 41.75 cm(-1) can be improved by 4-10 dB when the frequency band was changed from 0-7 to 7-14 MHz. Finally, a method to further enhance the contrast based on optimal weighting is also presented. The proposed method is of particular interest in photoacoustic molecular imaging.

Dynamical origin of near- and below-threshold harmonic generation of Cs in an intense mid-infrared laser field

Near- and below-threshold harmonic generation provides a potential approach to generate vacuum-ultraviolet frequency comb. However, the dynamical origin of in these lower harmonics is less understood and largely unexplored. Here we perform an ab initio quantum study of the near- and below-threshold harmonic generation of caesium (Cs) atoms in an intense 3,600-nm mid-infrared laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, the roles of multiphoton and multiple rescattering trajectories on the near- and below-threshold harmonic generation processes are clarified. We find that the multiphoton-dominated trajectories only involve the electrons scattered off the higher part of the combined atom-field potential followed by the absorption of many photons in near- and below-threshold regime. Furthermore, only the near-resonant below-threshold harmonic is exclusive to exhibit phase locked features. Our results shed light on the dynamic origin of the near- and below-threshold harmonic generation.

A new time-frequency method to reveal quantum dynamics of atomic hydrogen in intense laser pulses: Synchrosqueezing transform

This study introduces a new adaptive time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), to explore the dynamics of a laser-driven hydrogen atom at an ab initio level, upon which we have demonstrated its versatility as a new viable venue for further exploring quantum dynamics. For a signal composed of oscillatory components which can be characterized by instantaneous frequency, the SST enables rendering the decomposed signal based on the phase information inherited in the linear TF representation with mathematical support. Compared with the classical type of TF methods, the SST clearly depicts several intrinsic quantum dynamical processes such as selection rules, AC Stark effects, and high harmonic generation.

Image reconstruction in intravascular photoacoustic imaging

Intravascular photoacoustic (IVPA) imaging is a technique for visualizing atherosclerotic plaques with differential composition. Unlike conventional photoacoustic tomography scanning, where the scanning device rotates around the subject, the scanning aperture in IVPA imaging is enclosed within the imaged object. The display of the intravascular structure is typically obtained by converting detected photoacoustic waves into Cartesian coordinates, which can produce images with severe artifacts. Because the acquired data are highly limited, there does not exist a stable reconstruction algorithm for such imaging geometry. The purpose of this work was to apply image reconstruction concepts to explore the feasibility and efficacy of image reconstruction algorithms in IVPA imaging using traditional analytical formulas, such as a filtered back-projection (FBP) and the lambda-tomography method. Although the closed-form formulas are not exact for the IVPA system, a general picture of and interface information about objects are provided. To improve the quality of the reconstructed image, the iterative expectation maximization and penalized least-squares methods were adopted to minimize the difference between the measured signals and those generated by a reconstructed image. In this work, we considered both the ideal point detector and the acoustic transducers with finite- size aperture. The transducer effects including the spatial response of aperture and acoustoelectrical impulse responses were incorporated in the system matrix to reduce the aroused distortion in the IVPA reconstruction. Computer simulations and experiments were carried out to validate the methods. The applicability and the limitation of the reconstruction method were also discussed.

Simulations of photoacoustic wave propagation using a finite-difference time-domain method with Berenger’s perfectly matched layers

This study developed a numerical solution of the general photoacoustic generation equations involving the heat conduction theory and the state, continuity, and Navier-Stokes equations in 2.5D axisymmetric cylindrical coordinates using a finite-difference time-domain scheme. The numerical techniques included staggered grids and Berengers perfectly matched layers (PMLs), and linear-perturbation analytical solutions were used to validate the simulation results. The numerical results at different detection angles and durations of laser pulses agreed with the theoretical estimates to within an error of 2% in the absolute differences. It was also demonstrated that the simulator can be used to develop advanced photoacoustic imaging methods. The performance of Berengers PMLs was also assessed by comparisons with the traditional first-order Murs boundary condition. At the edges of the simulation domain, a ten-layer PML medium with polynomial attenuation coefficient grading from 0 to 5 x 10(6) m(3)/kg s was designed to reduce the reflection to as low as -60 and -32 dB in the axial and radial directions, respectively. The reflections at the axial and radial boundaries were 32 and 7 dB lower, respectively, for the ten-layer PML absorbing layer than for the first-order Murs boundary condition.

Design, fabrication and testing of a dual-band photoacoustic transducer.

Combining photoacoustic and ultrasonic imaging allows both optical and acoustic properties to be displayed simultaneously. In this paper, we describe a dual-band transducer for implementing such a multimodality imaging setup. The transducer exhibits two frequency bands so that it matches the frequency of interest in both imaging methods. An optical fiber is included in the center so that it is inherently coregistered. The transducer was fabricated from lithium niobate and comprises two concentric rings whose center frequencies are 4.9 MHz and 14.8 MHz. Pulse-echo measurements and phantom imaging were performed to demonstrate its performance characteristics.

Exploring laser-driven quantum phenomena from a time-frequency analysis perspective: a comprehensive study.

Time-frequency (TF) analysis is a powerful tool for exploring ultrafast dynamics in atoms and molecules. While some TF methods have demonstrated their usefulness and potential in several quantum systems, a systematic comparison among them is still lacking. To this end, we compare a series of classical and contemporary TF methods by taking hydrogen atom in a strong laser field as a benchmark. In addition, several TF methods such as Cohen class distribution other than the Wigner-Ville distribution, reassignment methods, and the empirical mode decomposition method are first introduced to exploration of ultrafast dynamics. Among these TF methods, the synchrosqueezing transform successfully illustrates the physical mechanisms in the multiphoton ionization regime and in the tunneling ionization regime. Furthermore, an empirical procedure to analyze an unknown complicated quantum system is provided, suggesting the versatility of TF analysis as a new viable venue for exploring quantum dynamics.

Effects of absorption properties on photoacoustic spectral characteristics: numerical analysis

In photoacoustics, characteristics of the absorbed optical energy are a major determining factor of the characteristics of the photoacoustic signal. In other words, objects with different absorption properties generate photoacoustic signals with different frequency contents. Therefore, effects of absorption properties on photoacoustic spectral characteristics need to be fully understood in order to devise optimal setup for photoacoustic signal detection. The main purpose of this paper is therefore to numerically investigate such effects using a finite-difference time-domain (FDTD) approach. Photoacoustic signal generation can be described by the thermal conduction equation, the continuity and the Navier-Stokes equations, and the state equations in the system. To reduce computational and storage requirements, an axis-symmetrical cylindrical coordinate system with the z-axis parallel to the laser irradiation direction is adopted in our study. Moreover, the MacCormack scheme, which is fourth-order accurate in space and second-order accurate in time, and the first-order Mur absorbing boundary conditions are introduced to implement the FDTD code. The absorption coefficient range from 1cm-1 to 100 cm-1 and the signals are detected in forward modes. Results show that the peak frequency of the signal with absorbing coefficient 1cm-1, 10cm-1, 20cm-1, 30cm-1, 50cm-1, and 100 cm-1 is 2.4MHz, 2.4MHz, 4.2MHz, 4.8MHz, 6MHz, and 7.8MHz when detected forwardly. Note that although the peak acoustic frequency increases with the absorption coefficient, but the linear relationship between the two parameters does not hold. The results also imply that photoacoustic image contrast can be improved by properly selecting the receiver signal bandwidth. Assuming two objects with absorption coefficient of 10cm-1 and 100cm-1, respectively, it is estimated that a 11.34dB contrast improvement is achievable in the forward mode, while a 8.13dB improvement is possible in the backward mode.

Investigation on anisotropy of elastic properties in tendon using shear wave elasticity imaging

Non-invasive evaluation of tendon structure and function is of great use clinically. We proposed that the shear wave elasticity imaging has a better potential in differentiating normal and destructed tendon tissue than high frequency sonography. Four in vitro porcine tendons were studied in this research. High frequency ultrasound could provide good spatial resolution to monitor the detail structure changes by collagenase alterations. By analyzing the speckle changes based on defining a signal to noise ratio (SNR), we could quantitatively estimate the structure differences. The SNR alteration in the region of interest (ROI) before and after collagenase injection is close to 4%. The changes induced by structure alterations are not obvious even through in high frequency ultrasound imaging. Further to analyze the changes of shear wave speed, the differences before and after collagenase injection in longitudinal and transverse section of tendon were 21.3% and 8.3%, respectively. From our results, we found the changes of shear wave speed were much more than speckle intensity alterations after collagenase injection. Moreover, the decrease ratio of shear wave speed in longitudinal section is much more than in transverse section. In other words, to diagnose the tendon disease could prior to investigate on the changes of mechanical property in longitudinal section of tendon. The changes of shear wave speed could provide a batter characteristic for differentiation of normal or diseased tissue.

Application of limited-view image reconstruction method to intravascular photoacoustic tomography

Intravascular photoacoustic (IVPA) imaging that aims to detect atherosclerotic plaques with differential composition is studied computationally and experimentally. IVPA images are usually reconstructed by simply aligning photoacoustic signals with scan conversion, which results in images with severe blurring and increases the difficulty in signal detection. The scanning aperture in IVPA, in contrast to other photoacoustic tomography applications, is enclosed within the imaged object. Consequently, quantitative image reconstruction becomes infeasible, as the data sufficiency condition for stable image reconstruction is not satisfied in such a limited-view scanning. However, useful information regarding certain plaque boundaries can still be reconstructed, which can facilitate plaque detection. In this study, strategies for limited-view reconstruction will be investigated for the IVPA scanning geometry. Computer simulations are carried out to validate the developed method.