Dual-Domain Reconstruction Network Incorporating Multi-Level Wavelet Transform and Recurrent Convolution for Sparse View Computed Tomography Imaging.

Sparse view computed tomography (SVCT) aims to reduce the number of X-ray projection views required for reconstructing the cross-sectional image of an object. While SVCT significantly reduces X-ray radiation dose and speeds up scanning, insufficient projection data give rise to issues such as severe streak artifacts and blurring in reconstructed images, thereby impacting the diagnostic accuracy of CT detection. To address this challenge, a dual-domain reconstruction network incorporating multi-level wavelet transform and recurrent convolution is proposed in this paper. The dual-domain network is composed of a sinogram domain network (SDN) and an image domain network (IDN). Multi-level wavelet transform is employed in both IDN and SDN to decompose sinograms and CT images into distinct frequency components, which are then processed through separate network branches to recover detailed information within their respective frequency bands. To capture global textures, artifacts, and shallow features in sinograms and CT images, a recurrent convolution unit (RCU) based on convolutional long and short-term memory (Conv-LSTM) is designed, which can model their long-range dependencies through recurrent calculation. Additionally, a self-attention-based multi-level frequency feature normalization fusion (MFNF) block is proposed to assist in recovering high-frequency components by aggregating low-frequency components. Finally, an edge loss function based on the Laplacian of Gaussian (LoG) is designed as the regularization term for enhancing the recovery of high-frequency edge structures. The experimental results demonstrate the effectiveness of our approach in reducing artifacts and enhancing the reconstruction of intricate structural details across various sparse views and noise levels. Our method excels in both performance and robustness, as evidenced by its superior outcomes in numerous qualitative and quantitative assessments, surpassing contemporary state-of-the-art CNNs or Transformer-based reconstruction methods.

On the use of deep learning for phase recovery.

Phase recovery (PR) refers to calculating the phase of the light field from its intensity measurements. As exemplified from quantitative phase imaging and coherent diffraction imaging to adaptive optics, PR is essential for reconstructing the refractive index distribution or topography of an object and correcting the aberration of an imaging system. In recent years, deep learning (DL), often implemented through deep neural networks, has provided unprecedented support for computational imaging, leading to more efficient solutions for various PR problems. In this review, we first briefly introduce conventional methods for PR. Then, we review how DL provides support for PR from the following three stages, namely, pre-processing, in-processing, and post-processing. We also review how DL is used in phase image processing. Finally, we summarize the work in DL for PR and provide an outlook on how to better use DL to improve the reliability and efficiency of PR. Furthermore, we present a live-updating resource ( https://github.com/kqwang/phase-recovery ) for readers to learn more about PR.

Label-free and dynamic monitoring of cell evolutions using wavelength-multiplexing surface plasmon resonance holographic microscopy.

Dynamic characterizations of intracellular variations and cell-substrate interactions under different external environments are critical to study cell behaviors and exploring biological applications. However, techniques that are capable of dynamically and simultaneously measuring multiple parameters of living cells in a wide-field manner have rarely been reported. Here, we present a wavelength-multiplexing surface plasmon resonance holographic microscopy which allows wide-field, simultaneous, and dynamic measurements of cell parameters, including cell-substrate distance and cytoplasm refractive index (RI). We use two lasers of 632.8 nm and 690 nm as light sources. Two beam splitters are employed in the optical setup to separately adjust the incident angle of two light beams. Then, surface plasmon resonance (SPR) can be excited for each wavelength under SPR angles. We demonstrate the advances of the proposed apparatus by systematically studying the cell responses to osmotic pressure stimuli from the environmental medium at the cell-substrate interface. The SPR phase distributions of the cell are firstly mapped at two wavelengths, then the cell-substrate distance and cytoplasm RI are retrieved using a demodulation method. Based on phase response differences between two wavelengths and monotonic changes of SPR phase with cell parameters, cell-substrate distance, and cytoplasm RI can be determined simultaneously using an inverse algorithm. This work affords a new optical measurement technique to dynamically characterize cell evolutions and investigate cell properties in various cellular activities. It may become a useful tool in the bio-medical and bio-monitoring areas.

Simultaneous measurement of near-water-film air temperature and humidity fields based on dual-wavelength digital holographic interferometry.

We present a method to reconstruct the near-water-film air temperature and humidity distributions synchronously by measuring the phase delays based on dual-wavelength digital holographic interferometry. A falling water film device was used to create a water film evaporation environment and generate axially uniform temperature and humidity fields. The relationship between air temperature, humidity and phase delay is derived from the Edlen equations. With such relationship, the temperature and humidity distributions can be solved directly according to phase delays of two different wavelengths. An edge phase enhancement method and an error elimination method with PSO are presented to improve the measurement accuracy. The temperature and humidity fields in the falling water film model were experimentally reconstructed with temperature deviation of 0.06% and relative humidity deviation of 2.61%.

Light-field focusing and modulation through scattering media based on dual-polarization-encoded digital optical phase conjugation.

Digital optical phase conjugation (DOPC) can be applied for light-field focusing and imaging through or within scattering media. Traditional DOPC only recovers the phase but loses the polarization information of the original incident beam. In this Letter, we propose a dual-polarization-encoded DOPC to recover the full information (both phase and polarization) of the incident beam. The phase distributions of two orthogonal polarization components of the speckle field coming from a multimode fiber are first measured by using digital holography. Then, the phase distributions are separately modulated on two beams and their conjugations are superposed to recover the incident beam through the fiber. By changing the phase difference or amplitude ratio between the two conjugate beams, light fields with complex polarization distribution can also be generated. This method will broaden the application scope of DOPC in imaging through scattering media.

Dual-wavelength surface plasmon resonance holographic microscopy for simultaneous measurements of cell-substrate distance and cytoplasm refractive index.

Studying the basic characteristics of living cells is of great significance in biological research. Bio-physical parameters, including cell-substrate distance and cytoplasm refractive index (RI), can be used to reveal cellular properties. In this Letter, we propose a dual-wavelength surface plasmon resonance holographic microscopy (SPRHM) to simultaneously measure the cell-substrate distance and cytoplasm RI of live cells in a wide-field and non-intrusive manner. Phase-contrast surface plasmon resonance (SPR) images of individual cells at wavelengths of 632.8 nm and 690 nm are obtained using an optical system. The two-dimensional distributions of cell-substrate distance and cytoplasm RI are then demodulated from the phase-contrast SPR images of the cells. MDA-MB-231 cells and IDG-SW3 cells are experimentally measured to verify the feasibility of this approach. Our method provides a useful tool in biological fields for dual-parameter detection and characterization of live cells.

Optical tweezers integrated surface plasmon resonance holographic microscopy for characterizing cell-substrate interactions under noninvasive optical force stimuli.

The rapid development of bio-mechanical research increases the significance of studying cell behaviors near the substrate under the force stimuli in a real-time manner. Here, we present an optical tweezers (OT) integrated surface plasmon resonance holographic microscopy (SPRHM) to realize the dynamical and in-situ characterizations of cell-substrate interactions with noninvasive optical force stimulations. Using the OT integrated SPRHM (OT-SPRHM), we dynamically manipulate the living cells by OT, and simultaneously, the phase-contrast surface plasmon resonance images of the living cells are obtained and the cell-substrate distance is determined via SPRHM. We show that OT-SPRHM has the advanced capabilities of measuring the optical force and its tiny variations applied to the K562 cells near the substrate. Also, we for the first time reveal the manipulation of the MC3T3-E1 cells by OT. Demonstrating its robustness, this technique provides a powerful tool to explore the responses of various biological specimens to the force stimuli along the cell-substrate interface in the bio-sensing area.

Compact polarization-resolved common-path digital holography based on the Pancharatnam-Berry phase.

We propose a compact polarization-resolved common-path digital holography for measuring the polarization distribution of a light field dynamically with high temporal stability. The designed experimental setup allows simultaneously recording, in a common-path manner, two holograms carrying the complex amplitude information of two orthogonal polarization components of the light field. Based on the theory of the Pancharatnam-Berry phase to retrieve the full Stokes parameters of the light field, we demonstrate the experiments with polarized optical elements, stressed glass plate, and micrometer-sized liquid crystal droplet. The measurement results verify the method's high accuracy and stability, and the capability of measuring light fields with sizes ranging from centimeters to micrometers. Owing to the stable and compact optical path structure, this method is conducive to instrumentation and is expected to find wide applications in many fields.

Dual-channel illumination surface plasmon resonance holographic microscopy for resolution improvement.

Surface plasmon resonance holographic microscopy (SPRHM), combining digital holographic microscopy with surface plasmon resonance (SPR), can simultaneously obtain the amplitude and phase distributions of the reflected beam carrying specimen information in SPR. Due to the decaying length of the surface plasmon wave as large as tens of micrometers, the spatial resolution of SPRHM is lower than that of ordinary optical microscopes. In this work, we propose a scheme to improve the spatial resolution of SPRHM by applying dual-channel SPR excitations. Through the polarization multiplexing technique, two holograms carrying the information of SPR excited in orthogonal directions are simultaneously acquired. Via a numerical reconstruction and filtering algorithm for holograms, the lateral spatial resolution of SPRHM can be effectively enhanced to reach nearly 1 µm at a wavelength of 632.8 nm. This is comparable to the resolution of traditional optical microscopes, while possessing the advantages of wide-field imaging and high measurement sensitivity of SPR.

Real-time and wide-field mapping of cell-substrate adhesion gap and its evolution via surface plasmon resonance holographic microscopy.

As one of the most common biological phenomena, cell adhesion plays a vital role in the cellular activities such as the growth and apoptosis, attracting tremendous research interests over the past decades. Taking the cell evolution under drug injection as an example, the dynamics of cell-substrate adhesion gap can provide valuable information in the fundamental research of cell contacts. A robust technique of monitoring the cell adhesion gap and its evolution in real time is highly desired. Herein, we develop a surface plasmon resonance holographic microscopy to achieve the novel functionality of real-time and wide-field mapping of the cell-substrate adhesion gap and its evolution in situ. The cell adhesion gap images of mouse osteoblast cells and human breast cancer cells have been effectively extracted in a dynamic and label-free manner. The proposed technique opens up a new avenue of revealing the cell-substrate interaction mechanism and renders the wide applications in the biosensing area.

Structured light beams created through a multimode fiber via virtual Fourier filtering based on digital optical phase conjugation.

Digital optical phase conjugation (DOPC) is a newly developed technique in wavefront shaping to control light propagation through complex media. Currently, DOPC has been demonstrated for the reconstruction of two- and three-dimensional targets and enabled important applications in many areas. Nevertheless, the reconstruction results are only phase conjugated to the original input targets. Herein, we demonstrate that DOPC could be further developed for creating structured light beams through a multimode fiber (MMF). By applying annular filtering in the virtual Fourier domain of the acquired speckle field, we realize the creation of the quasi-Bessel and donut beams through the MMF. In principle, arbitrary amplitude and/or phase circular symmetry filtering could be performed in the Fourier domain, thus generating the corresponding point spread functions. We expect that the reported technique can be useful for super-resolution endoscopic imaging and optical manipulation through MMFs.

Y-Net: a one-to-two deep learning framework for digital holographic reconstruction.

In this Letter, for the first time, to the best of our knowledge, we propose a digital holographic reconstruction method with a one-to-two deep learning framework (Y-Net). Perfectly fitting the holographic reconstruction process, the Y-Net can simultaneously reconstruct intensity and phase information from a single digital hologram. As a result, this compact network with reduced parameters brings higher performance than typical network variants. The experimental results of the mouse phagocytes demonstrate the advantages of the proposed Y-Net.

Complex refractive index measurement for atomic-layer materials via surface plasmon resonance holographic microscopy.

The optical characterization of atomic-layer materials is significant for the clarification of fundamental physical properties of newly emerging nanomaterials. Here we propose to utilize the surface plasmon resonance (SPR) holographic microscopy to measure the complex refractive index (RI) of atomic-layer materials (i.e., graphene). We unambiguously determine the complex RI of single-layer graphene and few-layer graphene by fitting the measured reflection phase shift difference with theoretical values under the five-layer SPR model. The measurement results of the graphene layer grown by chemical vapor deposition at the visible range agree with the previous reports. Our method offers a cost-effective and robust avenue to characterize the complex RI of atomic-layer materials with distinct optical absorption, particularly the two-dimensional materials.

Measurement of full polarization states with hybrid holography based on geometric phase.

We propose a method for measuring the full polarization states of a light field by using hybrid polarization-angular multiplexing digital holography based on geometric phase. Through acquiring the geometric phase distribution of the whole light field by only recording a composite hologram, and according to quantitative relationship between the geometric phase and polarization state, the Stokes parameters of a light field can be calculated. Compared with other methods, this method can be used to obtain the complex amplitude information of the light field simultaneously without requiring other complex devices or elements to be adjusted, thus enabling dynamic polarization state measurement. The measurement results of the light fields generated by standard polarized optical elements, vortex half-wave retarder, and liquid crystal depolarizer verified this method's feasibility and validity.