Improving the low-dose performance of aberration correction in single sideband ptychography.

To solve some astronomical problems and the problems of atmospheric sounding, it is necessary to apply high-quality optical antennas because the quanlity of the optics determines the error at the parameter measuring. An intrinsic feature of the process of optical pieces manufacturing is the control of the quality of their shape at the production. Interferometry is one of the most precise methods for optical testing. The main difficulties in the control of the shape of optical surfaces are connected to the compensation of the aberrations of optical systems used to form the preset wavefront on the surface of the piece under control. When the aberrations are present in the object branch, the deformation of the interferometric fringes occurs even in case of a complete correspondence of the wavefronts of the object wave and the comparison wave. The appearance of the residual aberration is conditioned by the error of the manufacture of the elements of the illuminating branches, the remainder of the calculation and adjustment error. It is impossible to completely compensate for the residual aberration by introducing stringent manufacture and adjustment tolerances. That is why an optical system that forms the reference wavefront should possess a limited minimum value of the residual wave aberration. It is suggested to use a versatile adaptive mirror based on a bimorphed piezoelectric element as technical means for residual aberration correction. By selecting suitable controlling stresses, one may reach a rather good simulation of the geometric and wave aberrations to decrease the value of the residual aberration of the illuminating branch of the interferometer, thus increasing the precision of the interferometric control. The experimental results in the design of a real interferometer are given and discussed.

An adaptive optics scanning laser ophthalmoscope (AO-SLO) using a liquid-crystal spatial light modulator was developed. For routine clinical applications, long-term stability of the AO system is very important because unavoidable eye movement may degrade the instrument's performance. We studied the long-term performance of the aberration correction with healthy human eyes. Retinal image acquisition and AO data collection were performed simultaneously for periods of several minutes. We confirmed that, for more than 90% of the periods, the root-mean-square errors of residual wavefront were below the Marechal criterion. Drifts and microsaccades of fixational eye movement were examined using retinal images and residual aberrations. The results showed significant correlation between the transverse shift of retinal image and the low-order residual wavefront aberration during the drifts.

Objective.Photon-counting computed tomography (CT) is an advanced imaging technique that detects individual photons by energy, offering spectral imaging. While its application in radiotherapy remains under-explored, photon-counting CT (PCCT) could provide quantitative accuracy, particularly in proton stopping-power ratio (SPR) estimation, while it also potentially enables consistent low-dose imaging. This study evaluates estimation accuracy for relative electron density (RED), effective atomic number (Zeff), and SPR under low-dose conditions. This could be beneficial for repeated imaging in radiotherapy.Approach.A Gammex Advanced Electron Density phantom (Sun Nuclear) was scanned on a PCCT at 120 kVp with varying imaging doses (5 mGy (low dose), 20 mGy, and 40 mGy). Virtual monoenergetic images (VMIs) were generated with energies of 40-70 and 150-190 keV (10 keV increments). Performance was compared to dual-energy CT (DECT) acquisition (80/140 kVp) using the SOMATOM Confidence scanner with similar dose levels, and identically VMIs were generated as for PCCT. Mean ± standard deviation (SD) of CT numbers was quantified for tissue-equivalent phantom inserts. Furthermore, the physical quantities, RED,Zeff, and SPR accuracies were determined under high- and low-dose imaging conditions by quantifying the root-mean-squared-error (RMSE).Main results.PCCT generally showed lower CT number SD than DECT, while for both modalities, the SD increased at lower imaging dose. In addition, PCCT retained consistent mean CT numbers (differences up to 15 Hounsfield Unit (HU)), while DECT showed large CT number variations at 5 mGy (up to 1000 HU) in bone compared to higher dose. The accuracy of the physical quantities was independent of the dose for PCCT (0.7%-0.9% SPR RMSE), whereas DECT demonstrated larger differences with dose (1.1%-4.6% SPR RMSE).Significance.This study demonstrated that PCCT retains quantitative accuracy in low imaging dose conditions, outperforming DECT. This could be valuable for pediatric patients and in repeated imaging to reduce the cumulative dose.

Low-dose imaging techniques have many important applications in diverse fields, from biological engineering to materials science. Samples can be protected from phototoxicity or radiation-induced damage using low-dose illumination. However, imaging under a low-dose condition is dominated by Poisson noise and additive Gaussian noise, which seriously affects the imaging quality, such as signal-to-noise ratio, contrast, and resolution. In this work, we demonstrate a low-dose imaging denoising method that incorporates the noise statistical model into a deep neural network. One pair of noisy images is used instead of clear target labels and the parameters of the network are optimized by the noise statistical model. The proposed method is evaluated using simulation data of the optical microscope, and scanning transmission electron microscope under different low-dose illumination conditions. In order to capture two noisy measurements of the same information in a dynamic process, we built an optical microscope that is capable of capturing a pair of images with independent and identically distributed noises in one shot. A biological dynamic process under low-dose condition imaging is performed and reconstructed with the proposed method. We experimentally demonstrate that the proposed method is effective on an optical microscope, fluorescence microscope, and scanning transmission electron microscope, and show that the reconstructed images are improved in terms of signal-to-noise ratio and spatial resolution. We believe that the proposed method could be applied to a wide range of low-dose imaging systems from biological to material science.

Ptychography provides a sophisticated means of retrieving the complex object function via coherent diffractive imaging. It has become successfully established in the x‐ray and visible light communities as a means of lensless imaging and for its super‐resolution capability. Super‐resolution was also the original use of the method in electron microscopy [1]. However the technique did not become popular in the high resolution electron microscopy community due to the difficulty of acquiring and processing the four dimensional datasets required. Recent advances in detector technology however have resulted in a resurgence of interest in the method. As aberration correction now provides atomic resolution in hardware without the need for super‐resolution techniques, interest in ptychography in scanning transmission electron microscopy (STEM) has shifted towards achieving efficient phase contrast imaging. STEM provides sensitivity to atomic number via Z‐contrast annular dark field (ADF) imaging. The approximately quadratic variation of the intensity in ADF images with atomic number provides relatively facile compositional interpretability as compared to phase contrast imaging. However a relatively small proportion of the beam current is scattered out to the high angles sampled by ADF detectors, particularly for thin samples composed of light elements. Most of the transmitted electrons are contained within the bright field (BF) disk. Ptychography has recently been shown to be more efficient than other phase contrast imaging methods used in STEM, including conventional BF, annular bright field (ABF), and differential phase contrast (DPC) [2,3]. It has also proven superior to these modes at revealing the positions of light elements hidden by the scattering of heavy elements in the ADF signal [4]. Furthermore, ptychographic phase imaging requires no aberrations to achieve contrast, meaning the electron probe can be tuned to maximum capability of the aberration corrector. Here we investigate the sensitivity of STEM ptychography for two different applications. The first makes use of the sensitivity of phase contrast imaging to electromagnetic fields to detect charge transfer. Such charge transfer sensitivity was demonstrated in conventional TEM by Meyer et. al. by making use of lens aberrations to reveal contrast changes in N‐doped graphene and hexagonal boron nitride (hBN) that only matched with simulations based on potentials including the effects of charge transfer produced by density functional theory (DFT) and not the neutral atom potentials. We will present the results of testing charge transfer sensitivity in STEM with ptychography and various low dimensional materials. Figure 1 compares the projected potentials of (hBN) simulated with and without charge transfer. Figure 2 shows an example of simultaneously acquired ADF and ptychographic phase images of a region of single layer hBN surrounded by a double layer taken with the microscope fully tuned with the aberration corrector. As residual aberrations can affect phase images, we will also investigate the use of post acquisition aberration quantification and correction applied to ptychographic datasets of samples with the relatively subtle contrast effects of charge transfer. The second application of the sensitivity of STEM ptychography is its use for beam sensitive samples. We will assess the dose effectiveness of the method through simulations of varies samples, including biological samples frozen in amorphous ice, and compare to conventional TEM imaging. Consideration will be made of the pixelated detector technologies currently available, as the sensitivity and speed of the detector directly influence the dose effectiveness of the ptychographic phase images.

Higher order geometrical aberration correctors for transmission electron microscopes are essential for atomic-resolution imaging, especially at low-accelerating voltages. We quantitatively calculated the residual aberrations of fifth-order aberration correctors to determine the dominant aberrations. The calculations showed that the sixth-order three-lobe aberration was dominant when fifth-order aberrations were corrected by using the double-hexapole or delta types of aberration correctors. It was also deduced that the sixth-order three-lobe aberration was generally smaller in the delta corrector than in the double-hexapole corrector. The sixth-order three-lobe aberration was counterbalanced with a finite amount of the fourth-order three-lobe aberration and 3-fold astigmatism. In the experiments, we used a low-voltage microscope equipped with delta correctors for probe- and image-forming systems. Residual aberrations in each system were evaluated using Ronchigrams and diffractogram tableaux, respectively. The counterbalanced aberration correction was applied to obtain high-resolution transmission electron microscopy images of graphene and WS2 samples at 60 and 15 kV, respectively.

Transmission electron microscopy without aberrations: Applications to materials science

Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes. Correction of the aberrations has been achieved electron-optically through a hexapole-based corrector and also indirectly by computational analysis of a focal or tilt series of images. These direct and indirect methods are complementary, and a combination of the two offers further advantages. Materials characterization has benefitted from the reduced delocalization and higher resolution in the corrected images. It is now possible, for example, to locate atomic columns at surfaces to higher accuracy and reliability. This article describes the JEM-2200FS in Oxford, which is equipped with correctors for both the image-forming and probe-forming lenses. Examples of the use of this instrument in the characterization of nanocrystalline catalysts are given together with initial results combining direct and indirect methods. The double corrector configuration enables direct imaging of the corrected probe, and a potential confocal imaging mode is described. Finally, modifications to a second generation instrument are outlined.

Measurement of axial geometrical aberrations of the probe-forming lens by means of the shadow image of fine particles

Phase unwrapping is critical in many applications. Its purpose is to retrieve continuous phase information from discrete unwrapped phases, eliminate periodic aliasing of phases, and generate absolute phases with uniform spatial distribution. The phase unwrapping has been widely used in various cutting-edge fields, such as interferometry, fringe projection, and MRI. Phase unwrapping is an ill-posed problem, and its key point is to obtain accurate periodic ordinal information. However, when there is a lot of noise and aliasing, effective phase unwrapping still faces huge challenges. This paper converts phase unwrapping into a semantic segmentation problem. We propose a new network architecture SA-Unet, which adopts an encoder-decoder structure and adds a spatial attention module. To train SA-Unet, a simulated dataset with noise is produced. In this paper, we introduce focal loss and L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> norm to avoid dataset class imbalance problem. Experiments show that the SA-Unet proposed in this paper has higher accuracy and better robustness than other phase unwrapping methods.

Phase unwrapping (PU) is a key step in the reconstruction of digital elevation models (DEMs) and the monitoring of surface deformation from interferometric synthetic aperture radar (SAR, InSAR) data. In this paper, an improved PU method that combines an amended matrix pencil model, an adaptive unscented kalman filter (AUKF), an efficient quality-guided strategy based on heapsort, and a circular median filter is proposed. PU theory and the existing UKFPU method are covered. Then, the improved method is presented with emphasis on the AUKF and the circular median filter. AUKF has been well used in other fields, but it is for the first time applied to interferometric images PU, to the best of our knowledge. First, the amended matrix pencil model is used to estimate the phase gradient. Then, an AUKF model is used to unwrap the interferometric phase based on an efficient quality-guided strategy based on heapsort. Finally, the key results are obtained by filtering the results using a circular median. The proposed method is compared with the minimum cost network flow (MCF), statistical cost network flow (SNAPHU), regularized phase tracking technique (RPTPU), and UKFPU methods using two sets of simulated data and two sets of experimental GF-3 SAR data. The improved method is shown to yield the greatest accuracy in the interferometric phase maps compared to the methods considered in this paper. Furthermore, the improved method is shown to be the most robust to noise and is thus most suitable for PU of GF-3 SAR data in high-noise and low-coherence regions.

Customized contact lenses are limited in their correction performance, especially on irregular corneas, owing to decentration and rotation of the lenses. To overcome this limitation, we proposed to customize the back surface of soft contact lenses to match the anterior irregular corneal surface. These lenses were designed to correct anterior corneal aberrations and to improve lens stability. Although in keratoconic eyes the anterior corneal aberrations were effectively corrected, significant residual aberrations were observed. The internal optics, especially the posterior surface of the cornea, was the main source of these residual aberrations. Compared with conventional soft contact lenses, lens stability, on average over three eyes, was improved by a factor of 2 for horizontal and vertical decentrations and a factor of 5 in rotational orientation with the back-surface customized lenses.

Although a compensation device can correct aberrations of human eyes, the effect will be degraded by its misalignment, especially for high-order aberration correction. We calculate the positioning tolerance of correction device for high-order aberrations, and within what degree the correcting effect is better than low-order aberration (defocus and astigmatism) correction. With fixed certain misalignment within the positioning tolerance, we calculate the residual wavefront rms aberration of the first-6 to first-35 terms along with the 3rd-5th terms of aberrations corrected, and the combined first-13 terms of aberrations are also studied under the same quantity of misalignment. However, the correction effect of high-order aberrations does not meliorate along with the increase of the high-order terms under some misalignment, moreover, some simple combined terms correction can achieve similar result as complex combinations. These results suggest that it is unnecessary to correct too much the terms of high-order aberrations which are difficult to accomplish in practice, and gives confidence to correct high-order aberrations out of the laboratory.

A hybrid method for MT-InSAR phase unwrapping for deformation monitoring in urban areas

Signposts in electron optics