Nonlinear structured-illumination enhanced temporal focusing multiphoton excitation microscopy with a digital micromirror device

Temporal focusing multiphoton microscopy with cross-modality multi-stage 3D U-Net for fast and clear bioimaging

Temporal focusing multiphoton excitation microscopy (TFMPEM) enables fast widefield biotissue imaging with optical sectioning. However, under widefield illumination, the imaging performance is severely degraded by scattering effects, which induce signal crosstalk and a low signal-to-noise ratio in the detection process, particularly when imaging deep layers. Accordingly, the present study proposes a cross-modality learning-based neural network method for performing image registration and restoration. In the proposed method, the point-scanning multiphoton excitation microscopy images are registered to the TFMPEM images by an unsupervised U-Net model based on a global linear affine transformation process and local VoxelMorph registration network. A multi-stage 3D U-Net model with a cross-stage feature fusion mechanism and self-supervised attention module is then used to infer in-vitro fixed TFMPEM volumetric images. The experimental results obtained for in-vitro drosophila mushroom body (MB) images show that the proposed method improves the structure similarity index measures (SSIMs) of the TFMPEM images acquired with a 10-ms exposure time from 0.38 to 0.93 and 0.80 for shallow- and deep-layer images, respectively. A 3D U-Net model, pretrained on in-vitro images, is further trained using a small in-vivo MB image dataset. The transfer learning network improves the SSIMs of in-vivo drosophila MB images captured with a 1-ms exposure time to 0.97 and 0.94 for shallow and deep layers, respectively.

Light-field microscopy with temporal focusing multiphoton illumination for scanless volumetric bioimaging

A temporal focusing multiphoton illumination (TFMI) method is proposed for achieving selective volume illumination (SVI) (i.e., illuminating only the volume of interest) in light-field microscopy (LFM). The proposed method minimizes the background noise of the LFM images and enhances the contrast, and thus improves the imaging quality. Three-dimensional (3D) volumetric imaging is achieved by reconstructing the LFM images using a phase-space deconvolution algorithm. The experimental results obtained using 100-nm fluorescent beads show that the proposed TFMI-LFM system achieves lateral and axial resolutions of 1.2 µm and 1.1 µm, respectively, at the focal plane. Furthermore, the TFMI-LFM system enables 3D images of the single lobe of the drosophila mushroom body with GFP biomarker (OK-107) to be reconstructed in a one-snapshot record.

Image improvement of temporal focusing multiphoton microscopy via superior spatial modulation excitation and Hilbert-Huang transform decomposition

Temporal focusing-based multiphoton excitation microscopy (TFMPEM) just provides the advantage of widefield optical sectioning ability with axial resolution of several micrometers. However, under the plane excitation, the photons emitted from the molecules in turbid tissues undergo scattering, resulting in complicated background noise and an impaired widefield image quality. Accordingly, this study constructs a general and comprehensive numerical model of TFMPEM utilizing Fourier optics and performs simulations to determine the superior spatial frequency and orientation of the structured pattern which maximize the axial excitation confinement. It is shown experimentally that the optimized pattern minimizes the intensity of the out-of-focus signal, and hence improves the quality of the image reconstructed using the Hilbert transform (HT). However, the square-like reflection components on digital micromirror device leads to pattern residuals in the demodulated image when applying high spatial frequency of structured pattern. Accordingly, the HT is replaced with Hilbert-Huang transform (HHT) in order to sift out the low-frequency background noise and pattern residuals in the demodulation process. The experimental results obtained using a kidney tissue sample show that the HHT yields a significant improvement in the TFMPEM image quality.

Photobleaching Imprinting Enhanced Background Rejection in Line-Scanning Temporal Focusing Microscopy

Compared with two-photon point-scanning microscopy, two-photon temporal focusing microscopy (2pTFM) provides a parallel high-speed imaging strategy with optical sectioning capability. Owing to out-of-focus fluorescence induced by scattering, 2pTFM suffers deteriorated signal-to-background ratio (SBR) for deep imaging in turbid tissue, Here, we utilized the photobleaching property of fluorophore to eliminate out-of-focus fluorescence. According to different decay rates in different focal depth, we extract the in-focus signals out of backgrounds through time-lapse images. We analyzed the theoretical foundations of photobleaching imprinting of the line-scanning temporal focusing microscopy, simulated implementation for background rejection, and demonstrated the contrast enhancement in MCF-10A human mammary epithelial cells and cleared Thy1-YFP mouse brains. More than 50% of total background light rejection was achieved, providing higher SBR images of the MCF-10A samples and mouse brains. The photobleaching imprinting method can be easily adapted to other fluorescence dyes or proteins, which may have application in studies involving relatively large and nontransparent organisms.

Temporal focusing multiphoton microscopy with optimized parallel multiline scanning for fast biotissue imaging

SIGNIFICANCE

Line scanning-based temporal focusing multiphoton microscopy (TFMPM) has superior axial excitation confinement (AEC) compared to conventional widefield TFMPM, but the frame rate is limited due to the limitation of the single line-to-line scanning mechanism. The development of the multiline scanning-based TFMPM requires only eight multiline patterns for full-field uniform multiphoton excitation and it still maintains superior AEC.

AIM

The optimized parallel multiline scanning TFMPM is developed, and the performance is verified with theoretical simulation. The system provides a sharp AEC equivalent to the line scanning-based TFMPM, but fewer scans are required.

APPROACH

A digital micromirror device is integrated in the TFMPM system and generates the multiline pattern for excitation. Based on the result of single-line pattern with sharp AEC, we can further model the multiline pattern to find the best structure that has the highest duty cycle together with the best AEC performance.

RESULTS

The AEC is experimentally improved to 1.7  μm from the 3.5  μm of conventional TFMPM. The adopted multiline pattern is akin to a pulse-width-modulation pattern with a spatial period of four times the diffraction-limited line width. In other words, ideally only four π  /  2 spatial phase-shift scans are required to form a full two-dimensional image with superior AEC instead of image-size-dependent line-to-line scanning.

CONCLUSIONS

We have demonstrated the developed parallel multiline scanning-based TFMPM has the multiline pattern for sharp AEC and the least scans required for full-field uniform excitation. In the experimental results, the temporal focusing-based multiphoton images of disordered biotissue of mouse skin with improved axial resolution due to the near-theoretical limit AEC are shown to clearly reduce background scattering.

Two-Photon Photoexcited Photodynamic Therapy with Water-Soluble Fullerenol Serving as the Highly Effective Two-Photon Photosensitizer Against Multidrug-Resistant Bacteria

Background

Multidrug-resistant (MDR) bacterial strain is a serious medical problem. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to many antibiotics and is often associated with several diseases such as arthritis, osteomyelitis, and endocarditis. The development of an alternative treatment for eliminating MDR bacteria such as MRSA has attracted a considerable amount of research attention. Moreover, the development of a material for highly efficient generation of reactive oxygen species (ROS) involving two-photon photodynamic therapy (PDT) is currently desirable.

Materials and Methods

We present an example demonstrating that the use of water-soluble C₆₀(OH)₃₀ fullerenol with a 0.89 singlet oxygen quantum yield serving as a photosensitizer in PDT has the superior ability in effectively generating ROS.

Results

It has ultra-low energy (228.80 nJ pixel^-1) and can perform 900 scans under two-photon excitation (TPE) in the near-infrared region (760 nm) to completely eliminate the MDR species. Furthermore, the favorable two-photon properties are absorption of approximately 760 nm in wavelength, absolute cross-section of approximately 1187.50 Göeppert-Mayer units, lifetime of 6.640 ns, ratio of radiative to nonradiative decay rates of approximately 0.053, and two-photon stability under TPE.

Conclusion

This enabled water-soluble C₆₀(OH)₃₀ fullerenol to act as a promising two-photon photosensitizer proceeding with PDT to easily eliminate MDR species.

Water-Soluble Fullerenol with Hydroxyl Group Dependence for Efficient Two-Photon Excited Photodynamic Inactivation of Infectious Microbes

We successfully prepared water-soluble fullerenol [C₆₀(OH)₄₆] that exhibited a high singlet oxygen quantum yield and efficiently generated reactive oxygen species. Additionally, the water-soluble C₆₀(OH)₄₆ with a higher composition of exposed hydroxyl groups had superior two-photon stability and characteristics compared with that with a lower composition of such groups. Therefore, the prepared fullerenol can be an effective two-photon photosensitizer. The water-soluble C₆₀(OH)₄₆ had favorable two-photon properties. During two-photon photodynamic therapy, the water-soluble C₆₀(OH)₄₆ had substantial antimicrobial activity against Escherichia coli at an ultralow-energy level of 211.2 nJ pixel^-1 with 800 scans and a photoexcited wavelength of 760 nm.

Amino-Functionalized Nitrogen-Doped Graphene-Quantum-Dot-Based Nanomaterials with Nitrogen and Amino-Functionalized Group Content Dependence for Highly Efficient Two-Photon Bioimaging

We fabricated nanomaterials comprising amino-functionalized and nitrogen-doped graphene quantum dots (amino-N-GQDs) and investigated their photostability and intrinsic luminescence in the near-infrared spectrum to determine their suitability as contrast agents in two-photon imaging (TPI). We observed that amino-N-GQDs with a higher amount of bonded nitrogen and amino-functionalized groups (6.2%) exhibited superior two-photon properties to those with a lower amount of such nitrogen and groups (4.9%). These materials were conjugated with polymers containing sulfur (polystyrene sulfonate, PSS) and nitrogen atoms (polyethylenimine, PEI), forming amino-N-GQD–PSS–PEI specimens (amino-N-GQD-polymers). The polymers exhibited a high quantum yield, remarkable stability, and notable two-photon properties and generated no reactive oxygen species, rendering them excellent two-photon contrast agents for bioimaging. An antiepidermal growth factor receptor (Ab_EGFR) was used for labeling to increase specificity. Two-photon imaging (TPI) of amino-N-GQD (6.2%)-polymer-Ab_EGFR-treated A431 cancer cells revealed remarkable brightness, intensity, and signal-to-noise ratios for each observation at a two-photon excitation power of 16.9 nJ pixel⁻¹ under 30 scans and a three-dimensional (3D) depth of 105 µm, indicating that amino-N-GQD (6.2%)-polymer-Ab_EGFR-treated cells can achieve two-photon luminescence with 71 times less power required for two-photon autofluorescence (1322.8 nJ pixel⁻¹ with 500 scans) of similar intensity. This economy can minimize photodamage to cells, rendering amino-N-GQD-polymers suitable for noninvasive 3D bioimaging.

Scanless two-photon excitation with temporal focusing

Temporal focusing, with its ability to focus light in time, enables scanless illumination of large surface areas at the sample with micrometer axial confinement and robust propagation through scattering tissue. In conventional two-photon microscopy, widely used for the investigation of intact tissue in live animals, images are formed by point scanning of a spatially focused pulsed laser beam, resulting in limited temporal resolution of the excitation. Replacing point scanning with temporally focused widefield illumination removes this limitation and represents an important milestone in two-photon microscopy. Temporal focusing uses a diffusive or dispersive optical element placed in a plane conjugate to the objective focal plane to generate position-dependent temporal pulse broadening that enables axially confined multiphoton absorption, without the need for tight spatial focusing. Many techniques have benefitted from temporal focusing, including scanless imaging, super-resolution imaging, photolithography, uncaging of caged neurotransmitters and control of neuronal activity via optogenetics.

Structured illumination imaging without grating rotation based on mirror operation on 1D Fourier spectrum

Structured illumination microscopy (SIM) is a rapidly developing a super-resolution optical microscopy technique. With SIM, the grating is needed in order to rotate several angles for illuminating the sample in different directions. Multiple rotations reduce the imaging speed and grating rotation angle errors damage the image recovery quality. We introduce mirror transformation on one-dimension (1D) Fourier spectrum to SIM for resolving the problems of low imaging speed and severe impact on image reconstruction quality by grating rotation angle errors. When mirror operation and SIM are combined, the grating is placed at an orientation for obtaining three shadow images. The three shadow images are acquired by CCD at three different phase shift for a direction of grating. Thus, the SIM imaging speed is faster and the effect on image reconstruction quality by grating rotation angle errors is greatly reduced.

Faster, sharper, and deeper: structured illumination microscopy for biological imaging

Structured illumination microscopy (SIM) allows rapid, super-resolution (SR) imaging in live specimens. We review recent technical advances in SR-SIM, with emphasis on imaging speed, resolution, and depth. Since its introduction decades ago, the technique has grown to offer myriad implementations, each with its own strengths and weaknesses. We discuss these, aiming to provide a practical guide for biologists and to highlight which approach is best suited to a given application.

Scattering reduction by structured light illumination in line-scanning temporal focusing microscopy

Line-scanning temporal focusing microscopy (LineTFM) is capable of imaging biological samples more than 10 times faster than two-photon laser point-scanning microscopy (TPLSM), while achieving nearly the same lateral and axial spatial resolution. However, the image contrast taken by LineTFM is lower than that by TPLSM because LineTFM is severely influenced by biological tissue scattering. To reject the scattered photons, we implemented LineTFM using both structured illumination and uniform illumination combined with the HiLo post-processing algorithm, called HiLL microscopy (HiLo-Line-scanning temporal focusing microscopy). HiLL microscopy significantly reduces tissue scattering and improves image contrast. We demonstrate HiLL microscopy with in vivo brain imaging. This approach could potentially find applications in monitoring fast dynamic events and in mapping high resolution structures over a large volume.

Fast and improved bioimaging via temporal focusing multiphoton excitation microscopy with binary digital-micromirror-device holography

Conventional temporal focusing-based multiphoton excitation microscopy (TFMPEM) can offer widefield optical sectioning with an axial excitation confinement of a few microns. To improve the axial confinement of TFMPEM, a binary computer-generated Fourier hologram (CGFH) via a digital-micromirror-device (DMD) was implemented to intrinsically improve the axial confinement by filling the back-focal aperture of the objective lens. Experimental results show that the excitation focal volume can be condensed and the axial confinement improved about 24% according to the DMD holography. In addition, pseudouniform MPE can be achieved using two complementary CGFHs with rapid pulse-width modulation switching via the DMD. Furthermore, bioimaging of CV-1 in origin with SV40 genes-7 cells demonstrates that the TFMPEM with binary DMD holography can improve image quality by enhancing axial excitation confinement and rejecting out-of-focus excitation.

Widefield compressive multiphoton microscopy

A single-pixel compressively sensed architecture is exploited to simultaneously achieve a 10× reduction in acquired data compared with the Nyquist rate, while alleviating limitations faced by conventional widefield temporal focusing microscopes due to scattering of the fluorescence signal. Additionally, we demonstrate an adaptive sampling scheme that further improves the compression and speed of our approach.

Temporal focusing-based widefield multiphoton microscopy with spatially modulated illumination for biotissue imaging

A developed temporal focusing-based multiphoton excitation microscope (TFMPEM) has a digital micromirror device (DMD) which is adopted not only as a blazed grating for light spatial dispersion but also for patterned illumination simultaneously. Herein, the TFMPEM has been extended to implement spatially modulated illumination at structured frequency and orientation to increase the beam coverage at the back-focal aperture of the objective lens. The axial excitation confinement (AEC) of TFMPEM can be condensed from 3.0 μm to 1.5 μm for a 50 % improvement. By using the TFMPEM with HiLo technique as two structured illuminations at the same spatial frequency but different orientation, reconstructed biotissue images according to the condensed AEC structured illumination are shown obviously superior in contrast and better scattering suppression. Picture: TPEF images of the eosin-stained mouse cerebellar cortex by conventional TFMPEM (left), and the TFMPEM with HiLo technique as 1.09 μm^-1 spatially modulated illumination at 90° (center) and 0° (right) orientations.

Contrast and resolution enhanced optical sectioning in scattering tissue using line-scanning two-photon structured illumination microscopy

Optical sectioning imaging with high spatial resolution deep inside scattering samples such as mammalian brain is of great interest in biological study. Conventional two-photon microscopy deteriorates in focus when light scattering increases. Here we develop an optical sectioning enhanced two-photon technique which incorporates structured illumination into line-scanning spatial-temporal focusing microscopy (LTSIM), and generate patterned illumination via laser intensity modulation synchronized with scanning. LTSIM brings scattering background elimination and in-focus contrast enhancement, and realizes nearly 2-fold increase in spatial resolution to ∼208 nm laterally and ∼0.94 µm axially. In addition, the intensity modulated line-scanning implementation of LTSIM enables fast and flexible generation of structured illumination, permitting adjustable spatial frequency profiles to optimize image contrast. The highly qualified optical sectioning ability of our system is demonstrated on samples including tissue phantom, C. elegans and mouse brain at depths over hundreds of microns.

Fast two-snapshot structured illumination for temporal focusing microscopy with enhanced axial resolution

We present a new two-snapshot structured light illumination (SLI) reconstruction algorithm for fast image acquisition. The new algorithm, which only requires two mutually π phase-shifted raw structured images, is implemented on a custom-built temporal focusing fluorescence microscope (TFFM) to enhance its axial resolution via a digital micromirror device (DMD). First, the orientation of the modulated sinusoidal fringe patterns is automatically identified via spatial frequency vector detection. Subsequently, the modulated in-focal-plane images are obtained via rotation and subtraction. Lastly, a parallel amplitude demodulation method, derived based on Hilbert transform, is applied to complete the decoding processes. To demonstrate the new SLI algorithm, a TFFM is custom-constructed, where a DMD replaces the generic blazed grating in the system and simultaneously functions as a diffraction grating and a programmable binary mask, generating arbitrary fringe patterns. The experimental results show promising depth-discrimination capability with an axial resolution enhancement factor of 1.25, which matches well with the theoretical estimation, i.e, 1.27. Imaging experiments on pollen grain and mouse kidney samples have been performed. The results indicate that the two-snapshot algorithm presents comparable contrast reconstruction and optical cross-sectioning capability than those adopting the conventional root-mean-square (RMS) reconstruction method. The two-snapshot method can be readily applied to any sinusoidally modulated illumination systems to realize high-speed 3D imaging as less frames are required for each in-focal-plane image restoration, i.e., the image acquisition speed is improved by 2.5 times for any two-photon systems.

A fully-automated multiscale kernel graph cuts based particle localization scheme for temporal focusing two-photon microscopy

The temporal focusing two-photon microscope (TFM) is developed to perform depth resolved wide field fluorescence imaging by capturing frames sequentially. However, due to strong nonignorable noises and diffraction rings surrounding particles, further researches are extremely formidable without a precise particle localization technique. In this paper, we developed a fully-automated scheme to locate particles positions with high noise tolerance. Our scheme includes the following procedures: noise reduction using a hybrid Kalman filter method, particle segmentation based on a multiscale kernel graph cuts global and local segmentation algorithm, and a kinematic estimation based particle tracking method. Both isolated and partial-overlapped particles can be accurately identified with removal of unrelated pixels. Based on our quantitative analysis, 96.22% isolated particles and 84.19% partial-overlapped particles were successfully detected.

Two-color temporal focusing multiphoton excitation imaging with tunable-wavelength excitation

Wavelength tunable temporal focusing multiphoton excitation microscopy (TFMPEM) is conducted to visualize optical sectioning images of multiple fluorophore–labeled specimens through the optimal two-photon excitation (TPE) of each type of fluorophore. The tunable range of excitation wavelength was determined by the groove density of the grating, the diffraction angle, the focal length of lenses, and the shifting distance of the first lens in the beam expander. Based on a consideration of the trade-off between the tunable-wavelength range and axial resolution of temporal focusing multiphoton excitation imaging, the presented system demonstrated a tunable-wavelength range from 770 to 920 nm using a diffraction grating with groove density of 830 ?? lines / mm . TPE fluorescence imaging examination of a fluorescent thin film indicated that the width of the axial confined excitation was 3.0 ± 0.7 ?? ? m and the shifting distance of the temporal focal plane was less than 0.95 ?? ? m within the presented wavelength tunable range. Fast different wavelength excitation and three-dimensionally rendered imaging of Hela cell mitochondria and cytoskeletons and mouse muscle fibers were demonstrated. Significantly, the proposed system can improve the quality of two-color TFMPEM images through different excitation wavelengths to obtain higher-quality fluorescent signals in multiple-fluorophore measurements.

Fast volumetric imaging with patterned illumination via digital micro-mirror device-based temporal focusing multiphoton microscopy

Temporal focusing multiphoton microscopy (TFMPM) has the advantage of area excitation in an axial confinement of only a few microns; hence, it can offer fast three-dimensional (3D) multiphoton imaging. Herein, fast volumetric imaging via a developed digital micromirror device (DMD)-based TFMPM has been realized through the synchronization of an electron multiplying charge-coupled device (EMCCD) with a dynamic piezoelectric stage for axial scanning. The volumetric imaging rate can achieve 30 volumes per second according to the EMCCD frame rate of more than 400 frames per second, which allows for the 3D Brownian motion of one-micron fluorescent beads to be spatially observed. Furthermore, it is demonstrated that the dynamic HiLo structural multiphoton microscope can reject background noise by way of the fast volumetric imaging with high-speed DMD patterned illumination.

Dual-color dynamic tracking of GM-CSF receptors/JAK2 kinases signaling activation using temporal focusing multiphoton fluorescence excitation and astigmatic imaging

The dual-color dynamic particle tracking approach that uses temporal focusing multiphoton fluorescence excitation and two-channel astigmatic imaging is utilized to track molecular trajectories in three dimensions to explore molecular interactions. Images of two fluorophores were obtained to extract their positions by optical sectioning excitation using a fast temporal focusing multiphoton excitation microscope (TFMPEM) and by the simultaneous collection of data in two channels. The presented pair of cylindrical lenses, which was used to adjust the astigmatism effect with the minimum shifting of the imaging plane, was more feasible and flexible than single cylindrical lens for aligning two separate detection channels in astigmatic imaging. The lateral and axial positioning resolutions were observed to be approximately 9-13 nm and 23-30 nm respectively, for the two fluorescence channels. The dynamic movement and binding behavior of clusters of GM-CSF receptors and JAK2 kinases in HeLa cells in the presence of GM-CSF ligands were observed. Therefore, the proposed dual-color tracking strategy is useful for the dynamic study of molecular interactions in living specimens with a fast frame rate, less photobleaching, better penetration depth, and minimum optical trapping force.

Wide-field three-dimensional optical imaging using temporal focusing for holographically trapped microparticles

A contemporary challenge across the natural sciences is the simultaneous optical imaging or stimulation of small numbers of cells or colloidal particles organized into arbitrary geometries. We demonstrate the use of temporal focusing with holographic optical tweezers in order to achieve depth-resolved two-photon imaging of trapped objects arranged in arbitrary three-dimensional (3D) geometries using a single objective. Trapping allows for the independent position control of multiple objects by holographic beam shaping. Temporal focusing of ultrashort pulses provides the wide-field two-photon depth-selective activation of fluorescent samples. We demonstrate the wide-field depth-resolved illumination of both trapped fluorescent beads and trapped HL60 cells in suspension with full 3D positioning control. These approaches are compatible with implementation through scattering media and can be beneficial for emergent studies in colloidal science and particularly optogenetics, offering targeted photoactivation over a wide area with micrometer-precision depth control.

High-throughput multiphoton-induced three-dimensional ablation and imaging for biotissues

In this study, a temporal focusing-based high-throughput multiphoton-induced ablation system with axially-resolved widefield multiphoton excitation has been successfully applied to rapidly disrupt biotissues. Experimental results demonstrate that this technique features high efficiency for achieving large-area laser ablation without causing serious photothermal damage in non-ablated regions. Furthermore, the rate of tissue processing can reach around 1.6 × 10(6) μm(3)/s in chicken tendon. Moreover, the temporal focusing-based multiphoton system can be efficiently utilized in optical imaging through iterating high-throughput multiphoton-induced ablation machining followed by widefield optical sectioning; hence, it has the potential to obtain molecular images for a whole bio-specimen.

Dynamic particle tracking via temporal focusing multiphoton microscopy with astigmatism imaging

A three-dimensional (3D) single fluorescent particle tracking strategy based on temporal focusing multiphoton excitation microscopy (TFMPEM) combined with astigmatism imaging is proposed for delivering nanoscale-level axial information that reveals 3D trajectories of single fluorospheres in the axially-resolved multiphoton excitation volume without z-axis scanning. Whereas other scanning spatial focusing multiphoton excitation schemes induce optical trapping interference, temporal focusing multiphoton excitation produces widefield illumination with minimum optical trapping force on the fluorospheres. Currently, the lateral and axial positioning resolutions of the dynamic particle tracking approach are about 14 nm and 21 nm in standard deviation, respectively. Furthermore, the motion behavior and diffusion coefficients of fluorospheres in glycerol solutions with different concentrations are dynamically measured at a frame rate up to 100 Hz. This TFMPEM with astigmatism imaging holds great promise for exploring dynamic molecular behavior deep inside biotissues via its superior penetration, reduced trapping effect, fast frame rate, and nanoscale-level positioning.