Deep tissue multiphoton microscopy using longer wavelength excitation

Discussion on the comparison of Raman spectroscopy and cardiovascular disease-related imaging techniques and the future applications of Raman technology: a systematic review

Cardiovascular disease (CVD) is a major cause of unnatural death worldwide, so timely diagnosis of CVD is crucial for improving patient outcomes. Although the traditional diagnostic tools can locate plaque and observe inner wall of blood vessel structure, they commonly have radioactivity and cannot detect the chemical composition of the plaque accurately. Recently emerging Raman techniques can detect the plaque composition precisely, and have the advantages of being fast, high-resolution and marker-free. This makes Raman have great potential for detecting blood samples, understanding disease conditions, and real-time monitoring. This review summarizes the origin and state-of-art of Raman techniques, including the following aspects: (a) the principle and technical classification of Raman techniques; (b) the applicability of Raman techniques and its comparison with traditional diagnostic tools at different diagnosis targets; (c) the applicability of Raman spectroscopy in advanced CVD. Lastly, we highlight the possible future applications of Raman techniques in CVD diagnosis.

Two-photon brightness of NIR-emitting, atomically precise DNA-stabilized silver nanoclusters

Near-infrared (NIR) emitters with high two-photon absorption (2PA) cross-sections are of interest to enable in vivo imaging in the tissue transparency windows. This study explores the potential of DNA-stabilized silver nanoclusters (Ag N -DNAs) as water-soluble two-photon absorbers. We investigate 2PA of four different atomically precise Ag N -DNA species with far-red to NIR emission and varying nanocluster and ligand compositions. 2PA cross-sections, σ 2, were determined by two-photon excited luminescence (2PEL) technique for a wide wavelength range from 810 to 1400 nm. The Ag N -DNAs exhibited reversed strength of corresponding transitions in the two-photon regime, as compared to one-photon, which further demonstrates the complex photophysics of these emitters. Maximal 2PA cross-section value (∼582 GM) was observed for (DNA)3[Ag21]15+, which is stabilized by 3 DNA oligomers. (DNA)2[Ag16Cl2]8+ presented distinct 2PA behavior from the Ag N -DNAs without chlorido ligands, with a high 2PA of 176 GM at 1050 nm. Our findings support the potential of Ag N -DNAs as NIR-to-NIR two-photon probes that are both excited and emit in the NIR. Their high σ 2 and fluorescence quantum yield values result in superior two-photon brightness on the order of ∼102 GM, significantly higher than water-soluble organic fluorophores.

In vivo three-photon fluorescence imaging of mouse brain vasculature labeled by Evans blue excited at the NIR-III window

Multiphoton fluorescence microscopy (MFM), renowned for its noninvasiveness and high spatiotemporal resolution, is extensively applied in brain structure imaging in vivo. Three-photon fluorescence (3PF) imaging, excited at the NIR-III window, can penetrate the deepest mouse cerebrovascular. Evans blue, a substance known for its low toxicity, high water solubility, and resistance to metabolism, is frequently employed to assess blood-brain barrier (BBB) permeability. However, its suitability for multiphoton fluorescence imaging of mouse cerebrovascular at the NIR-III window in vivo remains unexplored. In this paper, we conduct a comprehensive analysis of the multiphoton excitation and emission characterization of Evans blue when excited at the NIR-III window. Our findings indicate that 1) Evans blue can generate 3PF signals; 2) it exhibits a substantial three-photon action cross-section (ησ3 ) in plasma; 3) its three-photon emission spectrum measured in vivo agrees with that measured in plasma ex vivo. Drawing upon these findings, we successfully demonstrated the application of 3PF imaging of mouse brain vasculature labeled with Evans blue. Notably, the maximum depth of cerebrovascular is 1550 μm beneath the brain surface, spanning the entire gray matter layer and white matter layer and extending into the hippocampus. Evans blue is thus highly ideal for brain cerebrovascular 3PF imaging in vivo.

Polarization multiplexed 2200-nm soliton generation and its application to 3-photon microscopy

The 2200-nm window has recently been demonstrated as the longest excitation window for deep-tissue multiphoton microscopy (MPM). So far, MPM at this window exclusively uses a soliton laser source based on soliton self-frequency shift (SSFS). In order to boost the multiphoton signal level at this window, here we demonstrate a polarization multiplexed soliton source based on orthogonal polarized SSFS in a polarization maintaining large mode area (PM LMA) fiber. Using this technology, the effective pulse repetition rate, i.e., the pulse number per unit time, is twice higher than the 1550-nm pump laser with a fixed repetition rate. We further demonstrate comparative third harmonic generation (THG) and 3-photon fluorescence (3PF) imaging with this polarization multiplexed soliton source and single polarization soliton source. The ∼2 times higher 3-photon signal of the former indicates that this soliton multiplexing technology is promising for MPM at the 2200-nm window.

Super-resolved coherent anti-Stokes Raman scattering microscopy by coherent image scanning

We present super-resolved coherent anti-Stokes Raman scattering (CARS) microscopy by implementing phase-resolved image scanning microscopy, achieving up to two-fold resolution increase as compared with a conventional CARS microscope. Phase-sensitivity is required for the standard pixel-reassignment procedure since the scattered field is coherent, thus the point-spread function is well-defined only for the field amplitude. We resolve the complex field by a simple add-on to the CARS setup enabling inline interferometry. Phase-sensitivity offers additional contrast which informs the spatial distribution of both resonant and nonresonant scatterers. As compared with alternative super-resolution schemes in coherent nonlinear microscopy, the proposed method is simple, requires only low-intensity excitation, and is compatible with any conventional forward-detected CARS imaging setup.

Video-rate two-photon microendoscopy using second harmonic resonance fiber scanning

We present a 2.5-mm-diameter resonant fiber scanning two-photon microendoscope with a 30-mm long forward-viewing rigid probe tip that enables video-rate imaging (20 Hz frame rate) suitable for hand-held imaging of tissues without motion artifacts. Higher-order harmonic oscillation scanning techniques are developed to significantly increase the frame rate compared to prior published fiber scanning microendoscopy designs while maintaining the field-of-view (∼125 µm), the optical resolution (1.2 µm lateral and 10.9 µm axial resolution, full width at half maximum), and the spatial sampling (1250 circumferential pixels per spiral × 20 radial pixels over the diameter; 210 spirals per frame, ∼4 spiral samples per resolvable pixel) compared to a traditional scan using the fundamental resonance. 3D printed mounts were created to reduce the cost and simplify the fabrication for the fiber scanner without compromising performance or stability (<0.3 µm drift over 84 hours). A custom long-wavelength (∼1.08 µm) femtosecond fiber laser is coupled into several meters of fiber to realize a flexible, hand-held device for long-wavelength multiphoton microendoscopy.

In vivo spectrally unmixed multi-photon imaging of longitudinal axon-glia changes in injured spinal white matter

Understanding the sequence of cellular responses and their contributions to pathomorphogical changes in spinal white matter injuries is a prerequisite for developing efficient therapeutic strategies for spinal cord injury (SCI) as well as neurodegenerative and inflammatory diseases of the spinal cord such as amyotrophic lateral sclerosis and multiple sclerosis. We have developed several types of surgical procedures suitable for acute one-time and chronic recurrent in vivo multiphoton microscopy of spinal white matter [1]. Sophisticated surgical procedures were combined with transgenic mouse technology to image spinal tissue labeled with up to four fluorescent proteins (FPs) in axons, astrocytes, microglia, and blood vessels. To clearly separate the simultaneously excited FPs, spectral unmixing including iterative procedures was performed after imaging the diversely labeled spinal white matter with a custom-made 4-channel two-photon laser-scanning microscope. In our longitudinal multicellular studies of injured spinal white matter, we imaged axonal dynamics and invasion of microglia and astrocytes for a time course of over 200 days after SCI. Our methods offer ideal platforms for investigating acute and chronic cellular dynamics, cell-cell interactions, and metabolite fluctuations in health and disease as well as pharmacological manipulations in vivo.

Shot-Noise Limited Nonlinear Optical Imaging Excited With GHz Femtosecond Pulses and Denoised by Deep-Learning

Multiphoton fluorescence microscopy excited with femtosecond pulses at high repetition rates, particularly in the range of 100's MHz to GHz, offers an alternative solution to suppress photoinduced damage to biological samples, for example, photobleaching. Here, we demonstrate the use of a U-Net-based deep-learning algorithm for suppressing the inherent shot noise of the two-photon fluorescence images excited with GHz femtosecond pulses. With the trained denoising neural network, the image quality of the representative two-photon fluorescence images of the biological samples is shown to be significantly improved. Moreover, for input raw images with even SNR reduced to -4.76 dB, the trained denoising network can recover the main image structure from noise floor with acceptable fidelity and spatial resolution. It is anticipated that the combination of GHz femtosecond pulses and deep-learning denoising algorithm can be a promising solution for eliminating the trade-off between photoinduced damage and image quality in nonlinear optical imaging platforms.

Harmonic Imaging of Stem Cells in Whole Blood at GHz Pixel Rate

The pre-clinical validation of cell therapies requires monitoring the biodistribution of transplanted cells in tissues of host organisms. Real-time detection of these cells in the circulatory system and identification of their aggregation state is a crucial piece of information, but necessitates deep penetration and fast imaging with high selectivity, subcellular resolution, and high throughput. In this study, multiphoton-based in-flow detection of human stem cells in whole, unfiltered blood is demonstrated in a microfluidic channel. The approach relies on a multiphoton microscope with diffractive scanning in the direction perpendicular to the flow via a rapidly wavelength-swept laser. Stem cells are labeled with metal oxide harmonic nanoparticles. Thanks to their strong and quasi-instantaneous second harmonic generation (SHG), an imaging rate in excess of 10 000 frames per second is achieved with pixel dwell times of 1 ns, a duration shorter than typical fluorescence lifetimes yet compatible with SHG. Through automated cell identification and segmentation, morphological features of each individual detected event are extracted and cell aggregates are distinguished from isolated cells. This combination of high-speed multiphoton microscopy and high-sensitivity SHG nanoparticle labeling in turbid media promises the detection of rare cells in the bloodstream for assessing novel cell-based therapies.

Multiphoton fluorescence microscopy for in vivo imaging

Multiphoton fluorescence microscopy (MPFM) has been a game-changer for optical imaging, particularly for studying biological tissues deep within living organisms. MPFM overcomes the strong scattering of light in heterogeneous tissue by utilizing nonlinear excitation that confines fluorescence emission mostly to the microscope focal volume. This enables high-resolution imaging deep within intact tissue and has opened new avenues for structural and functional studies. MPFM has found widespread applications and has led to numerous scientific discoveries and insights into complex biological processes. Today, MPFM is an indispensable tool in many research communities. Its versatility and effectiveness make it a go-to technique for researchers investigating biological phenomena at the cellular and subcellular levels in their native environments. In this Review, the principles, implementations, capabilities, and limitations of MPFM are presented. Three application areas of MPFM, neuroscience, cancer biology, and immunology, are reviewed in detail and serve as examples for applying MPFM to biological research.

Circumscribing Laser Cuts Attenuate Seizure Propagation in a Mouse Model of Focal Epilepsy

In partial onset epilepsy, seizures arise focally in the brain and often propagate. Patients frequently become refractory to medical management, leaving neurosurgery, which can cause neurologic deficits, as a primary treatment. In the cortex, focal seizures spread through horizontal connections in layers II/III, suggesting that severing these connections can block seizures while preserving function. Focal neocortical epilepsy is induced in mice, sub-surface cuts are created surrounding the seizure focus using tightly-focused femtosecond laser pulses, and electrophysiological recordings are acquired at multiple locations for 3-12 months. Cuts reduced seizure frequency in most animals by 87%, and only 5% of remaining seizures propagated to the distant electrodes, compared to 80% in control animals. These cuts produced a modest decrease in cortical blood flow that recovered and left a ≈20-µm wide scar with minimal collateral damage. When placed over the motor cortex, cuts do not cause notable deficits in a skilled reaching task, suggesting they hold promise as a novel neurosurgical approach for intractable focal cortical epilepsy.

Quantitative Optical Imaging of Oxygen in Brain Vasculature

The intimate relationship between neuronal activity and cerebral oxygenation underpins fundamental brain functions like cognition, sensation, and motor control. Optical imaging offers a noninvasive approach to assess brain oxygenation and often serves as an indirect proxy for neuronal activity. However, deciphering neurovascular coupling─the intricate interplay between neuronal activity, blood flow, and oxygen delivery─necessitates independent, high spatial resolution, and high temporal resolution measurements of both microvasculature oxygenation and neuronal activation. This Perspective examines the established optical techniques employed for brain oxygen imaging, specifically functional near-infrared spectroscopy, photoacoustic imaging, optical coherence tomography, and two-photon phosphorescent lifetime microscopy, highlighting their fundamental principles, strengths, and limitations. Several other emerging optical techniques are also introduced. Finally, we discuss key technological challenges and future directions for quantitative optical oxygen imaging, paving the way for a deeper understanding of oxygen metabolism in the brain.

Lanthanide-based nanomaterials for temperature sensing in the near-infrared spectral region: illuminating progress and challenges

Being first proposed as a method to overcome limitations associated with conventional contact thermometers, luminescence thermometry has been extensively studied over the past two decades as a sensitive and fast approach to remote and minimally invasive thermal sensing. Herein, lanthanide (Ln)-doped nanoparticles (Ln-NPs) have been identified as particularly promising candidates, given their outstanding optical properties. Known primarily for their upconversion emission, Ln-NPs have also been recognized for their ability to be excited with and emit in the near-infrared (NIR) regions matching the NIR transparency windows. This sparked the emergence of the development of NIR-NIR Ln-NPs for a wide range of temperature-sensing applications. The shift to longer excitation and emission wavelengths resulted in increased efforts being put into developing nanothermometers for biomedical applications, however most research is still preclinical. This mini-review outlines and addresses the challenges that limit the reliability and implementation of luminescent nanothermometers to real-life applications. Through a critical look into the recent developments from the past 4 years, we highlight attempts to overcome some of the limitations associated with excitation wavelength, thermal sensitivity, calibration, as well as light-matter interactions. Strategies range from use of longer excitation wavelengths, brighter emitters through strategic core/multi-shell architectures, exploitation of host phonons, and a shift from double- to single-band ratiometric as well as lifetime-based approaches to innovative methods based on computation and machine learning. To conclude, we offer a perspective on remaining gaps and where efforts should be focused towards more robust nanothermometers allowing a shift to real-life, e.g., in vivo, applications.

Diving head-first into brain intravital microscopy

Tissue microenvironments during physiology and pathology are highly complex, meaning dynamic cellular activities and their interactions cannot be accurately modelled ex vivo or in vitro. In particular, tissue-specific resident cells which may function and behave differently after isolation and the heterogenous vascular beds in various organs highlight the importance of observing such processes in real-time in vivo. This challenge gave rise to intravital microscopy (IVM), which was discovered over two centuries ago. From the very early techniques of low-optical resolution brightfield microscopy, limited to transparent tissues, IVM techniques have significantly evolved in recent years. Combined with improved animal surgical preparations, modern IVM technologies have achieved significantly higher speed of image acquisition and enhanced image resolution which allow for the visualisation of biological activities within a wider variety of tissue beds. These advancements have dramatically expanded our understanding in cell migration and function, especially in organs which are not easily accessible, such as the brain. In this review, we will discuss the application of rodent IVM in neurobiology in health and disease. In particular, we will outline the capability and limitations of emerging technologies, including photoacoustic, two- and three-photon imaging for brain IVM. In addition, we will discuss the use of these technologies in the context of neuroinflammation.

Tunable liquid lens for three-photon excitation microscopy

We demonstrate a novel electrowetting liquid combination using a room temperature ionic liquid (RTIL) and a nonpolar liquid, 1-phenyl-1-cyclohexene (PCH) suitable for focus-tunable 3-photon microscopy. We show that both liquids have over 90% transmission at 1300 nm over a 1.1 mm pathlength and an index of refraction contrast of 0.123. A lens using these liquids can be tuned from a contact angle of 133 to 48° with applied voltages of 0 and 60 V, respectively. Finally, a three-photon imaging system including an RTIL electrowetting lens was used to image a mouse brain slice. Axial scans taken with an electrowetting lens show excellent agreement with images acquired using a mechanically scanned objective.

In vivo deep brain multiphoton fluorescence imaging emitting at NIR-I and NIR-II and excited at NIR-IV

Multiphoton microscopy (MPM) enables deep brain imaging. Three optical windows: NIR-I, NIR-II, and NIR-III are widely used. Recently, NIR-IV (the 2200 nm window) has been demonstrated to be the last and longest window for deep tissue MPM. However, so far MPM covers only two optical windows labeled by single fluorescent probe, one for emission and one for excitation. Here we demonstrate in vivo deep brain MPM covering three optical windows, with emission at NIR-I, NIR-II, and excitation at NIR-IV, labeled by ICG. The innovations include: (1) characterizing both 3-photon excitation and emission properties of ICG emitting at both NIR-I and NIR-II, in water, plasma, and circulating blood; (2) a home-built multiphoton microscope with simultaneous dual channel detection, with which we demonstrate deep brain MPM 950 μm (NIR-I) and 850 μm (NIR-II) into the mouse brain in vivo, verifying that multi-optical window MPM is promising for deep brain imaging.

Phototherapy for age-related brain diseases: Challenges, successes and future

Brain diseases present a significant obstacle to both global health and economic progress, owing to their elusive pathogenesis and the limited effectiveness of pharmaceutical interventions. Phototherapy has emerged as a promising non-invasive therapeutic modality for addressing age-related brain disorders, including stroke, Alzheimer's disease (AD), and Parkinson's disease (PD), among others. This review examines the recent progressions in phototherapeutic interventions. Firstly, the article elucidates the various wavelengths of visible light that possess the capability to penetrate the skin and skull, as well as the pathways of light stimulation, encompassing the eyes, skin, veins, and skull. Secondly, it deliberates on the molecular mechanisms of visible light on photosensitive proteins, within the context of brain disorders and other molecular pathways of light modulation. Lastly, the practical application of phototherapy in diverse clinical neurological disorders is indicated. Additionally, this review presents novel approaches that combine phototherapy and pharmacological interventions. Moreover, it outlines the limitations of phototherapeutics and proposes innovative strategies to improve the treatment of cerebral disorders.

Image restoration of degraded time-lapse microscopy data mediated by near-infrared imaging

Time-lapse fluorescence microscopy is key to unraveling biological development and function; however, living systems, by their nature, permit only limited interrogation and contain untapped information that can only be captured by more invasive methods. Deep-tissue live imaging presents a particular challenge owing to the spectral range of live-cell imaging probes/fluorescent proteins, which offer only modest optical penetration into scattering tissues. Herein, we employ convolutional neural networks to augment live-imaging data with deep-tissue images taken on fixed samples. We demonstrate that convolutional neural networks may be used to restore deep-tissue contrast in GFP-based time-lapse imaging using paired final-state datasets acquired using near-infrared dyes, an approach termed InfraRed-mediated Image Restoration (IR2). Notably, the networks are remarkably robust over a wide range of developmental times. We employ IR2 to enhance the information content of green fluorescent protein time-lapse images of zebrafish and Drosophila embryo/larval development and demonstrate its quantitative potential in increasing the fidelity of cell tracking/lineaging in developing pescoids. Thus, IR2 is poised to extend live imaging to depths otherwise inaccessible.

Numerical study of a convective cooling strategy for increasing safe power levels in two-photon brain imaging

Two-photon excitation fluorescence microscopy has become an effective tool for tracking neural activity in the brain at high resolutions thanks to its intrinsic optical sectioning and deep penetration capabilities. However, advanced two-photon microscopy modalities enabling high-speed and/or deep-tissue imaging necessitate high average laser powers, thus increasing the susceptibility of tissue heating due to out-of-focus absorption. Despite cooling the cranial window by maintaining the objective at a fixed temperature, average laser powers exceeding 100-200 mW have been shown to exhibit the potential for altering physiological responses of the brain. This paper proposes an enhanced cooling technique for inducing a laminar flow to the objective immersion layer while implementing duty cycles. Through a numerical study, we analyze the efficacy of heat dissipation of the proposed method and compare it with that of the conventional, fixed-temperature objective cooling technique. The results show that improved cooling could be achieved by choosing appropriate flow rates and physiologically relevant immersion cooling temperatures, potentially increasing safe laser power levels by up to three times (3×). The proposed active cooling method can provide an opportunity for faster scan speeds and enhanced signals in nonlinear deep brain imaging.

Comparison of the penetration depth in mouse brain in vivo through 3PF imaging using AIE nanoparticle labeling and THG imaging within the 1700 nm window

3-Photon microscopy (3PM) excited at the 1700 nm window features a smaller tissue attenuation and hence a larger penetration depth in brain imaging compared with other excitation wavelengths in vivo. While the comparison of the penetration depth quantified by effective attenuation length le with other excitation wavelengths have been extensively investigated, comparison within the 1700 nm window has never been demonstrated. This is mainly due to the lack of a proper excitation laser source and characterization of the in vivo emission properties of fluorescent labels within this window. Herein, we demonstrate detailed measurements and comparison of le through the 3-photon imaging of the mouse brain in vivo, at different excitation wavelengths (1600 nm, 1700 nm, and 1800 nm). 3PF imaging and in vivo spectrum measurements were performed using AIE nanoparticle labeling. Our results show that le derived from both 3PF imaging and THG imaging is the largest at 1700 nm, indicating that it enables the deepest penetration in brain imaging in vivo.

BF2-Functionalized Benzothiazole Amyloid Markers: Effect of Donor Substituents on One- and Two-Photon Properties

Investigation of amyloids with the aid of fluorescence microscopy provides crucial insights into the development of numerous diseases associated with the formation of aggregates. Here, we present a series of BF2-functionalized benzothiazoles with electron-donating methoxy group(s), which are tested as amyloid fluorescent markers. We evaluate how the position of donor functional group(s) influences optical properties (fluorescence lifetime (τ) and fluorescence quantum yield (FQY)) in a solution and upon binding to amyloids. We elucidate the importance of surrounding environmental factors (hydrogen-bonding network, polarity, and viscosity) on the observed changes in FQY and evaluate how the localization of a donor influences radiative and nonradiative decay pathways. We conclude that a donor attached to the benzothiazole ring contributes to the increment of radiative decay pathways upon binding to amyloids (kr), while the donor attached to the flexible part of a molecule (with rotational freedom) contributes to a decrease in nonradiative decay pathways (knr). We find that the donor-acceptor-donor architecture allows us to obtain 58 times higher FQY of the dye upon binding to bovine insulin amyloids. Finally, we measure two-photon absorption (2PA) cross sections (σ2) of the dyes and their change upon binding by the two-photon excited fluorescence (2PEF) technique. Measurements reveal that dyes that exhibit the increase/decrease of σ2 values when transferred from highly polar solvents to CHCl3 present a similar behavior upon amyloid binding. Our 2PA experimental values are supported by quantum mechanics/molecular mechanics (QM/MM) simulations. Despite this trend, the values of σ2 are not the same, which points out the importance of two-photon absorption measurements of amyloid-dye complexes in order to understand the performance of 2P probes upon binding.

Brewster mirror ultrafast optical parametric oscillator with high precision wavelength tuning

We demonstrate a synchronously-pumped optical parametric oscillator (OPO) with a cavity formed from high refractive index inverted prisms, also known as Brewster mirrors. Exploiting a single total internal reflection, this is the simplest device capable of deviating a laser beam by 180∘. The OPO produced a chirped signal output tunable from 1060 - 1570 nm with a maximum power of 114 mW. We investigate the geometric properties of ideal and imperfect Brewster mirror prisms and find the latter can provide ∼1000 × finer control of the signal wavelength when compared to cavity length tuning.

Simultaneous multimodal three-photon and optical coherence microscopy of the mouse brain in the 1700 nm optical window in vivo

Multimodal microscopy combining various imaging approaches can provide complementary information about tissue in a single imaging session. Here, we demonstrate a multimodal approach combining three-photon microscopy (3PM) and spectral-domain optical coherence microscopy (SD-OCM). We show that an optical parametric chirped-pulse amplification (OPCPA) laser source, which is the standard source for three-photon fluorescence excitation and third harmonic generation (THG), can be used for simultaneous OCM, 3-photon (3P) fluorescence and THG imaging. We validated the system performance in deep mouse brains in vivo with an OPCPA source operating at 1620 nm center wavelength. We visualized small structures such as myelinated axons, neurons, and large fiber tracts in white matter with high spatial resolution non-invasively using linear and nonlinear contrast at >1 mm depth in intact adult mouse brain. Our results showed that simultaneous OCM and 3PM at the long wavelength window can be conveniently combined for deep tissue imaging in vivo.

Advances in cellular resolution microscopy for brain imaging in rats

Rats are used in neuroscience research because of their physiological similarities with humans and accessibility as model organisms, trainability, and behavioral repertoire. In particular, rats perform a wide range of sophisticated social, cognitive, motor, and learning behaviors within the contexts of both naturalistic and laboratory environments. Further progress in neuroscience can be facilitated by using advanced imaging methods to measure the complex neural and physiological processes during behavior in rats. However, compared with the mouse, the rat nervous system offers a set of challenges, such as larger brain size, decreased neuron density, and difficulty with head restraint. Here, we review recent advances in in vivo imaging techniques in rats with a special focus on open-source solutions for calcium imaging. Finally, we provide suggestions for both users and developers of in vivo imaging systems for rats.

Pulse train gating to improve signal generation for in vivo two-photon fluorescence microscopy

Significance

Two-photon microscopy is used routinely for in vivo imaging of neural and vascular structures and functions in rodents with a high resolution. Image quality, however, often degrades in deeper portions of the cerebral cortex. Strategies to improve deep imaging are therefore needed. We introduce such a strategy using the gating of high repetition rate ultrafast pulse trains to increase the signal level.

Aim

We investigate how the signal generation, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR) improve with pulse gating while imaging in vivo mouse cerebral vasculature.

Approach

An electro-optic modulator with a high-power (6 W) 80 MHz repetition rate ytterbium fiber amplifier is used to create gates of pulses at a 1 MHz repetition rate. We first measure signal generation from a Texas Red solution in a cuvette to characterize the system with no gating and at a 50%, 25%, and 12.5% duty cycle. We then compare the signal generation, SNR, and SBR when imaging Texas Red-labeled vasculature using these conditions.

Results

We find up to a 6.73-fold increase in fluorescent signal from a cuvette when using a 12.5% duty cycle pulse gating excitation pattern as opposed to a constant 80 MHz pulse train at the same average power. We verify similar increases for in vivo imaging to that observed in cuvette testing. For deep imaging, we find that pulse gating results in a 2.95-fold increase in the SNR and a 1.37-fold increase in the SBR on average when imaging mouse cortical vasculature at depths ranging from 950 to 1050    μ m .

Conclusions

We demonstrate that a pulse gating strategy can either be used to limit heating when imaging superficial brain regions or used to increase signal generation in deep regions. These findings should encourage others to adopt similar pulse gating excitation schemes for imaging neural structures through two-photon microscopy.

A Fine-Scale and Minimally Invasive Marking Method for Use with Conventional Tungsten Microelectrodes

In neurophysiology, achieving precise correlation between physiological responses and anatomic structures is a significant challenge. Therefore, the accuracy of the electrode marking method is crucial. In this study, we describe a tungsten-deposition method, in which tungsten oxide is generated by applying biphasic current pulses to conventional tungsten electrodes. The electrical current used was 40-50 μA, which is similar to that used in electrical microstimulation experiments. The size of the markings ranged from 10 to 100 μm, corresponding to the size of the electrode tip, which is smaller than that of existing marking methods. Despite the small size of the markings, detection is easy as the marking appears in bright red under dark-field observation after Nissl staining. This marking technique resulted in low tissue damage and was maintained in vivo for at least two years. The feasibility of this method was tested in mouse and macaque brains.

Lanthanide doped nanoparticles for reliable and precise luminescence nanothermometry in the third biological window

In recent years, infrared emitting luminescent nanothermometers have attracted significant attention because their potential for the development of new diagnosis and therapy procedures. Despite their promising applications, concerns have been raised about their reliability due to the spectral distortions induced by tissues that are present even in the commonly used second biological window (1000-1370 nm). In this work, we present an innovative solution to this issue by demonstrating the effectiveness of shifting the operation range of these nanothermometers to the third biological window (1550-1850 nm). Through experimental evidence using ytterbium, erbium, and thulium tri-doped CaF₂ nanoparticles, we demonstrate that luminescence spectra acquired in the third biological window are minimally distorted by the presence of tissue, opening the way to reliable luminescence thermometry. In addition, advanced analysis (singular value decomposition) of emission spectra allows sub-degree thermal uncertainties to be achieved.

Dynamic Volumetric Imaging of Mouse Cerebral Blood Vessels In Vivo with an Ultralong Anti-Diffracting Beam

Volumetric imaging of a mouse brain in vivo with one-photon and two-photon ultralong anti-diffracting (UAD) beam illumination was performed. The three-dimensional (3D) structure of blood vessels in the mouse brain were mapped to a two-dimensional (2D) image. The speed of volumetric imaging was significantly improved due to the long focal length of the UAD beam. Comparing one-photon and two-photon UAD beam volumetric imaging, we found that the imaging depth of two-photon volumetric imaging (80 μm) is better than that of one-photon volumetric imaging (60 μm), and the signal-to-background ratio (SBR) of two-photon volumetric imaging is two times that of one-photon volumetric imaging. Therefore, we used two-photon UAD volumetric imaging to perform dynamic volumetric imaging of mouse brain blood vessels in vivo, and obtained the blood flow velocity.

Intravital imaging of osteocyte integrin dynamic with locally injectable fluorescent nanoparticles

Osteocytes are the resident mechanosensory cells in bone. They are responsible for skeletal homeostasis and adaptation to mechanical cues. Integrin proteins play a prominent role in osteocyte mechanotransduction, but the details are not well stratified. Intravital imaging with multiphoton microscopy presents an opportunity to study molecular level mechanobiological events in vivo and presents an opportunity to study integrin dynamics in osteocytes. However, fluorescent imaging limitations with respect to excessive optical scattering and low signal to noise ratio caused by mineralized bone matrix make such investigations non-trivial. Here, we demonstrate that ultra-small and bright fluorescent core-shell silica nanoparticles (<7 nm diameter), known as Cornell Prime Dots (C'Dots), are well-suited for the in vivo bone microenvironment and can improve intravital imaging capabilities. We report validation studies for C'Dots as a novel, locally injectable in vivo osteocyte imaging tool for both non-specific cellular uptake and for targeting integrins. The pharmacokinetics of C'Dots reveal distinct sex differences in nanoparticle intracellular dynamics and clearance in osteocytes, which represents a novel topic of study in bone biology. Integrin-targeted C'Dots were used to study osteocyte integrin dynamics. To the best of our knowledge, we report here the first evidence of osteocyte integrin endocytosis and recycling in vivo. Our results provide novel insights in osteocyte biology and will open up new lines of investigation that were previously unavailable in vivo.

Coherent Stokes Raman scattering microscopy (CSRS)

We report the first implementation of laser scanning coherent Stokes Raman scattering (CSRS) microscopy. To overcome the major challenge in CSRS imaging, we show how to suppress the fluorescence background by narrow bandpass filter and a lock-in based demodulation. Near background free CSRS imaging of polymer beads, human skin, onion cells, avocado flesh and the wing disc of a drosphila larva are presented. Finally, we explain and demonstrate numerically that CSRS solves a major obstacle of other coherent Raman techniques by sending a significant part (up to 100%) of the CSRS photons into the backward direction under tight focusing conditions. We believe that this discovery will pave the way for numerous technological advances, e.g., in epi-detected coherent Raman multi-focus imaging, real-time laser scanning based spectroscopy or efficient endoscopy.

Rabies virus-based labeling of layer 6 corticothalamic neurons for two-photon imaging in vivo

Neocortical layer 6 (L6) is less understood than other more superficial layers, largely owing to limitations of performing high-resolution investigations in vivo. Here, we show that labeling with the Challenge Virus Standard (CVS) rabies virus strain enables high-quality imaging of L6 neurons by conventional two-photon microscopes. CVS virus injection into the medial geniculate body can selectively label L6 neurons in the auditory cortex. Only three days after injection, dendrites and cell bodies of L6 neurons could be imaged across all cortical layers. Ca²⁺ imaging in awake mice showed that sound stimulation evokes neuronal responses from cell bodies with minimal contamination from neuropil signals. In addition, dendritic Ca²⁺ imaging revealed significant responses from spines and trunks across all layers. These results demonstrate a reliable method capable of rapid, high-quality labeling of L6 neurons that can be readily extended to other brain regions.

In vivo deep-brain 2-photon fluorescent microscopy labeled with near-infrared dyes excited at the 1700 nm window

2-Photon fluorescence microscopy (2PFM) is an indispensable imaging technology for neuroscience. However, the imaging depth is usually limited to the cortical layer in mouse brain in vivo. Here, we demonstrate deep brain 2PFM in vivo excited at the 1700 nm window, using IR780 and aza-IR780 as fluorescent labels. Our detailed characterization of the multiphoton excitation and emission properties of IR780 and aza-IR780 show that: (1) IR780 or aza-IR780 generate 2-photon fluorescence excited at the 1700 nm window and are promising for 2PFM; (2) aza-IR780 exhibits a larger ησ₂ with better anti-photobleaching property compared to IR780; The 2-photon action cross-sections of IR780 and aza-IR780 in plasma are an order-of-magnitude larger than those in PBS; (3) In vivo 2-photon emission spectra for both dyes show a notable red shift compared to those in vitro. Based on these characterization results, we demonstrate deep brain 2PFM labeled by them. A maximum imaging depth of 1585 μm (labeled by IR780) and 1800 μm (labeled by aza-IR780) into the mouse brain in vivo readily penetrates the subcortical region of hippocampus. Besides, a maximum of 1528 μm hemodynamic imaging depth is realized via 2PFM with aza-IR780 labeling, enabling us to measure blood flow speed in the hippocampus.

Deep Imaging to Dissect Microvascular Contributions to White Matter Degeneration in Rodent Models of Dementia

The increasing socio-economic burden of Alzheimer disease (AD) and AD-related dementias has created a pressing need to define targets for therapeutic intervention. Deficits in cerebral blood flow and neurovascular function have emerged as early contributors to disease progression. However, the cause, progression, and consequence of small vessel disease in AD/AD-related dementias remains poorly understood, making therapeutic targets difficult to pinpoint. Animal models that recapitulate features of AD/AD-related dementias may provide mechanistic insight because microvascular pathology can be studied as it develops in vivo. Recent advances in in vivo optical and ultrasound-based imaging of the rodent brain facilitate this goal by providing access to deeper brain structures, including white matter and hippocampus, which are more vulnerable to injury during cerebrovascular disease. Here, we highlight these novel imaging approaches and discuss their potential for improving our understanding of vascular contributions to AD/AD-related dementias.

Pulse train gating to improve signal generation for in vivo two-photon fluorescence microscopy

Significance

Two-photon microscopy is used routinely for in vivo imaging of neural and vascular structure and function in rodents with a high resolution. Image quality, however, often degrades in deeper portions of the cerebral cortex. Strategies to improve deep imaging are therefore needed. We introduce such a strategy using gates of high repetition rate ultrafast pulse trains to increase signal level.

Aim

We investigate how signal generation, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR) improve with pulse gating while imaging in vivo mouse cerebral vasculature.

Approach

An electro-optic modulator is used with a high-power (6 W) 80 MHz repetition rate ytterbium fiber amplifier to create gates of pulses at a 1 MHz repetition rate. We first measure signal generation from a Texas Red solution in a cuvette to characterize the system with no gating and at a 50%, 25%, and 12.5% duty cycle. We then compare signal generation, SNR, and SBR when imaging Texas Red-labeled vasculature using these conditions.

Results

We find up to a 6.73-fold increase in fluorescent signal from a cuvette when using a 12.5% duty cycle pulse gating excitation pattern as opposed to a constant 80 MHz pulse train. We verify similar increases for in vivo imaging to that observed in cuvette testing. For deep imaging we find pulse gating to result in a 2.95-fold increase in SNR and a 1.37-fold increase in SBR on average when imaging mouse cortical vasculature at depths ranging from 950 μm to 1050 μm.

Conclusions

We demonstrate that a pulse gating strategy can either be used to limit heating when imaging superficial brain regions or used to increase signal generation in deep regions. These findings should encourage others to adopt similar pulse gating excitation schemes for imaging neural structure through two-photon microscopy.

Generation of 1.3 µm femtosecond pulses by cascaded nonlinear optical gain modulation in phosphosilicate fiber

Nonlinear optical gain modulation (NOGM) is a simple and effective approach to generate highly coherent ultrafast pulses with a flexible wavelength. In this work, we demonstrate 34 nJ, 170 fs pulse generation at 1319 nm through a piece of phosphorus-doped fiber by two-stage cascaded NOGM with a 1064 nm pulsed pump. Beyond the experiment, numerical results show that 668 nJ, 391 fs pulses at 1.3 µm can be produced with up to 67% conversion efficiency by increasing the pump pulse energy and optimizing the pump pulse duration. This would offer an efficient method to obtain high-energy sub-picosecond laser sources for applications such as multiphoton microscopy.

In Vivo Deep-Brain 3- and 4-Photon Fluorescence Imaging of Subcortical Structures Labeled by Quantum Dots Excited at the 2200 nm Window

Multiphoton microscopy (MPM) is an enabling technology for visualizing deep-brain structures at high spatial resolution in vivo. Within the low tissue absorption window, shifting to longer excitation wavelengths reduces tissue scattering and boosts penetration depth. Recently, the 2200 nm excitation window has emerged as the last and longest window suitable for deep-brain MPM. However, multiphoton fluorescence imaging at this window has not been demonstrated, due to the lack of characterization of multiphoton properties of fluorescent labels. Here we demonstrate technologies for measuring both the multiphoton excitation and emission properties of fluorescent labels at the 2200 nm window, using (1) 3-photon (ησ₃) and 4-photon action cross sections (ησ₄) and (2) 3-photon and 4-photon emission spectra both ex vivo and in vivo of quantum dots. Our results show that quantum dots have exceptionally large ησ₃ and ησ₄ for efficient generation of multiphoton fluorescence. Besides, the 3-photon and 4-photon emission spectra of quantum dots are essentially identical to those of one-photon emission, which change negligibly subject to the local environment of circulating blood. Based on these characterization results, we further demonstrate deep-brain vasculature imaging in vivo. Due to the superb multiphoton properties of quantum dots, 3-photon and 4-photon fluorescence imaging reaches a maximum brain imaging depth of 1060 and 940 μm below the surface of a mouse brain, respectively, which enables the imaging of subcortical structures. We thus fill the last gap in multiphoton fluorescence imaging in terms of wavelength selection.

Tutorial: multiphoton microscopy to advance neuroscience research

Multiphoton microscopy (MPM) employs ultrafast infrared lasers for high-resolution deep three-dimensional imaging of live biological samples. The goal of this tutorial is to provide a practical guide to MPM imaging for novice microscopy developers and life-science users. Principles of MPM, microscope setup, and labeling strategies are discussed. Use of MPM to achieve unprecedented imaging depth of whole mounted explants and intravital imaging via implantable glass windows of the mammalian nervous system is demonstrated.

1.3/1.4 µm dual-wave band dissipative soliton resonance in a passively mode-locked Bi-doped phosphosilicate fiber laser

We report the 1.3/1.4 µm dual-wave band dissipative soliton resonance (DSR) in a passively mode-locked bismuth-doped phosphosilicate fiber (Bi-PSF) laser. The low-water-peak Bi-PSF with two bismuth active centers associated with silicon and phosphorus supports the O+E-band gain. Using a 1239 nm home-made Raman fiber laser as pump source and nonlinear amplifying loop mirror for initiating mode-locking, stable DSR operation at 1343 and 1406 nm is achieved with the spectral bandwidth of 12 and 16 nm. The pulse duration with the pump power increases from 62 to 270 ps with a repetition frequency of 4.069 MHz. The average power is 11.05 mW corresponding to the maximum energy of 2.7 nJ. This is, to the best of our knowledge, the first demonstration of a mode-locked fiber laser in the ∼1.38 µm water absorption band and the O+E dual-wave band operation for applications in all-spectral-band communications, bio-medical imaging, and terahertz difference frequency generation.

Intravital imaging to study cancer progression and metastasis

Navigation through the bulk tumour, entry into the blood vasculature, survival in the circulation, exit at distant sites and resumption of proliferation are all steps necessary for tumour cells to successfully metastasize. The ability of tumour cells to complete these steps is highly dependent on the timing and sequence of the interactions that these cells have with the tumour microenvironment (TME), including stromal cells, the extracellular matrix and soluble factors. The TME thus plays a major role in determining the overall metastatic phenotype of tumours. The complexity and cause-and-effect dynamics of the TME cannot currently be recapitulated in vitro or inferred from studies of fixed tissue, and are best studied in vivo, in real time and at single-cell resolution. Intravital imaging (IVI) offers these capabilities, and recent years have been a time of immense growth and innovation in the field. Here we review some of the recent advances in IVI of mammalian models of cancer and describe how IVI is being used to understand cancer progression and metastasis, and to develop novel treatments and therapies. We describe new techniques that allow access to a range of tissue and cancer types, novel fluorescent reporters and biosensors that allow fate mapping and the probing of functional and phenotypic states, and the clinical applications that have arisen from applying these techniques, reporters and biosensors to study cancer. We finish by presenting some of the challenges that remain in the field, how to address them and future perspectives.

Multiscale imaging informs translational mouse modeling of neurological disease

Multiscale neurophysiology reveals that simple motor actions are associated with changes in neuronal firing in virtually every brain region studied. Accordingly, the assessment of focal pathology such as stroke or progressive neurodegenerative diseases must also extend widely across brain areas. To derive mechanistic information through imaging, multiple resolution scales and multimodal factors must be included, such as the structure and function of specific neurons and glial cells and the dynamics of specific neurotransmitters. Emerging multiscale methods in preclinical animal studies that span micro- to macroscale examinations fill this gap, allowing a circuit-based understanding of pathophysiological mechanisms. Combined with high-performance computation and open-source data repositories, these emerging multiscale and large field-of-view techniques include live functional ultrasound, multi- and single-photon wide-scale light microscopy, video-based miniscopes, and tissue-penetrating fiber photometry, as well as variants of post-mortem expansion microscopy. We present these technologies and outline use cases and data pipelines to uncover new knowledge within animal models of stroke, Alzheimer's disease, and movement disorders.

Carbonaceous Nanoparticle Air Pollution: Toxicity and Detection in Biological Samples

Among the different air pollutants, particulate matter (PM) is of great concern due to its abundant presence in the atmosphere, which results in adverse effects on the environment and human health. The different components of PM can be classified based on their physicochemical properties. Carbonaceous particles (CPs) constitute a major fraction of ultrafine PM and have the most harmful effects. Herein, we present a detailed overview of the main components of CPs, e.g., carbon black (CB), black carbon (BC), and brown carbon (BrC), from natural and anthropogenic sources. The emission sources and the adverse effects of CPs on the environment and human health are discussed. Particularly, we provide a detailed overview of the reported toxic effects of CPs in the human body, such as respiratory effects, cardiovascular effects, neurodegenerative effects, carcinogenic effects, etc. In addition, we also discuss the challenges faced by and limitations of the available analytical techniques for the qualitative and quantitative detection of CPs in atmospheric and biological samples. Considering the heterogeneous nature of CPs and biological samples, a detailed overview of different analytical techniques for the detection of CPs in (real-exposure) biological samples is also provided. This review provides useful insights into the classification, toxicity, and detection of CPs in biological samples.

Dynamic Fluorescence Imaging in Experimental Epilepsy

The ability to develop effective new treatments for epilepsy may depend on improved understanding of seizure pathophysiology, about which many questions remain. Dynamic fluorescence imaging of activity at single-neuron resolution with fluorescent indicators in experimental model systems in vivo has revolutionized basic neuroscience and has the potential to do so for epilepsy research as well. Here, we review salient issues as they pertain to experimental imaging in basic epilepsy research, including commonly used imaging technologies, data processing and analysis, interpretation of results, and selected examples of how imaging-based approaches have revealed new insight into mechanisms of seizures and epilepsy.

Label-free measurement of wall shear stress in the brain venule and arteriole using dual-wavelength third-harmonic-generation line-scanning imaging

Wall shear stress (WSS) is of fundamental physiological and pathological significance. Current measurement technologies suffer from poor spatial resolution or cannot measure instantaneous values in a label-free manner. Here we demonstrate dual-wavelength third-harmonic-generation (THG) line-scanning imaging, for instantaneous wall shear rate and WSS measurement in vivo. We used the soliton self-frequency shift to generate dual-wavelength femtosecond pulses. Simultaneous acquisition of dual-wavelength THG line-scanning signals extract blood flow velocities at adjacent radial positions for instantaneous wall shear rate and WSS measurement. Our results show the oscillating behavior of WSS in brain venules and arterioles at micron spatial resolution in a label-free manner.

Two-photon calcium imaging of neuronal activity

In vivo two-photon calcium imaging (2PCI) is a technique used for recording neuronal activity in the intact brain. It is based on the principle that, when neurons fire action potentials, intracellular calcium levels rise, which can be detected using fluorescent molecules that bind to calcium. This Primer is designed for scientists who are considering embarking on experiments with 2PCI. We provide the reader with a background on the basic concepts behind calcium imaging and on the reasons why 2PCI is an increasingly powerful and versatile technique in neuroscience. The Primer explains the different steps involved in experiments with 2PCI, provides examples of what ideal preparations should look like and explains how data are analysed. We also discuss some of the current limitations of the technique, and the types of solutions to circumvent them. Finally, we conclude by anticipating what the future of 2PCI might look like, emphasizing some of the analysis pipelines that are being developed and international efforts for data sharing.

1.7 µm figure-9 Tm-doped ultrafast fiber laser

The evolution of multiphoton microscopy is critically dependent on the development of ultrafast laser technologies. The ultrashort pulse laser source at 1.7 µm waveband is attractive for in-depth three-photon imaging owing to the reduced scattering and absorption effects in biological tissues. Herein, we report on a 1.7 µm passively mode-locked figure-9 Tm-doped fiber laser. The nonreciprocal phase shifter that consists of two quarter-wave plates and a Faraday rotator introduces phase bias between the counter-propagating beams in the nonlinear amplifying loop mirror. The cavity dispersion is compensated to be slightly positive, enabling the proposed 1.7 µm ultrafast fiber laser to deliver the dissipative soliton with a 3-dB bandwidth of 20 nm. Moreover, the mode-locked spectral bandwidth could be flexibly tuned with different phase biases by rotating the wave plates. The demonstration of figure-9 Tm-doped ultrafast fiber laser would pave the way to develop the robust 1.7 µm ultrashort pulse laser sources, which could find important application for three-photon deep-tissue imaging.

A potent luminogen with NIR-IIb excitable AIE features for ultradeep brain vascular and hemodynamic three-photon imaging

Three-photon excited fluorescence microscopy (3PEFM) has emerged as a promising protocol for visualizing deep-brain vasculature and hemodynamics. However, the current situation is still far from satisfactory, due to small excitation action cross-section and short excitation wavelength of those previously reported 3PEFM luminogens. Herein, we manipulated molecular engineering by subtly regulating structural planarization/twisting to achieve ingenious integration of large three-photon absorption cross-section, high fluorescence quantum yield, ultralong near-infrared IIb excitation, and aggregation-induced emission features. The resulting molecule, namely DPCZ-BT, exhibited as high as 50.6% of fluorescence quantum yield and as large as 2.0 × 10-81 cm6s2/photon2 of three-photon absorption cross-section, which can be excited by 1665 nm fs laser and presents a recorded penetration depth of 1860 μm for deep-brain vascular structural imaging with high spatiotemporal resolution and signal-to-background ratio. Moreover, DPCZ-BT having good photostability and excellent biocompatibility is capable of impressively approaching 1600 μm depth in monitoring red blood cells flow velocity with extraordinary clarity for hemodynamics.

Assessment of angle-dependent spectral distortion to develop accurate hyperspectral endoscopy

Hyperspectral endoscopy has shown its potential to improve disease diagnosis in gastrointestinal tracts. Recent approaches in developing hyperspectral endoscopy are mainly focusing on enhancing image speed and quality of spectral information under a clinical environment, but there are many issues in obtaining consistent spectral information due to complicated imaging conditions, including imaging angle, non-uniform illumination, working distance, and low reflected signal. We quantitatively investigated the effect of imaging angle on the distortion of spectral information by exploiting a bifurcated fiber, spectrometer, and tissue-mimicking phantom. Spectral distortion becomes severe as increasing the angle of the imaging fiber or shortening camera exposure time for fast image acquisition. Moreover, spectral ranges from 450 to 550 nm are more susceptible to the angle-dependent spectral distortion than longer spectral ranges. Therefore, imaging angles close to normal and longer target spectral ranges with enough detector exposure time could minimize spectral distortion in hyperspectral endoscopy. These findings will help implement clinical HSI endoscopy for the robust and accurate measurement of spectral information from patients in vivo.

Towards super-resolved terahertz microscopy for cellular imaging

Biomedical imaging includes the use of a variety of techniques to study organs and tissues. Some of the possible imaging modalities are more spread at clinical level (CT, MRI, PET), while others, such as light and electron microscopy are preferred in life sciences research. The choice of the imaging modalities can be based on the capability to study functional aspects of an organism, the delivered radiation dose to the patient, and the achievable resolution. In the last few decades, spectroscopists and imaging scientists have been interested in the use of terahertz (THz) frequencies (30 μm to 3 mm wavelength) due to the low photon energy associated (E∼1 meV, not causing breaking of the molecular bonds but still interacting with some vibrational modes) and the high penetration depth that is achievable. THz has been already adopted in security, quality control and material sciences. However, the adoption of THz frequencies for biological and clinical imaging means to face, as a major limitation, the very scarce resolution associated with the use of such long wavelengths. To address this aspect and reconcile the benefit of minimal harmfulness for bioimaging with the achievable resolving power, many attempts have been made. This review summarises the state-of-the-art of THz imaging applications aimed at achieving super-resolution, describing how practical aspects of optics and quasi-optics may be treated to efficaciously implement the use of THz as a new low-dose and versatile modality in biomedical imaging and clinical research.

Enhanced Vascular Features in Porcine Gastrointestinal Endoscopy Using Multispectral Imaging

Endoscopic investigation is a predominant stan-dard while assessing the gastrointestinal tract. Even though it has been rigorously used in diagnostics for many decades, a high miss rate has been recorded. Advanced endoscopic imaging still has not found solutions to problems like early cancer detection, polyp generality, disease classification, etc. One of the less explored techniques to study early cancer detection is spectral imaging which deals with the absorption and reflection spectra of various wavelengths of light by different layers of tissue. To study tissues under various illumination, a multi-spectral light source unit that can be used along with an endoscopy system was developed with 10 different LEDs of very narrow bandwidths. Using this light source, a feasibility study was per-formed on an animal in which the upper GI tract of a porcine model was imaged and sample images were taken for processing from five different sections. Some wavelengths showed better contrast enhancements for visualization of vascular structures. Wavelength 420 nm (violet light) showed better contrast and the gradient of the line profile histogram showed the highest intensity change between the blood vessels and the surrounding mucosa. These enhancements showed that spectral imaging can potentially help in studying tissues for early cancer detection and improved visualization of the G I tract using endoscopy.

Cranial and Spinal Window Preparation for in vivo Optical Neuroimaging in Rodents and Related Experimental Techniques

Optical neuroimaging provides an effective neuroscience tool for multi-scale investigation of the neural structures and functions, ranging from molecular, cellular activities to the inter-regional connectivity assessment. Amongst experimental preparations, the implementation of an artificial window to the central nervous system (CNS) is primarily required for optical visualization of the CNS and associated brain activities through the opaque skin and bone. Either thinning down or removing portions of the skull or spine is necessary for unobstructed long-term in vivo observations, for which types of the cranial and spinal window and applied materials vary depending on the study objectives. As diversely useful, a window can be designed to accommodate other experimental methods such as electrophysiology or optogenetics. Moreover, auxiliary apparatuses would allow the recording in synchrony with behavior of large-scale brain connectivity signals across the CNS, such as olfactory bulb, cerebral cortex, cerebellum, and spinal cord. Such advancements in the cranial and spinal window have resulted in a paradigm shift in neuroscience, enabling in vivo investigation of the brain function and dysfunction at the microscopic, cellular level. This Review addresses the types and classifications of windows used in optical neuroimaging while describing how to perform in vivo studies using rodent models in combination with other experimental modalities during behavioral tests. The cranial and spinal window has enabled longitudinal examination of evolving neural mechanisms via in situ visualization of the brain. We expect transformable and multi-functional cranial and spinal windows to become commonplace in neuroscience laboratories, further facilitating advances in optical neuroimaging systems.