Polarization Second Harmonic Generation Discriminates Between Fresh and Aged Starch-Based Adhesives Used in Cultural Heritage

Widefield polarization-resolved second harmonic generation imaging of entire thyroid nodule sections for the detection of capsular invasion

The identification of tumor capsular invasion as a sign of malignancy is currently employed in traditional histopathology routines for thyroid nodules. However, its limitations are associated with the assessment criteria for invasion, which often lead to disagreements among observers. The aim of this paper is to introduce a widefield imaging technique combined with quantitative collagen analysis to identify areas of capsular invasion in thyroid neoplasms. In this study, we introduce the application of widefield polarization-resolved second harmonic generation microscopy for imaging entire thyroid nodule sections on histological slides. We employ a cylindrical collagen model to extract parameters associated with the ultrastructure and orientation of collagen within the entire capsule of the thyroid nodule. We showcase the effectiveness of these parameters in distinguishing between areas of nodule capsule invasion and unaffected regions of the capsule through statistical analysis of individual parameters and employing a machine learning technique that involves generating maps via cluster analysis. Our results suggest that quantitative analysis facilitated by polarization-resolved widefield second harmonic generation microscopy could prove beneficial for the automated evaluation of capsular invasion sites in thyroid pathology.

Photoacoustic speckle pattern interferometry for detecting cracks of different sizes

A system and method for non-destructive detection of cracks of different width and depths based on digital speckle interferometry coupled with pulsed laser excitation is introduced and tested. Based on photoacoustic effect, acoustic waves are induced onto the rear of the samples by pumping a pulsed laser beam on it. The generated mechanical wave propagates from the rear surface of the sample to the front while front surface is monitored by speckle interferometry. In order to acquire information about surface deformation, the front surface is illuminated by continuous wave laser and interference are imaged onto the camera as speckle images. After processing the produced fringe patterns, it indicates the presence and location of the cracks in qualitative way. In this study, the system and method mentioned above are validated by detecting medium density fiberboard with simulated cracks. The fringe patterns from areas with or without defects are compared and discussed. Besides, the system and method to distinguish and predict cracks sizes is proposed and validated.

Nondestructive Nonlinear Optical Microscopy Revealed the Blackening Mechanism of Ancient Chinese Jades

Jade is most valued in Chinese culture since ancient times. For unearthed jade artifacts, the alteration color resulting from weathering effects and human activities provides information for cultural heritage conservation, archaeology, and history. Currently, the noninvasive 3-dimensional characterization of jade artifacts with high chemical and spatial resolution remains challenging. In this work, we applied femtosecond pump-probe microscopy and second harmonic generation microscopy techniques to study the black alteration of an ancient jade artifact of the late Spring and Autumn period (546 to 476 BC). The direct cause of the "mercury alteration" phenomena was discovered to be the conversion of metacinnabar from buried cinnabar in the tomb. Furthermore, a 3-dimensional optical reconstruction of the black alteration was achieved, providing a high-resolution method for analyzing the blackening mechanism without the need of sample damage. Our approach opens up new opportunities to extract microscopic spatiochemical information for a broad range of alteration colors in jade artifacts.

PSHG-TISS: A collection of polarization-resolved second harmonic generation microscopy images of fixed tissues

Second harmonic generation (SHG) microscopy is acknowledged as an established imaging technique capable to provide information on the collagen architecture in tissues that is highly valuable for the diagnostics of various pathologies. The polarization-resolved extension of SHG (PSHG) microscopy, together with associated image processing methods, retrieves extensive image sets under different input polarization settings, which are not fully exploited in clinical settings. To facilitate this, we introduce PSHG-TISS, a collection of PSHG images, accompanied by additional computationally generated images which can be used to complement the subjective qualitative analysis of SHG images. These latter have been calculated using the single-axis molecule model for collagen and provide 2D representations of different specific PSHG parameters known to account for the collagen structure and distribution. PSHG-TISS can aid refining existing PSHG image analysis methods, while also supporting the development of novel image processing and analysis methods capable to extract meaningful quantitative data from the raw PSHG image sets. PSHG-TISS can facilitate the breadth and widespread of PSHG applications in tissue analysis and diagnostics.

Measurement(s)	Type I Collagen
Technology Type(s)	multi-photon laser scanning microscopy
Factor Type(s)	second order susceptibility tensor elements
Sample Characteristic - Organism	Homo sapiens
Sample Characteristic - Environment	laboratory environment
Sample Characteristic - Location	Romania

Combined TPEF and SHG Imaging for the Microstructural Characterization of Different Wood Species Used in Artworks

The morphological and chemical conformation of wood microstructures is characteristic of individual species and strongly influences the macromechanical properties of the material, as well as its sensitivity to deterioration factors. Noninvasive techniques enabling the visualization of wood microstructures, while simultaneously providing compositional information, can significantly facilitate the analysis of wooden artworks for conservation purposes. In this paper, we present the application of combined two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) imaging as a versatile diagnostic tool for the microcharacterization of three hardwood species never analyzed by this method. Multimodal mapping of the molecular constituents based on the detected nonlinear signals provides useful information for studying the biological and biochemical deterioration of wood, opening a new field of application for a well-established and widely used imaging technology.

Noninvasive quantitative assessment of collagen degradation in parchments by polarization-resolved SHG microscopy

Nondestructive and noninvasive investigation techniques are highly sought-after to establish the degradation state of historical parchments, which is up to now assessed by thermal techniques that are invasive and destructive. We show that advanced nonlinear optical (NLO) microscopy enables quantitative in situ mapping of parchment degradation at the micrometer scale. We introduce two parameters that are sensitive to different degradation stages: the ratio of two-photon excited fluorescence to second harmonic generation (SHG) signals probes severe degradation, while the anisotropy parameter extracted from polarization-resolved SHG measurements is sensitive to early degradation. This approach is first validated by comparing NLO quantitative parameters to thermal measurements on artificially altered contemporary parchments. We then analyze invaluable parchments from the Middle Ages and show that we can map their conservation state and assess the impact of a restoration process. NLO quantitative microscopy should therefore help to identify parchments most at risk and optimize restoration methods.

Three-dimensional characterization of collagen remodeling in cell-seeded collagen scaffolds via polarization second harmonic generation

In this study, we use non-linear imaging microscopy to characterize the structural properties of porous collagen-GAG scaffolds (CGS) seeded with human umbilical vein endothelial cells (HUVECs), as well as human mesenchymal stem cells (hMSCs), a co-culture previously reported to form vessel-like structures inside CGS. The evolution of the resulting tissue construct was monitored over 10 days via simultaneous two- and three-photon excited fluorescence microscopy. Time-lapsed 2- and 3-photon excited fluorescence imaging was utilized to monitor the temporal evolution of the vascular-like structures up to 100 µm inside the scaffold up to 10 days post-seeding. 3D polarization-dependent second harmonic generation (PSHG) was utilized to monitor collagen-based scaffold remodeling and determine collagen fibril orientation up to 200 µm inside the scaffold. We demonstrate that polarization-dependent second harmonic generation can provide a novel way to quantify the reorganization of the collagen architecture in CGS simultaneously with key biomechanical interactions between seeded cells and CGS that regulate the formation of vessel-like structures inside 3D tissue constructs. A comparison between samples at different days in vitro revealed that gradually, the scaffolds developed an orthogonal net-like architecture, previously found in real skin.

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

The non-invasive depth-resolved imaging of pictorial layers in paintings by means of linear optical techniques represents a challenge in the field of Cultural Heritage (CH). The presence of opaque and/or highly-scattering materials may obstruct the penetration of the radiation probe, thus impeding the visualization of the stratigraphy of paintings. Nonlinear Optical Microscopy (NLOM), which makes use of tightly-focused femtosecond pulsed lasers as illumination sources, is an emerging technique for the analysis of painted objects enabling micrometric three-dimensional (3D) resolution with good penetration capability in semi-transparent materials. In this work, we evaluated the potential of NLOM, specifically in the modality of Multi-Photon Excitation Fluorescence (MPEF), to probe the stratigraphy of egg-tempera mock-up paintings. A multi-analytical non-invasive approach, involving ultraviolet-visible-near infrared (UV-Vis-NIR) Fiber Optics Reflectance Spectroscopy, Vis-NIR photoluminescence, and Laser Induced Fluorescence, yielded key-information for the characterization of the constituting materials and for the interpretation of the nonlinear results. Furthermore, the use of three nonlinear optical systems allowed evaluation of the response of the analyzed paints to different excitation wavelengths and photon doses, which proved useful for the definition of the most suitable measurement conditions. The micrometric thickness of the paint layers, which was not measurable by means of Optical Coherence Tomography (OCT), was instead assessed by MPEF, thus demonstrating the effectiveness of this nonlinear modality in probing highly-scattering media, while ensuring the minimal photochemical disturbance to the examined materials.

Non-Linear Microscopy: A Well-Established Technique for Biological Applications towards Serving as a Diagnostic Tool for in situ Cultural Heritage Studies

A range of sophisticated imaging techniques have been developed in recent years that can reveal the surface structure of cultural heritage objects with varying precision. In combination with various spectroscopic methods, they allow the study of the chemical composition of the object; thus, conclusions can be drawn about the origin of the object or its initial components, method, or time of creation, authenticity, mechanisms of degradation, and ways of further conservation. At present, different techniques can be applied to a wide range of cultural heritage objects, such as varnishes, paintings, archaeological objects, binding media, paper-based documents, parchments, marbles, frescoes, as well as various objects made of leather, fabric, stone, ceramics and glass, wood, or metal. One of the main needs in the study of cultural heritage (CH) is the transportability/portability of the research equipment, since many pieces under investigation cannot be moved to the laboratory, either because of their size, inseparability (for example, frescoes on walls, mural paintings in caves), or the threat of damage. In this work, we briefly overview the main optical- and laser-based methods used for the study of cultural heritage objects indicating the scope of their application, and we focus on the applications of non-linear microscopic methods for the investigation of a series of artifacts. We also discuss all the requirements for the construction of a prototype transportable non-linear optical system that will be used as a novel diagnostic tool for in situ studies of CH assets. The availability of such a transportable workstation will significantly improve the study and characterization of various types of CH objects and will constitute an extremely useful diagnostic tool for heritage scientists dealing with a variety of investigations.

Combined multiphoton fluorescence microscopy and photoacoustic imaging for stratigraphic analysis of paintings

We demonstrate the effective combination of multiphoton and photoacoustic (PA) imaging for the high-resolution stratigraphic analysis of multilayered art objects with emphasis on paintings. A novel convolution-based algorithm is additionally applied for the precise discrimination of nonlinear signals, providing valuable information in regard to the thickness and composition of successive varnish and paint layers in the mock-up samples. On the other hand, PA contrast complements the extracted data by revealing well-hidden graphite underdrawings below the paint at high sensitivity levels. The final composite images are directly compared with cross-sectional brightfield observations, validating the capabilities of the bimodal diagnosis in terms of measurement accuracy and contrast specificity. The presented hybrid diagnostic approach has the potential to optimize delicate interventions in works of art such as the selective removal of aged materials, thus promoting a significantly improved restoration outcome.

Multi-scale characterization of thermoplastic starch structure using Second Harmonic Generation imaging and NMR

Starch granules can be extruded to obtain a thermoplastic material. Thermoplastic starch (TPS) usually requires a significant break down of the starch granular organization to form a continuous polysaccharide matrix. In this work, we extrude potato starch with and without a plasticizer and store samples at high humidity to generate recrystallization. A multi-scale investigation of the microstructure is performed by combining different techniques: WAXS and solid-state NMR to describe macromolecule organization and Second Harmonic Generation (SHG) imaging to provide spatial information. Finally, the ability of the material to swell and remain sound in water is assessed. Glycerol-plasticized samples swell the least despite many granules with native-like structure embedded in the starch matrix. Glycerol limits the fragmentation and melting of the granules and crystallites during extrusion but also reduces the proportion of starch molecules in constrained conformations, enabling the formation of a polymer network that can sustain the penetration of water.

Optical coherence tomography and non-linear microscopy for paintings - a study of the complementary capabilities and laser degradation effects

This paper examines for the first time the potential complementary imaging capabilities of Optical coherence tomography (OCT) and non-linear microscopy (NLM) for multi-modal 3D examination of paintings following the successful application of OCT to the in situ, non-invasive examination of varnish and paint stratigraphy of historic paintings and the promising initial studies of NLM of varnish samples. OCT provides image contrast through the optical scattering and absorption properties of materials, while NLM provides molecular information through multi-photon fluorescence and higher harmonics generation (second and third harmonic generation). OCT is well-established in the in situ non-invasive imaging of the stratigraphy of varnish and paint layers. While NLM examination of transparent samples such as fresh varnish and some transparent paints showed promising results, the ultimate use of NLM on paintings is limited owing to the laser degradation effects caused by the high peak intensity of the laser source necessary for the generation of non-linear phenomena. The high intensity normally employed in NLM is found to be damaging to all non-transparent painting materials from slightly scattering degraded varnish to slightly absorbing paint at the wavelength of the laser excitation source. The results of this paper are potentially applicable to a wide range of materials given the diversity of the materials encountered in paintings (e.g. minerals, plants, insects, oil, egg, synthetic and natural varnish).