Safe limits for the application of nonlinear optical microscopies to cultural heritage: A new method for in-situ assessment

Stratigraphy of Fresco Paintings: A New Approach with Photoacoustic and SORS Imaging

Photoacoustic (PA) imaging is a novel, powerful diagnostic technique utilized in different research fields. In particular, during recent years it has found several applications in Cultural Heritage (CH) diagnostics. PA imaging can be realized in transmittance or epi-illumination (reflectance) modes, obtaining variable levels of contrast and spatial resolution. In this work, we confirmed the applicability of the PA technique as a powerful tool for the imaging of one of the most challenging artwork objects, namely fresco wall paints, to obtain precise stratigraphic profiles in different layered fresco samples. In this regard, we studied some multi-layered fragments of the vault of San Giuseppe Church in Cagliari (1870 AD) and some mock-ups realized specifically to test the potentiality of this technique. Due to complex structures of the frescoes, we used the Spatially Off-set Raman Spectroscopy (SORS) technique to provide complementary information. The experimental results were in agreement for both techniques, even for the three-layered complex structure, and were confirmed with Scanning Electron Microscopy (SEM) analysis of cross-sections. The combined use of these two techniques proved useful to investigate detailed hidden information on the fresco samples.

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.

Analysis on the Characterization of Multiphoton Microscopy Images for Malignant Neoplastic Colon Lesion Detection under Deep Learning Methods

BACKGROUND

Colorectal cancer has a high incidence rate worldwide, with over 1.8 million new cases and 880,792 deaths in 2018. Fortunately, its early detection significantly increases the survival rate, reaching a cure rate of 90% when diagnosed at a localized stage. Colonoscopy is the gold standard technique for detection and removal of colorectal lesions with potential to evolve into cancer. When polyps are found in a patient, the current procedure is their complete removal. However, in this process, gastroenterologists cannot assure complete resection and clean margins which are given by the histopathology analysis of the removed tissue, which is performed at laboratory.

AIMS

In this paper, we demonstrate the capabilities of multiphoton microscopy (MPM) technology to provide imaging biomarkers that can be extracted by deep learning techniques to identify malignant neoplastic colon lesions and distinguish them from healthy, hyperplastic, or benign neoplastic tissue, without the need for histopathological staining.

MATERIALS AND METHODS

To this end, we present a novel MPM public dataset containing 14,712 images obtained from 42 patients and grouped into 2 classes. A convolutional neural network is trained on this dataset and a spatially coherent predictions scheme is applied for performance improvement.

RESULTS

We obtained a sensitivity of 0.8228 ± 0.1575 and a specificity of 0.9114 ± 0.0814 on detecting malignant neoplastic lesions. We also validated this approach to estimate the self-confidence of the network on its own predictions, obtaining a mean sensitivity of 0.8697 and a mean specificity of 0.9524 with the 18.67% of the images classified as uncertain.

CONCLUSIONS

This work lays the foundations for performing in vivo optical colon biopsies by combining this novel imaging technology together with deep learning algorithms, hence avoiding unnecessary polyp resection and allowing in situ diagnosis assessment.

Multiphoton Excitation Fluorescence Microscopy and Spectroscopic Multianalytical Approach for Characterization of Historical Glass Grisailles

The preservation of the integrity of artworks and cultural heritage items during characterization and conservation operations is of high priority, therefore, the application of non-invasive techniques is commonly suggested and recommended. Nonlinear optical microscopies (NLOM), based on the use of tightly focused pulsed femtosecond lasers, are emerging techniques for structural and chemical analysis of heritage objects with micrometric lateral and axial resolution. The results obtained with a set of optical and spectroscopic techniques for the chemical and physical characterization of grisaille paint layers on historical stained glasses, from different chronologies and provenance in Spain, are presented in this work. Optical behaviour and chemical composition were investigated by NLOM, using a laboratory set-up in the modality of Multi-Photon Excitation Fluorescence (MPEF), and by a multi-analytical combination of Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectrometry (FESEM-EDS), Laser Induced Breakdown Spectrosocopy (LIBS) and Laser Induced Fluorescence (LIF). Thicknesses values of the historical grisaille paint layers measured with MPEF were compared with those retrieved through FESEM, showing significant consistency and agreement. Under proper conditions, analysis via MPEF microscopy avoids the photochemical and physical damage to the examined materials, thus ensuring their preservation. This approach paves the way for future in-situ, non-invasive stratigraphic investigations on cultural heritage objects.

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.