A clinical instrument for combined raman spectroscopy-optical coherence tomography of skin cancers

Towards non-invasive monitoring of non-melanoma skin cancer using spatially offset Raman spectroscopy

BCC (basal cell carcinoma) and SCC (squamous cell carcinoma) account for the vast majority of cases of non-melanoma skin cancer (NMSC). The gold standard for the diagnosis remains biopsy, which, however, is an invasive and time-consuming procedure. In this study, we employed spatially offset Raman spectroscopy (SORS), a non-invasive approach, allowing the assessment of deeper skin tissue levels and collection of Raman photons with a bias towards the different layers of epidermis, where the non-melanoma cancers are initially formed and expand. Ex vivo Raman measurements were acquired from 22 skin biopsies using conventional back-scattering and a defocused modality (with and without a spatial offset). The spectral data were assessed against corresponding histopathological data to determine potential prognostic factors for lesion detection. The results revealed a positive correlation of protein and lipid content with the SCC and BCC types, respectively. By further correlating with patient data, multiple factor analysis (MFA) demonstrated a strong clustering of variables based on sex and age in all modalities. Specifically for the defocused modality (zero and 2 mm offset), further clustering occurred based on pathology. This study demonstrates the utility of the SORS technology in NMSC diagnosis prior to histopathological examination on the same tissue.

Integration of cellular-resolution optical coherence tomography and Raman spectroscopy for discrimination of skin cancer cells with machine learning

Significance

An integrated cellular-resolution optical coherence tomography (OCT) module with near-infrared Raman spectroscopy was developed on the discrimination of various skin cancer cells and normal cells. Micron-level three-dimensional (3D) spatial resolution and the spectroscopic capability on chemical component determination can be obtained simultaneously.

Aim

We experimentally verified the effectiveness of morphology, intensity, and spectroscopy features for discriminating skin cells.

Approach

Both spatial and spectroscopic features were employed for the discrimination of five types of skin cells, including keratinocytes (HaCaT), the cell line of squamous cell carcinoma (A431), the cell line of basal cell carcinoma (BCC-1/KMC), primary melanocytes, and the cell line of melanoma (A375). The cell volume, compactness, surface roughness, average intensity, and internal intensity standard deviation were extracted from the 3D OCT images. After removing the fluorescence components from the acquired Raman spectra, the entire spectra (600 to 2100    cm - 1 ) were used.

Results

An accuracy of 85% in classifying five types of skin cells was achieved. The cellular-resolution OCT images effectively differentiate cancer and normal cells, whereas Raman spectroscopy can distinguish the cancer cells with nearly 100% accuracy.

Conclusions

Among the OCT image features, cell surface roughness, internal average intensity, and standard deviation of internal intensity distribution effectively differentiate the cancerous and normal cells. The three features also worked well in sorting the keratinocyte and melanocyte. Using the full Raman spectra, the melanoma and keratinocyte-based cell carcinoma cancer cells can be discriminated effectively.

Multimodal Raman spectroscopy and optical coherence tomography for biomedical analysis

Optical techniques hold great potential to detect and monitor disease states as they are a fast, non-invasive toolkit. Raman spectroscopy (RS) in particular is a powerful label-free method capable of quantifying the biomolecular content of tissues. Still, spontaneous Raman scattering lacks information about tissue morphology due to its inability to rapidly assess a large field of view. Optical Coherence Tomography (OCT) is an interferometric optical method capable of fast, depth-resolved imaging of tissue morphology, but lacks detailed molecular contrast. In many cases, pairing label-free techniques into multimodal systems allows for a more diverse field of applications. Integrating RS and OCT into a single instrument allows for both structural imaging and biochemical interrogation of tissues and therefore offers a more comprehensive means for clinical diagnosis. This review summarizes the efforts made to date toward combining spontaneous RS-OCT instrumentation for biomedical analysis, including insights into primary design considerations and data interpretation.

Development of a coaxial DCF-GRIN fiberoptic Raman probe for enhancing in vivo epithelial tissue Raman measurements

We report on the development of a novel, to the best of our knowledge, coaxial double-clad-fiber (DCF) and graded-index (GRIN) fiberoptic Raman probe for enhancing epithelial tissue Raman measurements in vivo. The ultra-thin (140 µm outer diameter) DCF-GRIN fiberoptic Raman probe is designed and fabricated with an efficient coaxial optical configuration, whereby a GRIN fiber is spliced onto the DCF to enhance both the excitation/collection efficiency and depth-resolved selectivity. We demonstrate that the DCF-GRIN Raman probe can be used to acquire high-quality in vivo Raman spectra from various oral tissues (e.g., buccal mucosa, labial mucosa, gingiva, mouth floor, palate, and tongue) covering both the fingerprint (800-1800 cm^-1) and high-wavenumber (2800-3600 cm^-1) regions within sub-seconds. The subtle biochemical differences between different epithelial tissues in the oral cavity can also be detected with high sensitivity, suggesting the potential of the DCF-GRIN fiberoptic Raman probe for in vivo diagnosis and characterization in epithelial tissue.

Multimodal Handheld Probe for Characterizing Otitis Media - Integrating Raman Spectroscopy and Optical Coherence Tomography

Otitis media (OM) is a common disease of the middle ear, affecting 80% of children before the age of three. The otoscope, a simple illuminated magnifier, is the standard clinical diagnostic tool to observe the middle ear. However, it has limited contrast to detect signs of infection, such as clearly identifying and characterizing middle ear fluid or biofilms that accumulate within the middle ear. Likewise, invasive sampling of every subject is not clinically indicated nor practical. Thus, collecting accurate noninvasive diagnostic factors is vital for clinicians to deliver a precise diagnosis and effective treatment regimen. To address this need, a combined benchtop Raman spectroscopy (RS) and optical coherence tomography (OCT) system was developed. Together, RS-OCT can non-invasively interrogate the structural and biochemical signatures of the middle ear under normal and infected conditions.In this paper, in vivo RS scans from pediatric clinical human subjects presenting with OM were evaluated in parallel with RS-OCT data of physiologically relevant in vitro ear models. Component-level characterization of a healthy tympanic membrane and malleus bone, as well as OM-related middle ear fluid, identified the optimal position within the ear for RS-OCT data collection. To address the design challenges in developing a system specific to clinical use, a prototype non-contact multimodal handheld probe was built and successfully tested in vitro. Design criteria have been developed to successfully address imaging constraints imposed by physiological characteristics of the ear and optical safety limits. Here, we present the pathway for translation of RS-OCT for non-invasive detection of OM.

Co-localized line-field confocal optical coherence tomography and confocal Raman microspectroscopy for three-dimensional high-resolution morphological and molecular characterization of skin tissues ex vivo

Line-field confocal optical coherence tomography (LC-OCT) is an optical modality that provides three-dimensional (3D) images of the skin at cellular resolution. Confocal Raman microspectroscopy (CRM) is a label-free optical technique that can provide point measurement of the molecular content of the skin. This work presents a method to co-localize LC-OCT and CRM acquisitions for morpho-molecular analysis of ex vivo skin tissues at cellular level. The co-localization method allows acquisition of Raman spectra at specific locations in a sample identified from a 3D LC-OCT image, with an accuracy of ± 20 µm. The method was applied to the characterization of tattooed skin biopsies with adverse tattoo reactions. LC-OCT images allowed to target specific regions in the biopsies where the presence of tattoo ink was revealed by detection of the Raman signature of ink pigments. Micrometer-sized foreign bodies of various materials as well as inflammatory cells were also identified within the biopsies. From these results, we demonstrate the value of the LC-OCT-CRM co-localization method and its potential for future ex vivo analysis of suspicious skin lesions.

Optical coherence tomography-guided confocal Raman microspectroscopy for rapid measurements in tissues

We report a joint system with both confocal Raman spectroscopy (CRS) and optical coherence tomography (OCT) modules capable of quickly addressing the region of interest in a tissue for targeted Raman measurements from OCT. By using an electrically tunable lens in the Raman module, the focus of the module can be adjusted to address any specific depth indicated in an OCT image in a few milliseconds. We demonstrate the performance of the joint system in the depth dependent measurements of an ex vivo swine tissue and in vivo human skin. This system can be useful in measuring samples embedded with small targets, for example, to identify tumors in skin in vivo and assessment of tumor margins, in which OCT can be used to perform initial real-time screening with high throughput based on morphological features to identify suspicious targets then CRS is guided to address the targets in real time and fully characterize their biochemical fingerprints for confirmation.

Looking for a perfect match: multimodal combinations of Raman spectroscopy for biomedical applications

Raman spectroscopy has shown very promising results in medical diagnostics by providing label-free and highly specific molecular information of pathological tissue ex vivo and in vivo. Nevertheless, the high specificity of Raman spectroscopy comes at a price, i.e., low acquisition rate, no direct access to depth information, and limited sampling areas. However, a similar case regarding advantages and disadvantages can also be made for other highly regarded optical modalities, such as optical coherence tomography, autofluorescence imaging and fluorescence spectroscopy, fluorescence lifetime microscopy, second-harmonic generation, and others. While in these modalities the acquisition speed is significantly higher, they have no or only limited molecular specificity and are only sensitive to a small group of molecules. It can be safely stated that a single modality provides only a limited view on a specific aspect of a biological specimen and cannot assess the entire complexity of a sample. To solve this issue, multimodal optical systems, which combine different optical modalities tailored to a particular need, become more and more common in translational research and will be indispensable diagnostic tools in clinical pathology in the near future. These systems can assess different and partially complementary aspects of a sample and provide a distinct set of independent biomarkers. Here, we want to give an overview on the development of multimodal systems that use RS in combination with other optical modalities to improve the diagnostic performance.

Noninvasive and real-time in vivo characterization of Inflammation skin. A feasibility of animal study

BACKGROUND

Inflammatory skin diseases were the most common problem in dermatology. This study aimed to develop a circuit by using a simple method for noninvasive, objective, and real-time skin inflammation screening.

MATERIALS AND METHODS

Sprague-Dawley rats were used in this study. The rats were chemically induced to suffer from skin inflammation at the back of their left-hand side while the right-hand side of their back remained untreated serving as a control. Impedance (Z) spectrum of the rat's skin was recorded.

RESULTS

Two characteristic frequencies (4.5 and 48.3 kHz) were found. At the two frequencies, the impedance of inflammatory skin tissue (Z_IST ) was found to be significantly (P < .05) smaller than that of normal healthy skin tissue (Z_NHST ). Moreover, the ratio of the impedance measured at 4.5 kHz (Z_f = 4 ._5 kHz ) to the impedance measured at 48.3 kHz (Z_{f = 48.3 kHz} ), that is, Z_{f = 4.5 kHz} /Z_{f = 48.3 kHz} , was capable of skin inflammation screening. It was observed that the inflammatory skin tissue (IST) had the smaller value of Z_f = 4 ._5 kHz /Z_{f = 48.3 kHz} (value < 8.5) and normal healthy skin tissue (NHST) had the higher value of Z_f = 4 ._5 kHz /Z_{f = 48.3 kHz} (value ≈ 10) which almost remained constant.

CONCLUSION

A circuit was developed which was used for measuring the skin impedance accurately at the two characteristic frequencies for skin inflammation screening.

Introduction to Infrared and Raman-Based Biomedical Molecular Imaging and Comparison with Other Modalities

Molecular imaging has rapidly developed to answer the need of image contrast in medical diagnostic imaging to go beyond morphological information to include functional differences in imaged tissues at the cellular and molecular levels. Vibrational (infrared (IR) and Raman) imaging has rapidly emerged among the molecular imaging modalities available, due to its label-free combination of high spatial resolution with chemical specificity. This article presents the physical basis of vibrational spectroscopy and imaging, followed by illustration of their preclinical in vitro applications in body fluids and cells, ex vivo tissues and in vivo small animals and ending with a brief discussion of their clinical translation. After comparing the advantages and disadvantages of IR/Raman imaging with the other main modalities, such as magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography/single-photon emission-computed tomography (PET/SPECT), ultrasound (US) and photoacoustic imaging (PAI), the design of multimodal probes combining vibrational imaging with other modalities is discussed, illustrated by some preclinical proof-of-concept examples.

Deep learning architecture "LightOCT" for diagnostic decision support using optical coherence tomography images of biological samples

Optical coherence tomography (OCT) is being increasingly adopted as a label-free and non-invasive technique for biomedical applications such as cancer and ocular disease diagnosis. Diagnostic information for these tissues is manifest in textural and geometric features of the OCT images, which are used by human expertise to interpret and triage. However, it suffers delays due to the long process of the conventional diagnostic procedure and shortage of human expertise. Here, a custom deep learning architecture, LightOCT, is proposed for the classification of OCT images into diagnostically relevant classes. LightOCT is a convolutional neural network with only two convolutional layers and a fully connected layer, but it is shown to provide excellent training and test results for diverse OCT image datasets. We show that LightOCT provides 98.9% accuracy in classifying 44 normal and 44 malignant (invasive ductal carcinoma) breast tissue volumetric OCT images. Also, >96% accuracy in classifying public datasets of ocular OCT images as normal, age-related macular degeneration and diabetic macular edema. Additionally, we show ∼96% test accuracy for classifying retinal images as belonging to choroidal neovascularization, diabetic macular edema, drusen, and normal samples on a large public dataset of more than 100,000 images. The performance of the architecture is compared with transfer learning based deep neural networks. Through this, we show that LightOCT can provide significant diagnostic support for a variety of OCT images with sufficient training and minimal hyper-parameter tuning. The trained LightOCT networks for the three-classification problem will be released online to support transfer learning on other datasets.

Photoacoustic Imaging of Tattoo Inks: Phantom and Clinical Evaluation

Photoacoustic imaging (PAI) is a novel hybrid imaging modality that provides excellent optical contrast with the spatial resolution of ultrasound in vivo. The method is widely being investigated in the clinical setting for diagnostic applications in dermatology. In this report, we illustrate the utility of PAI as a non-invasive tool for imaging tattoos. Ten different samples of commercially available tattoo inks were examined for their optoacoustic properties in vitro. In vivo PAI of an intradermal tattoo on the wrist was performed in a healthy human volunteer. Black/gray, green, violet and blue colored pigments provided higher levels of PA signal compared to white, orange, red and yellow pigments in vitro. PAI provided excellent contrast and enabled accurate delineation of the extent of the tattoo in the dermis. Our results reveal the photoacoustic properties of tattoo inks and demonstrate the potential clinical utility of PAI for intradermal imaging of tattoos. PAI may be useful as a clinical adjunct for objective preoperative evaluation of tattoos and potentially to guide/monitor laser-based tattoo removal procedures.

Raman Spectroscopy: Guiding Light for the Extracellular Matrix

The extracellular matrix (ECM) consists of a complex mesh of proteins, glycoproteins, and glycosaminoglycans, and is essential for maintaining the integrity and function of biological tissues. Imaging and biomolecular characterization of the ECM is critical for understanding disease onset and for the development of novel, disease-modifying therapeutics. Recently, there has been a growing interest in the use of Raman spectroscopy to characterize the ECM. Raman spectroscopy is a label-free vibrational technique that offers unique insights into the structure and composition of tissues and cells at the molecular level. This technique can be applied across a broad range of ECM imaging applications, which encompass in vitro, ex vivo, and in vivo analysis. State-of-the-art confocal Raman microscopy imaging now enables label-free assessments of the ECM structure and composition in tissue sections with a remarkably high degree of biomolecular specificity. Further, novel fiber-optic instrumentation has opened up for clinical in vivo ECM diagnostic measurements across a range of tissue systems. A palette of advanced computational methods based on multivariate statistics, spectral unmixing, and machine learning can be applied to Raman data, allowing for the extraction of specific biochemical information of the ECM. Here, we review Raman spectroscopy techniques for ECM characterizations over a variety of exciting applications and tissue systems, including native tissue assessments (bone, cartilage, cardiovascular), regenerative medicine quality assessments, and diagnostics of disease states. We further discuss the challenges in the widespread adoption of Raman spectroscopy in biomedicine. The results of the latest discovery-driven Raman studies are summarized, illustrating the current and potential future applications of Raman spectroscopy in biomedicine.

Micro-Raman spectroscopy in medicine

A potential role of optical technologies in medicine including micro-Raman spectroscopy is diagnosis of bacteria, cells and tissues which is covered in this chapter. The main advantage of Raman-based methods to complement and augment diagnostic tools is that unsurpassed molecular specificity is achieved without labels and in a nondestructive way. Principles and applications of micro-Raman spectroscopy in the context of medicine will be described. First, Raman spectra of biomolecules representing proteins, nucleic acids, lipids and carbohydrates are introduced. Second, microbial applications are summarized with the focus on typing on species and strain level, detection of infections, antibiotic resistance and biofilms. Third, cytological applications are presented to classify single cells and study cell metabolism and drug–cell interaction. Fourth, applications to tissue characterization start with discussion of lateral resolution for Raman imaging followed by Raman-based detection of pathologies and combination with other modalities. Finally, an outlook is given to translate micro-Raman spectroscopy as a clinical tool to solve unmet needs in point-of-care applications and personalized treatment of diseases.

In vivo quantitative molecular absorption of glycerol in human skin using coherent anti-Stokes Raman scattering (CARS) and two-photon auto-fluorescence

The penetration of small molecules through the human skin is a major issue for both safety and efficacy issues in cosmetics and pharmaceutic domains. To date, the quantification of active molecular compounds in human skin following a topical application uses ex vivo skin samples mounted on Franz cell diffusion set-up together with appropriate analytical methods. Coherent anti-Stokes Raman scattering (CARS) has also been used to perform active molecule quantification on ex vivo skin samples, but no quantification has been described in human skin in vivo. Here we introduce and validate a framework for imaging and quantifying the active molecule penetration into human skin in vivo. Our approach combines nonlinear imaging microscopy modalities, such as two-photon excited auto-fluorescence (TPEF) and coherent anti-Stokes Raman scattering (CARS), together with the use of deuterated active molecules. The imaging framework was exemplified on topically applied glycerol diluted in various vehicles such as water and xanthan gel. In vivo glycerol quantitative percutaneous penetration over time was demonstrated, showing that, contrary to water, the xanthan gel vehicle acts as a film reservoir that releases glycerol continuously over time. More generally, the proposed imaging framework provides an enabling platform for establishing functional activity of topically applied products in vivo.

Raman Spectroscopy and Microscopy Applications in Cardiovascular Diseases: From Molecules to Organs

Noninvasive and label-free vibrational spectroscopy and microscopy methods have shown great potential for clinical diagnosis applications. Raman spectroscopy is based on inelastic light scattering due to rotational and vibrational modes of molecular bonds. It has been shown that Raman spectra provide chemical signatures of changes in biological tissues in different diseases, and this technique can be employed in label-free monitoring and clinical diagnosis of several diseases, including cardiovascular studies. However, there are very few literature reviews available to summarize the state of art and future applications of Raman spectroscopy in cardiovascular diseases, particularly cardiac hypertrophy. In addition to conventional clinical approaches such as electrocardiography (ECG), echocardiogram (cardiac ultrasound), positron emission tomography (PET), cardiac computed tomography (CT), and single photon emission computed tomography (SPECT), applications of vibrational spectroscopy and microscopy will provide invaluable information useful for the prevention, diagnosis, and treatment of cardiovascular diseases. Various in vivo and ex vivo investigations can potentially be performed using Raman imaging to study and distinguish pathological and physiological cardiac hypertrophies and understand the mechanisms of other cardiac diseases. Here, we have reviewed the recent literature on Raman spectroscopy to study cardiovascular diseases covering investigations on the molecular, cellular, tissue, and organ level.

Classification of burn injury using Raman spectroscopy and optical coherence tomography: An ex-vivo study on porcine skin

Accurate depth assessment of burn wounds is a critical task to provide the right treatment and care. Currently, laser Doppler imaging is able to provide better accuracy compared to the standard clinical evaluation. However, its clinical applicability is limited by factors like scanning distance, time, and cost. Precise diagnosis of burns requires adequate structural and functional details. In this work, we evaluated the combined potential of two non-invasive optical modalities, optical coherence tomography (OCT) and Raman spectroscopy (RS), to identify degrees of burn wounds (superficial partial-thickness (SPT), deep partial-thickness (DPT), and full-thickness (FT)). OCT provides morphological information, whereas, RS provides biochemical aspects. OCT images and Raman spectra were obtained from burns created on ex-vivo porcine skin. Algorithms were developed to segment skin region and extract textural features from OCT images, and derive spectral wave features from RS. These computed features were fed into machine learning classifiers for categorization of burns. Histological results obtained from trichrome staining were used as ground-truth. The combined performance of RS-OCT reported an overall average accuracy of 85% and ROC-AUC=0.94, in distinguishing the burn wounds. The significant performance on ex vivo skin motivates to assess the feasibility of combined RS-OCT in in vivo models.

In-vivo Raman spectroscopy: from basics to applications

For more than two decades, Raman spectroscopy has found widespread use in biological and medical applications. The instrumentation and the statistical evaluation procedures have matured, enabling the lengthy transition from ex-vivo demonstration to in-vivo examinations. This transition goes hand-in-hand with many technological developments and tightly bound requirements for a successful implementation in a clinical environment, which are often difficult to assess for novice scientists in the field. This review outlines the required instrumentation and instrumentation parameters, designs, and developments of fiber optic probes for the in-vivo applications in a clinical setting. It aims at providing an overview of contemporary technology and clinical trials and attempts to identify future developments necessary to bring the emerging technology to the clinical end users. A comprehensive overview of in-vivo applications of fiber optic Raman probes to characterize different tissue and disease types is also given.

Trimodal system for in vivo skin cancer screening with combined optical coherence tomography-Raman and colocalized optoacoustic measurements

A new multimodal system for rapid, noninvasive in vivo skin cancer screening is presented, combining optical coherence tomography (OCT) and optoacoustic (OA) modalities to provide precise tumor depth determination with a Raman spectroscopic modality capable of detecting the lesion type and, thus, providing diagnostic capability. Both OA and Raman setups use wide field skin illumination to ensure the compliance with maximum permissible exposure (MPE) requirements. The Raman signal is collected via the OCT scanning lens to maximize the signal-to-noise ratio of the measured signal while keeping radiation levels below MPE limits. OCT is used to optically determine the tumor thickness and for volumetric imaging whereas OA utilizes acoustic signals generated by optical absorption contrast for thickness determination at potentially higher penetration depths compared to OCT. Preliminary results of first clinical trials using our setup are presented. The measured lesion depth is in good agreement with histology results, while Raman measurements show distinctive differences between normal skin and melanocytic lesions, and, moreover, between different skin areas. In future, we will validate the setup presented for reliable detection of pathophysiological parameters, morphology and thickness of suspicious skin lesions.

Depth-resolved multimodal imaging: Wavelength modulated spatially offset Raman spectroscopy with optical coherence tomography

A major challenge in biophotonics is multimodal imaging to obtain both morphological and molecular information at depth. We demonstrate a hybrid approach integrating optical coherence tomography (OCT) with wavelength modulated spatially offset Raman spectroscopy (WM-SORS). With depth colocalization obtained from the OCT, we can penetrate 1.2-mm deep into strong scattering media (lard) to acquire up to a 14-fold enhancement of a Raman signal from a hidden target (polystyrene) with a spatial offset. Our approach is capable of detecting both Raman and OCT signals for pharmaceutical particles embedded in turbid media and revealing the white matter at depth within a 0.6-mm thick brain tissue layer. This depth resolved label-free multimodal approach is a powerful route to analyze complex biomedical samples.

Non-invasive diagnostic system and its opto-mechanical probe for combining confocal Raman spectroscopy and optical coherence tomography

Non-invasive optical diagnostic methods allow important information about studied systems to be obtained in a non-destructive way. Complete diagnosis requires information about the chemical composition as well as the morphological structure of a sample. We report on the development of an opto-mechanical probe that combines Raman spectroscopy (RS) and optical coherence tomography (OCT), two methods that provide all the crucial information needed for a non-invasive diagnosis. The aim of this paper is to introduce the technical design, construction and optimization of a dual opto-mechanical probe combining two in-house developed devices for confocal RS and OCT. The unique benefit of the probe is a gradual acquisition of OCT and RS data, which allows to use the acquired OCT images to pinpoint locations of interest for RS measurements. The parameters and the correct functioning of the probe were verified by RS scanning of various samples (silicon wafer and ex vivo tissue) based on their OCT images - lateral as well as depth scanning was performed. Both the OCT and RS systems were developed, optimized and tested with the ultimate aim of verifying the functionality of the probe. Picture: Schematic illustration and visualization of the developed RS-OCT probe.

Development of a combined OCT-Raman probe for the prospective in vivo clinical melanoma skin cancer screening

A combined optical coherence tomography (OCT)-Raman probe was designed and built into a spectral domain OCT head, and its performance was evaluated and compared to the most common Raman probe setups, based on a fiber bundle and confocal free space optics. Due to the use of the full field of view of an OCT scanning lens, the combined probe has a superior performance within maximum permissible exposure limits, compared to the other two probes. Skin Raman spectra, recorded in vivo, further prove the feasibility of the OCT-Raman probe for the future in vivo clinical applications in skin cancer screening.

Raman Plus X: Biomedical Applications of Multimodal Raman Spectroscopy

Raman spectroscopy is a label-free method of obtaining detailed chemical information about samples. Its compatibility with living tissue makes it an attractive choice for biomedical analysis, yet its translation from a research tool to a clinical tool has been slow, hampered by fundamental Raman scattering issues such as long integration times and limited penetration depth. In this review we detail the how combining Raman spectroscopy with other techniques yields multimodal instruments that can help to surmount the translational barriers faced by Raman alone. We review Raman combined with several optical and non-optical methods, including fluorescence, elastic scattering, OCT, phase imaging, and mass spectrometry. In each section we highlight the power of each combination along with a brief history and presentation of representative results. Finally, we conclude with a perspective detailing both benefits and challenges for multimodal Raman measurements, and give thoughts on future directions in the field.

Label-Free Molecular Imaging of Biological Cells and Tissues by Linear and Nonlinear Raman Spectroscopic Approaches

Raman spectroscopy is an emerging technique in bioanalysis and imaging of biomaterials owing to its unique capability of generating spectroscopic fingerprints. Imaging cells and tissues by Raman microspectroscopy represents a nondestructive and label-free approach. All components of cells or tissues contribute to the Raman signals, giving rise to complex spectral signatures. Resonance Raman scattering and surface-enhanced Raman scattering can be used to enhance the signals and reduce the spectral complexity. Raman-active labels can be introduced to increase specificity and multimodality. In addition, nonlinear coherent Raman scattering methods offer higher sensitivities, which enable the rapid imaging of larger sampling areas. Finally, fiber-based imaging techniques pave the way towards in vivo applications of Raman spectroscopy. This Review summarizes the basic principles behind medical Raman imaging and its progress since 2012.

Multimodal discrimination of immune cells using a combination of Raman spectroscopy and digital holographic microscopy

The ability to identify and characterise individual cells of the immune system under label-free conditions would be a significant advantage in biomedical and clinical studies where untouched and unmodified cells are required. We present a multi-modal system capable of simultaneously acquiring both single point Raman spectra and digital holographic images of single cells. We use this combined approach to identify and discriminate between immune cell populations CD4+ T cells, B cells and monocytes. We investigate several approaches to interpret the phase images including signal intensity histograms and texture analysis. Both modalities are independently able to discriminate between cell subsets and dual-modality may therefore be used a means for validation. We demonstrate here sensitivities achieved in the range of 86.8% to 100%, and specificities in the range of 85.4% to 100%. Additionally each modality provides information not available from the other providing both a molecular and a morphological signature of each cell.

Multimodal optical coherence tomography and fluorescence lifetime imaging with interleaved excitation sources for simultaneous endogenous and exogenous fluorescence

Multimodal imaging probes a variety of tissue properties in a single image acquisition by merging complimentary imaging technologies. Exploiting synergies amongst the data, algorithms can be developed that lead to better tissue characterization than could be accomplished by the constituent imaging modalities taken alone. The combination of optical coherence tomography (OCT) with fluorescence lifetime imaging microscopy (FLIM) provides access to detailed tissue morphology and local biochemistry. The optical system described here merges 1310 nm swept-source OCT with time-domain FLIM having excitation at 355 and 532 nm. The pulses from 355 and 532 nm lasers have been interleaved to enable simultaneous acquisition of endogenous and exogenous fluorescence signals, respectively. The multimodal imaging system was validated using tissue phantoms. Nonspecific tagging with Alexa Flour 532 in a Watanbe rabbit aorta and active tagging of the LOX-1 receptor in human coronary artery, demonstrate the capacity of the system for simultaneous acquisition of OCT, endogenous FLIM, and exogenous FLIM in tissues.

Pseudo-HE images derived from CARS/TPEF/SHG multimodal imaging in combination with Raman-spectroscopy as a pathological screening tool

Background

Due to the steadily increasing number of cancer patients worldwide the early diagnosis and treatment of cancer is a major field of research. The diagnosis of cancer is mostly performed by an experienced pathologist via the visual inspection of histo-pathological stained tissue sections. To save valuable time, low quality cryosections are frequently analyzed with diagnostic accuracies that are below those of high quality embedded tissue sections. Thus, alternative means have to be found that enable for fast and accurate diagnosis as the basis of following clinical decision making.

Methods

In this contribution we will show that the combination of the three label-free non-linear imaging modalities CARS (coherent anti-Stokes Raman-scattering), TPEF (two-photon excited autofluorescence) and SHG (second harmonic generation) yields information that can be translated into computational hematoxylin and eosin (HE) images by multivariate statistics. Thereby, a computational HE stain is generated resulting in pseudo-HE overview images that allow for identification of suspicious regions. The latter are analyzed further by Raman-spectroscopy retrieving the tissue’s molecular fingerprint.

Results

The results suggest that the combination of non-linear multimodal imaging and Raman-spectroscopy possesses the potential as a precise and fast tool in routine histopathology.

Conclusions

As the key advantage, both optical methods are non-invasive enabling for further pathological investigations of the same tissue section, e.g. a direct comparison with the current pathological gold-standard.

Development of a hybrid Raman spectroscopy and optical coherence tomography technique for real-time in vivo tissue measurements

We report on the development of a unique sideview handheld hybrid Raman spectroscopy (RS) and optical coherence tomography (OCT) technique for real-time in vivo tissue measurements. A sideview handheld RS-OCT optical probe is designed to coalign the optical paths of RS and OCT sampling arms, whereby a compact long-pass dichroic mirror (LPDM) is utilized to transmit the OCT signal through a gradient index rod lens and a reflection mirror, whereas the LPDM deflects the tissue Raman signal by 90°, leading to coaligned RS/OCT optical samplings on the tissue. Further study shows that the hybrid RS and OCT technique developed is capable of simultaneously acquiring both morphological and biochemical information about the oral tissue in vivo, facilitating real-time, in vivo tissue diagnoses and characterizations in the oral cavity.

Combined fiber probe for fluorescence lifetime and Raman spectroscopy

In this contribution we present a dual modality fiber optic probe combining fluorescence lifetime imaging (FLIm) and Raman spectroscopy for in vivo endoscopic applications. The presented multi-spectroscopy probe enables efficient excitation and collection of fluorescence lifetime signals for FLIm in the UV/visible wavelength region, as well as of Raman spectra in the near-IR for simultaneous Raman/FLIm imaging. The probe was characterized in terms of its lateral resolution and distance dependency of the Raman and FLIm signals. In addition, the feasibility of the probe for in vivo FLIm and Raman spectral characterization of tissue was demonstrated. Graphical Abstract An image comparison between FLIm and Raman spectroscopy acquired with the bimodal probe onseveral tissue samples.

In vivo study for the discrimination of cancerous and normal skin using fibre probe-based Raman spectroscopy

Raman spectroscopy has proved its capability as an objective, non-invasive tool for the detection of various melanoma and non-melanoma skin cancers (NMSC) in a number of studies. Most publications are based on a Raman microspectroscopic ex vivo approach. In this in vivo clinical evaluation, we apply Raman spectroscopy using a fibre-coupled probe that allows access to a multitude of affected body sites. The probe design is optimized for epithelial sensitivity, whereby a large part of the detected signal originates from within the epidermal layer's depth down to the basal membrane where early stages of skin cancer develop. Data analysis was performed on measurements of 104 subjects scheduled for excision of lesions suspected of being malignant melanoma (MM) (n = 36), basal cell carcinoma (BCC) (n = 39) and squamous cell carcinoma (SCC) (n = 29). NMSC were discriminated from normal skin with a balanced accuracy of 73% (BCC) and 85% (SCC) using partial least squares discriminant analysis (PLS-DA). Discriminating MM and pigmented nevi (PN) resulted in a balanced accuracy of 91%. These results lie within the range of comparable in vivo studies and the accuracies achieved by trained dermatologists using dermoscopy. Discrimination proved to be unsuccessful between cancerous lesions and suspicious lesions that had been histopathologically verified as benign by dermoscopy.

Advances in the in Vivo Raman Spectroscopy of Malignant Skin Tumors Using Portable Instrumentation

Raman spectroscopy has emerged as a promising tool for real-time clinical diagnosis of malignant skin tumors offering a number of potential advantages: it is non-intrusive, it requires no sample preparation, and it features high chemical specificity with minimal water interference. However, in vivo tissue evaluation and accurate histopathological classification remain a challenging task for the successful transition from laboratory prototypes to clinical devices. In the literature, there are numerous reports on the applications of Raman spectroscopy to biomedical research and cancer diagnostics. Nevertheless, cases where real-time, portable instrumentations have been employed for the in vivo evaluation of skin lesions are scarce, despite their advantages in use as medical devices in the clinical setting. This paper reviews the advances in real-time Raman spectroscopy for the in vivo characterization of common skin lesions. The translational momentum of Raman spectroscopy towards the clinical practice is revealed by (i) assembling the technical specifications of portable systems and (ii) analyzing the spectral characteristics of in vivo measurements.

Co-localized confocal Raman spectroscopy and optical coherence tomography (CRS-OCT) for depth-resolved analyte detection in tissue

We report the development of a combined confocal Raman spectroscopy (CRS) and optical coherence tomography (OCT) instrument (CRS-OCT) capable of measuring analytes in targeted biological tissues with sub-100-micron spatial resolution. The OCT subsystem was used to measure depth-resolved tissue morphology and guide the acquisition of chemically-specific Raman spectra. To demonstrate its utility, the instrument was used to accurately measure depth-resolved, physiologically-relevant concentrations of Tenofovir, a microbicide drug used to prevent the sexual transmission of HIV, in ex vivo tissue samples.

Multimodal Imaging Spectroscopy of Tissue

Advanced optical imaging technologies have experienced increased visibility in medical research, as they allow for a label-free and nondestructive investigation of tissue in either an excised state or living organisms. In addition to a multitude of ex vivo studies proving the applicability of these optical imaging approaches, a transfer of various modalities toward in vivo diagnosis is currently in progress as well. Furthermore, combining optical imaging techniques, referred to as multimodal imaging, allows for an improved diagnostic reliability due to the complementary nature of retrieved information. In this review, we provide a summary of ongoing multifold efforts in multimodal tissue imaging and focus in particular on in vivo applications for medical diagnosis. We also discuss the advantages and potential limitations of the imaging methods and outline opportunities for future developments.

Automated identification of basal cell carcinoma by polarization-sensitive optical coherence tomography

We report an automated classifier to detect the presence of basal cell carcinoma in images of mouse skin tissue samples acquired by polarization-sensitive optical coherence tomography (PS-OCT). The sensitivity and specificity of the classifier based on combined information of the scattering intensity and birefringence properties of the samples are significantly higher than when intensity or birefringence information are used alone. The combined information offers a sensitivity of 94.4% and specificity of 92.5%, compared to 78.2% and 82.2% for intensity-only information and 85.5% and 87.9% for birefringence-only information. These results demonstrate that analysis of the combination of complementary optical information obtained by PS-OCT has great potential for accurate skin cancer diagnosis.

Hybrid single-source online Fourier transform coherent anti-Stokes Raman scattering/optical coherence tomography

We demonstrate a multimodal optical coherence tomography (OCT) and online Fourier transform coherent anti-Stokes Raman scattering (FTCARS) platform using a single sub-12 femtosecond (fs) Ti:sapphire laser enabling simultaneous extraction of structural and chemical ("morphomolecular") information of biological samples. Spectral domain OCT prescreens the specimen providing a fast ultrahigh (4×12  μm axial and transverse) resolution wide field morphologic overview. Additional complementary intrinsic molecular information is obtained by zooming into regions of interest for fast label-free chemical mapping with online FTCARS spectroscopy. Background-free CARS is based on a Michelson interferometer in combination with a highly linear piezo stage, which allows for quick point-to-point extraction of CARS spectra in the fingerprint region in less than 125 ms with a resolution better than 4  cm(-1) without the need for averaging. OCT morphology and CARS spectral maps indicating phosphate and carbonate bond vibrations from human bone samples are extracted to demonstrate the performance of this hybrid imaging platform.

Depth-sensitive Raman spectroscopy combined with optical coherence tomography for layered tissue analysis

Complete characterization of a layered tissue requires probing both the biochemical and the morphological information from its different layers at various depths. We report the development of a combined Raman spectroscopy (RS) and optical coherence tomography (OCT) system that is capable of measuring depth-sensitive Raman signal from the tissue layers imaged by the OCT. The sample arm of a real-time time-domain OCT system was modified to allow for co-alignment of the OCT with the Raman probe beam. The depth sensitivity of Raman was obtained by incorporating confocal Raman configuration that minimized out-of-focus Raman scattered light. The system was first validated using a layered phantom prepared by depositing a thin layer of paraffin over acetaminophen. A good correlation was observed between the OCT images and the Raman signal. The system was also used to record OCT and Raman images of a resected mucosal tissue sample. While OCT image showed the presence of epithelial and stromal layers, Raman spectra measured from these layers confirmed the biochemical difference between the two.

Optical coherence tomography-current technology and applications in clinical and biomedical research

Optical coherence tomography (OCT) is a noninvasive imaging technique that provides real-time two- and three-dimensional images of scattering samples with micrometer resolution. By mapping the local reflectivity, OCT visualizes the morphology of the sample. In addition, functional properties such as birefringence, motion, or the distributions of certain substances can be detected with high spatial resolution. Its main field of application is biomedical imaging and diagnostics. In ophthalmology, OCT is accepted as a clinical standard for diagnosing and monitoring the treatment of a number of retinal diseases, and OCT is becoming an important instrument for clinical cardiology. New applications are emerging in various medical fields, such as early-stage cancer detection, surgical guidance, and the early diagnosis of musculoskeletal diseases. OCT has also proven its value as a tool for developmental biology. The number of companies involved in manufacturing OCT systems has increased substantially during the last few years (especially due to its success in opthalmology), and this technology can be expected to continue to spread into various fields of application.