X. K. Chen

Bio: X. K. Chen is an academic researcher from Simon Fraser University. The author has contributed to research in topics: Raman scattering & Raman spectroscopy. The author has an hindex of 1, co-authored 1 publications receiving 181 citations.

Raman spectroscopy of in vivo cutaneous melanin.

Abstract: We successfully acquire the in vivo Raman spectrum of melanin from human skin using a rapid near-infrared (NIR) Raman spectrometer. The Raman signals of in vivo cutaneous melanin are similar to those observed from natural and synthetic eumelanins. The melanin Raman spectrum is dominated by two intense and broad peaks at about 1580 and 1380 cm ˛1 , which can be interpreted as originating from the in-plane stretching of the aromatic rings and the linear stretching of the C-C bonds within the rings, along with some contributions from the C-H vibrations in the methyl and methylene groups. Variations in the peak frequencies and bandwidths of these two Raman signals due to differing biological environments have been observed in melanin from different sources. The ability to ac- quire these unique in vivo melanin signals suggests that Raman spec- troscopy may be a useful clinical method for noninvasive in situ analysis and diagnosis of the skin. © 2004 Society of Photo-Optical Instrumenta-

Photonic crystals cause active colour change in chameleons

Abstract: Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid colour changes during social interactions such as male contests or courtship. It is generally interpreted that these changes are due to dispersion/aggregation of pigment-containing organelles within dermal chromatophores. Here, combining microscopy, photometric videography and photonic band-gap modelling, we show that chameleons shift colour through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition, we show that a deeper population of iridophores with larger crystals reflects a substantial proportion of sunlight especially in the near-infrared range. The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection.

Automated Autofluorescence Background Subtraction Algorithm for Biomedical Raman Spectroscopy

Abstract: A significant advantage of Raman spectroscopy as a noninvasive optical technique is its ability to detect subtle molecular or biochemical signatures within tissue. One of the major challenges for biomedical Raman spectroscopy is the removal of intrinsic autofluorescence background signals, which are usually a few orders of magnitude stronger than those arising from Raman scattering. A number of methods have been proposed for fluorescence background removal including excitation wavelength shifting, Fourier transformation, time gating, and simple or modified polynomial fitting. The single polynomial and the modified multi-polynomial fitting methods are relatively simple and effective, and thus are widely used in biological applications. However, their performance in real-time in vivo applications and low signal-to-noise ratio environments is sub-optimal. An improved automated algorithm for fluorescence removal has been developed based on modified multi-polynomial fitting, but with the addition of (1) a peak-removal procedure during the first iteration, and (2) a statistical method to account for signal noise effects. Experimental results demonstrate that this approach improves the automated rejection of the fluorescence background during real-time Raman spectroscopy and for in vivo measurements characterized by low signal-to-noise ratios.

Near-infrared autofluorescence imaging of the fundus: visualization of ocular melanin.

Abstract: PURPOSE To evaluate the origin of the near-infrared autofluorescence (AF) of the fundus detected by scanning laser ophthalmoscopy and compare the distribution of this AF with that of lipofuscin. METHODS AF [787] fundus images (excitation [Exc.] 787 nm; emission [Emi.] >800 nm) were recorded with a confocal scanning laser ophthalmoscope, in 85 normal subjects (ages: 11-77 years) and in 25 patients with AMD and other retinal diseases. Standard AF [488] images (Exc. 488 nm; Emi. >500 nm) were recorded in a subset of the population. RESULTS The fovea exhibits higher AF[787] than the perifovea in an area approximately 8 degrees in diameter, roughly equivalent to the area of higher RPE melanin seen in AF[488] and color images. The ratio of foveal to perifoveal AF[787] decreases with age (P < 0.0001) and is higher in subjects with light irides (P = 0.04). Higher AF[787] emanates from hyperpigmentation, from the choroidal pigment (nevi, outer layers) and from the pigment epithelium and stroma of the iris. Low AF[787] is observed in geographic atrophy particularly in subjects with light irides. CONCLUSIONS AF[787] originates from the RPE and to a varying degree from the choroid. Oxidized melanin, or compounds closely associated with melanin, contributes substantially to this AF, but other fluorophores cannot be excluded at this stage. Confocal AF[787] imaging may provide a new modality to visualize pathologic features of the RPE and the choroid, and, together with AF[488] imaging, offers a new tool to study biological changes associated with aging of the RPE and pathology.

Diagnostic potential of near-infrared Raman spectroscopy in the stomach: differentiating dysplasia from normal tissue

Abstract: Raman spectroscopy is a molecular vibrational spectroscopic technique that is capable of optically probing the biomolecular changes associated with diseased transformation. The purpose of this study was to explore near-infrared (NIR) Raman spectroscopy for identifying dysplasia from normal gastric mucosa tissue. A rapid-acquisition dispersive-type NIR Raman system was utilised for tissue Raman spectroscopic measurements at 785 nm laser excitation. A total of 76 gastric tissue samples obtained from 44 patients who underwent endoscopy investigation or gastrectomy operation were used in this study. The histopathological examinations showed that 55 tissue specimens were normal and 21 were dysplasia. Both the empirical approach and multivariate statistical techniques, including principal components analysis (PCA), and linear discriminant analysis (LDA), together with the leave-one-sample-out cross-validation method, were employed to develop effective diagnostic algorithms for classification of Raman spectra between normal and dysplastic gastric tissues. High-quality Raman spectra in the range of 800–1800 cm−1 can be acquired from gastric tissue within 5 s. There are specific spectral differences in Raman spectra between normal and dysplasia tissue, particularly in the spectral ranges of 1200–1500 cm−1 and 1600–1800 cm−1, which contained signals related to amide III and amide I of proteins, CH3CH2 twisting of proteins/nucleic acids, and the C=C stretching mode of phospholipids, respectively. The empirical diagnostic algorithm based on the ratio of the Raman peak intensity at 875 cm−1 to the peak intensity at 1450 cm−1 gave the diagnostic sensitivity of 85.7% and specificity of 80.0%, whereas the diagnostic algorithms based on PCA-LDA yielded the diagnostic sensitivity of 95.2% and specificity 90.9% for separating dysplasia from normal gastric tissue. Receiver operating characteristic (ROC) curves further confirmed that the most effective diagnostic algorithm can be derived from the PCA-LDA technique. Therefore, NIR Raman spectroscopy in conjunction with multivariate statistical technique has potential for rapid diagnosis of dysplasia in the stomach based on the optical evaluation of spectral features of biomolecules.

Biomedical applications of Raman and infrared spectroscopy to diagnose tissues

Abstract: The objective of the article is to review biomedical applications which became possible after the development of sensitive and high throughput Raman and Fourier transform infrared spectrometers in the past decade. Technical aspects of the instrumentation are briefly described. Then the broad range of vibrational spectroscopic applications with the focus on imaging and fiber-optical methods are discussed to study mineralized tissue (bone, teeth), skin, brain, the gastrointestinal tract (mouth, pharynx, esophagus, colon), breast, arteries, cartilage, cervix uteri, the urinary tract (prostate, bladder), lung, ocular tissue, liver, heart and spleen. Experimental studies are summarized demonstrating the possibilities and prospects of these methods in various fields of biodiagnostics to detect and characterize diseases, tumors and other pathologies. Infrared (IR) and Raman spectroscopy have recently been applied to address various biomedical is- sues. The basis for these applications is that IR and Raman spectroscopy are vibrational spectroscopic techniques capable of providing details of the chemical composition and molecular structures in cells and tissues. In principle, diseases and other pathological anomalies lead to chemical and structural changes on the molecular level which also change the vibrational spectra and which can be used as sensitive, phenotypic markers of the disease. As these spectral changes are very specific and unique, they are also called fingerprint. The advantages of the methods include that they are non-destructive and do not require extrinsic contrast-enhancing agents. Early reports in the literature regarding the utility of IR and Raman spectroscopy to biomedical problems were based on macroscopic acquisition of spectral data only at sin- gle points which required an a priori knowledge of the location or a pre-selection of the probed position. Since the inhomogeneous nature of tissue was not considered in these early studies, an accurate corre- lation between the histopathology of the sampled area and the corresponding spectra was not possible. Therefore, many of the early results were spurious and they will not be presented here. Considerable progress was made in the past ten years because high throughput and more sensitive instruments be- came available for Raman and IR microspectroscopic imaging. They enable to microscopically collect larger number of spectra from larger sample populations in less time, improving statistical significance and spatial specificity. Simultaneously, fiber-optical probes have been developed for in vivo applications.