Malignancies and atherosclerotic plaque diagnosis--is laser induced fluorescence spectroscopy the ultimate solution?

Development of thin skin mimicking bilayer solid tissue phantoms for optical spectroscopic studies

In vivo spectroscopic measurements have the proven potential to provide important insight about the changes in tissue during the development of malignancies and thus help to diagnose tissue pathologies. Extraction of intrinsic data in the presence of varying amounts of scatterers and absorbers offers great challenges in the development of such techniques to the clinical level. Fabrication of optical phantoms, tailored to the biochemical as well as morphological features of the target tissue, can help to generate a spectral database for a given optical spectral measurement system. Such databases, along with appropriate pattern matching algorithms, could be integrated with in vivo measurements for any desired quantitative analysis of the target tissue. This paper addresses the fabrication of such soft, photo stable, thin bilayer phantoms, mimicking skin tissue in layer dimensions and optical properties. The performance evaluation of the fabricated set of phantoms is carried out using a portable fluorescence spectral measurement system. The alterations in flavin adenine dinucleotide (FAD)-a tissue fluorophore that provides important information about dysplastic progressions in tissues associated with cancer development based on changes in emission spectra-fluorescence with varied concentrations of absorbers and scatterers present in the phantom are analyzed and the results are presented. Alterations in the emission intensity, shift in emission wavelength and broadening of the emission spectrum were found to be potential markers in the assessment of biochemical changes that occur during the progression of dysplasia.

Wide-field spectral imaging of human ovary autofluorescence and oncologic diagnosis via previously collected probe data

With no sufficient screening test for ovarian cancer, a method to evaluate the ovarian disease state quickly and nondestructively is needed. The authors have applied a wide-field spectral imager to freshly resected ovaries of 30 human patients in a study believed to be the first of its magnitude. Endogenous fluorescence was excited with 365-nm light and imaged in eight emission bands collectively covering the 400- to 640-nm range. Linear discriminant analysis was used to classify all image pixels and generate diagnostic maps of the ovaries. Training the classifier with previously collected single-point autofluorescence measurements of a spectroscopic probe enabled this novel classification. The process by which probe-collected spectra were transformed for comparison with imager spectra is described. Sensitivity of 100% and specificity of 51% were obtained in classifying normal and cancerous ovaries using autofluorescence data alone. Specificity increased to 69% when autofluorescence data were divided by green reflectance data to correct for spatial variation in tissue absorption properties. Benign neoplasm ovaries were also found to classify as nonmalignant using the same algorithm. Although applied ex vivo, the method described here appears useful for quick assessment of cancer presence in the human ovary.

Hyperspectral imaging of atherosclerotic plaques in vitro

Vulnerable plaques constitute a risk for serious heart problems, and are difficult to identify using existing methods. Hyperspectral imaging combines spectral- and spatial information, providing new possibilities for precise optical characterization of atherosclerotic lesions. Hyperspectral data were collected from excised aorta samples (n = 11) using both white-light and ultraviolet illumination. Single lesions (n = 42) were chosen for further investigation, and classified according to histological findings. The corresponding hyperspectral images were characterized using statistical image analysis tools (minimum noise fraction, K-means clustering, principal component analysis) and evaluation of reflectance/fluorescence spectra. Image analysis combined with histology revealed the complexity and heterogeneity of aortic plaques. Plaque features such as lipids and calcifications could be identified from the hyperspectral images. Most of the advanced lesions had a central region surrounded by an outer rim or shoulder-region of the plaque, which is considered a weak spot in vulnerable lesions. These features could be identified in both the white-light and fluorescence data. Hyperspectral imaging was shown to be a promising tool for detection and characterization of advanced atherosclerotic plaques in vitro. Hyperspectral imaging provides more diagnostic information about the heterogeneity of the lesions than conventional single point spectroscopic measurements.

New developments in fluorescence diagnostics

In the last decade, significant advances have been achieved in the direct viewing of the skin. Non-invasive analysis of various skin diseases in vivo has become possible by special skin display devices, allowing the physician to view the structure and properties of the skin in greater detail than can be achieved by simple visual examination. We review the last 100 years of fluorescence imaging development from clinical observation to advanced spectral imaging, addressing the role of fluorescence diagnostics (FD) in modern dermatology as well as the detection of autofluorescence.

Detection and photodynamic therapy of inflamed atherosclerotic plaques in the carotid artery of rabbits

Photodynamic therapy (PDT) has been applied in the treatment of artery restenosis following balloon injury. This study aimed to detect the accumulation of 5-aminolevulinic acid (ALA)-derived protoporphyrin IX (PpIX) in inflamed atherosclerotic plaque in rabbit model and evaluate the efficacy of PDT. The inflamed atherosclerotic plaque in the common carotid artery was produced by combination of balloon denudation injury and high cholesterol diet. After intravenous administration of ALA, the fluorescence of PpIX in plaque was detected. At the peak time, the correlation between the fluorescence intensity of PpIX and the macrophage infiltration extent in plaque was analyzed. Subsequently, PDT (635nm at 50J/cm(2)) on the atherosclerotic plaques (n=48) was performed and its effect was evaluated by histopathology and immunohistochemistry. The fluorescence intensity of PpIX in the plaque reached the peak 2h after injection and was 12 times stronger than that of adjacent normal vessel segment, and has a positive correlation with the macrophage content (r=0.794, P<0.001). Compared with the control group, the plaque area was reduced by 59% (P<0.001) at 4week after PDT, the plaque macrophage content decreased by 56% at 1week and 64% at 4week respectively, the smooth muscle cells (SMCs) was depleted by 24% at 1week (P<0.05) and collagen content increased by 44% at 4week (P<0.05). It should be pointed out that the SMC content increased by 18% after PDT at 4week compared with that at 1week (P<0.05). Our study demonstrated that the ALA-derived PpIX can be detected to reflect the macrophage content in the plaque. ALA mediated PDT could reduce macrophage content and inhibit plaque progression, indicating a promising approach to treat inflamed atherosclerotic plaques.

In vivo detection of macrophages in a rabbit atherosclerotic model by time-resolved laser-induced fluorescence spectroscopy

Accumulation of numerous macrophages in the fibrous cap is a key identifying feature of plaque inflammation and vulnerability. This study investigates the use of time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) as a potential tool for detection of macrophage foam cells in the intima of atherosclerotic plaques. Experiments were conducted in vivo on 14 New Zealand rabbits (6 control, 8 hypercholesterolemic) following aortotomy to expose the intimal luminal surface of the aorta. Tissue autofluorescence was induced with a nitrogen pulse laser (337 nm, 1 ns). Lesions were histologically classified by the percent of collagen or macrophage foam cells as well as thickness of the intima. Using parameters derived from the time-resolved fluorescence emission of plaques, we determined that intima rich in macrophage foam cells can be distinguished from intima rich in collagen with high sensitivity (>85%) and specificity (>95%). This study demonstrates, for the first time, that a time-resolved fluorescence-based technique can differentiate and demark macrophage content versus collagen content in vivo. Our results suggest that TR-LIFS technique can be used in clinical applications for identification of inflammatory cells important in plaque formation and rupture.

Noninvasive in situ evaluation of osteogenic differentiation by time-resolved laser-induced fluorescence spectroscopy

The clinical implantation of bioengineered tissues requires an in situ nondestructive evaluation of the quality of tissue constructs developed in vitro before transplantation. Time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) is demonstrated here to noninvasively monitor the formation of osteogenic extracellular matrix (ECM) produced by putative stem cells (PLA cells) derived from human adipose tissue. We show that this optical spectroscopy technique can assess the relative expression of collagens (types I, III, IV, and V) within newly forming osteogenic ECM. The results are consistent with those obtained by conventional histochemical techniques (immunofluorescence and Western blot) and demonstrate that TR-LIFS is a potential tool for monitoring the expression of distinct collagen types and the formation of collagen cross-links in intact tissue constructs.

Fiber optic probes for biomedical optical spectroscopy

Fiber optic probes are a key element for biomedical spectroscopic sensing. We review the use of fiber optic probes for optical spectroscopy, focusing on applications in turbid media, such as tissue. The design of probes for reflectance, polarized reflectance, fluorescence, and Raman spectroscopy is illustrated. We cover universal design principles as well as technologies for beam deflecting and reshaping.

Artificial neural networks for discriminating pathologic from normal peripheral vascular tissue

The identification of the state of human peripheral vascular tissue by using artificial neural networks is discussed in this paper. Two different laser emission lines (He-Cd, Ar+) are used to excite the chromophores of tissue samples. The fluorescence spectrum obtained, is passed through a nonlinear filter based on a high-order (HO) neural network neural network (NN) [HONN] whose weights are updated by stable learning laws, to perform feature extraction. The values of the feature vector reveal information regarding the tissue state. Then a classical multilayer perceptron is employed to serve as a classifier of the feature vector, giving 100% successful results for the specific data set considered. Our method achieves not only the discrimination between normal and pathologic human tissue, but also the successful discrimination between the different types of pathologic tissue (fibrous, calcified). Furthermore, the small time needed to acquire and analyze the fluorescence spectra together with the high rates of success, proves our method very attractive for real-time applications.

The role of laser-induced fluorescence in myocardial tissue characterization: an experimental in vitro study

OBJECTIVE

The fluorescence of tissue when stimulated by a laser beam is a well-known phenomenon. The resulting emission spectra depend on the biochemical and structural composition of the tissue. In this study, we examined the spectra of laser-induced fluorescence emitted by myocardial tissue.

METHODS

We used an argon-ion laser to stimulate the myocardium of 20 intact sheep hearts. For each spectral emission, we calculated the intensity in specific regions in order to characterize the spectra and to reveal intercavitary and intracavitary morphologic differences.

RESULTS

The statistical analysis showed significant differences in the emission spectra intensity between atria and ventricles. The intensity was higher in the atria than in the ventricles (p < 0.001). The atrial emission spectra were morphologically different from those of the ventricles. There was no difference in the intensity or morphology of emission spectra within each chamber. All measurements showed good reproducibility after a short period of time.

CONCLUSIONS

Laser-induced fluorescence of myocardial tissue seems to have the characteristics necessary for tissue recognition. This might prove useful in identifying cardiomyopathies and transplant rejection, as well as for myocardial mapping, assisting electrophysiologists in discovering fibrotic arrhythmogenic foci.

Single and double wavelength excitation of laser-induced fluorescence of normal and atherosclerotic peripheral vascular tissue

Laser-induced fluorescence spectra were recorded from the exposure of peripheral vascular tissue to both helium-cadmium and argon-ion laser radiation. Spectral analysis was based on simple algebraic expressions constructed using the intensity difference of the various spectral regions. The above methods were developed in order to determine the degree of atherosclerosis according to the laser-induced fluorescence signal. Similar results with single wavelength excitation were observed during in vivo irradiation of peripheral vessels.

Multispectral imaging autofluorescence microscopy for the analysis of lymph-node tissues

Although histochemical and immunohistochemical methods are the standard procedures in diagnosis of lymphoproliferative disorders, useful improvements in evidencing histopathologic manifestations can be obtained with the introduction of tissue autofluorescence analyses. We used microspectrofluorometry and a Multispectral Imaging Autofluorescence Microscopy (MIAM) technique to analyze lymph-node biopsies from patients with lymphoadenopathy of different origins. Images of tissue autofluorescence were obtained by excitation at 365 nm of lymph-node sections and sequential detection with interference filters (50 nm bandwidth) peaked at 450, 550 and 658 nm. Monochrome images were combined together in a single red-green-blue color image. Most of the fluorescence was observed within the blue spectral band because of large contributions from extracellular collagen and elastin fibers as well as from reduced form of intracellular nicotinamide adenine dinucleotide (phosphate). Autofluorescence imaging shows morphological differences between neoplastic and non-neoplastic tissues. The reactive hyperplasia samples show the typical lymph-node organization with weak fluorescent follicles separated by high fluorescent connective trabeculae. In the neoplastic lymph nodes the loss of follicle organization is observed. Consequently, MIAM permits to discriminate between non-neoplastic and neoplastic tissues on the basis of their autofluorescence pattern. Multispectral imaging of tissue autofluorescence may present some advantages with respect to standard histochemical microscopy since it (1) does not require any chemical manipulation of samples; (2) gives real-time results performing the analysis immediately upon specimen resection; and (3) supplies a representation of the biological structure organization linked to endogenous fluorophores.

Fluorescence spectroscopy of neoplastic and non-neoplastic tissues

Fast and non-invasive, diagnostic techniques based on fluorescence spectroscopy have the potential to link the biochemical and morphologic properties of tissues to individual patient care. One of the most widely explored applications of fluorescence spectroscopy is the detection of endoscopically invisible, early neoplastic growth in epithelial tissue sites. Currently, there are no effective diagnostic techniques for these early tissue transformations. If fluorescence spectroscopy can be applied successfully as a diagnostic technique in this clinical context, it may increase the potential for curative treatment, and thus, reduce complications and health care costs. Steady-state, fluorescence measurements from small tissue regions as well as relatively large tissue fields have been performed. To a much lesser extent, time-resolved, fluorescence measurements have also been explored for tissue characterization. Furthermore, sources of both intrinsic (endogenous fluorophores) and extrinsic fluorescence (exogenous fluorophores) have been considered. The goal of the current report is to provide a comprehensive review on steady-state and time-resolved, fluorescence measurements of neoplastic and non-neoplastic, biologic systems of varying degrees of complexity. First, the principles and methodology of fluorescence spectroscopy are discussed. Next, the endogenous fluorescence properties of cells, frozen tissue sections and excised and intact bulk tissues are presented; fluorescence measurements from both animal and human tissue models are discussed. This is concluded with future perspectives.

Effect of liquid-nitrogen and formalin-based conservation in the in vitro measurement of laser-induced fluorescence from peripheral vascular tissue

In order to investigate the effects of conservation in liquid nitrogen and formalin on peripheral vascular tissue (abdominal aortic, femoral, flank, ham, fibular and tibial artery tissue), laser-induced fluorescence spectra have been recorded during the exposure of these tissues to helium-cadmium and argon ion radiation. The spectral distribution of tissue fluorescence allows the development of simple algorithms based on the intensity difference in order to discriminate the tissue samples when they are fresh and after they have been stored for 24 and 28 h in liquid nitrogen or formalin.

Near infrared diffuse reflection and laser-induced fluorescence spectroscopy for myocardial tissue characterisation

In order to evaluate the potential of cardiovascular tissue characterisation using near-infrared (NIR) spectroscopy, spectra in a previously unexplored wavelength region 0.8-2.3 micron were recorded from various pig heart tissue samples in vitro: normal myocardium (with and without endo/epicardium), aorta, fatty and fibrous heart tissue. The spectra were analysed with principal component analysis (PCA), revealing several spectroscopically characteristic features enabling tissue classification. Several of the identified spectral features could be attributed to specific tissue constituents by comparing the tissue signals with spectra obtained from water, elastin, collagen and cholesterol as well as with published data. The results obtained with the NIR spectroscopy technique in terms of its potential to classify different tissue types were compared with those from laser-induced fluorescence (LIF) using 337 nm excitation. LIF and NIR spectroscopy can in combination with PCA be used to discriminate between all previously mentioned tissue groups, apart from fatty versus fibrous tissue (LIF) and aorta versus fibrous tissue (NIR), respectively. The NIR analysis was improved by focusing the PCA to the wavelength segment 2.0-2.3 microns, resulting in successful spectral characterisation of all cardiovascular tissue groups.

Spectral variations of laser-induced tissue emission during in vivo detection of malignancies in the female genital tract

A laser-based fluorescence-guided biopsy system has been developed for the screening and early detection of malignancies in the female inner/outer genital tract. Fluorescence spectra were recorded during in vivo exposure of normal and malignant tissue to He-Cd laser (442 nm) radiation. Spectral distribution of tissue natural fluorescence allowed for the development of simple algorithms, based on spectral intensity variations. A subsequent index of discrimination between normal and various malignant tissues has been calculated. This study focuses on the variability of the experimental data and the possible sources of spectral variations. These results suggest that monitoring of this index during colposcopy could enhance selective detection of the malignant tissue, reducing the risk of leaving pathologic tissue untreated during standard exploratory surgical procedures.

Ultraviolet and visible spectroscopies for tissue diagnostics: fluorescence spectroscopy and elastic-scattering spectroscopy

We review the application of fluorescence spectroscopy and elastic-scattering spectroscopy, over the ultraviolet-to-visible wavelength range, to minimally invasive medical diagnostics. The promises and hopes, as well as the difficulties, of these developing techniques are discussed.

In vivo fluorescence imaging for tissue diagnostics

Non-invasive fluorescence imaging has the potential to provide in vivo diagnostic information for many clinical specialties. Techniques have been developed over the years for simple ocular observations following UV excitation to sophisticated spectroscopic imaging using advanced equipment. Much of the impetus for research on fluorescence imaging for tissue diagnostics has come from parallel developments in photodynamic therapy of malignant lesions with fluorescent photosensitizers. However, the fluorescence of endogenous molecules (tissue autofluorescence) also plays an important role in most applications. In this paper, the possibilities of imaging tissues using fluorescence spectroscopy as a mean of tissue characterization are discussed. The various imaging techniques for extracting diagnostic information suggested in the literature are reviewed. The development of exogenous fluorophores for this purpose is also presented. Finally, the present status of clinical evaluation and future directions are discussed.

Quantitative optical spectroscopy for tissue diagnosis

The interaction of light within tissue has been used to recognize disease since the mid-1800s. The recent developments of small light sources, detectors, and fiber optic probes provide opportunities to quantitatively measure these interactions, which yield information for diagnosis at the biochemical, structural, or (patho)physiological level within intact tissues. However, because of the strong scattering properties of tissues, the reemitted optical signal is often influenced by changes in biochemistry (as detected by these spectroscopic approaches) and by physiological and pathophysiological changes in tissue scattering. One challenge of biomedical optics is to uncouple the signals influenced by biochemistry, which themselves provide specificity for identifying diseased states, from those influenced by tissue scattering, which are typically unspecific to a pathology. In this review, we describe optical interactions pursued for biomedical applications (fluorescence, fluorescence lifetime, phosphorescence, and Raman from cells, cultures, and tissues) and then provide a descriptive framework for light interaction based upon tissue absorption and scattering properties. Finally, we review important endogenous and exogenous biological chromophores and describe current work to employ these signals for detection and diagnosis of disease.