Intravascular ultrasound combined with Raman spectroscopy to localize and quantify cholesterol and calcium salts in atherosclerotic coronary arteries

Label-Free Fiber-Optic Raman Spectroscopy for Intravascular Coronary Atherosclerosis and Plaque Detection

The rupture of atherosclerotic plaques remains one of the leading causes of morbidity and mortality worldwide. The plaques have certain pathological characteristics including a fibrous cap, inflammation, and extensive lipid deposition in a lipid core. Various invasive and noninvasive imaging techniques can interrogate structural aspects of atheroma; however, the composition of the lipid core in coronary atherosclerosis and plaques cannot be accurately detected. Fiber-optic Raman spectroscopy has the capability of in vivo rapid and accurate biomarker detection as an emerging omics technology. Previous studies demonstrated that an intravascular Raman spectroscopic technique may assess and manage the therapeutic and medication strategies intraoperatively. The Raman spectral information identified plaque depositions consisting of lipids, triglycerides, and cholesterol esters as the major components by comparing normal region and early plaque formation region with histology. By focusing on the composition of plaques, we could identify the subgroups of plaques accurately and rapidly by Raman spectroscopy. Collectively, this fiber-optic Raman spectroscopy opens up new opportunities for coronary atherosclerosis and plaque detection, which would assist optimal surgical strategy and instant postoperative decision-making. In this paper, we will review the advancement of label-free fiber-optic Raman probe spectroscopy and its applications of coronary atherosclerosis and atherosclerotic plaque detection.

Multimodal intravascular imaging technology for characterization of atherosclerosis

Early detection of vulnerable plaques is the critical step in the prevention of acute coronary events. Morphology, composition, and mechanical property of a coronary artery have been demonstrated to be the key characteristics for the identification of vulnerable plaques. Several intravascular multimodal imaging technologies providing co-registered simultaneous images have been developed and applied in clinical studies to improve the characterization of atherosclerosis. In this paper, the authors review the present system and probe designs of representative intravascular multimodal techniques. In addition, the scientific innovations, potential limitations, and future directions of these technologies are also discussed.

Multimodal Intravascular Photoacoustic and Ultrasound Imaging

The rupture of atherosclerotic plaques is the leading cause of death in developed countries. Early identification of vulnerable plaque is the essential step in preventing acute coronary events. Intravascular photoacoustic (IVPA) technology is able to visualize chemical composition of atherosclerotic plaque with high specificity and sensitivity. Integrated with intravascular ultrasound (IVUS) imaging, this multimodal intravascular IVPA/IVUS imaging technology is able to provide both structural and chemical compositions of arterial walls for detecting and characterizing atherosclerotic plaques. In this paper, we present representative multimodal IVPA/IVUS imaging systems and discuss current scientific innovations, potential limitations, and prospective improvements for characterization of coronary atherosclerosis.

Calibration transfer of a Raman spectroscopic quantification method from at-line to in-line assessment of liquid detergent compositions

The industrial production of liquid detergent compositions entails delicate balance of ingredients and process steps. In order to assure high quality and productivity in the manufacturing line, process analytical technology tools such as Raman spectroscopy are to be implemented. Marked chemical specificity, negligible water interference and high robustness are ascribed to this process analytical technique. Previously, at-line calibration models have been developed for determining the concentration levels of the being studied liquid detergents main ingredients from Raman spectra. A strategy is now proposed to transfer such at-line developed regression models to an in-line set-up, allowing real-time dosing control of the liquid detergent composition under production. To mimic in-line manufacturing conditions, liquid detergent compositions are created in a five-liter vessel with an overhead mixer. Raman spectra are continuously acquired by pumping the detergent under production via plastic tubing towards a Raman superhead probe, which is incorporated into a metal frame with a sapphire window facing the detergent fluid. Two at-line developed partial least squares (PLS) models are aimed at transferring, predicting the concentration of surfactant 1 and polymer 2 in the examined liquid detergent composition. A univariate slope/bias correction (SBC) is investigated, next to three well-acknowledged multivariate transformation methods: direct, piecewise and double-window piecewise direct standardization. Transfer is considered successful when the magnitude of the validation sets root mean square error of prediction (RMSEP) is similar to or smaller than the corresponding at-line prediction error. The transferred model offering the most promising outcome is further subjected to an exhaustive statistical evaluation, in order to appraise the applicability of the suggested calibration transfer method. Interval hypothesis tests are thereby performed for method comparison. It is illustrated that the investigated transfer approach yields satisfactory results, provided that the original at-line calibration model is thoroughly validated. Both SBC transfer models return lower RMSEP values than their corresponding original models. The surfactant 1 assay met all relevant evaluation criteria, demonstrating successful transfer to the in-line set-up. The in-line quantification of polymer 2 levels in the liquid detergent composition could not be statistically validated, due to the poorer performance of the at-line model.

Visualization of the biochemical markers of atherosclerotic plaque with the use of Raman, IR and AFM

In this work, we describe a methodology to visualize the biochemical markers of atherosclerotic plaque in cross sections of brachiocephalic arteries (BCA) taken from ApoE/LDLR(-/-) mice. The approach of the visualization of the same area of atherosclerotic plaque with the use of Raman, IR and AFM imaging enables the parallel characterisation of various features of atherosclerotic plaques. This support to the histochemical staining is utilized mainly in studies on mice models of atherosclerotic plaques, where micro and sub-micro resolutions are required. This work presents the methodology of the measurement and visualization of plaque features important for atherosclerosis development and plaques vulnerability analysis. Label-free imaging of cholesterol, cholesteryl esters, remodeled media, heme, internal elastic lamina, fibrous cap and calcification provides additional knowledge to previously presented quantitative measurements of average plaque features. AFM imaging enhanced the results obtained with the use of vibrational microspectroscopies with additional topographical information of the sample. To the best of our knowledge, this is the first work which demonstrates that co-localized measurement of atherosclerotic plaque with Raman, IR and AFM imaging provides a comprehensive insight into the biochemical markers of atherosclerotic plaques, and can be used as an integrated approach to assess vulnerability of the plaque.

Characterization of atherosclerotic plaque depositions by Raman and FTIR imaging

Spectroscopy-based imaging techniques can provide useful biochemical information about tissue samples. Here, we employ Raman and Fourier transform infrared (IR) imaging to characterize composition and constitution of atherosclerotic plaques of rabbits, fed with a high cholesterol diet. The results were compared with conventional light microscopy after staining with hematoxylin eosin, and elastica van Gieson. The spectral unmixing algorithm vertex component analysis was applied for data analysis and image reconstruction. IR microscopy allowed for differentiation between lipids and proteins in plaques of full aortic cross sections. Raman microscopy further discriminated cholesterol esters, cholesterol and triglycerides. FTIR and Raman images were recorded at a resolution near 20 micrometer per pixel for a large field of view. High resolution Raman images at 1 micrometer per pixel revealed structural details at selected regions of interest. The intima-media and the lipid-protein ratio were determined in five specimens for quantitation. These results correlate well with histopathology. The described method is a promising tool for easy and fast molecular imaging of atherosclerosis.

An FPGA-based open platform for ultrasound biomicroscopy

Ultrasound biomicroscopy (UBM) has been extensively applied to preclinical studies in small animal models. Individual animal study is unique and requires different utilization of the UBM system to accommodate different transducer characteristics, data acquisition strategies, signal processing, and image reconstruction methods. There is a demand for a flexible and open UBM platform to allow users to customize the system for various studies and have full access to experimental data. This paper presents the development of an open UBM platform (center frequency 20 to 80 MHz) for various preclinical studies. The platform design was based on a field-programmable gate array (FPGA) embedded in a printed circuit board to achieve B-mode imaging and directional pulsed-wave Doppler. Instead of hardware circuitry, most functions of the platform, such as filtering, envelope detection, and scan conversion, were achieved by FPGA programs; thus, the system architecture could be easily modified for specific applications. In addition, a novel digital quadrature demodulation algorithm was implemented for fast and accurate Doppler profiling. Finally, test results showed that the platform could offer a minimum detectable signal of 25 μV, allowing a 51 dB dynamic range at 47 dB gain, and real-time imaging at more than 500 frames/s. Phantom and in vivo imaging experiments were conducted and the results demonstrated good system performance.

Intravascular imaging of vulnerable coronary plaque: current and future concepts

Advances in coronary imaging are needed to enable the early detection of plaque segments considered to be 'vulnerable' for causing clinical events. Pathological studies have contributed to our current understanding of these vulnerable or unstable segments of plaque. Intravascular ultrasonography (IVUS) has provided insights into the morphology of atherosclerosis, the mediators of plaque progression and the factors associated with acute coronary syndrome (ACS). In addition, the demonstration of pancoronary arterial instability has highlighted that ACS involves a multifocal disease process. Various second-generation intravascular imaging technologies-employing advanced processing of ultrasound radiofrequency backscatter signals, light-based imaging, spectroscopic imaging and molecular targeting-possess inherent advantages for the identification of meaningful surrogates of plaque instability. The fusion of these imaging technologies within a single imaging catheter is likely to allow for greater synergism in image quality and early disease detection. However, natural-history studies to validate the use of these novel imaging tools for enhanced risk prediction are needed before these strategies can be incorporated into mainstream clinical practice.

Differentiating atherosclerotic plaque burden in arterial tissues using femtosecond CARS-based multimodal nonlinear optical imaging

A femtosecond CARS-based nonlinear optical microscope was used to simultaneously image extracellular structural proteins and lipid-rich structures within intact aortic tissue obtained from myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits (WHHLMI). Clear differences in the NLO microscopic images were observed between healthy arterial tissue and regions dominated by atherosclerotic lesions. In the current ex-vivo study, we present a single parameter based on intensity changes derived from multi-channel NLO image to classify plaque burden within the vessel. Using this parameter we were able to differentiate between healthy regions of the vessel and regions with plaque, as well as distinguish plaques relative to the age of the WHHLMI rabbit.

Vulnerable plaque: definition, diagnosis, and treatment

This article provides a systematic approach to vulnerable plaques. It is divided into 4 sections. The first section is devoted to definition, incidence, anatomic distribution, and clinical presentation. The second section is devoted to plaque composition, setting up the foundations to understand plaque vulnerability. The third section relates to invasive plaque imaging. The fourth section is devoted to therapy, from conservative pharmacologic options to aggressive percutaneous coronary intervention alternatives.

Current status of vulnerable plaque detection

Critical coronary stenoses have been shown to contribute to only a minority of acute coronary syndromes (ACS) and sudden cardiac death. Autopsy studies have identified a subgroup of high-risk patients with disrupted vulnerable plaque and modest stenosis. Consequently, a clinical need exists to develop methods to identify these plaques prospectively before disruption and clinical expression of disease. Recent advances in invasive and noninvasive imaging techniques have shown the potential to identify these high-risk plaques. The anatomical characteristics of the vulnerable plaque such as thin cap fibroatheroma and lipid pool can be identified with angioscopy, high frequency intravascular ultrasound, intravascular MRI, and optical coherence tomography. Efforts have also been made to recognize active inflammation in high-risk plaques using intravascular thermography. Plaque chemical composition by measuring electromagnetic radiation using spectroscopy is also an emerging technology to detect vulnerable plaques. Noninvasive imaging with MRI, CT, and PET also holds the potential to differentiate between low and high-risk plaques. However, at present none of these imaging modalities are able to detect vulnerable plaque neither has been shown to definitively predict outcome. Nevertheless in contrast, there has been a parallel development in the physiological assessment of advanced atherosclerotic coronary artery disease. Thus recent trials using fractional flow reserve in patients with modest non flow-limiting stenoses have shown that deferral of PCI with optimal medical therapy in these patients is superior to coronary intervention. Further trials are needed to provide more information regarding the natural history of high-risk but non flow-limiting plaque to establish patient-specific targeted therapy and to refine plaque stabilizing strategies in the future.

Spectroscopy to improve identification of vulnerable plaques in cardiovascular disease

Many apparent healthy persons die from cardiovascular disease, despite major advances in prevention and treatment of cardiovascular disease. Traditional cardiovascular risk factors are able to predict cardiovascular events in the long run, but fail to assess current disease activity or nearby cardiovascular events. There is a clear relation between the occurrence of cardiovascular events and the presence of so-called vulnerable plaques. These vulnerable plaques are characterized by active inflammation, a thin cap and a large lipid pool. Spectroscopy is an optical imaging technique which depicts the interaction between light and tissues, and thereby shows the biochemical composition of tissues. In recent years, impressive advances have been made in spectroscopy technology and intravascular spectroscopy is able to assess the composition of plaques of interest and thereby to identify and actually quantify plaque vulnerability. This review summarizes the current evidence for spectroscopy as a measure of plaque vulnerability and discusses the potential role of intravascular spectroscopic imaging techniques.

Biomechanical issues in endovascular device design

The biomechanical nature of the arterial system and its major disease states provides a series of challenges to treatment strategies. Endovascular device design objectives have mostly centered on short-term challenges, such as deployability and immediate restoration of reliable flow channels. The resulting design features may be at odds with long-term clinical success. In-stent restenosis, endoleaks, and loss of device structural integrity (e.g., strut fractures) are all manifestations of a lack of compatibility between the host vessel biomechanical environment and the implant design. Initial attempts to adapt device designs for increased compatibility, including drug-eluting and bioabsorbable stents, barely begin to explore the ways in which implant design can be modulated in time to minimize risk of failure. Biomechanical modeling has the potential to provide a virtual vascular environment in which new designs can be tested for their implications on long-term tissue reaction. These models will be based on high quality, highly resolved imaging information, as well as mechanobiology experiments from the cellular to the whole tissue level. These models can then be extended to incorporate biodegradation mechanics, facilitating the next generations of devices whose designs (including drug delivery profiles) change with time to enhance healing. The possibility of initiating changes in device design or drug release according to information on vascular healing (through clinical intervention or automated methods) provides the opportunity for truly individualized dynamic device design optimization.

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.

Development of a dual-modal tissue diagnostic system combining time-resolved fluorescence spectroscopy and ultrasonic backscatter microscopy

We report a tissue diagnostic system which combines two complementary techniques of time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) and ultrasonic backscatter microscopy (UBM). TR-LIFS evaluates the biochemical composition of tissue, while UBM provides tissue microanatomy and enables localization of the region of diagnostic interest. The TR-LIFS component consists of an optical fiber-based time-domain apparatus including a spectrometer, gated multichannel plate photomultiplier, and fast digitizer. It records the fluorescence with high sensitivity (nM concentration range) and time resolution as low as 300 ps. The UBM system consists of a transducer, pulser, receiving circuit, and positioning stage. The transducer used here is 45 MHz, unfocused, with axial and lateral resolutions 38 and 200 microm. Validation of the hybrid system and ultrasonic and spectroscopic data coregistration were conducted both in vitro (tissue phantom) and ex vivo (atherosclerotic tissue specimens of human aorta). Standard histopathological analysis of tissue samples was used to validate the UBM-TRLIFS data. Current results have demonstrated that spatially correlated UBM and TR-LIFS data provide complementary characterization of both morphology (necrotic core and calcium deposits) and biochemistry (collagen, elastin, and lipid features) of the atherosclerotic plaques at the same location. Thus, a combination of fluorescence spectroscopy with ultrasound imaging would allow for better identification of features associated with tissue pathologies. Current design and performance of the hybrid system suggests potential applications in clinical diagnosis of atherosclerotic plaque.

Catheters: instrumental advancements in biomedical applications of optical fibers

This review is focused on the advancements in biomedical engineering regarding the elaboration of new prototypes of optical fiber catheters to be applied in spectroscopic analysis, such as Raman and fluorescence spectroscopy. Our group has contributed to the development of new prototypes with interesting properties, such as side-viewing signal excitation and collection, distal tip with bending control, and Raman scattering minimization from the optical fiber. In addition, several groups have contributed to other new catheter-improving properties of this spectroscopic device. However, a relatively small number of studies has been published in the literature, due to industrial interest in this interdisciplinary and multidisciplinary area. To our knowledge, no review that has focused on the applications of catheters to several modes of spectroscopy has been published. In this work we revised this topic, analyzing the advancements and limitations of the recent biomedical catheters.

Evaluation of collagen in atherosclerotic plaques: the use of two coherent laser-based imaging methods

Acute coronary events such as myocardial infarction are frequently caused by the rupture of unstable atherosclerotic plaque. Collagen plays a key role in determining plaque stability. Methods to measure plaque collagen content are invaluable in detecting unstable atherosclerotic plaques. Recently, novel coherent laser-based imaging techniques, such as polarization-sensitive optical coherence tomography (PSOCT) and laser speckle imaging (LSI) have been investigated, and they provide a wealth of information related to collagen content and plaque stability. Additionally, given their potential for intravascular use, these technologies will be invaluable for improving our understanding of the natural history of plaque development and rupture and, hence, enable the detection of unstable plaques. In this article we review recent developments in these techniques and potential challenges in translating these methods into intra-arterial use in patients.

Diagnosis and treatment of coronary vulnerable plaques

Thin-capped fibroatheroma is the morphology that most resembles plaque rupture. Detection of these vulnerable plaques in vivo is essential to being able to study their natural history and evaluate potential treatment modalities and, therefore, may ultimately have an important impact on the prevention of acute myocardial infarction and death. Currently, conventional grayscale intravascular ultrasound, virtual histology and palpography data are being collected with the same catheter during the same pullback. A combination of this catheter with either thermography capability or additional imaging, such as optical coherence tomography or spectroscopy, would be an exciting development. Intravascular magnetic resonance imaging also holds much promise. To date, none of the techniques described above have been sufficiently validated and, most importantly, their predictive value for adverse cardiac events remains elusive. Very rigorous and well-designed studies are compelling for defining the role of each diagnostic modality. Until we are able to detect in vivo vulnerable plaques accurately, no specific treatment is warranted.

Evaluation of pancreatic cancer with Raman spectroscopy in a mouse model

OBJECTIVES

Detection of neoplastic changes using optical spectroscopy has been an active area of research in recent times. Raman spectroscopy is a vibrational spectroscopic technique that can be used to diagnose various tumors with high sensitivity and specificity. We evaluated the ability of Raman spectroscopy to differentiate normal pancreatic tissue from malignant tumors in a mouse model.

METHODS

We collected 920 spectra, 475 from 31 normal pancreatic tissue and 445 from 29 tumor nodules using a 785-nm near-infrared laser excitation. Discriminant function analysis was used for classification of normal and tumor samples.

RESULTS

Using principal component analysis, we were able to highlight subtle chemical differences in normal and malignant tissue. Using histopathology as the gold standard, Raman analysis gave sensitivities between 91% and 96% and specificities between 88% and 96%.

CONCLUSIONS

Raman spectroscopy along with discriminant function analysis is a useful method to detect cancerous changes in the pancreas. Pancreatic tumors were characterized by increased collagen content and decreased DNA, RNA, and lipids components compared with normal pancreatic tissue.

Use of near-infrared Raman spectroscopy for identification of atherosclerotic plaques in the carotid artery

OBJECTIVES

The aim of this work was to identify the presence of atherosclerotic plaque in the human carotid artery using near infrared Raman spectroscopy.

BACKGROUND DATA

Atherosclerosis is the most common and serious pathology of the cardiovascular system. Raman spectroscopy is an analytical tool that can be used to gather information about both the morphology and chemical composition of tissues.

METHODS

A Ti:sapphire laser operating at the near-infrared wavelength of 830 nm pumped by an argon laser was used for excitation of the samples, and the Raman scattering was detected by an optical spectrometer with a liquid-nitrogen-cooled CCD detector. Carotid artery samples were classified into five groups: normal, intimal thickening, fatty plaque, fibrous-fatty plaque, and fibrous-calcified plaque.

RESULTS

It was observed that the Raman spectrum of atheromatous plaque was different that that of normal tissue. The spectra of atheromatous plaques had bands due to the presence of cholesterol and its esters, with major bands at 1439 and 1663 cm(1), respectively. In normal tissues a peak related to C-H bending appears at 1451 cm(1). Calcified atheromatous plaques had primary bands at 961 and 1071 cm(1), which were due to the presence of phosphate and carbonate in the accumulated calcium. Peaks were seen at 1451 and 1655 cm(1) in the non-atherosclerotic tissue, which were shifted to 1439 and 1663 cm(1) in the atherosclerotic plaque.

CONCLUSIONS

Our results indicate that this technique could be used to detect the presence of atherosclerotic plaques in carotid arterial tissue.

Identification of calcifications in cardiac valves by near infrared Raman spectroscopy

OBJECTIVE

The objective of this work was to detect calcification in cardiac valves using near infrared Raman spectroscopy (NIRS). A Ti:sapphire laser pumped by an argon-ion laser operating at a wavelength of 830 nm was used for excitation of the valve samples, and Raman emission was detected by an optical spectrometer with a liquid nitrogen-cooled CCD detector.

BACKGROUND

Cardiac valves are subjected to highly repetitive mechanical stresses, due to their over 40 million cardiac cycles per year. These structures may suffer cumulative lesions, complicated by the deposition of calcium phosphate, which can lead to clinically significant diseases. NIRS can provide important information about biological tissue composition and has been used for diagnosis of some types of human pathology.

METHODS

Samples of normal and pathologic tissues 5 mm in size were analyzed.

RESULTS

It was observed that the Raman spectrum of calcified cardiac valves presented different behavior when compared with normal valves. Differences were observed at the intensity of 960, 1,260, 1,452, and 1,660 cm(1) peaks.

CONCLUSIONS

These results suggest that this technique could be used to detect calcium phosphate mineral deposition in cardiac valves.

Imaging techniques for the vulnerable coronary plaque

The goal of this article is to illustrate the main invasive and noninvasive diagnostic modalities to image the vulnerable coronary plaque, which is responsible for acute coronary syndrome. The main epidemiologic and histological issues are briefly discussed in order to provide an adequate background. Comprehensive coronary atherosclerosis imaging should involve visualization of the entire coronary artery tree and plaque characterization, including three-dimensional morphology, relationship with the lumen, composition, vascular remodelling and presence of inflammation. No single technique provides such a comprehensive description, and no available modality extensively identifies the vulnerable plaque. In particular, we describe multislice computed tomography, which at present seems to be the most promising noninvasive tool for an exhaustive image-based quantification of coronary atherosclerosis.

Intravascular Raman spectroscopic catheter for molecular diagnosis of atherosclerotic coronary disease

An intravascular catheter for Raman spectroscopic detection and analysis of coronary atherosclerotic disease has been developed. The catheter, having an outer diameter of 2 mm, consisted of a side-view-type micro-Raman probe, an imaging fiber bundle, a working channel (injection drain), and a balloon. By inflating the balloon, the probe was brought close to the inner wall of a modeled blood flow system and detected a phantom target buried in the wall. Results obtained demonstrate the possibility of using the spectroscopic catheter for molecular diagnosis of coronary lesions.

Noninvasive detection of coronary atherosclerotic plaque by multidetector row computed tomography

BACKGROUND

Coronary artery disease continues to be one of the leading causes of death and disability around the globe, challenging the efficacy of currently applied schemes to predict the risk for future coronary events. In fact, algorithms such as the Framingham risk score that are based on traditional risk factors like hypertension and dyslipidemia are not very sensitive, leaving a majority of the population at intermediate risk.

METHODS

Advances in multidetector computed tomography (MDCT) technology with submillimeter slice collimation (approximately 0.6 mm) and high temporal resolution now permit contrast-enhanced imaging of the coronary artery lumen and wall in a single breath hold. The current generation of MDCT provided in-plane resolution of 0.5 mm and a temporal resolution of 210 ms. The simultaneous acquisition of 16/64 parallel cross-sections reduces image acquisition time to about 10-20s using 60-80 ml of contrast agents to opacify the coronary artery lumen. CT imaging for coronary calcification is an established method with low radiation exposure. The amount of calcification is expressed as an Agatston Score (AS).

RESULTS

The presence and amount of coronary calcification significantly increases the relative risk for future coronary events, independent from traditional risk factors (risk ratio 8.7 [95% Cl, 2.7-28.1]). Especially, individuals with a high AS (>400) who are at intermediate 10-y Framingham event risk may benefit from this additional risk stratification. However, calcification is rarely present in children and adolescents. However, there is a growing body of evidence suggesting that contrast-enhanced MDCT can detect both calcified and noncalcified plaques with high sensitivity and specificity for the detection of plaques > 0.5 mm when compared to intravascular ultrasound. Moreover, initial data suggest that plaque characteristics such as plaque area, volume, quantify and coronary plaque remodeling index can be quantified in good agreement with IVUS. The composition of noncalcified plaque may be further stratified into predominantly fibrous or lipid-rich plaque. Noncalcified plaque may be present already in children and adolescents with multiple risk factors.

CONCLUSION

The available data indicate that high resolution MDCT can reliably detect, quantify and characterize calcified and noncalcified coronary atherosclerotic plaque. With MDCT, we now have a unique opportunity to study the natural history and response to therapy of noncalcified coronary plaques, which may be already present in obese children or children with multiple risk factors.

Micro-optical fiber probe for use in an intravascular Raman endoscope

We believe that we have developed the narrowest optical-fiber Raman probe ever reported, 600 microm in total diameter, that can be inserted into coronary arteries. The selection of suitable optical fibers, filters, and a processing method is discussed. Custom-made filters attached to the front end of a probe eliminate the background Raman signals of the optical fiber itself. The experimental evaluation of various optical fibers is carried out for the selection of suitable fibers. Measurement of the Raman spectra of an atherosclerotic lesion of a rabbit artery in vitro demonstrates the excellent performance of the micro-Raman probe.

Imaging of atherosclerosis -- can we predict plaque rupture?

Rupture of so-called vulnerable or unstable atherosclerotic lesions is responsible for a significant proportion of myocardial infarcts and strokes. However, timely identification of such plaques, in order to allow for aggressive local and systemic therapy, remains problematic. In order to address this problem, there is a need to develop techniques that can image the cellular, biochemical, and molecular components that typify the vulnerable plaque. In this article, both techniques that are in current clinical use and those being evaluated in clinical trials are reviewed with regard to their ability to identify unstable lesions at risk of rupture.

Raman spectroscopy for diagnosis of atherosclerosis: a rapid analysis using neural networks

Near-infrared Raman spectroscopy (NIRS) is one of the novel techniques that has a potential for in vivo diagnosis of atherosclerosis in human arteries. For such real time clinical applications, a rapid collection and analysis of the data is needed. One of the major problems with the fast data collection is that the noise generated by the detector has the same level as the Raman signal from the tissue, which makes the analysis difficult. In this work, NIRS measurements have been carried out on a total of 60 samples from human coronary arteries. Raman spectral data with the correlated histopathological analysis have been used as a basis to stimulate the cases of severe noise conditions. The main objective of this paper is the comparison of different processing algorithms that have been developed based on either wavelet transformation or principal component analysis for compressing the Raman spectral vectors and a rapid data classification based on different neural network architectures. The developed algorithms found to provide promising diagnosis results with classification errors smaller than 5%, even in the cases of Raman data with collection times as small as 20 ms. It has been concluded that the developed algorithms would be very much useful in the development of Raman spectroscopy systems for in vivo biological applications.

Vulnerable plaque: Definition, detection, treatment, and future implications

Atherosclerosis continues to account for significant morbidity and mortality in most of the world. The major proportion of atherosclerosis mortality is related to atherosclerotic coronary artery disease, yet there still is not an optimal method for making the diagnosis of vulnerable plaque in vivo. The search for such an undefined method, along with studies on amelioration of currently available technology, gains special significance when the association between the qualitative definition of lesions in an individual and cardiovascular risks are considered. We, therefore, start by defining the critical lesion of coronary atherosclerosis and review the advantages and potential for clinical use of various methods to detect the vulnerable plaque and comment on possible future implications in this field.

Evaluation of a soft atherosclerotic lesion in the rabbit aorta by an invasive IVUS method versus a non-invasive MRI technology

The intravascular ultrasound (IVUS) modality has rapidly gained acceptance for the measurement of arterial plaque thickness and for anatomical characterization. In view, however, of the growing interest in the direct assessment of plaque size after therapeutic modalities directly reducing plaque burden, a non-invasive method such as magnetic resonance imaging (MRI) may be of help for repeated evaluations. The two methods were compared directly on a focal plaque developed at the abdominal aortic level by a combination of local electric lesion followed by a hypercholesterolemic diet. The plaque was fully characterized histopathologically at intervals up to 120 days from lesion induction, and maximal plaque formation was detected at 90 days from electrical injury. Plaques could be well assessed by IVUS at each time point analyzed and data correlated very well to histopathologic findings (r = 0.969, P = 0.0014). The MRI technology provided reliable determinations only at 90 days after lesion induction, i.e. at maximal plaque formation, with excellent correspondence to IVUS determinations (r = 0.989, P = 0.0111). Altogether these findings indicate that the non-invasive MRI technology, when applied to the analysis of arterial plaques of adequate size, can be used successfully for plaque determination, with results comparable to the invasive IVUS technique.

Side-viewing fiberoptic catheter for biospectroscopy applications

Utilization of fiberoptic catheters can turn the Raman and fluorescence spectroscopy systems into powerful bio-medical diagnostic probes. An in vivo bio-chemical diagnosis of some important organs like the esophagus, intestine, lung branches, artery, etc., can be possible by developing fiber-probes with good signal collection capabilities, a good flexibility to scan different spatial regions of the sample and less background signals generated in the probes themselves. An in vivo diagnosis of endoluminal inner walls utilizing front-viewing catheters (FVC) is very difficult because the internal diameter of these organs do not allow (excitation and collection) flexibility to access the different spatial regions of the sample. In this work we have developed, different side-viewing catheter (SVC) probes with a very small distal tip (semi sphere, phi approximately 1.5 mm) and micro mirrors allow beam steering of the excitation and collected radiation at a 90 degree angle. Preliminary results of spectroscopic applications have been presented. Reflectance, fluorescence and Raman scattering measurements have been used to compare the efficiency of SVC with traditional FVC probes. The results demonstrate that the SVC probes not only exhibit more flexibility but also similar spectral characteristics and signal collection efficiencies in comparison with conventional FVC probes.

Characterization and application of Raman labels for confocal Raman microspectroscopic detection of cellular proteins in single cells

A method using confocal Raman microspectroscopy for the detection of cellular proteins in single intact cells was developed. Two approaches were used to improve the detection of these cellular components. First, compounds with high Raman scattering were investigated for potential use as Raman labels. Raman labels were conjugated to either biomolecules or biotin and used as markers in the detection of cellular enzymes and receptors. Second, silver colloids were used to increase the surface-enhanced Raman scatter (SERS) of these Raman labels. Cresyl violet and dimethylaminoazobenzene are Raman labels that provide very sensitive SERS detection by a confocal Raman microscope with a HeNe laser at wavelength of 632.8 nm. The detection of 12-lipoxygenase and cyclooxygenase-1 in single bovine coronary artery endothelial cells and the binding of angiotensin II to its receptors in zona glomerulosa cells was demonstrated.

On-line detection of cholesterol and calcification by catheter based Raman spectroscopy in human atherosclerotic plaque ex vivo

BACKGROUND

Raman spectroscopy has the unique potential to detect and quantify cholesterol and calcification in an atherosclerotic plaque in vivo.

OBJECTIVE

To evaluate the sensitivity and specificity of this technique for detecting cholesterol or calcification in human coronary artery and aorta specimens ex vivo, using a compact clinical fibreoptic based Raman system developed for in vivo applications.

DESIGN

From nine coronary arteries and four aorta specimens, 114 sites were evaluated for the presence of cholesterol and calcification by Raman spectroscopy and standard histology. Raman spectra were acquired and evaluated on-line in around five seconds.

RESULTS

The correlation between Raman spectroscopy and histology was r = 0.68 for cholesterol and r = 0.71 calcification in the plaque (p < 0.0001). Sensitivity and specificity for detecting cholesterol and calcification were excellent: receiver operating characteristic (ROC) analysis for each of the components revealed areas under the curves of > 0.92 (p < 0.0001). At the optimal cut-off values determined by ROC analysis, positive predictive values of > 80% and negative predictive values of > 90% were obtained.

CONCLUSIONS

On-line real time catheter based Raman spectroscopy detects accumulation of cholesterol and calcification in atherosclerotic plaque with high sensitivity and specificity.

Detection of high-risk atherosclerotic coronary plaques by intravascular spectroscopy

Multiple technologies are under development to identify plaque composition and vulnerability. This review article is intended to provide basic knowledge to the interventional cardiologist and the clinician about spectroscopy. The concept of light, the wavelength unit and the electromagnetic spectrum are discussed. Different types of spectra analysis including nuclear magnetic resonance, Raman, fluorescence and diffuse reflectance near-infrared spectroscopy are then carefully reviewed. Experimental data to identify atherosclerotic plaque composition for each of these techniques is provided. Potential benefits and challenges are addressed. Finally, diffuse reflectance near-infrared spectroscopy is discussed in more detail as a promising technique to characterize plaque vulnerability in humans.

Determination of the cholesterol-collagen ratio of arterial atherosclerotic plaques using near infrared spectroscopy as a possible measure of plaque stability

Particular danger associated with an arteriosclerotic plaque consists in the possible rupture of its cap, dependent on the thickness of the cap covering the lipid core, its composition and different inflammatory changes. The purpose of this study was to compare the total cholesterol and collagen contents of arterial walls, both measured by near infrared spectroscopy (NIRS), and to test whether the ratios of cholesterol to collagen correlate with histochemical parameters possibly being indicators for plaque stability. NIR spectra of 118 sections from 36 human aortas were measured at 1000-2500 nm. Evaluation was performed by the partial least squares method (PLS), the chemical reference analysis by HPLC. Acceptable results were achieved for calibrations. With these calibrations 38 further aortic sections taken at autopsy were NIR-spectroscopically analysed and ordered in relation to histological findings of fatty deposits, cap thickness over the lipid core, and the ratio of fatty deposits to cap thickness. Correlations were found to exist between the spectroscopically determined total cholesterol concentrations and the histologically estimated fatty deposits (r=0.887), between the spectroscopically determined collagen concentrations and the cap thickness over the lipid core (r=0.441), and between the ratios total cholesterol to collagen and the ratios fatty deposits to cap thickness (r=0.575).

Atherothrombotic plaques and the need for imaging

The assessment of atherothrombotic plaques by imaging techniques is essential for the in vivo identification of vulnerable plaques. Several invasive and noninvasive imaging techniques are available to assess atherothrombotic disease. The use of some of the available imaging modalities for the study of regression and progression of atherothrombosis are described in more detail in the subsequent articles.

Medical applications of Raman spectroscopy: from proof of principle to clinical implementation

Raman spectroscopy has recently been applied ex vivo and in vivo to address various biomedical issues such as the early detection of cancers, monitoring of the effect of various agents on the skin, determination of atherosclerotic plaque composition, and rapid identification of pathogenic microorganisms. This leap in the number of applications and the number of groups active in this field has been facilitated by several technological advancements in lasers, CCD detectors, and fiber-optic probes. However, most of the studies are still at the proof of concept stage. We present a discussion on the status of the field today, as well as the problems and issues that still need to be resolved to bring this technology to hospital settings (i.e., the medical laboratory, surgical suites, or clinics). Taken from the viewpoint of clinicians and medical analysts, the potential of Raman spectroscopic techniques as new tools for biomedical applications is discussed and a path is proposed for the clinical implementation of these techniques.

In vivo determination of the molecular composition of artery wall by intravascular Raman spectroscopy

Atherosclerotic plaque vulnerability is suggested to be determined by its chemical composition. However, at present there are no in vivo techniques available that can adequately type atherosclerotic plaques in terms of chemical composition. Previous in vitro experiments have shown that Raman spectroscopy can provide such information in great detail. Here we present the results of in vitro and in vivo intravascular Raman spectroscopic experiments, in which dedicated, miniaturized fiber-optic probes were used to illuminate the blood vessel wall and to collect Raman scattered light. The results make clear that an important hurdle to clinical application of Raman spectroscopy in atherosclerosis has been overcome, namely, the ability to obtain in vivo intravascular Raman spectra of high quality. Of equal importance is the finding that the in vivo intravascular Raman signal obtained from a blood vessel is a simple summation of signal contributions of the blood vessel wall and of blood. It means that detailed information about the chemical composition of a blood vessel wall can be obtained by adapting a multiple least-squares fitting method, which was developed previously for the analysis of in vitro spectra, to account for signal contributions of blood.

Imaging techniques to predict cardiovascular risk

Conventional cardiovascular imaging, with a focus on identifying flow-limiting stenoses, does not directly image the atherosclerotic lesion. Recent clinical and pathobiologic data indicate that stenosis severity does not dictate cardiovascular risk and that there are functional, structural, and biologic features of atherosclerosis that are associated with cardiovascular events. Imaging technologies, such as ultrasound, light, x-ray, magnetic resonance, and targeted contrast agents, have been developed to characterize directly the atherosclerotic vessel wall. They provide promising approaches to predict cardiovascular risk and facilitate further study of the mechanisms of atherosclerosis progression and its response to therapy.