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Background
Many heart attacks occur when a fatty coronary plaque ruptures, forming dangerous blood clots. Lipid content within coronary artery plaques are believed to correlate with the probability of plaque rupture as well as with the rate of progression of atherosclerosis. Pathologic studies of patients who died from a heart attack have identified a large lipid core among features of coronary artery disease that were associated with plaque rupture and thrombosis.
García-García et al (2008) stated that detection of coronary vulnerable plaques in-vivo is essential for studying their natural history and assessing potential treatment modalities and, therefore, may have an important impact on the prevention of acute myocardial infarction and death. Research is currently underway to determine how coronary artery plaques that are prone to rupture can best be identified before they cause a heart attack and several invasive and non-invasive techniques have been developed to assess the vulnerable plaque. Most of the techniques show exciting features, but none have proven their value in an extensive in-vivo validation study and all have a lack of prospective data (Schaar et al, 2007).
Near-infrared (NIR) spectroscopy is a catheter-based technique that is currently under investigation for detecting coronary vulnerable plaques (Lau et al, 2004). It identifies the chemical composition of substances based on the differential absorption of light in the NIR spectrum by different molecules. An important feature of NIR light is that it can penetrate tissue and can therefore identify a tissue despite the presence of blood between the detector and the target. This is an important advantage for imaging within the human coronary artery. The application of NIR spectroscopy to identify lipid deposition within coronary arteries has shown promising results in pre-clinical ex-vivo studies (Moreno et al, 2002; Gardner et al, 2008). A limitation of NIR spectroscopy is that it is influenced by flowing blood, and its lack of structural definition restricts its independent use in vulnerable plaque detection (Cassis, 1993; Lau et al, 2004).
Waxman et al (2009) reported their initial results from the SPECTACL (SPECTroscopic Assessment of Coronary Lipid) trial, a parallel first-in-human multi-center, phase II-III study designed to demonstrate the applicability of the lipid core-containing plaques (LCP) detection algorithm in living patients. Intra-coronary NIR spectroscopy was performed in patients undergoing percutaneous coronary intervention. Acquired spectra were blindly compared with autopsy NIR spectroscopy signals with multi-variate statistics. To meet the end point of spectral similarity, at least 2/3 of the scans were required to have greater than 80 % of spectra similar to the autopsy spectra. A total of 106 patients were enrolled. Spectroscopic data could not be obtained in 17 (16 %) patients due to technical limitations, leaving 89 patients for analysis. Spectra from 30 patients were un-blinded to test the calibration of the LCP detection algorithm. Of the remaining 59 blinded cases, after excluding 11 due to inadequate data, spectral similarity was demonstrated in 40 of 48 spectrally adequate scans (83 % success rate, 95 % confidence interval: 70 % to 93 %, median spectral similarity/pullback: 96 %, inter-quartile range 10 %). The LCP was detected in 58 % of 60 spectrally similar scans from both cohorts. There were no serious adverse events attributed to NIR spectroscopy. The authors concluded that the intra-vascular NIR spectroscopy system safely obtained spectral data in patients that were similar to those from autopsy specimens and that the results demonstrate the feasibility of invasive detection of coronary LCP with this novel system.
The LipiScan Coronary Imaging System (InfraReDx, Inc., Burlington, MA) uses intra-vascular NIR spectroscopy during coronary angiography to take images of lesions within coronary arteries in order to detect the lipid content within coronary artery plaques. It was cleared for marketing through the U.S. Food and Drug Administration (FDA) 510(k) process in April 2008. The system measures light delivered through the blood and reflected from the artery wall and produces a map of the artery's chemical composition. The reflected wavelengths vary depending on how much fat and other substances are in the plaque in the illuminated portion of the wall.
Although initial results from the SPECTACL trial showed that signals obtained in patients were spectrally similar to those obtained in autopsy-validated lesions, there is insufficient evidence of how NIR spectroscopy impacts the clinical outcomes of patients with coronary artery disease. Prospective studies demonstrating the effectiveness of NIR spectroscopy in improving clinical outcomes are needed.
In a review on imaging the vulnerable carotid artery plaque, Hermus and colleagues (2009) stated that imaging plays a key role in the selection of patients for carotid artery surgery. Indication for carotid endarterectomy or stenting is based on symptomatology and degree of stenosis as determined by angiography, duplex ultrasonography or computed tomographic angiography. Degree of stenosis has long time been assumed the most reliable predictor of stroke-risk in patients with carotid artery stenosis and accordingly, traditional imaging methods were focused on luminal stenosis. There is, however, growing evidence that other factors than degree of stenosis determine whether a carotid plaque will result in acute neurologic events or not. Various morphological characteristics and molecular processes have proven to be highly related to carotid plaque instability and symptomatology. As a result, the focus of imaging techniques in carotid artery disease is more and more shifting towards identification of the vulnerable plaque rather than the high-grade stenosis. In traditional imaging modalities, new insights of imaging beyond degree of stenosis have been explored and may be able to detect morphological characteristics of plaque vulnerability. In addition, advanced molecular imaging methods have been developed and are able to identify molecular and cellular processes in the vulnerable carotid artery plaque. It is clear that recent developments in carotid imaging are of great potential in the identification of the vulnerable carotid plaque. Near-infrared spectroscopy was not mentioned as a modality for imaging carotid plaques.
Yamaki et al (2011) examined whether the pre-operative level of deoxygenated hemoglobin (HHb) in the calf muscle during light-intensity exercise is useful for identifying patients at risk of developing deep vein thrombosis (DVT) after total knee or hip arthroplasty. A total of 68 patients undergoing total knee or total hip arthroplasty were enrolled. The Caprini risk assessment model was used to stratify patients into Caprini 5 to 6, Caprini 7 to 8, and Caprini greater than 8 groups. The pre-operative diameter of each venous segment was measured, and the time-averaged velocity (TAV) and time-averaged flow (TAF) of the popliteal vein (POPV) were assessed. Moreover, the prevalence of venous reflux in the POPV was evaluated pre-operatively. Near-infrared spectroscopy was used to measure the calf muscle HHb level. The calf venous blood filling index (FI-HHb) was calculated on standing, and then the calf venous ejection index (EI-HHb) was obtained after 1 tip-toe movement and the venous retention index (RI-HHb) after 10 tip-toe movements. All patients received low-dose unfractionated heparin pre-operatively and fondaparinux for post-operative thrombo-prophylaxis. Patients with arterial insufficiency, those who had pre-operative DVT, and those who developed bilateral DVT after surgery were excluded from the study. Four patients were excluded on the basis of the exclusion criteria. Among the 64 patients evaluated, 14 (21.9 %) were found to have DVT post-operatively. Among the risk factors for DVT, only the previous DVT was significantly predominant in patients who developed DVT (p = 0.001). The diameter of the popliteal vein was significantly smaller in patients who developed post-operative DVT than in those who did not (p = 0.001). Similarly, the diameter of the gastrocnemius vein was significantly larger in patients with post-operative DVT than in those without (p = 0.010). TAV and TAF were significantly increased in the popliteal vein in patients who developed post-operative DVT (p = 0.043, 0.046, respectively). Both groups showed a similar prevalence of reflux in the POPV (p = 0.841). The pre-operative NIR spectroscopy-derived RI was significantly increased in patients who developed DVT relative to those who did not (p = 0.004). The RI increased as the Caprini score progressed; however, there were no statistically significant differences between the 3 categories. Using ultrasound- and NIR spectroscopy-derived parameters of significance as a unit of analysis, an optimal RI cut-off point of greater than 2.3 showed the strongest ability to predict post-operative DVT, followed by a cut-off point greater than 0.25 cm for the diameter of the gastrocnemius vein (GV). The authors concluded that NIR spectroscopy-derived RI greater than 2.3 may be a promising parameter for identifying patients at risk of developing post-operative DVT despite pharmacologic DVT prophylaxis. A GV diameter of greater than 0.25 cm also seems to contribute to the development of post-operative DVT. The authors stated that these results might be helpful to physicians for deciding which patients require more intensive thrombo-prophylaxis.
Aries et al (2012) noted that there is uncertainty whether bilateral NIR spectroscopy can be used for monitoring of patients with acute stroke. In a pilot study, these researchers examined the NIR spectroscopy responsiveness to systemic and stroke-related changes over-night by assessing the effects of brief peripheral arterial oxygenation and mean arterial pressure alterations in the affected versus non-affected hemisphere in 9 patients with acute stroke. Significantly more NIR spectroscopy drops were registered in the affected compared with the non-affected hemisphere (477 drops versus 184, p < 0.001). In the affected hemispheres, nearly all peripheral arterial oxygenation drops (n = 128; 96 %) were detected by NIR spectroscopy; in the non-affected hemispheres only 23 % (n = 30; p = 0.17). Only a few mean arterial pressure drops were followed by a significant NIR spectroscopy drop. However, this was significantly different between both hemispheres (32 % versus 13 %, p = 0.01). The authors concluded that this pilot study found good responsiveness of NIR spectroscopy signal to systemic and stroke-related changes at the bedside; however, these findings require confirmation in a larger sample.
Cross et al (2009) examined if NIR could detect water concentration changes or edema formation in acute partial-thickness burn injuries. Adult burn patients within 72 hours post-injury, thermal etiology, partial-thickness burn depth, and less than 20 % total body surface area (TBSA) were included. Burn wounds were stratified into partial-thickness superficial or deep wounds based on histology and wound healing time. NIR devices were used to quantify edema in a burn and respective control sites. The sample population consisted of superficial (n = 12) and deep (n = 5) partial-thickness burn injuries. The patients did not differ with respect to age (40 +/- 15 years), TBSA (5 +/- 4 %), and mean time for edema assessment (2 days). Water content increased 15 % in burned tissue compared with the respective control regions. There were no differences in water content at the control sites. At 48 hours, deep partial-thickness injuries showed a 23 % increase in water content compared with 18 % superficial partial-thickness burns. NIR could detect differences in water content or edema formation in partial-thickness burns and unburned healthy regions. The authors concluded that NIR holds promise as a non-invasive, portable clinical tool to quantify water content or edema in burn wounds.
In a pilot study, Weingarten et al (2010) examined the effectiveness of in-vivo diffuse NIR spectroscopy in predicting wound healing in diabetic foot ulcers. A total of 16 chronic diabetic wounds were followed and assessed for subsurface oxy-hemoglobin concentration using the NIR device. Weekly measurements were conducted until there was wound closure, limb amputation, or 20 completed visits without healing. Digital photography measured wound size, and the degree of wound contraction was compared with the NIR results. In the 16 patients followed, 7 wounds healed, 6 limbs were amputated, and 3 wounds remained opened after 20 visits. The initial values in subsurface hemoglobin concentration in all wounds were higher than the non-wound control sites. Healed wounds showed a consistent reduction of hemoglobin concentration several weeks before closure that approached control site values. In wounds that did not heal or resulted in amputation of the limb, the hemoglobin concentration remained elevated. In some cases, these non-healing wounds appeared to be improving clinically. A negative slope for the rate of change of hemoglobin concentration was indicative of healing across all wounds. The authors concluded that evaluation of wounds using NIR may provide an effective measurement of wound healing. These preliminary findings need to be validated by well-designed studies.
In June 2010, NIR-intravascular ultrasound (IVUS) coronary imaging (e.g., the InfraReDx LipiScan™ IVUS Imaging System [InfraReDx, Inc., Burlington, MA]) was cleared by the FDA for marketing via the 510(k) process. The system combines both NIR and IVUS technologies. According to the 510(k) summary the modifications from the LipiScan Coronary Imaging System to the LipiScan IVUS Imaging System are the inclusion of ultrasound imaging within the same dimensions of the catheter and an expanded indication for use (ultrasound examination of coronary intravascular pathology). The InfraReDx LipiScan IVUS imaging system utilizes the same basic catheter design and the same operating principle as the predicate LipiScan Coronary Imaging system, while the ultrasound capabilities are functionally equivalent to the iLab™ Ultrasound Imaging System (Boston Scientific Corp., Fremont, CA). However, there is currently insufficient evidence to support the clinical value of near-infrared intravascular ultrasound imaging systems. Well designed studies are needed to ascertaining the effectiveness of near-infrared intravascular ultrasound coronary imaging in improving clinical outcomes.
Mallas et al (2011) stated that the use of intravascular imaging modalities for the detection and assessment of atherosclerotic plaque is becoming increasingly useful. Current clinical invasive modalities assess the presence of plaque using anatomical information and include IVUS and optical coherence tomography (OCT). However, such modalities cannot take into account underlying functional biological information, which can however be revealed with the use of molecular imaging. Consequently, intravascular molecular imaging is emerging as a powerful approach. These researchers have developed such a near-infrared fluorescence (NIRF) imaging system and showcased, in phantom as well as in-vivo (rabbit) experiments, its potential to successfully detect inflamed atherosclerotic plaques, using appropriate fluorescent probes. In this article, the authors discussed some limitations of the current system and suggested the combined use of the NIRF and IVUS imaging systems as a means for more accurate assessment of atherosclerotic plaque. They included some results and models that showcase the potential power of this kind of hybrid imaging.
Madder et al (2011) noted that recent studies emphasized the importance of direct intra-coronary imaging techniques that provide insights regarding not only lesion architecture but also plaque composition, especially the presence or absence of lipid-core plaque (LCP). A recently introduced catheter provides simultaneous near-infrared spectroscopy (NIRS) spectral data co-registered with standard IVUS images in a single intra-coronary pullback. The case-series study illustrated the data obtained by this combined NIRS-IVUS device and highlighted its potential clinical applications.
Zynda et al (2012) examined if there was a relationship between angiographic lesion complexity and the extent of LCP identified by catheter-based NIRS. A total of 78 patients who underwent coronary angiography and target-vessel NIRS were selected from the Chemometric Observations of Lipid Core Containing Plaques of Interest in Native Coronary Arteries Registry, an industry sponsored registry to collate clinical findings in all patients undergoing NIRS evaluation. A lipid-core burden index (LCBI) was obtained from the scan of the proximal 50 mm of the target vessel. Three-vessel SYNTAX (total, tSYN) and target single-vessel (only NIRS-interrogated vessel) SYNTAX (1vSYN) scores were calculated and compared to LCBI. High LCBI was defined as (greater than 110) and was compared to tertile scores for 1vSYN score (low: 0 to 5, intermediate: 6 to 10, high: greater than or equal to11) and previously established tertiles for tSYN score (low: 0 to22, intermediate: 23 to 32, high: greater than or equal to 33). Patients had mean age of 63 years with prevalence of females (10 %), diabetes mellitus (28 %), hypertension (88 %), and smoking history (72 %); 1vSYN and tSYN scores correlated poorly with LCBI [(r(2) = 0.25; p = 0.02; n = 78) and (r(2) = 0.24; p = 0.04; n = 78), respectively]. Mean LCBI did not differ significantly across all tertiles of 1vSYN or tSYN scores. The authors concluded that angiographic SYNTAX score only weakly correlated with LCBI. It is of interest as well that high LCBI was also present in cases of low SYNTAX scores. The disparity between the degree of angiographic complexity and the amount of LCP supports postulated mechanisms of the adverse event propensity even in patients who demonstrate low angiographic complexity. They stated that future studies are needed to ascertain the clinical significance of high LCBI in patients with low-to-intermediate angiographic complexity and their potential for percutaneous coronary intervention-related complications.
Pu et al (2012) tested the hypothesis that NIRS combined with IVUS would provide novel information of human coronary plaque characterization. Greyscale-IVUS, virtual histology (VH)-IVUS, and NIRS were compared in 131 native lesions (66 vessels) that were interrogated during catheterization by all 3 modalities. Greyscale-IVUS detected attenuated and echo-lucent plaques correlated with NIRS-detected lipid-rich areas. Attenuated plaques contained the highest NIRS probability of lipid core, followed by echo-lucent plaques. By VH-IVUS, 93.5 % of attenuated plaques contained confluent necrotic core (NC) and were classified as VH-derived fibro-atheromas (FAs). Although 75.0 % of echo-lucent plaques were classified as VH-FAs, VH-NC was seen surrounding an echo-lucent zone, but not within any echo-lucent zone. Furthermore, echo-lucent zones themselves contained fibro-fatty and/or fibrous tissue. All calcified plaques with arc greater than 90° contained greater than 10 % VH-NC (range of 16.0 % to 41.2 %) and were classified as calcified VH-FAs, but only 58.5 % contained NIRS-detected lipid core. A positive relationship between VH-derived % NC and NIRS-derived LCBI was found in non-calcified plaques, but not in calcified plaques. The authors concluded that combining NIRS with IVUS contributes to the understanding of plaque characterization in-vivo. They stated that further studies are needed to examine if combining NIRS and IVUS will contribute to the assessment of high-risk plaques to predict outcomes in patients with coronary artery disease.
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