Intravascular multispectral optoacoustic tomography of atherosclerosis: Prospects and challenges.

The progression of atherosclerosis involves complex changes in the structure, composition and biology of the artery wall. Currently, only anatomical plaque burden is routinely characterized in living patients, whereas compositional and biological changes are mostly inaccessible. However, anatomical imaging alone has proven to be insufficient for accurate diagnostics of the disease. Multispectral optoacoustic tomography offers complementary data to anatomical methods and is capable of imaging both tissue composition and, via the use of molecular markers, the biological activity therein. In this paper we review recent progress in multispectral optoacoustic tomography imaging of atherosclerosis with specific emphasis on intravascular applications. The potential capabilities of multispectral optoacoustic tomography are compared with those of established intravascular imaging techniques and current challenges on the road towards a clinically viable imaging modality are discussed

Intravascular optoacoustic catheter with extended sensitivity field.

To extend sensitivity field for effective optoacoustic imaging, a novel concept of a non-mechanical point spread function (PSF) adjustment is proposed. Method was validated on phantoms and showed to be useful for distance-adaptive imaging

Near-infrared fluorescence catheter system for two-dimensional intravascular imaging in vivo.

Detection of high-risk coronary arterial plaques prior to rupture remains an unmet clinical challenge, in part due to the stringent resolution and sensitivity requirements for in vivo human coronary arterial imaging. To address this need, we have developed a near-infrared (NIR) fluorescence imaging catheter system for intra-vascular molecular imaging of atherosclerosis in coronary artery-sized vessels, capable of resolving two-dimensional fluorescence activity in hollow organs, such as blood vessels. Based on a rotational fiber design, the catheter system illuminates and detects perpendicular to the rotational axis, while an automated pullback mechanism enables visualization along blood vessels with a scan speed of up to 1.5 mm/sec. We demonstrate the previously undocumented capacity to produce intravascular NIR fluorescence images of hollow organs in vivo and showcase the performance metrics of the system developed using blood vessel mimicking phantoms. This imaging approach is geared toward in vivo molecular imaging of atherosclerotic biomarkers and is engineered to allow seamless integration into the cardiac catheterization laboratory

Improving quantification of intravascular fluorescence imaging using structural information.

Intravascular near-infrared fluorescence (iNIRF) imaging can enable the in vivo visualization of biomarkers of vascular pathology, including high-risk plaques. The technique resolves the bio-distribution of systemically administered fluorescent probes with molecular specificity in the vessel wall. However, the geometrical variations that may occur in the distance between fibre-tip and vessel wall can lead to signal intensity variations and challenge quantification. Herein we examined whether the use of anatomical information of the cross-section vessel morphology, obtained from co-registered intravascular ultrasound (IVUS), can lead to quantification improvements when fibre-tip and vessel wall distance variations are present. The algorithm developed employs a photon propagation model derived from phantom experiments that is used to calculate the relative attenuation of fluorescence signals as they are collected over 360° along the vessel wall, and utilizes it to restore accurate fluorescence readings. The findings herein point to quantification improvements when employing hybrid iNIRF, with possible implications to the clinical detection of high-risk plaques or blood vessel theranostics

Double-cladding-fiber-based detection system for intravascular mapping of fluorescent molecular probes.

Early detection of high-risk coronary atherosclerosis remains an unmet clinical challenge. We have previously demonstrated a near-infrared fluorescence catheter system for two-dimensional intravascular detection of fluorescence molecular probes [1]. In this work we improve the system performance by introducing a novel high resolution sensor. The main challenge of the intravascular sensor is to provide a highly focused spot at an application relevant distance on one hand and a highly efficient collection of emitted light on the other. We suggest employing a double cladding optical fiber (DCF) in combination with focusing optics to provide a sensor with both highly focused excitation light and highly efficient fluorescent light collection. The excitation laser is coupled into the single mode core of DCF and guided through a focusing element and a right angle prism. The resulting side-fired beam exhibits a small spot diameter (50 μm) throughout a distance of up to 2 mm from the sensor. This is the distance of interest for intravascular coronary imaging application, determined by an average human coronary artery diameter. At the blood vessel wall, an activatable fluorescence molecular probe is excited in the diseased lesions. Next light of slightly shifted wavelength emits only in the places of the inflammations, associated with dangerous plaques [2]. The emitted light is collected by the cladding of the DCF, with a large collection angle (NA=0.4). The doublecladding acts as multimodal fiber and guides the collected light to the photo detection elements. The sensor automatically rotates and pulled-back, while each scanned point is mapped according to the amount of detected fluorescent emission. The resulting map of fluorescence activity helps to associate the atherosclerotic plaques with the inflammation process. The presented detection system is a valuable tool in the intravascular plaque detection and can help to differentiate the atherosclerotic plaques based on their biological activity, identify the ones that prone to rupture and therefore require more medical attention

<em>In vivo</em> near infrared fluorescence (NIRF) intravascular molecular imaging of inflammatory plaque, a multimodal approach to imaging of atherosclerosis.

The vascular response to injury is a well-orchestrated inflammatory response triggered by the accumulation of macrophages within the vessel wall leading to an accumulation of lipid-laden intra-luminal plaque, smooth muscle cell proliferation and progressive narrowing of the vessel lumen. The formation of such vulnerable plaques prone to rupture underlies the majority of cases of acute myocardial infarction. The complex molecular and cellular inflammatory cascade is orchestrated by the recruitment of T lymphocytes and macrophages and their paracrine effects on endothelial and smooth muscle cells.(1) Molecular imaging in atherosclerosis has evolved into an important clinical and research tool that allows in vivo visualization of inflammation and other biological processes. Several recent examples demonstrate the ability to detect high-risk plaques in patients, and assess the effects of pharmacotherapeutics in atherosclerosis.(4) While a number of molecular imaging approaches (in particular MRI and PET) can image biological aspects of large vessels such as the carotid arteries, scant options exist for imaging of coronary arteries.(2) The advent of high-resolution optical imaging strategies, in particular near-infrared fluorescence (NIRF), coupled with activatable fluorescent probes, have enhanced sensitivity and led to the development of new intravascular strategies to improve biological imaging of human coronary atherosclerosis. Near infrared fluorescence (NIRF) molecular imaging utilizes excitation light with a defined band width (650-900 nm) as a source of photons that, when delivered to an optical contrast agent or fluorescent probe, emits fluorescence in the NIR window that can be detected using an appropriate emission filter and a high sensitivity charge-coupled camera. As opposed to visible light, NIR light penetrates deeply into tissue, is markedly less attenuated by endogenous photon absorbers such as hemoglobin, lipid and water, and enables high target-to-background ratios due to reduced autofluorescence in the NIR window. Imaging within the NIR &#39;window&#39; can substantially improve the potential for in vivo imaging.(2,5) Inflammatory cysteine proteases have been well studied using activatable NIRF probes(10), and play important roles in atherogenesis. Via degradation of the extracellular matrix, cysteine proteases contribute importantly to the progression and complications of atherosclerosis(8). In particular, the cysteine protease, cathepsin B, is highly expressed and colocalizes with macrophages in experimental murine, rabbit, and human atheromata.(3,6,7) In addition, cathepsin B activity in plaques can be sensed in vivo utilizing a previously described 1-D intravascular near-infrared fluorescence technology(6), in conjunction with an injectable nanosensor agent that consists of a poly-lysine polymer backbone derivatized with multiple NIR fluorochromes (VM110/Prosense750, ex/em 750/780nm, VisEn Medical, Woburn, MA) that results in strong intramolecular quenching at baseline.(10) Following targeted enzymatic cleavage by cysteine proteases such as cathepsin B (known to colocalize with plaque macrophages), the fluorochromes separate, resulting in substantial amplification of the NIRF signal. Intravascular detection of NIR fluorescence signal by the utilized novel 2D intravascular NIRF catheter now enables high-resolution, geometrically accurate in vivo detection of cathepsin B activity in inflamed plaque. In vivo molecular imaging of atherosclerosis using catheter-based 2D NIRF imaging, as opposed to a prior 1-D spectroscopic approach,(6) is a novel and promising tool that utilizes augmented protease activity in macrophage-rich plaque to detect vascular inflammation. (11,12) The following research protocol describes the use of an intravascular 2-dimensional NIRF catheter to image and characterize plaque structure utilizing key aspects of plaque biology. It is a translatable platform that when integrated with existing clinical imaging technologies including angiography and intravascular ultrasound (IVUS), offers a unique and novel integrated multimodal molecular imaging technique that distinguishes inflammatory atheromata, and allows detection of intravascular NIRF signals in human-sized coronary arteries

Progress on multimodal molecular / anatomical intravascular imaging of coronary vessels combining near infrared fluorescence and ultrasound.

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 Intravascular Ultrasound (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. We have developed such a Near-Infrared Fluorescence (NIRF) imaging system and showcased, in both phantom and in-vivo (rabbit) experiments, its potential to successfully detect inflamed atherosclerotic plaques, using appropriate fluorescent probes. Here, we discuss some limitations of the current system and suggest the combined use of the NIRF and IVUS imaging systems as a means for more accurate assessment of atherosclerotic plaque. We include some results and models that showcase the potential power of this kind of hybrid imaging

Atheroma susceptible to thrombosis exhibit impaired endothelial permeability in vivo as assessed by nanoparticle-based fluorescence molecular imaging.

BACKGROUND: The role of local alterations in endothelial functional integrity in atherosclerosis remains incompletely understood. This study used nanoparticle-enhanced optical molecular imaging to probe in vivo mechanisms involving impaired endothelial barrier function in experimental atherothrombosis. METHODS AND RESULTS: Atherosclerosis was induced in rabbits (n=31) using aortic balloon injury and high-cholesterol diet. Rabbits received ultrasmall superparamagnetic iron oxide nanoparticles (CLIO) derivatized with a near-infrared fluorophore (CyAm7) 24 hours before near-infrared fluorescence imaging. Rabbits were then either euthanized (n=9) or underwent a pharmacological triggering protocol to induce thrombosis (n=22). CLIO-CyAm7 nanoparticles accumulated in areas of atheroma (P&lt;0.05 versus reference areas). On near-infrared fluorescence microscopy, CLIO-CyAm7 primarily deposited in the superficial intima within plaque macrophages, endothelial cells, and smooth muscle cells. Nanoparticle-positive areas further exhibited impaired endothelial barrier function as illuminated by Evans blue leakage. Deeper nanoparticle deposition occurred in areas of plaque neovascularization. In rabbits subject to pharmacological triggering, plaques that thrombosed exhibited significantly higher CLIO-CyAm7 accumulation compared with nonthrombosed plaques (P&lt;0.05). In thrombosed plaques, nanoparticles accumulated preferentially at the plaque-thrombus interface. Intravascular 2-dimensional near-infrared fluorescence imaging detected nanoparticles in human coronary artery-sized atheroma in vivo (P&lt;0.05 versus reference segments). CONCLUSIONS: Plaques that exhibit impaired in vivo endothelial permeability in cell-rich areas are susceptible to subsequent thrombosis. Molecular imaging of nanoparticle deposition may help to identify biologically high-risk atheroma

Real-time catheter molecular sensing of inflammation in proteolytically active atherosclerosis.

To enable intravascular detection of inflammation in atherosclerosis, we developed a near-infrared fluorescence (NIRF) catheter-based strategy to sense cysteine protease activity during vascular catheterization. METHODS AND RESULTS: The NIRF catheter design was based on a clinical coronary artery guidewire. In phantom studies of NIRF plaques, blood produced only a mild (&lt;30%) attenuation of the fluorescence signal compared with saline, affirming the favorable optical properties of the NIR window. Catheter evaluation in vivo used atherosclerotic rabbits (n=11). Rabbits received an injection of a cysteine protease-activatable NIRF imaging agent (Prosense750; excitation/emission, 750/770 nm) or saline. Catheter pullbacks through the blood-filled iliac artery detected NIRF signals 24 hours after injection of the probe. In the protease agent group, the in vivo peak plaque target-to- BACKGROUND: &lt;0.05). Ex vivo fluorescence reflectance imaging corroborated these results (target-to- BACKGROUND: &lt;0.01). In the protease group only, saline flush-modulated NIRF signal profiles further distinguished atheromata from normal segments in vivo (P&lt;0.01). Good correlation between the in vivo and ex vivo plaque target-to- BACKGROUND: =0.82, P&lt;0.01). Histopathological analyses demonstrated strong NIRF signal in plaques only from the protease agent group. NIRF signals colocalized with immunoreactive macrophages and the cysteine protease cathepsin B. CONCLUSIONS: An intravascular fluorescence catheter can detect cysteine protease activity in vessels the size of human coronary arteries in real time with an activatable NIRF agent. This strategy could aid in the detection of inflammation and high-risk plaques in small arteries