Stuart L. Houser

Characterization of Human Atherosclerosis by Optical Coherence Tomography

Background—High-resolution visualization of atherosclerotic plaque morphology may be essential for identifying coronary plaques that cause acute coronary events. Optical coherence tomography (OCT) is an intravascular imaging modality capable of providing cross-sectional images of tissue with a resolution of 10 &mgr;m. To date, OCT imaging has not been investigated in sufficient detail to assess its accuracy for characterizing atherosclerotic plaques. The aim of this study was to establish objective OCT image criteria for atherosclerotic plaque characterization in vitro. Methods and Results—OCT images of 357 (diseased) atherosclerotic arterial segments obtained at autopsy were correlated with histology. OCT image criteria for 3 types of plaque were formulated by analysis of a subset (n=50) of arterial segments. OCT images of fibrous plaques were characterized by homogeneous, signal-rich regions; fibrocalcific plaques by well-delineated, signal-poor regions with sharp borders; and lipid-rich plaques by signal-poor regions with diffuse borders. Independent validation of these criteria by 2 OCT readers for the remaining segments (n=307) demonstrated a sensitivity and specificity ranging from 71% to 79% and 97% to 98% for fibrous plaques, 95% to 96% and 97% for fibrocalcific plaques, and 90% to 94% and 90% to 92% for lipid-rich plaques, respectively (overall agreement, &kgr;=0.83 to 0.84). The interobserver and intraobserver reliabilities of OCT assessment were high (&kgr; values of 0.88 and 0.91, respectively). Conclusions—Objective OCT criteria are highly sensitive and specific for characterizing different types of atherosclerotic plaques. These results represent an important step in validating this new intravascular imaging modality and will provide a basis for the interpretation of intracoronary OCT images obtained from patients.

Visualization of Coronary Atherosclerotic Plaques in Patients Using Optical Coherence Tomography: Comparison With Intravascular Ultrasound

OBJECTIVES The aim of this study was to evaluate the feasibility and the ability of intravascular optical coherence tomography (OCT) to visualize the components of coronary plaques in living patients. BACKGROUND Disruption of a vulnerable coronary plaque with subsequent thrombosis is currently recognized as the primary mechanism for acute myocardial infarction. Although such plaques are considered to have a thin fibrous cap overlying a lipid pool, imaging modalities in current clinical practice do not have sufficient resolution to identify thin (< 65 microm) fibrous caps. Optical coherence tomography is a new imaging modality capable of obtaining cross-sectional images of coronary vessels at a resolution of approximately 10 microm. METHODS The OCT images and corresponding histology of 42 coronary plaques were compared to establish OCT criteria for different types of plaques. Atherosclerotic lesions with mild to moderate stenosis were identified on angiograms in 10 patients undergoing cardiac catheterization. Optical coherence tomography and intravascular ultrasound (IVUS) images of these sites were obtained in all patients without complication. RESULTS Comparison between OCT and histology demonstrated that lipid-rich plaques and fibrous plaques have distinct OCT characteristics. A total of 17 IVUS and OCT image pairs obtained from patients were compared. Axial resolution measured 13 +/- 3 microm with OCT and 98 +/- 19 microm with IVUS. All fibrous plaques, macrocalcifications and echolucent regions identified by IVUS were visualized in corresponding OCT images. Intimal hyperplasia and echolucent regions, which may correspond to lipid pools, were identified more frequently by OCT than by IVUS. CONCLUSIONS Intracoronary OCT appears to be feasible and safe. Optical coherence tomography identified most architectural features detected by IVUS and may provide additional detailed structural information.

In Vivo Characterization of Coronary Atherosclerotic Plaque by Use of Optical Coherence Tomography

Background—The current understanding of the pathophysiology of coronary artery disease is based largely on postmortem studies. Optical coherence tomography (OCT) is a high-resolution (≈10 μm), catheter-based imaging modality capable of investigating detailed coronary plaque morphology in vivo. Methods and Results—Patients undergoing cardiac catheterization were enrolled and categorized according to their clinical presentation: recent acute myocardial infarction (AMI), acute coronary syndromes (ACS) constituting non–ST-segment elevation AMI and unstable angina, or stable angina pectoris (SAP). OCT imaging was performed with a 3.2F catheter. Two observers independently analyzed the images using the previously validated criteria for plaque characterization. Of 69 patients enrolled, 57 patients (20 with AMI, 20 with ACS, and 17 with SAP) had analyzable images. In the AMI, ACS, and SAP groups, lipid-rich plaque (defined by lipid occupying ≥2 quadrants of the cross-sectional area) was observed in 90%, 75%, and 59%, respectively (P=0.09). The median value of the minimum thickness of the fibrous cap was 47.0, 53.8, and 102.6 μm, respectively (P=0.034). The frequency of thin-cap fibroatheroma (defined by lipid-rich plaque with cap thickness ≤65 μm) was 72% in the AMI group, 50% in the ACS group, and 20% in the SAP group (P=0.012). No procedure-related complications occurred. Conclusions—OCT is a safe and effective modality for characterizing coronary atherosclerotic plaques in vivo. Thin-cap fibroatheroma was more frequently observed in patients with AMI or ACS than SAP. This is the first study to compare detailed in vivo plaque morphology in patients with different clinical presentations.

Quantification of Macrophage Content in Atherosclerotic Plaques by Optical Coherence Tomography

Background—Macrophage degradation of fibrous cap matrix is an important contributor to atherosclerotic plaque instability. An imaging technology capable of identifying macrophages in patients could provide valuable information for assessing plaque vulnerability. Optical coherence tomography (OCT) is a new intravascular imaging modality that allows cross-sectional imaging of tissue with a resolution of ≈10 &mgr;m. The aim of this study was to investigate the use of OCT for identifying macrophages in fibrous caps. Methods and Results—OCT images of 26 lipid-rich atherosclerotic arterial segments obtained at autopsy were correlated with histology. Cap macrophage density was quantified morphometrically by immunoperoxidase staining with CD68 and smooth muscle actin and compared with the standard deviation of the OCT signal intensity at corresponding locations. There was a high degree of positive correlation between OCT and histological measurements of fibrous cap macrophage density (r =0.84, P <0.0001) and a negative correlation between OCT and histological measurements of smooth muscle actin density (r =−0.56, P <0.005). A range of OCT signal standard deviation thresholds (6.15% to 6.35%) yielded 100% sensitivity and specificity for identifying caps containing >10% CD68 staining. Conclusions—The high contrast and resolution of OCT enables the quantification of macrophages within fibrous caps. The unique capabilities of OCT for fibrous cap characterization suggest that this technology may be well suited for identifying vulnerable plaques in patients.

Heart transplantation in baboons using α1,3-galactosyltransferase gene-knockout pigs as donors: Initial experience

Hearts from α1,3-galactosyltransferase knockout pigs (GalT-KO, n = 8) were transplanted heterotopically into baboons using an anti-CD154 monoclonal antibody–based regimen. The elimination of the galactose-α1,3-galactose epitope prevented hyperacute rejection and extended survival of pig hearts in baboons for 2–6 months (median, 78 d); the predominant lesion associated with graft failure was a thrombotic microangiopathy, with resulting ischemic injury. There were no infectious complications directly related to the immunosuppressive regimen. The transplantation of hearts from GalT-KO pigs increased graft survival over previous studies.

Evaluation of intracoronary stenting by intravascular optical coherence tomography

Background: Conventional contrast cineangiography and intravascular ultrasound (IVUS) provide a limited definition of vessel microstructure and are unable to evaluate dissection, tissue prolapse, and stent apposition on a size scale less than 100 μm. Objective: To evaluate the use of intravascular optical coherence tomography (OCT) to assess the coronary arteries in patients undergoing coronary stenting. Methods: OCT was employed in patients having percutaneous coronary interventions. Images were obtained before initial balloon dilatation and following stent deployment, and were evaluated for vessel dissection, tissue prolapse, stent apposition, and stent asymmetry. IVUS images were obtained before OCT, using an automatic pull back device. Results: 42 stents were imaged in 39 patients without complications. Dissection, prolapse, and incomplete stent apposition were observed more often with OCT than with IVUS. Vessel dissection was identified in eight stents by OCT and two by IVUS. Tissue prolapse was identified in 29 stents by OCT and 12 by IVUS; the extent of the prolapse (mean (SD)) was 242 (156) μm by OCT and 400 (100) μm by IVUS. Incomplete stent apposition was observed in seven stents by OCT and three by IVUS. Irregular strut separation was identified in 18 stents by both OCT and IVUS. Conclusions: Intracoronary OCT for monitoring stent deployment is feasible and provides superior contrast and resolution of arterial pathology than IVUS.

α1,3-galactosyltransferase gene-knockout pig heart transplantation in baboons with survival approaching 6 months

Background. The recent generation of α1,3-galactosyltransferase gene-knockout (GalT-KO) pigs has allowed investigation of the survival of GalT-KO pig organs in nonhuman primates. Methods. Heterotopic heart transplantation from GalT-KO pigs was carried out in baboons (n=8) using a human antihuman CD154 monoclonal antibody-based immunosuppressive regimen. Results. In six of the eight cases, graft survival extended to between approximately 2 and 6 months. All grafts developed thrombotic microangiopathy (TM). In particular, the clinical course of one baboon in which the graft functioned for 179 days is summarized. This baboon received aspirin (40 mg on alternate days) from day 4 in addition to heparin, which may have been a factor in the delay of onset and progression of TM and in prolonged graft survival. Maintenance therapy with anti-CD154 mAb, mycophenolate mofetil, and methylprednisolone was associated with persistently low numbers of CD3+CD4+ and CD3+CD8+ cells. Despite persisting depletion of these cells, no infectious complications occurred. Conclusions. It remains to be established whether TM is related to a very low level of natural preformed or T-cell-induced antibody deposition on the graft, inducing endothelial activation and injury, or to molecular incompatibilities in the coagulation mechanisms between pig and baboon, or to both. However, function of a pig organ in a baboon for a period approaching six months, which has not been reported previously, lends encouragement that the barriers to xenotransplantation will eventually be overcome.

Percutaneous transvenous cellular cardiomyoplasty

Objectives The study evaluated a nonsurgical means of intramyocardial cell introduction using the coronary venous system for direct myocardial access and cell delivery. Background Direct myocardial cell repopulation has been proposed as a potential method to treat heart failure. Methods We harvested bone marrow from Yorkshire swine (n = 6; 50 to 60 kg), selected culture-flask adherent cells, labeled them with the gene for green fluorescence protein, expanded them in culture, and resuspended them in a collagen hydrogel. Working through the coronary sinus, a specialized catheter system was easily delivered to the anterior interventricular coronary vein. The composite catheter system (TransAccess) incorporates a phased-array ultrasound tip for guidance and a sheathed, extendable nitinol needle for transvascular myocardial access. A microinfusion (IntraLume) catheter was advanced through the needle, deep into remote myocardium, and the autologous cell–hydrogel suspension was injected into normal heart. Animals were sacrificed at days 0 (n = 2), 14 (n = 1, + 1 control/collagen biogel only), and 28 (n = 2), and the hearts were excised and examined. Results We gained widespread intramyocardial access to the anterior, lateral, septal, apical, and inferior walls from the anterior interventicular coronary vein. No death, cardiac tamponade, ventricular arrhythmia, or other procedural complications occurred. Gross inspection demonstrated no evidence of myocardial perforation, and biogel/black tissue dye was well localized to sites corresponding to fluoroscopic landmarks for delivery. Histologic analysis demonstrated needle and microcatheter tracts and accurate cell–biogel delivery. Conclusions Percutaneous intramyocardial access is safe and feasible by a transvenous approach through the coronary venous system. The swine offers an opportunity to refine approaches used for cellular cardiomyoplasty.

Porcine coronary imaging in vivo by optical coherence tomography.

OBJECTIVE A high-resolution coronary artery imaging modality has the potential to address important diagnostic and management problems in cardiology. Optical coherence tomography (OCT) is a promising new optical imaging technique with a resolution of approximately 10 microm. The purpose of this study was to use a new OCT catheter to demonstrate the feasibility of performing OCT imaging of normal coronary arteries, intimal dissections, and deployed stents in vivo. METHODS AND RESULTS Normal coronary arteries, intimal dissections, and stents were imaged in five swine with OCT and compared with intravascular ultrasound (IVUS). In the normal coronary arteries, visualization of all of the layers of the vessel wall was achieved with a saline flush, including the intima which was not identified by IVUS. Following dissection, detailed layered structures including intimal flaps, intimal defects, and disruption of the medial wall were visualized by OCT. IVUS failed to show clear evidence of intimal and medial disruption. Finally, the microanatomic relationships between stents and the vessel walls were clearly identified only by OCT. CONCLUSIONS In this preliminary experiment, we have demonstrated that in vivo OCT imaging of normal coronary arteries, intimal dissections, and deployed stents is feasible, and allows identification of clinically relevant coronary artery morphology with high-resolution and contrast.

Suppression of Natural and Elicited Antibodies in Pig-to-Baboon Heart Transplantation Using a Human Anti-Human CD154 mAb-Based Regimen

Natural and elicited antipig antibodies (Abs) lead to acute humoral xenograft rejection (AHXR). Ten baboons underwent heterotopic heart transplantation (Tx) from human decay‐accelerating factor (hDAF) pigs. Depletion of anti‐Galα1, 3Gal (Gal) Abs was achieved by the infusion of a Gal glycoconjugate from day – 1. Immunosuppression included induction of antithymocyte globulin, thymic irradiation, and cobra venom factor, and maintenance with a human antihuman CD154 mAb, mycophenolate mofetil, and methylprednisolone; heparin and prophylactic ganciclovir were also administered. Pig heart survival ranged from 4 to 139 (mean 37, median 27) days, with three functioning for >50 days. Graft failure (n = 8) was from classical AHXR [ 4], thrombotic microangiopathy [ 3], or intragraft thrombosis [ 1], with death (n = 2) from pneumonia [ 1], or possible drug toxicity (with features of thrombotic microangiopathy) [ 1]. Anti‐Gal Abs (in μg/mL) were depleted by Gal glycoconjugate before graft implantation from means of 41.3 to 6.3 (IgM) and 12.4–4.6 (IgG), respectively, and at graft excision were 6.3 and 1.7 μg/mL, respectively. No elicited Abs developed, and no cellular infiltration was seen. The treatment regimen was effective in maintaining low anti‐Gal Ab levels and in delaying or preventing AHXR. The combination of costimulatory blockade and heparin with Tx of a Gal‐negative pig organ may prolong graft survival further.