Red Thrombus Research Articles

Intravascular ultrasound, angioscopic and histopathological characterisation of heterogeneous patterns of restenosis after sirolimus-eluting stent implantation: insights into potential “thromborestenosis” phenomenon

Histopathology of DES restenosis remains unclear. The purpose is to examine restenotic tissue characteristics after sirolimus-eluting stent (SES) and comparing with that after bare-metal stent (BMS). Intravascular ultrasound (IVUS), coronary angioscopy (CAS), and directional coronary atherectomy were performed simultaneously in 21 patients who presented restenosis after SES (n=13) and BMS (n=8). Mean time of restenosis was 10.8 months in the SES versus 7.5 months in the BMS. Typical "black hole'', echolucent appearance by IVUS was observed in one SES case, and corresponded to a fibrin rich tissue by histology which appeared translucent tissue by CAS. CAS did not reveal red thrombus, but showed white thrombus in six SES versus two BMS (46.2% vs. 25.0%, p=0.597). Histology demonstrated various patterns after SES including thrombus, fibrin, inflammatory infiltrate, and collagen-matrix rich tissue, while thrombus component was not detected in BMS. Thrombus and fibrin deposition detected by either CAS or histopathology were observed more frequently in SES than in BMS group (92.3% vs. 25.0%, p=0.007). Restenosis after SES and BMS have different clinical and histological patterns. SES restenosis may be frequently associated with thrombus component.

Treatment of a large thrombus containing lesion with the MGuard™ protective net coronary stent system: optical coherence tomographic evidence of complete plaque sealing

Avoidance of distal embolization is the key of success in the percutaneous treatment of patients with ST elevation myocardial infarction. Embolization may lead to microvascular obstruction, and consequently impairment of myocardial perfusion with increased infarct size and mortality [1, 2]. Different types of devices have been proposed to avoid this issue. Prospective randomized clinical trials have shown the beneficial effect of thrombus aspiration devices in the setting of primary percutaneous coronary intervention (PCI) [3, 4]. Recently, the use of MGuard net coronary stent system (MGS, Inspire-MD, Tel Aviv, Israel) in the setting of primary PCI has shown high rate of myocardial reperfusion and angiographic procedural success [5]. MGS is a new stainless-steel closed-cell design stent concept. Other than conventional available bare-metal stents (BMS), it has an ultrathin flexible polyethylene terephthalate mesh sleeve that is anchored to the external surface of the struts. This biocompatible microfiber net (string diameter 10–22 lm) has minimal effects on stent crossing profile and deliverability (Fig. 1). During stent deployment, the net stretches and slides over the expanding stent struts, creating custom designed pores of &200 lm in diameter. The idea of this design is to entrap the thrombus burden between stent mesh and the wall of the artery, isolating the high-thrombogenic intima components and preventing the ‘‘snow plug effect’’. This is the first time, at our knowledge, that the optical coherence tomography (OCT) assessment of MGS implantation for the treatment of a large thrombus burden right coronary artery (RCA) lesion is being reported. A 78-year-old female with mild hypertension and dislipidemia, previously treated with dual chamber pacemaker implant, was admitted at our hospital for coronary angiography following an inferior ST elevation myocardial infarction treated with a partial effective systemic thrombolytic therapy with reteplase. At the time of coronary angiography, the patient was already on double antiplatelet therapy (600 mg loading dose of clopidogrel and 300 mg loading dose of acetylsalicylic acid). The angiography showed a significant mid-RCA lesion with large thrombus burden (Fig. 2a), while there were no lesions on the left coronary system. PCI was then performed using the standard approach. A bolus of 80 UI/kg of unfractioned heparin was given at the start of procedure. No glycoprotein IIb/IIIa inhibitors were given during the procedure to avoid bleeding complications related to the already administered anti-platelet therapy and patient’s age. After cannulation of the RCA with a 7 French side holes Judking right 4 guiding catheter, a BMW guide wire (Abbott Vascular, Redwood City, CA, USA) was advanced until the distal postero-lateral branch and an OCT scan using the C7XR OCT image system (LightLab Imaging Inc., Westford, MA, USA) was then performed using the nonocclusive flushing technique [6]. The OCT pullback showed a tight stenosis of RCA with a very large intraluminal stratified red thrombus (Fig. 2b). Subsequently, it was decided to perform a direct stenting with 4.0/24 mm MGS implantation, which was deployed at 16 atm. The OCT was repeated thereafter showing a malapposition at the mid part of the stent. Therefore, post-dilatation was A. La Manna S. D. Tomasello C. Tamburino Clinical Division of Cardiology, Department of Internal Medicine and Systemic Disease, Ferrarotto Hospital, University of Catania, Catania, Italy

High incidence of thrombus formation at 18 months after paclitaxel-eluting stent implantation: Angioscopic comparison with sirolimus-eluting stent

Difference of neointimal formational pattern and incidence of thrombus formation between sirolimus-eluting stent (SES) and paclitaxel-eluting stent (PES) at 18 months after stent implantation has not previously been reported. We retrospectively enrolled 35 event-free patients who received SES (15 patients, 18 stents) or PES (20 patients, 23 stents) implantation with 18-month follow-up coronary angioscopy. We divided our patients into SES or PES groups and compared neointimal coverage pattern and incidence of red mural thrombus formation between the 2 groups. Neointimal coverage grades were classified into 4 categories. Minimum neointimal coverage grade, maximum grade, and heterogeneity score were assessed in each stent. Heterogeneity score was calculated by subtracting minimum from maximum grade within one stent. Minimum neointimal coverage grade of PES was significantly lower than that of SES (0.70 +/- 0.64 vs 1.33 +/- 0.69, P = .005), whereas maximum grade was not significantly different (2.48 +/- 0.73 vs 2.22 +/- 0.73, P = .218). Heterogeneity score and incidence of red mural thrombus of PES were higher than those of SES (1.78 +/- 0.80 vs 0.89 +/- 0.76, P = .002 and 70% vs 11%, P < .001). The present study revealed that PES shows more heterogeneous neointimal coverage and higher incidence of thrombus formation as compared with SES at 18 months after stent implantation.

Comparison of Hemostatic Durability between N-Butyl Cyanoacrylate and Gelatin Sponge Particles in Transcatheter Arterial Embolization for Acute Arterial Hemorrhage in a Coagulopathic Condition in a Swine Model

This study was designed to compare the efficacy of transcatheter arterial embolization (TAE) with N-butyl cyanoacrylate (NBCA) or gelatin sponge particles (GSP) for acute arterial bleeding in a coagulopathic condition using a swine model. Four healthy swine were divided into two coagulopathic conditions: mild and severe. Five hemorrhages were created in each swine (10 hemorrhages per coagulopathy). Mild coagulopathy was achieved by bloodletting 10% of the total circulatory whole blood and preserving activated clotting time (ACT) less than 200 s (ACT < 200 s state); severe coagulopathy was achieved by bloodletting 30% and preserving ACT > 400 s (ACT > 400-second state). For each state, of ACT < 200 s or ACT > 400 s, TAE was conducted with GSP or NBCA to control five hemorrhages arising from artificially created renal and splenic injuries. Angiography immediately after TAE with GSP or NBCA showed complete occlusion in both coagulopathic conditions. In the ACT < 200-second state, follow-up angiography at 5-30 min after TAE with GSP or NBCA showed no evidence of recurrent hemorrhage. In the ACT > 400-second state, follow-up angiography showed recurrent hemorrhage in four (80%) of the five hemorrhages embolized with GSP and in one (20%) of the five hemorrhages embolized with NBCA. Microscopically, red thrombi were observed densely surrounding GSP in mild coagulopathy but were scarce in severe coagulopathy. In a condition with severe coagulopathy, TAE with NBCA was more effective in durability to cease active arterial bleeding than with GSP.

Idiopathic left atrial dissection mimicking cardiac malignancy

Fig. 1. Computed tomography, plane (A) and enhanced. Computed tomography revealed a relatively heterogenous mass with a well defined margin, and the mass was not enhance with contrast material. Intra-operative exploration was the undefined bulge of the intact intima, like a subintimal tumor, andwe found a block of red thrombus within the left atrial wall. Histological examination confirmed no malignant cells in it.

Difference of Neointimal Formational Pattern and Incidence of Thrombus Formation Among 3 Kinds of Stents: An Angioscopic Study

The purpose of this study is to compare the neointimal formational pattern and incidence of thrombus formation among sirolimus-eluting (SES), paclitaxel-eluting (PES), and bare-metal stents (BMS) with coronary angioscopy.Neointimal formation and incidence of mural thrombus are different with the type of stent.One hundred successive patients who received 43 SES, 40 PES, or 32 BMS implantation underwent 6-month follow-up coronary angioscopy. We evaluated angioscopic parameters, including minimum and maximum neointimal grade; presence and number of red mural thrombus; neointimal grade around thrombus; and heterogeneity score, which is defined by subtracting minimum from maximum grade within 1 stent by classifying angioscopic neointimal coverage grades into 4 categories. We compared these parameters among 3 kinds of stent groups.Heterogeneity scores of SES, PES, and BMS were 0.79 +/- 0.60, 1.27 +/- 0.75, and 1.03 +/- 0.82, respectively (p = 0.011). The PES showed the highest incidence of angioscopic red mural thrombus (50% in PES, 12% in SES, and 3% in BMS, p < 0.001), and the number of thrombus observed within 1 stent in the PES group tended to be larger than those in the SES and BMS groups.At 6 months after stent implantation, PES showed the most heterogeneous neointimal formation and the highest incidence of thrombus formation compared with SES and BMS.

Very late stent thrombosis of a NIR Advance bare metal stent nine years after implantation

This is an image of a 52 year old man with a past history of CABG and multiple PCIs in 2000 and 2008. During the most recent diagnostic catheterisation LIMA to Lad and LCX stent were patent. The patient denied having interrupted dual antiplatelet therapy with aspirin and plavix. He presented to his cardiologist with recurrent chest pain and positive cardiac enzymes. Angiography revealed a patent SVG to OM stent, and an occlusion of the LCX NIR Advance bare metal stent which had been placed nine years previously (Figure 1A). The Pilot 50 wire crossed the lesion easily. Thrombectomy was performed and a large amount of red thrombus was aspirated (Figure 2). Optical frequency domain imaging (OFDI) with the C7XR (LightLab Imaging Inc., Westford, MA, USA ) system revealed large amounts of soft material within the stent (Figure 3A). After predilation with a 2.0x15 mm Voyager balloon (Abbott), the

M1 Susceptibility Vessel Sign on T2* as a Strong Predictor for No Early Recanalization After IV-t-PA in Acute Ischemic Stroke

In acute stroke patients treated with intravenous tissue plasminogen activator (t-PA), early recanalization of occluded arteries can improve the clinical outcome. The magnetic susceptibility effect of deoxygenated hemoglobin in red thrombi can present as hypointense signals on T2*-weighted gradient echo imaging. We investigated whether the gradient echo imaging M1 susceptibility vessel sign (M1 SVS) can predict no early recanalization after t-PA infusion. Patients with internal carotid artery and M1 occlusion were prospectively studied. MRI studies, including DWI, T2*, and MRA, were performed before and within 30 minutes and 24 hours after t-PA infusion. The NIHSS score was obtained before and 7 days after t-PA administration. The relationship between the presence of the M1 SVS and no early recanalization and patient outcome was examined. A total of 48 patients (29 men; mean age, 74.6+/-11.2 years) were enrolled. M1 SVS was present in 13 (27.1%) patients and absent in 35 (72.9%) patients. There were no significant differences in clinical characteristics between the 2 groups. Follow-up MRA within 30 minutes after t-PA infusion revealed that 20 (57.1%) of the 35 patients without the M1 SVS had early recanalization, but that none of the 13 patients with the M1 SVS had early recanalization (P=0.0002). Seven days after t-PA infusion, dramatic improvement was more frequently observed in patients without the M1 SVS (51.4%) than in those with the M1 SVS (0%, P=0.0007). The M1 SVS on T2* appears to be a strong predictor for no early recanalization after t-PA therapy.

“Intracoronary Whirling Current Phenomenon” and Thrombus Formation After Sirolimus-Eluting Stent Implantation Visualized by Optical Coherence Tomography

We present a case of imaged “intracoronary whirling current phenomenon.” A 60-year-old Japanese man was treated with 2 sirolimus-eluting stents (SES; 3.0×18 and 3.5×33 mm) in the right coronary artery (RCA) for chronic total occlusion in October 2007. After percutaneous coronary intervention, he was taking 200 mg of aspirin and 200 mg of ticlopidine. However, he had recurrent transient ischemic chest pain beginning at 7 months. A second coronary angiography at 8 months after SES implantation showed reocclusion at a site just proximal to the SES. A repeat percutaneous coronary intervention was performed for RCA chronic total occlusion (Figure 1). After crossing the guide wire to the distal portion of the RCA, the target lesion was dilated using a 1.25×10 mm balloon catheter. After minimum balloon dilatation, optical coherence tomography (OCT) was performed to evaluate in-stent reocclusion from the distal portion to the proximal portion (Figure 2). Two different types of tissue coverage (may be consistent with thrombus formations of different ages) were observed in the stent reocclusion.1 Moreover, in the patent distal portion of the RCA, an intracoronary whirling current …

Eosinophils May Be Involved in Thrombus Growth in Acute Coronary Syndrome Histologic Examination of Aspiration Samples

Thrombus aspiration therapy allows for the examination of thrombus and atheroma fragments in acute coronary syndrome (ACS). Inflammatory cells and platelet activation play key roles in thrombus formation in ACS. However, histopathologic evaluation of thrombi in ACS has not been adequately addressed. We performed histologic analysis of tissue samples obtained by thrombus aspiration therapy. We studied 165 samples from patients with ACS. The area of each sample, percentage of red thrombus, and percentage of white thrombus were measured. Samples were stained immunohistochemically with antibodies against macrophages, activated platelets, and interleukin (IL)-5. Seventy-six samples included atheroma fragments. Macrophages, neutrophils, and activated platelets were observed in thrombi and in atheroma fragments. Eosinophil infiltration was also observed predominantly in the area between white thrombus and red thrombus in 106 samples. We categorized all samples into 3 groups according to the grade of eosinophil infiltration (eos-, eos+, eos++ group). Sample area in the eos++ group was greater than that in the eos- group (P < 0.0001). In addition, the percentage of the red thrombus areas in the eos++ group and the eos+ group was greater than that in the eos- group (P < 0.009, P < 0.02, respectively). However, there was no difference in the percentage of white thrombus area between the 3 groups. Staining for IL-5 was identified in inflammatory cells within thrombi. Eosinophils may play an important role in coronary occlusion by promoting thrombus growth.

Fibrin Clot Visualized by Optical Coherence Tomography

Optical coherence tomography (OCT) is a recently developed optical imaging technique that provides high-resolution (≈10 to 20 μm) cross-sectional images of vessels.1 It has been reported that OCT images of white and red thrombus were characterized as backscattering protrusions in the coronary lumen.2 However, OCT characteristics of fibrin clot composed of fibrin material and few blood cells have not been described. The coronary arteries of a 75-year-old man …

Distal embolization during intracranial carotid artery stenting

Intracranial stenting, with or without balloon angioplasty, has emerged as an effective alternative to medical therapy for intracranial atherosclerosis. However, this strategy appears to involve the potential risk of distal embolization, which may lead to disability and death. Here, the authors have described a successful case of distal balloon-protected intracranial carotid artery stenting for clinically and angiographically unstable intracarotid artery stenosis revealing aspirated embolic materials containing yellow plaque, red thrombus, and white debris.

Effects of cilostazol and k-134 on reconstructive surgery using prosthetic grafts in the abdominal aorta of beagle dogs

Problems associated with prosthetic graft replacement are stenosis at the anastomosis site and thrombus formation on the inner surface. Cilostazol is known to have antiplatelet activity and inhibit vascular smooth muscle cell proliferation and neointima thickening. A cilostazol derivative, (-)-6-[3-[3-cyclopropyl-3-[(1R,2R)-2-hydroxycyclohexyl]ureido]-propoxy]-2-(1H)-quinolinone (K-134), has more potent anti-platelet activity and anti-neointimal thickening activity than cilostazol in the in-vitro platelet aggregation and in-vivo anti-hyperplastic activity assay. The aim of this study was to investigate effects of cilostazol and K-134 on thrombus formation and neointimal thickening at the site of prosthetic graft replacement. Beagle dogs underwent infrarenal abdominal aortic resection with straight Dacron graft replacement, which were allocated to the control, cilostazol, and K-134 groups. Two dogs were dead without confirming the cause of death. After 6 months, all dogs were necropsied. All prosthetic grafts were patent in each group. Ratios of red thrombus to prosthetic graft area were 0.3 ± 6.4%, and 3.3 ± 4.5% in the cilostazol and K-134 groups, respectively, which were significant different from that in the control group (24.4 ± 16.8%). However, no clear difference was seen among the 3 groups with respect to neointimal thickness (control group, 0.70 ± 0.13 mm; cilostazol group, 0.59 ± 0.14 mm; K-134 group, 0.67 ± 0.14 mm). Cilostazol and K-134 significantly inhibited thrombus formation on the inner surface of the prosthetic graft at 6 months after graft replacement. Neointimal thickening on the inner surface was slight even in control-group animals, and the effects of cilostazol and K-134 on such thickening were unclear.

Sind mit der Mehrschicht-Computertomografie Dichtemessungen auch bei geringer Schichtdicke und niedrigem Röhrenstrom verlässlich? Ergebnisse einer Phantomstudie

To test whether CT with low slice thickness and low tube current provides reliable attenuation measurements. Using multi-slice CT and a phantom, we measured the attenuation values of thrombi with different proportions of erythrocytes, using a slice thickness of 1.25 mm, 2.5 mm, and 5 mm with tube currents of 200 mA, 300 mA, and 400 mA and a slice thickness of 0.625 mm with tube currents of 150 mA, 175 mA, and 200 mA. Differences in attenuation values and pixel noise between the three thrombi for tube current and slice thickness were statistically analyzed. The attenuation values of all thrombi increased (p<0.05) when the slice thickness decreased using a tube current of 200 mA or when the tube current decreased using a slice thickness of 1.25 mm. With higher tube currents and thicker slices, the CT values depended on the type of thrombus and the slice thickness. In slices with a thickness of 0.625 mm, the CT values decreased with the tube current in the mixed thrombus with a low proportion of erythrocytes and in the red thrombus (p<0.05). The maximal difference in mean attenuation values was 4.3 HU with a slice thickness of 0.625 mm and 2.2 HU with a slice thickness of 1.25 mm. The pixel noise increased as the slice thickness decreased (p<0.05) with the exception of the red thrombus, if reduced to 0.625 mm. The pixel noise also increased as the tube current decreased (p<0.05) except in mixed thrombi measured with 0.625 mm. The maximal difference in mean standard deviation was 1.8 HU with a slice thickness of 1.25 mm. The accuracy of attenuation values as determined by CT with low slice thickness and low tube current with a maximal difference of 4.3 HU suffices for the purposes of clinical routine. A reduction of slice thickness from 1.25 mm to 0.625 mm yields the greatest differences in CT values.

Stroke prevention in atrial fibrillation – things can only get better

Stroke prevention in atrial fibrillation – things can only get better

Serial Angioscopic Evidence of Incomplete Neointimal Coverage After Sirolimus-Eluting Stent Implantation

The time course of neointimal formation after stent implantation has not been studied extensively by angioscopy in the drug-eluting stent era. Serial angioscopic findings at first follow-up (3.6+/-1.1 months), second follow-up (10.5+/-1.6 months), and third follow-up (21.2+/-2.2 months) after stent implantation were compared between sirolimus-eluting stents (SES, n=17) and bare-metal stents (BMS, n=11). Neointimal coverage, thrombus, and presence of yellow plaques underneath the stents were assessed. Neointimal coverage was graded as follows: grade 0, stent struts were fully visible; grade 1, struts bulged into the lumen, although they were covered; grade 2, struts were embedded by the neointima but were seen translucently; or grade 3, struts were fully embedded and invisible. Neointimal coverage was remarkably different between SES and BMS at each follow-up point. Neointimal coverage grade was 1.1+/-0.5 in SES versus 2.9+/-0.3 in BMS at the first follow-up (P<0.0001), 1.1+/-0.5 in SES versus 3.0+/-0.0 in BMS (P<0.0001) at the second follow-up, and 1.3+/-0.5 in SES versus 3.0+/-0.0 in BMS at the third follow-up (P=0.0009). No significant serial changes in coverage grade were noted in the BMS group, whereas coverage grade slightly but significantly increased at the third follow-up in the SES group (P<0.05). Thrombi were detected in 4 SES: a red thrombus was seen from the first to the third follow-up in 2; another was detected only at the third follow-up; and the fourth was seen at the first follow-up but disappeared at the second follow-up, associated with a new white thrombus despite dual antiplatelet therapy. Yellow plaques had disappeared by the time of the second follow-up in BMS. In contrast, yellow plaques were exposed in 71% of SES at the first follow-up and remained exposed until the third follow-up. Neointimal coverage grades correlated with thrombi (P=0.002) and with yellow plaques (P<0.0001). Serial angioscopic findings up to 2 years after SES implantation were markedly different from those after BMS. Neointimal coverage was completed by 3 to 6 months in BMS. In contrast, SES demonstrated the presence of thrombi and yellow plaques even as much as 2 years after implantation.

Distinguishing characteristics of erythrocyte-rich and platelet-rich thrombus by intravascular ultrasound catheter system

Acute coronary syndromes are associated with platelet-rich, white thrombi (WT) and erythrocyte-rich, red thrombi (RT), but their ultrasonic characteristics are not well defined. To determine whether intravascular ultrasound (IVUS) could be used to detect specific characteristics of WT and RT, two experiments were performed. An in-vitro experiment evaluated five WT and five RT and an ex-vivo experiment evaluated thrombi from 17 atherosclerotic rabbits with disrupted plaques and overlying thrombi. Specimen were mounted flat, immersed in a saline bath and examined from the intimal surface. Thrombi were classified as WT (n = 69) or RT (n = 40) by gross inspection and histology. IVUS was performed using a 1 mm, 20 MHz transducer in a 4.8F catheter. Images were digitally converted and points integrated to account for angular and depth resolution. Sampling was performed at the water-tissue interface and four other sites at 0.3 mm radial depth increments. Signals from each depth were standardized by obtaining the ratio of each energy level to the level at the water-tissue interface. The average energy ratio backscattered by RT was constant with increasing tissue depth while it attenuated for WT (P < 0.005; 2-way ANOVA). RT was less homogeneous and had more backscatter compared to WT. Light and electron microscopy corroborated these observations showing WT as densely homogenous and RT with loose cellular elements. WT may be detected by its attenuated ultrasound pattern versus a non-attenuated pattern for RT by IVUS. This technique has potential for characterizing WT and RT.

Thrombotic occlusion proximal to plaque rupture in acute myocardial infarction: Evaluation by intravascular ultrasound and coronary angioscopy

A 56-year-old man was admitted because of acute myocardial infarction for 10 h of onset. Emergent coronary angiography showed a total occlusion at the middle portion of the right coronary artery. After aspirating thrombus, high-grade stenosis was present in the distal segment. Angioscopy showed only a white thrombus, but not a yellow plaque nor a red thrombus at the initially occluded site. On the other hand, IVUS demonstrated large attenuation indicating a lipid core and cavity obstruction and angioscopy revealed a glittering yellow plaque and red thrombi in the distal segment. Precipitation of thrombus from the distal site to the middle site might have occurred. IVUS and angioscopy might be effective for detecting and evaluating the infarct-related vulnerable plaque and for making the therapeutic strategy when percutaneous coronary interventions are performed.

Hydrogel-coated and platinum coils for intracranial aneurysm embolization compared in three experimental models using computerized angiographic and histologic morphometry

Angiographic and histologic results of hydrogel-coated (HydroCoil®) and platinum coil-treated canine sidewall, canine bifurcation, and rabbit elastase aneurysms (2–52 weeks follow-ups) were quantified by computer-assisted morphometry. Angiograms were analyzed using density determination to differentiate embolic mass and contrast medium filling. Aneurysm total and occluded areas, embolic material and red thrombus areas were determined from images of surface stained, plastic embedded ground sections. Long-term stable occlusion was observed in all three models with the use of hydrogel-coated coils. However, platinum coils were as effective only in the canine sidewall model. With the use of hydrogel-coated coils, a two-fold increase in the volumetric filling of the aneurysm sac resulted in improved outcomes.

Classification of flow-limiting thrombus in acute coronary syndrome by acoustic microscopy

Virtual histology (VH), based on a power spectral analysis of radio-frequency signal from intravascular ultrasound (IVUS) with neural network, can detect an unstable plaque from all the regions of coronary atherosclerosis. However, it provides limited information on plaque vulnerability because the resolution is larger than 100 m and thrombus is not classified precisely. The objective of the present study is to measure the acoustic properties of flow-limiting thrombus responsible for acute coronary syndrome (ACS) at the microscopic level. A newly developed acoustic microscope with 100-MHz pulsed ultrasound was equipped to measure two-dimensional distribution of attenuation and sound speed. The tissues were obtained from coronary arteries of 26 patients with ACS by intracoronary aspiration catheter. They were sectioned at 5-mm thickness and mounted onto glass slides without cover slips or staining and used acoustic microscopy study. The values of attenuation and sound speed of the red thrombus were significantly higher than those of the platelets-rich thrombus. Both acoustic parameters of old red thrombus were significantly lower than those of the organized thrombus. The acoustic properties provided by acoustic microscopy can classify the flow-limiting thrombus at different pathological stages. The criteria will provide further information for development of a VH-IVUS algorithm.