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April/May 2009
OCT: Ready for Prime Time?
Clinical applications of optical coherence tomography.
Optical coherence tomography (OCT) is an optical analogue of intravascular ultrasound (IVUS). OCT uses near-infrared light to create images. The greatest advantage is its extraordinarily high resolution. OCT has an axial resolution of 10 µm and a lateral resolution of 20 µm, which is approximately 10 times higher than that of IVUS. This new intravascular imaging method can provide more detailed structural information about the coronary artery wall compared to conventional imaging methods (Table 1).1
PLAQUE CHARACTERIZATION
OCT allows for the identification of the boundary of the intima and media within the coronary arterial wall, which currently cannot be distinguished by IVUS. In an OCT image, the intima is observed as the signal-rich layer nearest to the lumen, and media is visualized as the signal-poor middle layer. The OCT measurement of intimal thickness is well correlated to histological examination.2 OCT has the ability to evaluate subtle intimal thickening in vivo, which may indicate the early phase of coronary atherosclerosis.
Yabushita et al have developed objective OCT image criteria for differentiating distinct components of atherosclerotic tissue in a large series of autopsy specimens.3 In their histology-controlled OCT study, fibrous plaques were characterized by homogeneous, signal-rich regions; fibrocalcific plaques were characterized by signal-poor regions with sharp borders; and lipid-rich plaques were characterized by signal-poor regions with diffuse borders. Validation testing revealed good intraobserver and interobserver reliability (k = 0.83–0.84), as well as excellent sensitivity and specificity: 71%–79% and 97%–98% for fibrous plaques, 95%–96% and 97% for fibrocalcific plaques, and 90%–94% and 90%–92% for lipid-rich plaques, respectively. These definitions have formed the basis of plaque composition interpretation in clinical OCT studies (Figure 1).
VULNERABLE PLAQUE DETECTION
It has been reported that OCT might be the best tool available to detect vulnerable plaque.4-7 To assess the ability of each imaging method to detect the specific characteristics of vulnerable plaque, Kubo et al performed OCT, IVUS, and angioscopy in patients with acute myocardial infarction.8,9 Their research showed that OCT was superior in detecting plaque rupture (73% vs 40% vs 43%; P = .021), erosion (23% vs 0% vs 3%; P = .003), and thrombus (100% vs 33% vs 100%; P < .001) compared to IVUS and angioscopy. Intraobserver and interobserver variability yielded acceptable concordance for these characteristics (k = 0.61–0.83). Sawada et al evaluated the feasibility of OCT and virtual histology IVUS for detecting thin-capped fibroatheroma.10 The high resolution of OCT allows us to identify the thin fibrous cap (< 65 µm).11,12 Although the positive ratio of virtual histology IVUS for detecting thin-capped fibroatheroma was 45.9%, that of OCT was 77.8%.10 Kume et al demonstrated the abilities of OCT to differentiate thrombus from the vessel wall and to evaluate thrombus type. Red thrombi (red-blood-cell-rich) were identified as high-backscattering protrusions with signal-free shadowing inside the lumen of the artery, and white thrombi (platelet-rich) were detected as low-backscattering projections.13
Furthermore, Tearney et al proposed the potential of OCT to assess macrophage distribution within fibrous caps.14 There was a high degree of positive correlation between OCT and histological measurements of fibrous cap macrophage density (r < 0.84; P < .0001). 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.
GUIDANCE OF CORONARY INTERVENTION
Considering the high resolution of OCT, it is not surprising that it provides more detailed morphologic information for monitoring stent deployment than conventional imaging methods. OCT has the ability to detect stent-edge dissection, tissue protrusion, and stent malapposition at a level that is two to three times better than that of IVUS.15 Moreover, OCT is capable of visualizing the thin-neointimal hyperplasia after drug-eluting stent implantation (Figure 2). Recent studies demonstrated that the rate of sirolimus-eluting stent struts with neointimal coverage was > 90% at 12-month follow-up, and most of them were covered by thin neointima of < 100 µm.15-18 To determine when antiplatelet therapy could be discontinued, OCT provides important information about chronic drug-eluting stent status.
LIMITATIONS
The present OCT image-acquisition process requires vessel occlusion by means of gentle balloon inflation plus vessel flushing with saline infusion, because the near-infrared light signals are attenuated by red blood cells. This technique is rather cumbersome and time consuming and does not encourage its routine use. To overcome this limitation, Prati et al have developed a simplified technique for coronary blood removal that is achieved through continuous nonionic contrast administration.19 This nonocclusive technique of OCT image acquisition is safe and effective and promises to reduce the procedural time.20 A further limitation of OCT is the relatively shallow axial penetration depth of 2 mm; the OCT signal does not reach the back wall of thick atherosclerotic lesions. However, the current OCT is well suited for the assessment of the plaque morphologies within 500 µm of the luminal surface.
CURRENT TECHNOLOGICAL CHALLENGES
Recently, a second-generation OCT technology, termed frequency-domain OCT, has been developed and solves the current time-domain OCT problems by imaging at much higher frame rates.21 In combination with a short, nonocclusive saline flush and rapid spiral pullback, the higher frame rates generated by frequency-domain OCT enable imaging of the three-dimensional microstructure of long segments of coronary arteries. In addition, frequency-domain OCT facilitates the acquisition of spectroscopic and polarization data for plaque characterization. When this technology is fully utilized, it has the potential to dramatically change the way that physicians and researchers understand coronary artery disease and to better diagnose and treat the disease.
CONCLUSION
The high resolution of OCT provides histology-grade definition of the microstructure of coronary plaque in vivo. OCT provides a greater understanding of the pathophysiology of coronary artery disease and guidance for the appropriate patient-specific therapeutic approach. Although more clinical research and development of this imaging technology are required, we believe OCT will play an important role in the future of cardiology.
Takashi Kubo, MD, PhD, is Assistant Professor, Department of Cardiovascular Medicine, Wakayama Medical University in Wakayama, Japan. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Kubo may be reached at +81 73 441 0621; takakubo@wakayama-med.ac.jp.
Takashi Akasaka, MD, PhD, is Professor, Department of Cardiovascular Medicine, Wakayama Medical University in Wakayama, Japan. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Akasaka may be reached at +81 73 441 0621; akasat@wakayama-med.ac.jp.
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