The Ability of Optical Coherence Tomography to Monitor Percutaneous Coronary Intervention: Detailed Comparison with Intravascular Ultrasound

ABSTRACT: Background. We investigated the usefulness of optical coherence tomography (OCT) to evaluate vessel response after stent implantation by comparing with that of intravascular ultrasound (IVUS). Methods and Results. Eighteen cases undergoing percutaneous coronary intervention (PCI) who provided consent for both IVUS and OCT usage pre- and post-PCI procedure were enrolled. The lumen area at the distal site of the culprit lesion was smaller on OCT images than on IVUS images due to proximal vessel occlusion, whereas the lumen area at the proximal site of the lesion did not differ between OCT and IVUS images (distal site: 4.6 ± 2.0 vs. 5.0 ± 1.8 mm²; p = 0.0004; proximal site: 5.5 ± 2.3 vs. 5.6 ± 2.3 mm²; p = 0.8160). Stent malapposition was more frequently observed by OCT (30%) than by IVUS (5%, p = 0.0381). Stent edge dissection was not detected by IVUS, but was detected in 10% by OCT. Tissue prolapse was identified in all stents by OCT and in 5% by IVUS. Thrombus was observed in 15% by OCT and in 5% by IVUS. Conclusions. Proximal coronary occlusion during OCT imaging was possibly related to underestimation of vessel sizing at distal reference. Our data suggested that OCT might provide more detailed information on the presence of tissue prolapse, thrombus formation and edge dissection than IVUS. Further study is warranted to assess its clinical utility. J INVASIVE CARDIOL 2010;22:541–545 ———————————————————— Intravascular ultrasound (IVUS) has been used to understand coronary structures in clinical practice. IVUS provides useful information on vessel size, plaque area, and other important morphological changes after stent implantation. Also, several studies have shown that IVUS guidance improves clinical outcome after bare-metal and drug-eluting stent placement.^1–3 The resolution of IVUS (100–150 µm), however, limits its ability to detect detailed structures, such as intimal tears, thrombus, stent malapposition, and tissue prolapse during stent strut insertion. Optical coherence tomography (OCT) is a novel intracoronary diagnostic technique that has an axial resolution of 10 to 20 µm, approximately ten times greater than that of IVUS.⁴ Although OCT has its well-known disadvantage in its relatively poor penetration depth, the high resolution of OCT allows for visualization of microstructural features such as intimal tears, thrombus, stent malapposition, and tissue prolapse before and after PCI.^5–8 Therefore, OCT may be useful for optimizing stenting procedures in clinical practice. Recently, a new generation OCT; frequency-domain OCT (FD-OCT) has been developed. Although FD-OCT offers faster image acquisition speeds and greater scan depth without sacrificing its resolution, the FD-OCT system is not available in Japan for clinical use. Therefore, in this study, we evaluated usefulness of clinically available TD-OCT for the assessment of vessel response after stent deployment by comparing with that of IVUS.

Methods

From November 2008 to May 2009, 18 cases (20 lesions) with stable angina pectoris and unstable angina pectoris underwent PCI, and provided consent for both the IVUS and OCT examinations. PCI was performed to a target native coronary lesion with stenosis of more than 75% using a 0.014″ guidewire through a 6 or 7 Fr guide catheter. PCI included balloon angioplasty and stenting. Both IVUS and OCT imaging were performed in the target-related coronary artery before and after PCI, and we compared the IVUS and OCT images. This study was approved by the Ethics Committee of Kobe University and all the patients enrolled in the study provided their written informed consent. Angiographic analysis. Cine angiograms were analyzed by a computer-assisted automated edge detection algorithm (QCA-CMS; Medis, Leesburg, Virginia) with standard protocols. Quantitative measurements included reference lumen diameter, lesion length and % diameter stenosis (reference lumen diameter – minimal luminal diameter / reference lumen diameter) X 100).⁹IVUS examination. After administration of 0.2 mg nitroglycerine into the coronary artery, the IVUS catheter (Eagle Eye Gold^™ 2.9 Fr 20-MHz; Volcano Corp , Rancho Cordova, California) was inserted into the target vessel and pulled back with an automatic pullback device moving at 0.5 mm/s to image the artery retrograde to the coronary ostium before and after PCI. Acquired gray-scale IVUS images were stored on a CD-ROM for off-line analysis. OCT examination. OCT images were obtained with a M2 OCT system (LightLab Imaging, Inc., Westford, Massachusetts). Before and after PCI, OCT examination was performed as previously described. Briefly, an over-the-wire type occlusion balloon catheter (Helious^™, LightLab Imaging Inc.) and an OCT imaging probe (ImagingWire^™, LightLab Imaging Inc.) were inserted into the target vessel. To clear blood from the imaging site, the occlusion balloon was inflated to 0.5 atm and Lactated Ringer’s solution was infused into the coronary artery from the distal tip of the occlusion balloon at 0.5 mL/s. Motorized pullback OCT imaging was performed at a rate of 1.0 mm/s for a maximum length of 50 mm. The images were saved on a CD-ROM for off-line analysis. IVUS and OCT image analysis. For quantitative measurements, QCU-CMS software (Medis) was used for IVUS analysis and proprietary software provided by Light Lab was used for OCT analysis. Cross-sectional IVUS and OCT images were analyzed at 1-mm intervals for the entire lesion. The evaluated indices were lumen cross-sectional area (CSA; the area bounded by the luminal border), maximum lumen diameter (the longest diameter through the center point of the lumen), and minimum lumen diameter (the shortest diameter through the center point of the lumen) at the minimal lumen site, proximal site and distal site of the culprit lesion. Before stent implantation, we defined the proximal and distal site in both IVUS and OCT images as the site corresponding to the proximal and distal edge of the implanted stent. We also measured minimum stent CSA (inner stent area), maximum stent diameter (the longest diameter through the center point of the stent), and minimum stent diameter (the shortest diameter through the center point of the stent) from both IVUS and OCT images. Measurements in IVUS were performed according to the American College of Cardiology Clinical Expert Consensus standards for acquisition, measurement, and reporting of IVUS studies.¹⁰ OCT measurements were obtained as previously reported.^11,12 For qualitative analysis, we evaluated the presence of stent malapposition, stent edge dissection, tissue prolapse and thrombus. Stent malapposition was identified as a clear separation between at least one stent strut and the vessel wall in the IVUS images, and was defined as a distance between the center reflection of the strut and the vessel wall of greater than the actual stent thickness +20 µm (OCT resolution limit) in the OCT images.^5,7,8,12Stent edge dissection was defined as disruption of the luminal vessel surface in the edge segments.⁵Tissue prolapse was defined as protrusion of tissue between stent struts extending inside a circular arc connecting adjacent struts in both IVUS and OCT images.⁵Thrombus was defined as an irregular low echoic mass, often mobile and extruding into the vessel lumen and sometimes becoming detached from the vessel wall based on IVUS.¹³ On OCT image, intracoronary thrombus was defined as a protruding mass beyond the stent strut into the lumen with significant attenuation behind the mass.¹⁴ To differentiate thrombus from plaque protrusion, we excluded protruding masses without significant signal attenuation and surface irregularity from the category of thrombus.¹⁵ Representative images of stent malapposition, stent edge dissection, tissue prolapse, and thrombus are shown in Figure 1. Statistical analysis. Qualitative data are expressed as frequencies, and quantitative data are shown as mean values ± SD. Statistical analysis was performed with StatView 5.0 software (SAS Institute, Cary, North Carolina). Comparison of the data between IVUS and OCT regarding measurements of the lumen and stent parameters was performed with Wilcoxon signed rank sum test. Correlation between lumen parameters of proximal site and minimum stent CSA (cross-sectional area) in IVUS and OCT images was examined using Spearman’s correlation coefficient. Comparison of the frequencies of qualitative findings between IVUS and OCT findings was performed using a Chi-Square test. For all comparisons, a p-value

Results

The baseline characteristics and procedural summary of 20 lesions from 18 cases (65 ± 10 years of age) are shown in Table 1. Imaged coronary arteries included 8 in the left anterior descending coronary artery, 6 in the circumflex coronary artery, and 6 in the right coronary artery. Mean stent diameter was 3.0 ± 0.3 mm and stent length was 21.5 ± 6.7 mm. Comparison of OCT and IVUS images. In all 18 cases, both OCT and IVUS catheters were successfully advanced before and after PCI. Lumen CSA, maximum lumen diameter and minimum lumen diameter at the proximal site, minimal lumen site and distal site before PCI are shown in Table 2. Lumen parameters at the proximal site of the culprit lesion with OCT imaging were not different from that with IVUS imaging (lumen CSA: 5.5 ± 2.3 mm² vs. 5.6 ± 2.3 mm², maximum luminal diameter: 2.8 ± 0.6 mm vs 2.9 ± 0.5 mm, minimum luminal diameter: 2.5 ± 0.5 mm vs. 2.5 ± 0.6 mm). Lumen parameters on OCT images were well correlated with those of IVUS (lumen CSA: r = 0.968; p = 0.0002; maximum lumen diameter: r = 0.954; p = 0.0006; minimum lumen diameter: r = 0.921; p = 0.0009). On the other hand, lumen parameters at the distal site of culprit lesion on OCT images were smaller than those of IVUS (lumen CSA : 4.6 ± 2.0 mm² vs. 5.0 ± 1.8 mm²; p = 0.0004, maximum luminal diameter: 2.7 ± 0.5 mm vs 2.9 ± 0.5 mm; p = 0.0302, minimum lumen diameter, 2.3 ± 0.4 mm vs. 2.4 ± 0.4 mm; p = 0.0692). Lumen parameters at the minimum luminal site of OCT were smaller than those of IVUS (lumen CSA: 1.6 ± 0.8 mm² vs. 2.5 ± 0.5 mm²; p = 0.0051; maximum luminal diameter: 1.6 ± 0.5 mm vs. 1.9 ± 0.2 mm; p = 0.0092; minimum luminal diameter: 1.2 ± 0.3 mm vs. 1.7 ± 0.1 mm; p = 0.0014). A representative case of lumen area measurements by OCT and IVUS is shown in Figure 2. When there was a major branch between a Helios^™ balloon occlusion site and minimal lumen site (4 lesions), lumen CSA at the proximal site of culprit lesion on OCT images was smaller than that on IVUS images, although the number of lesions was too small to allow for a meaningful statistical assessment (Figure 3). Table 2 shows post-procedure minimum stent CSA, maximum stent diameter and minimum stent diameter in both OCT and IVUS images. These stent parameters measured with OCT were equivalent to those measured by IVUS (stent CSA: r = 0.999; p = 0.0001; maximum stent diameter: r = 0.968; p = 0.0002; minimum stent diameter: r = 0.982; p = 0.0002). Stent malapposition and tissue prolapse were observed significantly more often with OCT images than with IVUS images (Table 3). Immediately after PCI, stent malapposition was observed in 14 stents (70%) by OCT and 8 stents (40%) by IVUS. In the final result after additional balloon inflation, stent malapposition was observed in 6 stents (30%) by OCT and 1 stent (5%) by IVUS imaging. The characteristics of these 6 malapposed stents are shown in Table 4. There was no stent edge dissection detected with IVUS images, but 2 stent edge dissections (10%) were detected in OCT images. Tissue prolapse was identified in all stents in OCT images but in only 1 stent (5%) in IVUS images. Thrombus was observed in 3 stents (15%) by OCT imaging and in 1 stent (5%) by IVUS imaging.

Discussion

The present study demonstrated the following: (1) Before PCI, lumen parameters at the proximal site of the culprit lesion based on OCT were almost identical to those based on with IVUS. The lumen parameters at the distal site of the culprit lesion, however, were smaller based on OCT than on IVUS. (2) OCT was effective for detecting stent malapposition, edge dissection, tissue prolapse, and thrombus without significant procedural complications. Yamaguchi et al reported that minimal lumen area measured on OCT images correlated with those measured on IVUS images.¹⁶ However, they also reported detection of lumen border was more clearly visualized by OCT than IVUS in the tight stenosis lesion due to wedging of the IVUS imaging catheter. We believe the more accurate delineation of the lumen by OCT is associated with greater measurement accuracy. We examined lumen parameters in the proximal and distal sites of culprit lesions with OCT and IVUS images. Lumen parameters in the proximal site of the culprit lesion measured on OCT images were almost identical to those based on IVUS. On the other hand, lumen parameters at the distal site of the culprit lesion on OCT images, were smaller than those measured on IVUS images. This may be related to the decrease in intracoronary pressure during OCT imaging resulting from proximal vessel occlusion with a balloon. Also, while infusing Lactated Ringer’s solution, flow obstruction by the culprit lesion may play a role in producing higher intracoronary pressure at the proximal side compared with distal side by increasing the filling pressure at the proximal side, and decreasing filling pressure at the distal side. If there was a major branch between the balloon occlusion site and the culprit lesion, the pressure from the Lactated Ringer’s flushing was spread toward the branch, thus the lumen parameters of the proximal site measured on OCT images were smaller than those measured on IVUS images. The difference in lumen diameter between OCT and IVUS measurements is likely due to the narrowing of elastic arteries, particularly in the lesions with decreased coronary filling pressure at the distal site.¹⁶ This limitation of OCT imaging should be considered when stent size is determined based on OCT. Immediately after PCI, stent CSA, maximum stent diameter, and minimum stent diameter in OCT images were not different from those in IVUS images. A previous animal study demonstrated that these stent parameters were not different between OCT and IVUS images.¹¹ The CSA measured by OCT imaging may be an alternative as the endpoint of PCI. Stent malaposition and tissue prolapse were better detected on OCT images than on IVUS images. Immediately after stenting, OCT detected several cases with trivial malapposed struts, which were not detected by IVUS. Based on our OCT findings of stent malapposition, the conditions most likely to lead to stent malapposition were: coronary aneurysm, severely calcified and tortuous lesions, proximal edge of the long stents, LMT-bifurcated stenting, and no post ballooning due to slow flow. Further study is warranted by using serial OCT study to evaluate time course and clinical significance of malapposed struts and tissue prolapse seen on OCT imaging after stenting. An apparent thrombus was observed in 3 cases with OCT imaging. One case was in a patient with unstable angina, and OCT before PCI visualized a fibroatheroma with massive intracoronary thrombus, and the thrombus could be also detected in an IVUS image of this case. The other 2 cases were patients with stable angina. In one case, thrombus was observed immediately after ballooning. In the other case, the thrombus was flattened against the surface of the stent struts. Although several single-center studies have reported that OCT is superior to IVUS for visualization of microscopic structures of coronary arteries,^4,17–20 there is an inherent limitation of this technique because it requires that the blood is displaced during OCT image acquisition.^4,21,22 Therefore, for PCI guidance, it is important to evaluate the safety of this technique in a clinical setting. In our study population, transient cardiac symptoms were more frequently observed with OCT than IVUS. Bradycardia during balloon occlusion in OCT and ST-T changes on ECG were immediately resolved, and no major complications or adverse events were observed in PCI cases. Although newly developed FD-OCT system may not be suitable for severely calcified small vessel and very tight lesions due to its larger catheter size (FD-OCT: 2.7 Fr vs. TD-OCT: 0.16 Fr),²³ the FD-OCT system lacks these limitations as it does not require coronary artery occlusion and has faster pullback speed.²⁴ These advances will enable us to use OCT not only for PCI-guidance but also as a multivessel diagnostic modality, and may also provide a truly representative assessment of the entire coronary tree.^{24Study limitations.} There were several limitations to our study. First, this is a non-randomized retrospective study based on a relatively limited sample size, raising the possibility of selection bias. A study involving a larger population would be needed to establish and refine the clinical applications and safety of the OCT imaging system. Second, OCT has limited ability to visualize certain lesions, such as ostial lesions due to the risks associated with producing a blood-free environment by occlusion balloon. Non-occlusion flushing technique may be an alternative for visualizing proximal lesion, however, it has a limitation for scanning length.²⁵ Also, severely calcified tortuous vessels could not be imaged with OCT due to the difficulty of passing the occlusion balloon through the lesion. Finally, the current OCT system has a limited penetration depth, which can be a disadvantage in visualizing whole vessel structure. Therefore, if a new imaging device can achieve greater penetration depth without sacrificing its resolution (e.g., combined imaging device of IVUS and OCT), this may provide more comprehensive information, possibly offering more benefit during PCI. However, even with current generation OCT system, Kume et al reported that OCT had high sensitivity and specificity for characterizing the different types of atherosclerotic plaque as compared with IVUS. Hence, we believe that current OCT system can be at an advantage in terms of plaque characterization during PCI, which may be able to enhance the quality of PCI.

Conclusion

OCT is a feasible method for the evaluation of PCI procedure without serious complications. OCT could detect several stent deployment issues (e.g., stent malapposition, dissection, tissue prolapse and thrombus) more clearly than IVUS, although OCT was limited for assessing lumen size especially at the distal site of a culprit lesion. A new generation of OCT system (FD-OCT) without the need for proximal vessel occlusion may potentially provide more accurate assessment of lumen size at the distal site of a culprit lesion without transient myocardial ischemia.

References

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———————————————————— From the Kobe University Graduate School of Medicine, Division of Cardiovascular Medicine, Department of Internal Medicine, Kobe, Japan. Relevant disclosures: Drs. Shite and Shinke are consultants to the Goodman Co. Manuscript submitted June 21, 2010, provisional acceptance given July 19, 2010, final version accepted September 7, 2010. Address for correspondence: Junya Shite, MD, Associate Professor, Kobe University Graduate School of Medicine, Division of Cardiovascular Medicine, Department of Internal Medicine,7-5-1 Kusunoki-cho, Chuo-ku, Kobe, Hyogo, 650-0017, Japan. E-mail: shite@med.kobe-u.ac.jp