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May/June 2011
How Do OCT and IVUS Differ?
A comparison and assessment of these modern imaging modalities.
By Joshua Waggoner, MD, and Marc D. Feldman, MD, FACC, FSCAI
Optical coherence tomography (OCT) is a new, light-based, intravascular imaging technique that provides high-resolution, cross-sectional images of coronary artery anatomy. OCT image acquisition is analogous to intravascular ultrasound (IVUS), except it uses near-infrared light instead of ultrasound. This use of light gives OCT an approximate 10-fold higher resolution than IVUS (10–15 µm compared to 100–150 µm), allowing for increased ability to visualize vessel wall anatomy, characterize plaque, and assist with short- and long-term follow-up of coronary interventions.
Recent advances in OCT technology have allowed faster image acquisition, precluding the need for proximal balloon occlusion and making it easier and safer to use. This ease in acquisition has led many to believe that OCT will replace IVUS as the gold standard for intravascular imaging. However, OCT remains inferior to IVUS in regard to its depth of penetration (1.5 mm compared to 5 mm), which limits its ability to assess plaque burden and vessel remodeling, and in its inability to image ostial disease (eg, unable to clear blood in the ostium). This article focuses on the different uses of OCT and its advantages and limitations compared to IVUS.
PLAQUE CHARACTERIZATION
Arterial Morphology
OCT has proven to be a powerful tool in visualizing coronary
anatomy. Due to its superior resolution, OCT has the
ability to identify pathology that could be missed by IVUS.
For example, intimal thickening is an early phase of atherosclerosis
and provides important prognostic information.1-6
IVUS imaging can only indirectly measure intimal thickness
due to its inability to identify the internal elastic lamina and,
therefore, the intima media border.7 OCT is more accurate
in measuring intima media thickness than IVUS and can
visualize intimal thickness, intimal hyperplasia, and the internal
and external elastic lamina.8,9 However, in the presence
of heavy plaque burden, OCT lacks the depth of penetration to visualize the external elastic lamina, whereas IVUS
easily can.10 Thus, plaque burden, an important predictor
of clinical outcome, is more readily quantitated with IVUS.
Plaque Composition
Both IVUS and OCT have shown effectiveness in characterizing
plaque composition. Originally, conventional
IVUS was limited in its assessment of plaque composition
to fibrofatty, calcified, or “soft.” However, new applications
of IVUS, such as integrated backscatter, wavelet
analysis, and virtual histology, have allowed IVUS to characterize
plaques as lipid, fibrous tissue, calcification, or
necrotic core with high accuracy.11-15 The effectiveness of
OCT in this arena was first described in 2002, when
Yabushita et al showed that OCT is highly sensitive and
specific at characterizing plaque type.16
OCT's superior resolution—its ability to see plaque microstructures and tissues adjacent to calcium—makes it superior to both grayscale IVUS and radiofrequency IVUS in characterizing plaque type. 8,17-19 This superiority is especially apparent when identifying lipid-rich plaques. 17,18 However, OCT's limited depth of penetration precludes the full visualization of large plaques and causes OCT to occasionally misclassify thick-capped plaques as fibrous or fibrocalcific. 17 IVUS, on the other hand, can accurately quantify large lipid pools and visualize the entire vessel wall, even in the presence of large plaque burden. 10,1
Plaque Vulnerability
Rupture and subsequent thrombosis of coronary
plaque is believed to be the cause of most acute coronary
syndromes based on autopsy studies.20-22 Therefore,
identifying plaque features that impart vulnerability has
emerged as a potential tool in preventing acute coronary
syndromes. Pathohistologically, vulnerable plaques have
been described as hypocellular, lipid-rich with necrotic
cores, and covered by a thin cap (< 65 µm). OCT and
IVUS have different strengths when evaluating vulnerability.
OCT is superior to IVUS in visualizing thin caps (Figure 1)8,24 and has even shown the ability to visualize
cap composition, which can influence cap stability. For
example, both collagen content and macrophage density
are important markers of cap structural integrity.25-31
Tearney et al found that OCT has the potential to quantify
cap macrophage content,32 although the specificity of
this finding is unclear, and microcalcifications are known
to have a similar appearance.33 Several groups have also
shown OCT's ability to quantify cap collagen content
and smooth muscle density with the use of polarization-
sensitive OCT.34,35 Moreover, OCT has better accuracy
in identifying lipid-rich plaques,17-19 and it can visualize
and qualify intracoronary thrombus as being platelet
or red blood cell predominant.10,36
However, due to its depth of resolution, IVUS can assess plaque burden and vessel remodeling (important components of vulnerability), whereas OCT cannot. Additionally, OCT has the propensity to misinterpret signal-poor regions in the deeper vessel wall as a necrotic core, which can lead to false labeling of plaques as vulnerable.23 Due to these differing strengths, some contend that the use of combined IVUS and OCT to evaluate plaque vulnerability is superior to any one technique alone.37,38
VESSEL SIZE
Several studies have been performed comparing luminal measurements between OCT and IVUS, with conflicting results. Kawase et al performed measurements on stented pig arteries and reported no differences in lumen areas and volumes between the two imaging techniques. 39 However, several other studies performed in stented arteries have found minimal to no correlation between the two modalities for luminal measurements.40-42 In these studies, OCT is consistently found to have smaller minimal lumen area measurements than IVUS. One possible reason for this discrepancy is the superior ability of OCT to visualize the lumen-intima interface compared with IVUS,9 therefore allowing OCT to visualize the true lumen dimensions and causing IVUS to overestimate.
Supporting this theory is the fact that IVUS is known to overestimate lumen area compared to quantitative angiography measurements.43-45 Another possible explanation for this discrepancy is the fact that most of these studies were performed with time-domain OCT, which requires proximal balloon occlusion. Balloon occlusion causes a decrease in coronary perfusion pressure that is not compensated by continuous flush injection, therefore causing partial vessel collapse.
In support of this hypothesis, Gonzalo et al showed that acquisition technique appears to have an impact on lumen measurements.46 This study found greater differences in minimal lumen area measurements between IVUS and OCT with balloon occlusion than between IVUS and OCT without balloon occlusion (Figure 2). Further complicating this issue, they also found that both IVUS and OCT (with either acquisition technique) overestimate lumen area compared with histology. The proposed explanation is removal of water content from histology specimens causing vessel shrinkage and thereby falsely lowering luminal areas. Due to OCT consistently underestimating lumen area compared with IVUS, caution should be used before using literature-validated IVUS parameters to assess lesion significance by OCT. Further studies are needed to both validate OCT lumen measurements and to correlate OCT measurements with lesion significance.
USE IN PERCUTANEOUS CORONARY INTERVENTION
IVUS has been the gold standard for intravascular imaging during percutaneous intervention and monitoring the vascular response to coronary stenting. However, the low resolution of IVUS and the nature of soundbased imaging limit the ability to accurately assess fine architectural changes and vascular responses to coronary stents.39 Furthermore, the ability to study the vessel wall adjacent to the struts is impeded by the bright signal reflection of the metal struts.39 Due to its higher resolution and the fact that most of the relevant structures during this evaluation are located within OCT's depth of penetration (< 500 µm), OCT has the potential to be superior to IVUS in this specific patient population.47
Stent Apposition
Incomplete stent apposition is defined as separation of
at least one stent strut from the vessel wall.48 Several IVUS studies have suggested a possible correlation
between incomplete stent apposition and subsequent
stent thrombosis.50-52 Recent studies have shown that
very-small-sized stent malappositions can be detected by
OCT,53,54 and further studies have proven its superiority
to IVUS in detecting them.55,56 However, the long-term
clinical implications of stent malapposition discovered by
OCT are not clearly understood.
In an OCT study, Kim et al found that the surfaces in almost all minimally malapposed struts were covered by neointima and, therefore, are unlikely to cause clinically significant thrombosis (Figure 3).49 Furthermore, they found that minimal stent malapposition, which is not detectable by IVUS, may disappear or decrease in followup and may be benign.49 In contrast, Ozaki et al discovered that thrombus was seen in a significantly greater proportion of stent struts that were characterized as malapposed by OCT at 10-month follow-up.56 To further elucidate whether these stent malappositions visualized by OCT bear any clinical consequences, more large, longterm prospective studies will be necessary.
Intimal Coverage After Drug-Eluting Stent
Implantation
IVUS has been used extensively to assess neointimal
coverage months after percutaneous coronary intervention.
57 However, drug-eluting stents inhibit neointimal
proliferation to such an extent that it may not be
detectable by IVUS.58-60 On the other hand, the higher
resolution of OCT allows for the visualization and measurement
of thin layers of tissue growth covering
stents.61,62 In a 6-month follow-up of 34 patients with
drug-eluting stents, Matsumoto et al found that 65% of
the struts were covered by a thin neointima undetectable
by IVUS (< 100-µm thickness), with a median
tissue thickness of 52 µm (Figure 4).53 Further studies
have shown that OCT is superior to IVUS in detecting a
small amount of in-stent neointima.42
Due to its high resolution, in addition to quantifying neointimal hyperplasia (NIH) thickness, OCT can also be used for qualitative assessments of NIH (to determine if it is homogeneous, heterogeneous, or layered). 63 This could help in understanding the mechanism of in-stent restenosis. An attempt to use IVUS to assess different NIH morphology resulted in disappointing results. Kwon et al found that IVUS cannot distinguish patterns of NIH morphology when < 15% of the lumen cross-sectional area is occupied.64
Furthermore, IVUS measurements only had a moderate correlation with OCT results, and the assessments differed in approximately 30% of lesions.64 Histological validation of these measurements has not been performed, and the long-term prognostic implications are unknown, but this assessment remains a promising area of future research.
Complications in Percutaneous
Coronary Intervention
IVUS imaging has played an important role in understanding
failure and optimizing outcomes after stent
implantation. However, the lack of resolution and artifacts
caused by stent struts make visualization of small morphologic
changes in vessel structure after stent implantation
difficult. In contrast, the high resolution of OCT
allows the visualization of these small defects, such as tissue
prolapse and small-edge dissections, with improved
accuracy over IVUS.55,65 Although the clinical implications
of these findings are unclear, the ability of OCT to clearly
visualize these phenomena will allow researchers to further
study their impact on clinical outcomes.
BIODEGRADABLE STENTS
The recent results of the ABSORB trial and the subsequent European approval of the first biodegradable everolimus-eluting stent (Absorb BVS, Abbott Vascular, Santa Clara, CA) is potentially ushering in a new era in interventional cardiology. However, with this new technology come new challenges pertaining to imaging of these new stents. The Absorb stent is translucent and radiolucent, thus making visualizing these stents with traditional modalities difficult. The potential of OCT to quantitatively assess strut thickness and biodegradation makes it an ideal imaging modality for monitoring these stents.
Recent studies have proven the ability of OCT to precisely characterize stent apposition and stent strut coverage and to demonstrate structural changes in the bioresorbable drug-eluting stent over time (Figure 5).66 Furthermore, a head-to-head study comparing the ability to measure stent length proved OCT superior to both IVUS and quantitative coronary angiography, and its measurements correlate well with the known length.67
In the ABSORB trial, OCT was able to show serial changes in the optical properties of the struts over time,68,69 hypothesized to be reflecting the bioresorption process. However, in a follow-up study attempting to validate this hypothesis, Onuma et al found that OCT might not be sensitive enough to determine polymer degradation because of a lack of correlation between OCT images and histology.69 Given these findings, OCT will likely be the imaging modality of choice when assessing biodegradable stents.
ARTIFACTS
Both IVUS and OCT are prone to artifacts during image acquisition. Certain artifacts are shared by both IVUS and OCT, whereas others are unique to their respective imaging technique. IVUS catheters are afflicted by artifacts related to sound-based imaging (ring down and blood speckle), whereas OCT experiences artifacts that are unique to light-based imaging (saturation artifact and artifacts related to residual blood in the artery). The artifacts shared by both techniques are mostly related to mechanical issues, such as motion artifacts and nonuniform rotational distortion (mechanical IVUS). OCT's smaller profile and simplified rotational mechanics make it less susceptible to these mechanical artifacts.
A recent study using a phantom model found that OCT experienced less nonuniform rotational distortion than IVUS, particularly in a more tortuous vessel.70 Interestingly, the artifacts seen by OCT due to eccentric wire position, such as sunflower and merrygo- round effect, likely occur with rotational IVUS, but the resolution of OCT allows clearer visualization of these image misrepresentations.
CONCLUSION
Although the resolution of OCT allows for superior ability to characterize plaques, assess vessel size, and provide short- and long-term follow-up of coronary stenting, its use does not completely eliminate the need for IVUS imaging technologies. Due to the superior depth of penetration of IVUS, its use is still necessary to characterize and measure plaque burden, assess vessel remodeling, and view deep vascular structures, such as coronary adventitia.
Joshua Waggoner, MD, is a Fellow in Cardiovascular Diseases with the Department of Medicine, Briscoe Division of Cardiology at the University of Texas Health Sciences Center at San Antonio in San Antonio, Texas. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Waggoner may be reached at waggonerj@uthscsa.edu.
Marc D. Feldman, MD, FACC, FSCAI, is Professor of Medicine & Engineering, and Director of Cardiac Catheterization Laboratories at the University of Texas Health Science Center at San Antonio in San Antonio, Texas. He has disclosed that he receives grant/research funding from Volcano Corporation.
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