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July/August 2010
High-Resolution Lesion Assessment With OCT
What every interventional cardiologist needs to know about this FDA-approved imaging modality.
Cardiovascular optical coherence tomography (OCT) uses light and its reflection to generate high-resolution images of the arterial wall. Although this technology has been available outside of the United States for some time, the recent US Food and Drug Administration approval of the C7-XR OCT device (LightLab Imaging Inc., a St. Jude Medical company, St. Paul, MN) has brought this new technology into the hands of interventional cardiologists across the United States. Thus, OCT is poised to provide a new perspective on intravascular imaging and lesion assessment in the United States. This article elucidates the basic principles of OCT, defines possible calibration and imaging artifacts with OCT, and identifies potential clinical applications of this technology for operators who are unfamiliar with its use.
PRINCIPLES OF OCT
OCT was developed from concepts of light reflectometry,
initially in one dimension and then in two dimensions.
1,2 The technique was adopted in a variety of biomedical
applications including high-resolution retinal
scanning.3 Intravascular OCT was subsequently developed
with a fiber optic wire that both emits near-infrared light
(1,250–1,350 nm) and records its reflection while rotating
360°.4 At these wavelengths, tissue penetration ranges
between 1 to 3 mm, in contradistinction to the 4- to 8-mm
penetration achieved with intravascular ultrasound (IVUS).5
Despite the lower penetration depth, OCT has an axial resolution
of 12 to 18 µm compared to 150 to 200 µm for IVUS,
with a lateral resolution of 20 to 90 µm compared to 150 to
300 µm for IVUS.5 The biophotonics and physics of OCT
are well reviewed elsewhere, but it is generally held that,
in a typical human coronary artery, the use of OCT
results in a gain in resolution when compared to IVUS
(Figure 1).1,5-9
To obtain these images, blood must be cleared from the vessel and replaced with a solution with a known index of refraction. Typically, saline or contrast must be injected into the artery to obtain a clear image. Early versions of OCT imaging required balloon occlusion of an artery for up to 30 seconds during image acquisition. However, subsequent generations of OCT technology with faster pullback speeds have reduced image acquisition times to < 3 seconds per artery, which is approximately the same duration as that of a standard coronary injection for diagnostic angiography.5,10 Naturally, this advance in image acquisition speed obviates the need for long ischemic times to obtain OCT images.
COMMERCIAL OCT SYSTEMS AVAILABLE
IN THE UNITED STATES
Although earlier-generation, time-domain OCT systems
have been available in Europe, Asia, and South America, the
first OCT system approved by the US Food and Drug
Administration (granted on May 5, 2010) is the frequency
domain C7-XR, with its companion C7 Dragonfly catheter.
The C7-XR system acquires images at a rate of 100 frames/s
at a pullback speed of up to 10 to 20 mm/s, with the potential
to scan up to a 5-cm length of a coronary artery in
under 3 seconds.10,11 Rapid pullback speeds enable short,
low-volume bolus injections of contrast to sufficiently clear
the selected coronary artery for high-resolution OCT image
acquisition. The C7-XR is used with the Dragonfly imaging
catheter, a monorail 2.7-F catheter system that is compatible
with standard 6-F guide catheters and has a light source in
an optical fiber that is encased in a rotating torque wire.
Image acquisition is accomplished after coaxial engagement of the guide catheter, advancement of a standard 0.014-inch coronary guidewire into the distal coronary artery, and advancement of the OCT catheter beyond the area of interest in the vessel. Typically, automated powerinjection of contrast (14 mL in the left coronary circulation and 10 mL in the right coronary circulation, as is the protocol in our catheterization laboratory) over 3 seconds will sufficiently clear blood from the vessel to adequately image the vessel with OCT. Contrast is preferred to saline because its viscosity more effectively clears blood from the vessel. Once armed, the C7-XR console will automatically sense clearance of blood from the vessel and will initiate an automated pullback at 10 mm/s.10 Images will then be displayed on the console unit for consideration by the operator.10 Previous generations of OCT have proven to be safe and comparable to IVUS in clinical evaluation.12-14 However, as with IVUS, the OCT catheter can cause transient occlusion of the vessel in areas of tight stenosis and cause ischemia or injury to the vessel. In addition to the C7-RX system, Terumo Interventional Systems and Volcano Corporation are also developing OCT systems.5 The differences between OCT and IVUS are depicted in Figure 1 and Table 1.
OCT IMAGE INTERPRETATION: CALIBRATION
AND IMAGING ARTIFACTS
Although intravascular imaging principles are familiar to
most interventional cardiologists given the prevalence of
IVUS, several key features are unique to OCT and should be
understood by practitioners who are considering its use.
One term that may be new to those uninitiated in OCT
imaging is the “Z-offset,” which is an operator-adjustable
manual image calibration. This parameter is particularly
important for obtaining accurate measurements. In the C7-
XR system, four crescent-shaped marks in the OCT image
delineate the outer boundary of the OCT catheter in situ,
and proper alignment of these markers coincident with the
catheter image will ensure accurate intravascular measurements.
5 Calibration by adjusting the Z-offset is critically
important—previous work in our OCT imaging core laboratory
has shown that 1% differences in Z-offset calibrations
resulted in 12% to 14% differences in vascular area calculations.
5 Thus, during the initial training period with OCT,
particular attention must be paid to this parameter.7
In addition to calibration concerns, there are several possible
imaging artifacts that may be observed in OCT imaging.
As with IVUS, there are no substitutes for experience
and training. We list some common imaging artifacts for
reference here and depict them in Figure 2:
- Residual blood is the most common artifact and can obscure imaging of the vascular wall.
- Nonuniform rotational distortion is a result of rotational bias of the imaging system with resultant image distortion similar to that seen with IVUS.
- Sew-up artifact results in misalignment of subsequent images from pullback.
- Saturation artifact is observed when particularly dense objects, such as stent struts, are imaged and appear bright or “bloom” on OCT imaging.
- Fold-over artifact results in signal dropout at the sites of side branches or vessels larger in diameter than the dynamic range of OCT.
Vascular measurements with OCT are similar to those made via IVUS. Some common measurements are listed in Table 2. Although real-world validation has not yet been performed, typically, a minimal luminal area of 4 mm2 found in an epicardial coronary artery (not the left main) is thought to represent a significant stenosis when OCT imaging is undertaken, as it would be with IVUS.15
POTENTIAL CLINICAL
APPLICATIONS OF OCT
The images obtained with OCT are often breathtaking.
However, from a clinical perspective, these high-resolution
images have very practical applications in both preinterventional
assessment of atherosclerosis and in guiding
interventions. Thus, it is in these two areas that OCT is likely
to gain a foothold in clinical practice.
IMAGING OF ATHEROSCLEROSIS
Given the extremely high-resolution of OCT images,
diagnostic lesion assessment with this technology can
provide startling images of the atherosclerotic plaque.
Imaging the various biological components of active atherosclerotic
plaque has been reported by many investigators,
with each constituent element of the plaque having
particular imaging characteristics on OCT (Table 3).16-22 A
great deal of investigation has assessed the role of the
thin-capped fibroatheroma in the pathogenesis of acute
coronary syndromes.6,19,23-25 These lesions typically consist
of a thin fibrous cap, an inflammatory cell infiltrate at the
margins of the cap, and a lipid core.6 Although OCT cannot
always penetrate very deeply into the vascular wall, its
luminal resolution provides extremely reliable imaging of
the thin fibrous cap, which may identify a vulnerable
lesion that is at risk for rupture. Although the definition of
vulnerable plaque is controversial, OCTcan identify ruptured
plaques with high degrees of accuracy (Figure 3).5,17,26
Vascular inflammation with macrophage-rich infiltrates
have previously been identified with OCT; however, it
remains unclear if quantitative assessment of an inflammatory
cell burden may be imputed from an OCT
image.11,19,22 Unlike IVUS, the penetration of vascular calcium
by light results in images with well-defined boundaries
of calcium (Figure 3), and sensitivity and specificity
for identification of calcium have been determined to be
96% and 97%, respectively.17 Finally, although some operators
claim to be able to differentiate red from white
thrombus, our experience with this is less definitive.5,27
To summarize, OCT provides extremely high-resolution
images of atherosclerotic plaque, particularly the fibrous
cap with possible plaque rupture and the presence of calcium,
thrombus, and a lipid core. For lesion assessment,
these characteristics will demonstrate superior image clarity
and accuracy when evaluating coronary stenoses,
which should improve decision making when planning
percutaneous coronary intervention (PCI) or an alternate
therapy.
OCT-GUIDED PCI
During PCI, OCT imaging can be particularly helpful in
lesion sizing (both diameter and length), and the added resolution
of OCT can identify culprit plaque rupture and
stent malapposition, as previously discussed. Better lumen
quantification remains the primary advantage of intravascular
imaging over angiography in PCI because the sharp contrast
between the lumen and the vessel wall obtained with
OCT has practical advantages over IVUS. The images are
often easier to interpret, and fully automated lumen segmentation
reduces guesswork in lesion length determination.
The measurements previously mentioned (Table 3) will
generally facilitate correct selection of stent diameter and
length in PCI. OCT has substantial advantage over IVUS in
PCI in its ability to image individual stent struts, both for
apposition in the setting of performing an intervention or
for reviewing stent coverage after stent placement in the setting
of restenosis or thrombosis. Figure 4 depicts a common
scenario in interventional cardiology in which a stented segment
has both an edge dissection and malapposed struts,
both of which were missed on angiography. Further postdilation
in this case ensured proper stent apposition and sealing
of the edge dissection, which was not previously visible
on angiography. In fact, in clinical trials, OCT imaging has
increasingly been used to assess stent apposition and individual
strut coverage as a safety endpoint.5,7,28-31 Whether
these correlations will further enhance our understanding of
the biology of the vascular response to injury remains to be
seen.26,31
FUTURE APPLICATIONS OF
CARDIOVASCULAR OCT
In addition to the research applications of OCT in clinical
trials, it is hoped that future applications of OCT will focus
on enhanced imaging of atherosclerotic plaque constituents
via image processing algorithms. Practically speaking, the
advent of bioabsorbable stents will likely mandate the use of
OCT because IVUS does not have the resolution or capability
of imaging entire polymeric stent struts.5,32-36 Moreover,
contrast-enhanced and molecular OCT remain areas of active investigation at our institution and others. Finally, the
combination of physiologic with anatomic imaging, such as
Doppler-like signal processing from the OCT catheters, may
well be available in the future.5 This potent combination of
physiology with high-resolution lesion morphometry will
likely again transform our understanding of interventional
lesion assessment.
CONCLUSION
Given the current state of OCT technology, there appear
to be clear advantages over IVUS in the assessment of atherosclerotic
plaque morphology and the outcomes of PCI
with respect to stent apposition. However, like any new
technology, OCT will require a learning curve, and consistent
experience with multiple imaging modalities will further
improve our ability to reliably obtain and interpret
OCT images and optimize treatment of our patients. We
welcome OCT into our armamentarium for lesion assessment
and look forward to the evolution of this technology
and its clinical application.
Sahil A. Parikh, MD, is Assistant Professor of Medicine, Case Western Reserve University School of Medicine; Director, Experimental Interventional Cardiology Laboratory at the Harrington-McLaughlin Heart and Vascular Institute, University Hospitals Case Medical Center in Cleveland, Ohio. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Parikh may be reached at (216) 844-5170; sahil.parikh@uhhospitals.org.
Marco A. Costa, MD, PhD, is Professor of Medicine, Case Western Reserve University School of Medicine; Director, Interventional Cardiovascular Center; Director, Center for Research and Innovation at the Harrington-McLaughlin Heart and Vascular Institute, University Hospitals Case Medical Center in Cleveland, Ohio. He has disclosed that he receives consulting honoraria and research funding from LightLab Imaging, Inc. Dr. Costa may be reached at (216) 844-5840; marco.costa@uhhospitals.org.
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