by Farhad Abtahian MD, PhD, Koji Kato, MD, and Ik-Kyung Jang MD, PhD
Coronary angiography is the standard technique for the diagnosis of
coronary artery disease and assessment of coronary plaques. Angiography also
guides the vast majority of endovascular interventions. Unfortunately,
angiography has significant limitations in assessing physically small but
pathologically significant coronary lesions and identifying complications after
implantation of endovascular stents.
As a result, there has been significant interest in the development of
new intravascular imaging techniques.
Figure 1. The three
general plaque types: a thin fibrous cap (A); a lipid plaque (B); and a
fibro-calcific plaque (C).
Images: Farhad Abtahian, MD, PhD
Currently, the most commonly utilized method for endovascular imaging is
IVUS. IVUS allows visualization of coronary artery to a depth of 10 mm with a
resolution of 150 mcm.
Optical coherence tomography (OCT), the newest endovascular imaging
technique, allows significantly more detailed imaging, but at lower tissue
penetration. The function of OCT is analogous to IVUS with light as the imaging
modality instead of sound. It uses near-infrared light and measures the
magnitude and echo time delay of reflected light. As a result, tissue is imaged
to a depth of 2 mm to 3 mm on a micrometric (ie, sub-cellular) scale.
OCT advantages, limitations
Although the use of light allows far clearer distinction of vessel lumen
and wall, coronary plaque components and endovascular stents compared with
sound, it also results in the main limitation of OCT — the lack of
penetration into the vessel wall beyond 3 mm. The vessel adventitia cannot be
imaged and total plaque burden cannot be determined. Because blood scatters
light, the vessel being imaged must be flushed with either saline or contrast
as images are obtained. Current OCT systems utilizing a rapid automated
pull-back system can image vessels at a rate of 20 mm/second, minimizing the
flush time required during image acquisition. They can readily be incorporated
into catheterization procedures using a 6 French guiding catheter with low
A singular advantage of OCT is the detailed characterization of
superficial coronary plaque components. Features of plaques that are believed
to predispose to rupture include a thin fibrous cap, a lipid core and
accumulation of macrophages. The high resolution of OCT permits the
identification of the plaques that contain these features.
Histological analysis of autopsy specimens has revealed three general
plaque types: fibrous, fibro-calcific and lipid-rich. OCT has showed ex vivo to
accurately identify the three plaque types (Figure 1). Sensitivity of OCT for
fibro-calcific and lipid-rich plaques is notably higher than for fibrous
plaques. Plaque rupture occurs most often at sites where the fibrous cap is
thinnest. OCT is unique among intravascular imaging techniques at measuring cap
thickness. Cadaveric studies have confirmed the ability of OCT to accurately
measure the thickness of collagen caps. In vivo studies of patients presenting
with either acute coronary syndrome or stable angina revealed thinner plaque
caps (as measured by OCT) in ACS patients. As with thinner caps, increasing
plaque lipid content tends to correlate with plaque instability and is more
commonly seen in patients presenting with ACS.
Figure 2. A well-apposed
OCT is also significantly more sensitive than IVUS for the detection of
such lipid-rich plaques and allows identification of plaque macrophage density.
Increased density of macrophages, especially near points of plaque rupture, has
been seen in patients presenting with ACS. Because it can identify many of the
key characteristics of vulnerable plaques, OCT is a promising tool both to
study the pathobiology of atherosclerosis and potentially to guide treatment.
OCT and PCI
Besides its role in characterizing atherosclerotic plaques, OCT can
provide significant information to guide coronary interventions. Before
percutaneous coronary intervention, OCT can accurately determine the reference
vessel diameter and minimal luminal diameter of the target vessel. The length
of the lesion can be measured and the lesion characteristics (such as lipid and
calcium content) can be defined before intervention. These plaque
characteristics, as defined by OCT, can be predictive of post-procedural MI.
During intervention and immediately post-intervention, OCT can identify
stent malapposition, tissue prolapsed and both in-stent and edge dissection
with significantly higher sensitivity than IVUS (Figures 2 and 3). OCT is also
beneficial in assessing stent apposition with overlapping stents.
Figure 3. A malapposed
The clinical significance of malapposition and dissection as visualized
by OCT has not been firmly established. In small studies, malapposition of
stents appears to correlate with poor endothelialization of the stent and may
be a risk factor for late stent thrombosis. During long-term follow-up after
PCI, OCT can assess the degree of late-acquired malapposition, strut tissue
coverage and neointimal hyperplasia. As a result, OCT has been utilized to
compare various stent platforms in terms of stent deployment,
endothelialization and in-stent restenosis. For example, paclitaxel-eluting
(Taxus, Boston Scientific) and zotarolimus-eluting (Endeavor, Medtronic) stents
have been shown to have lower rates of malapposition and fewer exposed stents
at 9 months compared with sirolimus-eluting stents (Cypher, Cordis). OCT can
characterize different pathological processes resulting in-stent restenosis and
Recent studies have shown that late in-stent restenosis, unlike early
in-stent restenosis, is a result of de novo atherosclerosis rather than
neointimal hyperplasia. Although OCT is clarifying our understanding of
coronary biology and intervention, outcome data showing the clinical
significance of pathology seen by OCT is still lacking. Clinical trial and
registry data showing clinical significance are needed to transform OCT from a
research tool into a clinical tool.
Barlis P. EuroIntervention.
Barlis P. Eur Heart J.
Bouma BE. Heart. 2003;89:317-320.
Burke AP. N Engl J Med.
Cook S. Circulation.
Diaz-Sandoval LJ. Catheter Cardiovasc
Herrero-Garibi J. Rev Esp Cardiol.
Huang D. Science. 1991;254:1178-1181.
Hou J. Heart. 2010;96:1187-1190.
Jang IK. Circulation.
Jang IK. Circulation. 2001;104:2754.
Kawasaki M. J Am Coll Cardiol.
Kim JS. Int J Cardiovasc Imaging.
2011; doi:10.1007/5 10554-011-9943-X.
Kim JS. Circ J. 2010;74:320-326.
Kim JS. Heart. 2009;95:1907-1912.
Kume T. Am Heart J. 2006;152:755:
MacNeill BD. J Am Coll Cardiol.
Sawada T. J Cardiol. 2008;52:290-295.
Suh WM. Circ Cardiovasc Imaging.
Tanigawa J. EuroIntervention.
Tearney GJ. Circulation.
Yabushita H. Circulation.
Yonetsu T. Int J Cardiol.
Farhad Abtahian, MD, PhD, is a cardiology fellow
at Massachusetts General Hospital, Boston; Ik-Kyung Jang, MD, PhD, is
the director of the Clinical Trials Program at Massachusetts General Hospital
and is a board member of Cardiology Today Intervention; Koji
Kato, MD, is a research fellow in the Cardiology Division of the
Massachusetts General Hospital.
Disclosure: Drs. Abtahian and Kato report no
relevant financial disclosures; Dr. Jang has received research grant and
consulting fees from St. Jude Medical-LightLab.