IVUS

INTRAVASCULAR
ULTRASOUND
An overview

WHAT WE’RE GOING TO DISCUSS
• Basic principles of IVUS
• IVUS technology
• Interpretation of an image
• Diagnostic applications
• Interventional applications
• Advanced IVUS technology

INTRODUCTION
• Contrast angiography has been the gold standard of coronary artery imaging for
over six decades
• However, angiograms only delineate the coronary lumen with no direct imaging or
examination of the arterial wall

INTRODUCTION
• Coronary arteries are constantly moving structures with complex, three-dimensional
lumen shapes and atherosclerotic lesions of various distributions and compositions
• Critically important for the angiographer to understand the limitations of the
technique
• Fluoroscopy and cine angiography have limited resolution that is not adequate to
delineate all the information needed from a coronary angiogram
• Interpretation of coronary angiograms has traditionally been marked by significant
interobserver and intraobserver variability

INTRODUCTION
• Intravascular ultrasound (IVUS) imaging fills a gap in our understanding of
coronary disease
• It allows in vivo examination of both the arterial wall and the lumen of the coronary
arteries with high resolution
• IVUS imaging remains the most mature adjunctive intravascular imaging modality
• There is a large body of literature to support its application

CORONARY ATHEROSCLEROSIS
• Because angiograms are essentially contrast silhouettes of the lumen, it is not
possible to directly detect arterial wall thickening
• This information is inferred by a reduction in the diameter of the luminogram in one
segment compared to an adjacent one
• This is based on the assumption that narrower segments are diseased, while
adjacent larger segments are the normal size of the artery or at least of its lumen
• But autopsy studies have shown that atherosclerosis is diffuse, affecting almost all
segments of an artery

CORONARY ATHEROSCLEROSIS
• Therefore “focal” lesions are really the more diseased sites, and the “normal” or
“mildly irregular” segments are almost always diseased as well
• This leads to angiographic underestimation of disease severity and degree of
narrowing at the worst lesion sites

ARTERIAL REMODELLING
• First described by Glagov et al
• Describes the outward displacement of the arterial wall with increasing plaque size
• Compensatory enlargement allows the artery to accommodate a certain amount of
plaque volume without affecting the size of the lumen
• Stenoses develop only when this mechanism is overwhelmed by increasing plaque
size
• Therefore, by definition, interpretation of contrast luminograms is not helpful in
recognizing early, “well-compensated” stages of disease
• Remodeled arterial segments with significant plaque burden can thus be described
as “angiographically normal”

ARTERIAL REMODELLING
• “Negative remodeling” can also contribute to development of luminal narrowing in a
reverse manner
• The reduction in the size of the artery itself contributes more to the reduction in
lumen size than what the size of the plaque would indicate
• Remodeling index is calculated by
dividing the EEM area at the lesion site
by that of the proximal reference segment

COMPLEXITY OF CORONARY ANATOMY
• Arteries give off branches in different planes in a three-dimensional space
• Lesions may be
eccentric in distribution,
at bifurcation points,
ostial in location,
calcified, or
have complex luminal topography
• It takes a perfect orthogonal angiographic projection to delineate accurate
information – rarely possible

IVUS IMAGING
BASIC IMAGE,
AND MEASUREMENTS

IVUS IMAGING
• Imaging is performed using a miniaturized ultrasound transducer
• Placed on the tip of a catheter
• Usually 3/3.5 Fr and compatible with standard 0.014 inch wires
• Usually can be used in a 6 Fr guide

IVUS IMAGING
• Sector images are generated by emitting and receiving reflected ultrasound beams
• Two different technologic approaches are used
a. mechanical systems in which the transducer is a single large piezoelectric crystal
that is rotated at high speed to acquire images from all sectors of the circumference
b. phased array systems in which transducers are made of multiple small crystals
that are sequentially activated to image adjacent sectors of the arterial cross section
(Solid-state system)

CATHETER DESIGN
• Both types of catheters generate a
360-degree,
cross-sectional image plane that is
perpendicular to the catheter tip
• The reflected ultrasound waveforms
are processed into grayscale images
and
the sectors are reconstructed into
the full tomographic cross section of
the artery

SOLID-STATE SYSTEM
• In the solid-state approach, the
individual elements of a
circumferential array of
transducer elements mounted
near the tip of the catheter are
activated with different time
delays, to create an ultrasound
beam that sweeps the
circumference of the vessel

SOLID-STATE SYSTEM
• As the number of elements has increased,
there have been progressive improvements in
lateral resolution
• Complex miniaturized integrated circuits in
the catheter tip control the timing and
integration of the transducer activation and
route the resulting echocardiographic
information to a computer, where cross-
sectional images are reconstructed and
displayed in real time

MECHANICALLY ROTATING SINGLE-
TRANSDUCER SYSTEM
• In the mechanical approach, a single
transducer element is rotated inside the tip
of a catheter via a ﬂexible torque cable spun
by an external motor drive unit attached to
the proximal end of the catheter

MECHANICALLY ROTATING SINGLE-
TRANSDUCER SYSTEM
• Use of mechanical catheters is similar to the use of solid-state catheters, except that
mechanical catheters require ﬂushing with saline before insertion to eliminate any
air in the path of the beam.
• Incomplete ﬂushing can leave microbubbles adjacent to the transducer, resulting in
poor image quality once the catheter is inserted

RING DOWN ARTIFACT
• The solid-state transducer has a zone of
“ring-down artifact” encircling the
catheter, an extra step is required to
form a mask of the artifact and
subtract this from the image
• “Ring-down” artifacts produced by
acoustic oscillations in the piezoelectric
transducer that obscure the near field,
resulting in an acoustic catheter size
larger than its physical size

RING DOWN ARTIFACT
• The mask is usually acquired by disengaging the guiding catheter from the ostium
and positioning the tip of the imaging catheter free in the aorta
• The imaging element is then advanced distal to the area of interest, and the length
of the target vessel is scanned by a motorized or manual withdrawal of the entire
catheter over a standard 0.014-inch angioplasty guide wire

HALO ARTIFACT
• A “halo” or a series of bright rings
immediately around the mechanical
intravascular ultrasound (IVUS)
catheter (arrow)
• Usually caused by air bubbles that
need to be ﬂushed out

NOISE ARTIFACT
• Radiofrequency noise (arrows)
appears as alternating radial
spokes or random white dots in the
far-ﬁeld
• he interference is usually caused by
other electrical equipment in the
catheterization laboratory

WHITE CAP ARTIFACT
• “White cap” artifacts caused by side
lobe echoes (arrows) originate from a
strong reﬂecting surface, such as
metal stent struts or calciﬁcation.
• The smearing of the strut image can
lead to the mistaken impression that
the struts are protruding into the
lumen, potentially interfering with
area measurements and the
assessment of apposition, dissection,
and so on

NURD ARTIFACT
• Non-uniform rotational
distortion (NURD) results in
a wedge-shaped, smeared
appearance in one or more
segments of the image
(between 9 and 4 o’clock in
this example)

NURD ARTIFACT
• NURD can occur when bending of
the drive cable interferes with
uniform transducer rotation,
causing a wedge-shaped, smeared
image to appear in one or more
segments of the image.
• This may be corrected by
straightening the catheter and
motor drive assembly, lessening
tension on the guiding catheter, or
loosening the hemostatic valve of
the Y-adapter

HEAD-TO-HEAD COMPARISONS
• Mechanical transducers have traditionally offered advantages in image quality
compared with the solid-state systems
• Mechanical catheters have excellent near-ﬁeld resolution and do not require the
subtraction of a mask
• At the procedure, the short rail design of the mechanical systems may not track as
well as the solid-state catheter

HEAD-TO-HEAD COMPARISONS
• Mechanical catheters with a stationary outer sheath are easy to use with a
motorized pullback device, allowing the transducer to be moved through a segment
of interest in a precise and controlled manner
• The use of motorized pullback is important for two reasons :
a. It gives the ability to measure the length of a given segment or register the
position of a given cross-section for repeat studies
b. It provides the potential for a reasonably accurate longitudinal or three-
dimensional representation of a segment.

IMAGE INTERPRETATION
• The interpretation of IVUS images relies on the fact that the layers of a diseased
arterial wall can be identiﬁed separately
• The relative echo-lucency of media compared with intima and adventitia gives rise
to a three-layered appearance (bright-dark-bright)

• The lower ultrasound reflectance of the media is due to the presence of less collagen
and elastin than in the neighboring layers
• Because the intimal layer reflects ultrasound more strongly than the media, a
spillover effect, known as “blooming,” is seen in the image
• This results in a slight overestimation of the thickness of the intima and a
corresponding underestimation of the medial thickness
• The media/adventitia border is accurately rendered, because a step-up in echo
reflectivity occurs at this boundary and no blooming appears

• In normal coronary arteries from young patients, echo reﬂectivity of the intima
and internal lamina may not be sufﬁcient to resolve a clear inner layer
• The intima is so thin (<300μm in thickness) that it leads to signal dropout
• The traditional trilaminar appearance is replaced by a monolayer

APPARENTLY NORMAL CORONARIES
• IVUS helps reveal the true nature of disease burden
• In angiographically “normal” arteries, plaque burden is accurately visualized by
IVUS

minimal
plaque
accumulation
between 2 and
9 o’clock.
an eccentric plaque between 12 and 6
o’clock that is not evident on the
angiogram
a more concentric plaque

CALCIFIC PLAQUE
• Obstructs the penetration of ultrasound (acoustic shadowing)
• Only the leading edge is detected and thickness cannot be determined
• Regions of calciﬁcation are very brightly echo-reﬂective and create a dense shadow more
peripherally from the catheter

CALCIFIC PLAQUE
• Shadowing prevents determination of the true thickness of a calciﬁc deposit and
precludes visualization of structures in the tissue beyond the calcium
• Reverberation artifact causes the appearance of multiple ghost images of the leading
calcium interface, spaced at regular intervals radially

CALCIFIC PLAQUE
• Calcium is classified by its location
within the plaque
• Superficial calcium is closer to the
lumen than to the adventitia
• Deep calcium is closer to the adventitia
than to the lumen

FIBROUS VS. FATTY PLAQUE
• Densely ﬁbrotic tissue gives a bright appearance on the ultrasound scan and can
cause shadowing
• The extent of shadowing depends on both the thickness and the density of the
ﬁbrotic region as well as the transducer strength
• Majority of atherosclerotic lesions are fibrotic;
Very dense, fibrous plaques may cause so much acoustic shadowing that they could
be misclassified as calcified

FIBROUS VS. FATTY PLAQUE
• Fatty plaque is less echogenic than ﬁbrous plaque
• An area of plaque that images darker than the adventitia is fatty
• In an image of extremely good quality, the presence of a lipid pool can be inferred
from the appearance of a dark region within the plaque

IMAGE ORIENTATION
• One important aspect of image interpretation is determining the position of the
imaging plane within the artery
• The IVUS beam penetrates beyond the artery, providing images of perivascular
structures, including the cardiac veins, myocardium, and pericardium
• The branching patterns of the arteries are also distinctive and help identify the
position of the transducer

IMAGE ORIENTATION
• In the proximal portion of the
left main coronary artery, a clear
echo-free space ﬁlled with
pericardial ﬂuid, called the
transverse sinus, is found
adjacent to the artery,
immediately outside of the left
lateral aspect of the aortic root

• In this distal cross-section from the
left anterior descending coronary
artery (LAD), the right (R) and left
(L) branches of the anterior
interventricular vein (AIV) are seen
• The pericardium appears as a
typical bright stripe with rays
emitting from it (arrows)

• At the level of the middle
right coronary artery, the
veins arc over the artery,
typically at a position just
adjacent to the right
ventricular (RV) marginal
branches

AMBIGUOUS ANGIOGRAMS AND INDETERMINATE
CORONARY LESIONS
• Vessel overlap,
vessel tortuosity,
eccentric lesions,
ostial or bifurcation lesions, and
severe calcification are the main reasons for suboptimal angiographic visualization
of the lumen
• Particularly important when the lesions are of intermediate (40%-70%) severity in
patients with mild or atypical symptoms

CORONARY LESIONS
• When an angiographically intermediate lesion (50%-70% diameter stenosis) is encountered,
interobserver and intraobserver variability is high
• Further evaluation can be either by functional assessment (FFR measurement) or by
anatomic assessment using IVUS or OCT imaging
• Measurement of FFR accurately defines hemodynamically significant lesions
• IVUS measures used to define hemodynamically significant lesions have mostly been
benchmarked against established FFR cutoff thresholds

CORONARY LESIONS
• In a small study of 51 lesions, a lesion was considered hemodynamically significant when
FFR was <0.75
• IVUS measurements that identified such a lesion were
Minimum lumen area (MLA)<3.0mm2 (sensitivity, 83.0%; specificity, 92.3%) and
Area stenosis >60% (sensitivity, 92.0%; specificity, 88.5%)
• Combination of both criteria (MLA <3.0mm2 and area stenosis <60%) had 100% sensitivity
and specificity

CORONARY LESIONS
• In another study of 53 lesions,
Minimal luminal diameter (MLD) of <1.8mm,
MLA of ≤4mm2, and the
Cross-sectional area stenosis of >70% were the best indicators of hemodynamic significance,
as determined by an FFR <0.75

CORONARY LESIONS
• Retrospective analysis of 205 angiographically intermediate lesions was performed using
both IVUS and FFR measurements
• FFR was <0.8 in 26% of lesions
• There was moderate correlation between FFR and IVUS measurements, including MLA,
lesion length, and area stenosis
• MLA >4.0mm2 had an excellent negative predictive value (>94%), and MLA <3.09mm2 was
the best determinant of lesions with FFR <0.8 (sensitivity, 69.2%; specificity, 79.5%)

CORONARY LESIONS
• Depending on the reference vessel diameter, threshold MLA values for ischemia producing
lesions (FFR <0.8) are as follows:
MLA <2.4 mm2 in small vessels,
MLA <2.7 mm2 in medium-sized vessels, and
MLA <3.6 mm2 in large vessels

EVALUATION OF THE LEFT MAIN
CORONARY ARTERY
• Angiographic severity of left main coronary artery (LMCA) lesions is almost always
difficult to quantify
• Visualizing the ostium and most proximal part of the vessel depends on the “reflux”
of contrast into the aortic cusp.
• “Streaming” of contrast can give a false impression of luminal narrowing near the
tip of the injecting catheter
• When the whole length of the LMCA is diseased, the absence of a near normal
reference segment complicates visual quantification of stenosis severity

B1The caudal angiographic projection reveals a mild distal left main narrowing (arrow) in a patient with an
early positive stress test
B2, IVUS imaging reveals a significant eccentric plaque with an MLA below the threshold of hemodynami
significance typically used (<5.9mm2)

B3, an ostial left main lesion is seen on the angiogram without evidence of pressure
dampening and in a patient with a marginally positive stress test
B4, IVUS imaging reveals an eccentric calcified plaque but a large uncompromised lumen

EVALUATION OF THE LEFT MAIN
CORONARY ARTERY
• Similar to non–left main coronary lesions, a few studies were based on correlating
cutoff values of IVUS measurements with FFR measures of hemodynamic
significance of LMCA stenosis
• IVUS measurements that correlated best with hemodynamically significant lesions
as determined by FFR were
MLD <2.8mm (sensitivity and specificity of 93% and 98%, respectively) and
MLA <5.9 mm2 (sensitivity, 93%; specificity, 95%)
• A minimal lumen area 7.5 mm2 suggests that revascularization may be safely
deferred

EVALUATION OF TRANSPLANT
VASCULOPATHY
• Cardiac allograft vasculopathy (CAV) is a disease of unclear etiology that affects the
coronary arteries of the transplanted heart
• Characterized by progressive intimal proliferation of coronary arteries
• Due to its diffuse nature, the sensitivity of coronary angiography for detection of
CAV is low
• CAV is commonly defined by IVUS as a site where the intimal thickness is ≥0.5mm

IVUS IN INTERVENTIONS
• After rapid adoption of IVUS in the 90s, knowledge gained by operators from IVUS
imaging became assimilated in the technical approaches
• Widespread and routine use of IVUS imaging during interventional procedures has
tapered down to a more selective approach: situations when specific questions
cannot be accurately answered on the basis of angiography alone

VESSEL SIZING
• Use of properly sized balloons, stents, atherectomy burrs, etc is essential to achieve
optimal procedural outcomes
• Underestimation of reference vessel size and the use of undersized devices increase
risk of suboptimal outcomes such as early recoil, restenosis due to inadequate
procedural lumen gain, or stent underexpansion and/or strut malapposition

VESSEL SIZING
• Certain clinical scenarios are associated with significant underestimation of
angiographic reference vessel size
• PCI in setting of acute myocardial infarction (higher degree of coronary vasomotor
tone)
• PCI in chronic total occlusion – CTO (distal segments are underfilled and diffusely
diseased)
• Adequate coronary vasodilators and IVUS imaging can accurately delineate the
reference vessel size

VESSEL SIZING
• Reference vessel size is determined by the lumen diameter in the reference
segments, that is, those segments that have no or minimal disease in the vicinity of
the lesion to be treated
• Using the EEM dimension at the reference or at the site of the lesion will result in
overestimation of the vessel size, since EEM diameters reflect a degree of positive
remodeling in most cases
• This can result in device oversizing and should be avoided
• The operator should focus on lumen size, with some adjustment depending on
plaque burden

IVUS IMAGING IN CORONARY STENTING
• Initial IVUS observations in the early 1990s revealed that the technique of stents
was frequently inadequately expanded and not fully opposed to the arterial wall
despite satisfactory angiographic results
• This to major refinements in the technique of stenting, most notably higher pressure
postdilatations and the use of upsized balloons
• Coupled with dual antiplatelet therapy, these technical improvements resulted in a
reduction in the incidence of stent thrombosis

• Stent expansion refers to the size of the stent lumen achieved after the stent is
deployed and postdilated
• Can be expressed in terms of minimum stent lumen diameter (MLD) or minimum
lumen area (MLA)
• Expansion can also be indexed to the size of the reference segments
• MLD or MLA is divided by the mean diameter or area of the reference vessel to
express a percentage of the reference
• Stent MLA is an important predictor of restenosis

• Stent or strut apposition describes the contact between the stent struts and the
underlying arterial wall
• Qualitative metric of stenting that is detected with advanced imaging such as IVUS
or OCT
• Not synonymous with stent expansion.
• It is common to see a well opposed but under expanded stent
• For eg, when the stent is slightly undersized or if a rigid lesion is not adequately
predilated

IVUS TO OPTIMIZE ACUTE STENT RESULTS

STRUT MALAPPOSITION
• The clinical impact of strut malapposition may be significant, and a lower threshold
for IVUS examination may be justifiable after stenting

MANAGEMENT OF STENT RESTENOSIS
• True in-stent restenosis (ISR) is caused by neointimal hyperplastic response that is
initiated at the time of arterial wall injury caused by stent implantation
• Approximately 20% of stent “restenosis” results from initial underexpansion at the
time of deployment,
defined as a final in-stent MLA <80% of the average reference lumen area

ASSESSMENT OF COMPLICATIONS
AFTER INTERVENTION
• Coronary dissection is the most common cause of acute arterial closure during PCI
• Can result in serious complications, including MI and emergency bypass surgery
• IVUS is more sensitive in the detection of dissections as compared to angiography
and can localize the extension of dissection in the arterial wall
• IVUS classification of dissections is primarily based on the depth of the dissection
(i.e., intimal, medial, or extending to the adventitia)
• Also helpful in identifying intramural hematoma that may accompany a dissection

IVUS IMAGING IN
CHRONIC TOTAL OCCLUSION ANGIOPLASTY
• Application of IVUS imaging in CTO cases is in accurate vessel sizing
• After crossing the occluded segment and due to diffuse disease and chronic
underfilling, the distal vessel commonly appears smaller on angiography than its
true size
• IVUS imaging can delineate the burden of diffuse disease and allow the selection of
a larger, more appropriately sized stent, which in turn can reduce the risk of
restenosis

IVUS IMAGING IN
• Other applications of IVUS in CTO cases are more complex
• IVUS can be used to define the location of the occluded artery when the CTO is
“stumpless” or has a vague stump on angiography
• This can be performed if the occlusion is in the proximity of a small branch or if the
occlusion is at the ostium of a coronary artery
• The IVUS catheter is advanced into the patent branch and a slow pullback is
performed until the location of the occluded vessel is identified as a sector
interruption of the wall, as typically seen at bifurcation points
• The location is marked by filming the transducer on cine angiography

IVUS IMAGING IN
• The IVUS transducer can be left in place to guide passage of the wire into the
occluded branch and can eventually help determine whether the wire is in the true
lumen of the CTO by examining the wire location in the far field

ADVANCED IVUS TECHNOLOGY
• There has been growing interest in better understanding of plaque composition and
its role in defining plaque vulnerability
• Grayscale IVUS imaging for tissue characterization has modest accuracy and is not
reliable clinically
• In addition, resolution of IVUS is limited and may not allow detection of features
considered essential to the diagnosis of plaque vulnerability
• Advanced ultrasound-based tissue characterization technology, virtual histology
(VH) can allow the identification of the thin-cap fibroatheroma (TCFA)

IVUS-DERIVED VIRTUAL HISTOLOGY
• Depends on the analysis of the radio frequency backscatter of the ultrasound signal,
using a larger number of parameters
• Simplified color-coded display that broadly distinguishes lipidic, fibrotic, necrotic,
and calcific regions

IVUS-DERIVED VIRTUAL HISTOLOGY
• TCFA is defined as a
lipid-rich plaque (>10% confluent lipid core) with
a very thin cap (<100μm in thickness, not visible by VH) and with
an area stenosis that is usually >40%
• These plaques frequently contain speckles of calcification and are more prevalent in
patients presenting with acute coronary syndromes, even in nonculprit segments

CONCLUSION
• IVUS adds significant information to the assessment of coronary artery anatomy
over conventional catheter angiography
• IVUS helps in differentiating functionally significant disease segments from benign
cosmetic lesions
• Decision-making including need for intervention, sizing of devices and assessment of
complications and management of complications are significantly improved with
IVUS

IVUS

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