Atherothrombosis is the major cause of morbidity and mortality worldwide. Atherosclerosis is characterized by lipid deposition (along with other inﬂammatory material) within the arterial wall, leading to plaque formation. At the early stages of atherogenesis, the lumen of the vessel is not affected because of the “positive remodeling” of the external elastic membrane and eccentric plaque growth; therefore, the luminogram of the atherosclerotic vessel may appear to be “normal.” Hence, imaging modalities that are capable of accurately visualizing the entire vessel wall have gained enormous interest in order to quantify the atherosclerotic burden.
is a Cardiologist and is part of a multidisciplinary research
team with a special focus in Cardiovascular Imaging at Mount
Sinai School of Medicine, New York, NY. He received his MD in
1993 from The Universidad Complutense de Madrid in Madrid, Spain.
He performed his Residency and Fellowship in Cardiology at
Fundacion Jimenez Diaz, Madrid, Spain. He received several honors
and awards from the Spanish Society of Cardiology. He has
published a number of articles and has received Young
Investigator award nominations at the European Society of
Cardiology Annual meeting in 2007 and the North American Society
for Cardiac Imaging in 2005.
The assessment of changes in atherosclerotic plaque volume is
increasingly used as a surrogate endpoint in clinical trials
evaluating the effects of antiatherogenic therapies. Currently,
intravascular ultrasound, cardiovascular magnetic resonance, and
carotid intima media thickening are the modalities usually used.
The utility of multidetector computed tomography (MDCT) for this
purpose has not yet been assessed. This article presents the
results of a pilot study in rabbits to evaluate the use of MDCT
to monitor changes in atheroma volume.
Abdominal-aorta atherosclerotic lesions were induced in rabbits
(n = 7), which were then randomized to 2 doses of placebo or a
drug with known plaque-regressing properties (apoA-IMilano). Pre-
and posttreatment 64-slice CT angiographic studies of the aorta
Plaque burden by MDCT regressed by 29% in apoA-IMilano-treated
animals while it did not significantly change in the placebo
group. In the last MDCT study, plaque volume was 10.6% smaller in
apoA-IMilano animals, which corresponded to the postmortem
histologic analysis (11.5% smaller plaque in the apoA-IMilano
Multidetector CT is a promising tool to assess changes in
Atherothrombosis is the major cause of morbidity and mortality
worldwide. Atherosclerosis is characterized by lipid deposition
(along with other inflammatory material) within the arterial wall,
leading to plaque formation. At the early stages of atherogenesis,
the lumen of the vessel is not affected because of the "positive
remodeling" of the external elastic membrane and eccentric plaque
growth; therefore, the luminogram of the atherosclerotic vessel may
appear to be "normal." Hence, imaging modalities that are capable
of accurately visualizing the entire vessel wall have gained
enormous interest in order to quantify the atherosclerotic
Until recently, atherogenesis was envisioned as a progressive
cumulative phenomenon of lipid deposition; however, today it is
well known that lipid-lowering interventions are able to halt
or even to regress atheroma.
Although there is no data correlating atheroma regression with a
reduction in clinical events, there is a large body of indirect
evidence highly suggesting this association.
If this hypothesis is confirmed definitely, future clinical trials
will use imaging-based studies as a surrogate for clinical
endpoints. Current imaging modalities that are used to monitor
changes in atheroma burden include intravascular ultrasound (IVUS),
cardiovascular magnetic resonance (CMR),
and vessel wall intima media thickening (IMT) by surface
Multidetector computed tomography (MDCT) is an emerging
noninvasive imaging modality that combines high spatial and
temporal resolution with a very short scan time. MDCT has recently
gained significant attention because of its ability to accurately
depict not only arterial calcification and lumen size, but also the
presence and morphology of nonstenotic, noncalcified plaques.
Hence, MDCT may represent an attractive alternative imaging
modality to noninvasively track changes in the atheroma burden.
However, to date, there are no available data on the utility of
MDCT to accurately monitor atherosclerosis progression or
regression. We have recently shown, in a CMR-based preclinical
study, that the administration of the experimental drug
apoA-IMilano (ApoA-IM) induces marked plaque regression in just 4
days, which may offer a useful tool to explore the ability of MDCT
to monitor changes in atheroma burden.
The aim of this study was to assess whether MDCT is able to
detect and quantify the changes in the atheroma burden of the
abdominal aorta that are associated with the administration of
apoA-IM in an established animal model of atherosclerosis.
Abdominal aorta atherosclerotic plaques were induced in rabbits
(n = 7) by a combination of 9 months of 0.2% cholesterol-enriched
diet and 2 aortic balloon denudations. The atherosclerosis
induction protocol is described in detail elsewhere.
To replicate the clinical scenario of statin therapy, the
cholesterol content of the atherogenic diet was then reduced by 50%
immediately before the first administration of apoA-IM or placebo.
Afirst (baseline) MDCT aortic angiography study was performed, and
animals were then randomized to receive 2 intravenous injections, 4
days apart, of 75 mg/kg of ApoA-IM (n = 4) or an equal volume of
placebo (n = 3). Four days after the last dose, a second (final)
MDCT angiography was performed. Subsequently, the animals were
euthanized and the abdominal aorta was fixed for further
morphologic analysis. Serial sections were obtained at 3-mm
intervals to match the corresponding MDCT images, as previously
Specimens were embedded in paraffin, and 5-µm-thick sections were
cut and stained with combined Masson's trichrome elastin stain. The
detailed protocol is explained elsewhere.
The study protocol was approved by an institutional animal
research committee, and all animals received humane care in
compliance with the "Guide for the Care and Use of Laboratory
Noninvasive MDCT protocol
Multidetector CT studies were performed with a 64-slice MDCT
scanner (Sensation 64, Siemens Medical Solutions, Forchheim,
Germany). For the scans, the rabbits were sedated by intramuscular
injection of ketamine (30 mg/kg) and xylazine (2.2 mg/kg). An
intravenous line was placed in the central vein of each rabbit's
ear with a 21-gauge line. Before initiation of the protocol,
different dilutions of the nonionic, low-osmolar, iodinated
contrast agent ioversol (Optiray 370, Tyco Healthcare/Mallinckrodt,
St. Louis, MO) were tested in an animal not included in the study.
The purpose was to determine the optimal concentration leading to
intra-aortic lumen attenuation similar to that routinely obtained
in clinical human coronary CT angiography examinations (250 to 350
HU). The mixture used in this study was a 1:2 dilution of the
contrast agent with saline, yielding a concentration of 123.3 mg
I/mL. The animals were imaged in the craniocaudal direction and in
the supine position. An initial localizer image served to confirm
an adequate position of the animal and prescribe the angiographic
study covering the abdominal aorta. The contrast dilution was then
infused (8 mL at a rate of 0.5 mL/sec), and scanning was initiated
after a fixed delay of 10 seconds was used to trigger image
acquisition. Imaging parameters were as follows: 120 kV, 180 mA,
rotation time 330 msec, 32 × 0.6 mm collimation, and pitch 0.45.
The total acquisition time ranged from 6.5 to 7.5 seconds. Axial
images (3-mm thickness with no overlap) were reconstructed using a
sharp kernel (B46f), a field of view of 160 × 160 mm, and a 512 ×
512 matrix, resulting in an in-plane spatial resolution of 400 ×
MDCT data analysis
The MDCT images were transferred to a dedicated workstation for
analysis. The 17 consecutive 3-mm slices immediately distal to the
left renal artery were prespecified as the area of study for the
pre- and posttreatment MDCT studies. Because plaque visualization
is affected by the attenuation of the lumen,
a region of interest (2 to 2.5 cm
) was placed in the center of the vessel in each of the slices, and
the average attenuation was recorded. The mean luminal attenuation
in each study was calculated by averaging the values of the 17
individual slices. Afterward, the display settings were manipulated
as follows: for lumen quantification, the window level was brought
to 65% of the mean luminal attenuation, and the window width was
reduced to 1. For detecting outer vessel boundaries, the window
level was kept unchanged, and the window width was brought to 155%
of the mean luminal attenuation. These settings were chosen because
they have been shown to represent the optimal values for
plaque-size measurement when compared with coronary IVUS
This combination generates a "black and white" image for the
detection of the lumen area and a discernible outer border of the
vessel wall depiction (Figure 1). Each of the 17 axial sections
were manually traced by 2 independent researchers who were blinded
to the time of the study and to the treatment arm. Plaque volume of
the individual segments was calculated as the difference between
total vessel volume (3 × vessel area) and lumen volume (3 × lumen
The primary endpoint of the study was the change in plaque
burden be-tween the initial and final MDCT studies. Plaque burden
was quantified as the total plaque volume, calculated by adding the
plaque volumes of each of the individual slices within the aortic
segment of interest. In addition, total lumen volume (the sum of
individual slice lumen volumes) and total vessel volume (the sum of
individual slice vessel volumes) were computed.
A secondary analysis included the change in the individual
(slice) plaque volumes. For this comparison, pre- and posttreatment
images were matched by its distance from the left renal artery.
An observer who was blinded to the treatment arm performed the
immunohistochemical analysis using a computer-based validated
semiautomatic quanti-fication method programmed on ImageJ (National
Institutes of Health, Bethesda, MD) and determined the plaque area
(vessel area-lumen area).
Continuous variables are expressed as the mean ± standard error
of the mean (SEM). Statistical comparisons of means were made by
Wilcoxon Signed Rank test. A value of
<0.05 (2-tailed) was considered statistically significant. All
statistical analyses were performed with the statistical software
package SPSS 11.0 (SPSS Inc., Chicago, IL).
Contrast-enhanced 64-slice MDCT angiography was performed
successfully in all animals, resulting in adequate image quality
for analysis. Figure 2 shows the results of the primary analysis of
the study. A statistical significant plaque burden regression
= 0.031) was observed in the apoA-IM group between pre- and
posttreatment MDCT studies, while no significant change was
observed in the placebo animals.
A nonsignificant reduction in total lumen volume was observed in
the apoA-IM group, while no effect was found in the placebo group
(Figure 3A). A change in the total vessel volume showed a strong
trend toward external elastic membrane shrinkage in the apoA-IM
group (20% regression versus pretreatment MDCT,
= 0.1), whereas no change was documented in the placebo group
(Figure 3B). Figure 4 illustrates individual changes in total
At the slice level, changes in plaque size followed a comparable
pattern. The mean segmental plaque volume regressed in the apoA-IM
group (-28% versus pretreatment MDCT,
= 0.03), whereas no change was observed in the placebo group
(Figure 5). Overall, there was good agreement between the final
MSCT study and the histological analysis. In the posttreatment
MDCT, the plaque burden of the apoA-IM-treated animals was 11.5%
smaller compared with that of the animals that received placebo,
although this comparison did not reach statistical significance. At
the slice level, the mean plaque volume of the apoA-IM animals was
11.5% smaller than that of placebo animals (
= 0.018). In the histologic analysis, the mean plaque area was
10.6% smaller in the apoA-IM group (
In the current study, the hypothesis that MDCT is able to assess
changes in atheroma burden has been tested. Abdominal aorta
atherosclerotic lesions were induced in a well-established research
model of atherosclerosis.
All animals underwent 2 imaging studies, and, thus, every animal
served as its own control. Plaque burden was significantly reduced
in animals receiving apoA-IM, while nonsignificant changes were
seen in the placebo animals.
Change in plaque volume is currently one of the major surrogate
endpoints used in clinical trials. The advent of new imaging
modalities made it possible to accurately monitor changes in plaque
volume, while before only the vessel lumen was quantifiable. Such
imaging modalities include IVUS, CMR, and carotid IMT. All these
imaging modalities have certain issues that limit their global
acceptance. Intravascular ultrasound can accurately depict coronary
atheroma, and it has been shown to reliably monitor changes in
atheroma burden; however, its invasive nature makes its use
limited. Cardiovascular MR is an extremely accurate tool to
quantify and monitor changes in atheroma burden, but currently the
coronary territory is not well depicted because of its limited
temporal resolution. Carotid IMT is a noninvasive, safe, and
reproducible imaging modality that can precisely examine changes in
wall thickness. However, its usage is limited to the carotid
region, and the type of lesions developed in that area are widely
different from those in the coronary and aortic arteries.
Multidetector CT is a noninvasive new modality that potentially can
overtake all limitations stated above. In fact, few studies have
reported the accuracy of MDCT in quantifying the atherosclerotic
plaque burden. Those studies validated the MDCT analysis with
well-established imaging modalities: IVUS or CMR.
However, so far, there is no data on the value of MDCT to monitor
changes in atheroma burden. To the best of our knowledge, this is
the first report showing serial imaging studies for the
quantification of the atheroma burden.
Currently, there are several lipid-altering interventions that
have been proven to regress plaque burden; however, the time needed
to achieve this effect is very long.
ApoA-IM is a mutant form of apoA-I that has been shown to
significantly regress atheroma volume in all the animal studies so
It has also been tested in humans, showing a significant coronary
plaque regression in just 5 weeks.
We have recently reported, using the same atherosclerosis model as
described here, that only 2 infusions of apoA-IM in 4 days induced
a significant reduction in abdominal aorta plaque volume, as
assessed by CMR.
Due to its unequivocally known rapid plaque regressing effects,
apoA-IM represents the ideal tool to explore the utility of MDCT to
monitor changes in plaque burden. Usually, the effect of a new
intervention is assessed with an established imaging modality;
here, we have done it the other way around: using a drug with known
plaque-regressing effects, we have tested the feasibility of using
MDCT to monitor changes in plaque burden.
Besides the analysis of coronary stenosis and calcification,
nonstenotic plaque quantification by MDCT is an emerging area of
interest both in the research and the clinical arenas. Early
studies heated the enthusiasm on the ability of MDCT to quantify
and characterize atherosclerotic plaques.
Several studies have correlated the plaque measurements with that
of IVUS. Achenbach et al
reported a sensitivity of 82% and a specificity of 88% of MDCT in
detecting any kind of plaque. Those values were higher for
calcified than for noncalcified plaques. Also, the same group
reported that in selected images with good quality, MDCT can assess
the remodeling index of coronary artery lesions, correlating well
to IVUS measurements.
Therefore, MDCT seems to be a reliable modality to assess the lumen
and the external vessel wall boundaries. Leber et al
showed that 64-slice CT can accurately depict coronary artery
plaque burden, showing a significant moderate correlation with
IVUS. However, the interobserver variability for plaque burden
quantification has been reported to be high, varying from 17% to
37%, according to the size of the vessel and to the amount of
disease (the higher the amount of disease, the lower the
Since plaque visualization is affected by the attenuation of the
we decided to use a display setting that takes this parameter into
account. However, at this point we cannot be sure that the vessel
wall measurements could have been randomly affected by this
In an animal model of atherosclerosis, the administration of
apoA-IM (a drug with known rapid atheroma-regressing effects) was
used to explore MDCT utility to monitor changes in atheroma burden.
Atheroma burden in animals receiving apoA-IM was 30% significantly
smaller in the posttreatment MDCT study compared with the
pretreatment study. Placebo administration resulted in no effect on
plaque burden. Posttreatment plaque burden in apoA-IM animals was
10.6% significantly smaller than in those treated with placebo, in
agreement with the 11.5% significant smaller plaques found in
histology. These results suggest that MDCT may serve as a tool to
explore serial changes in plaque volume.
The author is indebted to the mentorship from Drs. Juan J.
Badimon, Mario J. Garcia, and Valentin Fuster. In addition, the
great work by Drs. Javier Sanz and Susanna Prat-González, which was
crucial for the proper study design and performance, should be
acknowledged. Drs. Carlos G. Santos-Gallego and Juan
Benezet-Mazuecos significantly contributed to the performance and
analysis of the data. Finally, the author would like to thank Dr.
Badimon's laboratory at Mount Sinai School of Medicine for all the
help and support.
The drug administered to the animals (apoA-IMilano, ETC-216) was
provided by Pfizer Research and Development, USA.