J Cardiol 2007 Oct; 50（4）: 223 – 228

Histological Characteristics ofPlaque With Ultrasonic Attenuation : A Comparison Between Intravascular Ultrasound and Histology
RyotaroYAMADA, MD

Hiroyuki

OKURA, MD

Teruyoshi

KUME, MD

Yoji

NEISHI, MD

Takahiro
Nozomi

KAWAMOTO, MD
WATANABE, MD

Eiji TOYOTA, MD
Kiyoshi

YOSHIDA, MD, FJCC

Abstract ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Objectives. Intermediate echogenic plaque without acoustic shadow on intravascular ultrasound（IVUS）
imaging has been recognized as fibrous plaque. Such echogenic plaque with ultrasonic attenuation may
have higher risk for distal flow disturbance（slow flow/no-reflow）
during percutaneous coronary intervention. However, histological evaluation of plaque with ultrasonic attenuation has not been performed. This
study evaluated the histological characteristics of plaque with ultrasonic attenuation assessed by IVUS.
Methods. By using IVUS, 36 samples of human cadaveric coronary arterial echogenic plaque（percentage plaque area ＞ 40%）
without calcium were selected, and classified into the attenuation group ; plaquewith ultrasonic attenuation, and the non-attenuation group ; plaque without attenuation. These plaques
were classified for fibrous, fibrofatty, calcium, and necrotic core areas by histological examination.
Results. True fibrous plaque was found in 91.7% of the non-attenuation group, but only 68.0% of theattenuation group（p ＜ 0.01）. On the other hand, the percentage fibrofatty and necrotic core plaque areas in
the attenuation group were significant larger than those in the non-attenuation group（fibrofatty : 16.3 ±
13.8% vs 2.7 ± 3.1% , p ＜ 0.01 ; necrotic core : 13.0 ± 19.4% vs 3.9 ± 8.0%, p ＝ 0.03）.
Conclusions. Plaque with ultrasonic attenuation contains more fibrofatty tissue and necrotic core compared to fibrous plaque without attenuation.
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────J Cardiol 2007 Oct; 50（4）: 223−228

Key Words■ Intravascular ultrasound（intracoronary imaging）■ Plaque（fibrofatty, necrotic core, histological analysis）

INTRODUCTION
Intravascular ultrasound（IVUS）is widely usedduring percutaneous coronary intervention（PCI）inthe cardiac catheterization laboratory. IVUS provides not only morphometric but also morphological lesion characteristics. According to previous

validation studies, intermediate echogenic plaquewithout acoustic shadow by IVUS imaging has
been recognized as hard or fibrous plaque. 1,2）
Although the fibrous rather than the fatty type ofatherosclerotic plaque seems more stable, several
studies have shown that such echogenic fibrousplaque with ultrasonic attenuation may have para-──────────────────────────────────────────────
川崎医科大学 循環器内科 : 〒 701−0192 岡山県倉敷市松島 577
Department of Cardiology, Kawasaki Medical School, Okayama
Address for correspondence : OKURA H, MD, Department of Cardiology, Kawasaki Medical School, Matsushima 577, Kurashiki,
Okayama 701−0192 ; E-mail : hokura@fides.dti.ne.jp
Manuscript received May 14, 2007 ; revised June 8, 2007 ; accepted June 26, 2007

223

224

Yamada, Okura, Kume et al

doxically higher risk for distal flow disturbance（slow flow/no-reflow phenomenon）during PCI.3,4）
However, the mechanism of slow flow/no-reflow
caused by the fibrous plaque has not been examined. Therefore, this study evaluated the histological characteristics of the plaque with ultrasonicattenuation assessed by IVUS to estimate the possible mechanism of distal flow disturbance.
SUBJECTS AND METHODSOne hundred and seven coronary arteries were
harvested from the heart in a series of 36 humancadavers. Of these 36 cadavers, 6 had symptomatic
cardiovascular disease（16.7%）. The study protocol
was approved by the ethics committee of Kawasaki
Medical School, and written informed consent was
obtained.
Hypercholesterolemia was defined as total cholesterol level ＞− 240 mg/dl or current medication for
hypercholesterolemia. Hypertension was defined as
systolic blood pressure ＞− 140 mmHg, diastolic＞blood pressure − 90 mmHg, or current antihypertensive medication. Diabetic patients were receiving insulin or some kind of oral hypoglycemic
agent. All blood samples and blood pressure data
were obtained during hospitalization.
Samples about 5 cm long of the proximal site of
the three major coronary arteries, including the left
anterior descending coronary artery, left circumflex
artery, and right coronary artery were obtained
from the cadaver at autopsy within 3 hr of death.
Surrounding soft tissues were dissected from eachspecimen. Small arterial perforators and theirbranches were tied off with sutures, and the distal
end of the arteries was occluded with a large cork.
A 7F sheath was sewn into the proximal end of thearteries to ensure complete closure. Saline（0.9%）
solution at 37 °C was infused through the side armof the sheath. The pressure inside the coronary
artery was maintained at the physiologic level（60to 80 mmHg）with a syringomanometer connectedto the infusion. An IVUS catheter（Atlantis SR Pro2.5F, 40-MHz ; Boston Scientific）was inserted
sequentially through the diaphragm of the sheath.
Serial IVUS images were obtained using an automatic pullback device at 0.5 mm/sec.
IVUS images were analyzed off-line by commercially available image processing software（Netra
3D IVUS system, ScImage）. Cross-sectional areas（CSA）of the external elastic membrane（EEM）,
lumen, and plaque plus media（P ＋ M）were mea-

sured. Percentage plaque area was calculated as（P ＋ M CSA/EEM CSA）× 100%. Lesions withpercentage plaque area ＞ 40% were consideredsignificant and selected for this study.5）By using
IVUS, fibrous plaque lesions, assessed according to
the standards of the American College of
Cardiology, were collected and classified into twogroups, the attenuation group ; plaque with ultrasonic attenuation of more than 90 degrees of theplaque, and the non-attenuation group ; plaque without attenuation.3）There were no significant differences in fibrous plaque with attenuation plaquebetween cadavers with
（2 in 6）and without cardiovascular disease（7 in 30）．The arc of attenuation
was measured and classified into two groups,
attenuation ＞ 120 °, and attenuation ＜.
− 120 °
After IVUS imaging, each coronary artery waspressure-fixed in 10% neutral buffered-formalin.
Following 48 hr of fixation, standard paraffin
embedding was performed. For every 400μm ofeach coronary artery, three series of 4-μm thicksections were cut and stained with hematoxylin andeosin, Masson s trichrome, and elastica van Gieson
stains. The image slice with the attenuationassessed by IVUS was compared with the corresponding histological images. Experienced
observers unaware of the IVUS image used quantitative computerized planimetry to analyze thefibrous, fibrofatty, calcium, and necrotic core areas.
By using National Institutes of Health（NIH）image
software, the measured areas were standardized by
the total plaque area and expressed as percentage
areas. Furthermore, the plaque histological characteristics of the segments within the ultrasonic attenuation were also investigated in the attenuationgroup.
Statistical analysis
Statistical analysis was performed with StatView
5.0 version（SAS Institute）. Continuous variables
are reported as mean ± SD. Student s t-test wasused to differentiate two sets of data with normal
distribution. If no significance was found byStudent s t-test, then the Mann-Whitney U-test was
performed. A p value ＜ 0.05 was considered statistically significant.
RESULTS
Baseline patient characteristics
Of these 36 patients, 20 fibrous plaques withattenuation from 9 patients and 19 fibrous plaques
J Cardiol 2007 Oct; 50（4）: 223 – 228

Histological Analysis of Attenuated PlaqueTable 1 Baseline patient characteristics
Attenuation Non-attenuation
p value
group
group
（n＝9）
（n＝12）＊

Age（yr, mean±SD）

71±14

63±17

0.08

Male

55.6%

75.0%

0.40

Hypertension＊

22.2%

33.3%

1.00

Diabetes mellitus

33.3%

33.3%

1.00

Hypercholesterolemia†

22.2%

16.6%

0.58

Current smoking

33.3%

33.3%

1.00

（％）
100

225

p ≤0.01
Non-attenuation group
Attenuation group7550

p ≤0.01p =0.0325

p =0.69

Hypertension was defined as systolic blood pressure＞140 mmHg, diastolic blood pressure ＞90 mmHg, or use of−an antihypertensive drug.

−

0

†

Hypercholesterolemia was defined as total cholesterol level＞240 mg/dl or medication use.
−

Table 2 Histological analysis
Attenuation Non-attenuation
p value
group
group
EEM CSA（mm2）

8.51±3.04

7.52±1.93

0.37

Lumen CSA（mm2）

3.01±1.54

2.70±1.30

0.38

Plaque plus media
CSA
（mm2）

5.50±2.29

4.85±1.20

0.42

Plaque plus media
64.8±15.0
CSA / EEM CSA（%）

65.2±9.5

0.79

Values are mean±SD.
EEM＝external elastic membrane ; CSA＝cross-sectional area.

without attenuation from 12 patients were selected.
Baseline patient characteristics are listed in Table
1. There were no significant differences in age, sex,
occurrence of diabetes, hypertension, hyperlipidemia, and current smoking between the twogroups.
Histological examination
Morphometrical analysis from the histological
specimens of the selected lesions is shown in Table
2. There were no significant differences in EEM,
lumen, and P ＋ M CSAs between the attenuationgroup and the non-attenuation group. As a result,
percentage plaque area was similar between the twogroups. Fig. 1 shows the morphological analysis of
the histological specimens. True fibrous plaque wasfound in 91.7% of the non-attenuation group, but
only 68.0% of the attenuation group（p ＜ 0.01）. On
the other hand, percentage fibrofatty and necroticcore tissue areas in the attenuation group were significant larger than in the non-attenuation group
J Cardiol 2007 Oct; 50（4）: 223 – 228

Fibrous

Fibrofatty Dense-calcium Necrotic coreFig. 1 Histological analysis of the plaque with andwithout ultrasonic attenuation
Percentage fibrofatty and necrotic core plaque areas in
the attenuation group（white bars）were significant larger that in the non-attenuation group（black bars）.

（fibrofatty : 16.3 ± 13.8% vs 2.7 ± 3.1%, p ＜0.01 ; necrotic core : 13.0 ± 19.4% vs 3.9 ± 8.0% ,p ＝ 0.03）. Fig. 2 shows a representative IVUS
image of the plaque with attenuation and the corresponding histological image. A large necrotic corecan be seen in the attenuated part of the plaque. The
percentage plaque burden area of the attenuationgroup was 70.9 ± 16.5%.
A significantly large percentage plaque burden
area（greater than 75%）
occurred in 12 of 39 specimens（31%）. Percentage fibrous plaque area in theattenuation ＞ 120 °group was significantly smaller（75.8 ±
than that in the attenuation ＜− 120 °group20.8% vs 56.3 ± 18.6%, p ＝ 0.01）. On the other
hand, percentage necrotic core plaque area in theattenuation ＞ 120 °group was significantly larger（5.4 ±
than that in the attenuation ＜− 120 °group9.0% vs 24.4 ± 25.3%, p ＝ 0.03）.
DISCUSSION
The present study investigated the histologicalcharacteristics of plaque with ultrasonic attenuationassessed by IVUS. Histological examination
revealed that fibrofatty plaque and necrotic core
were more frequently found in echogenic plaquewith attenuation. This study is the first report toevaluate histological characteristics of whole coronary sections with ultrasonic attenuation by IVUS.
In general, lipid components are important as acause of ultrasonic signal attenuation. The large

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Yamada, Okura, Kume et al

Fig. 2 Representative intravascular ultrasound image of attenuation（left）
and photograph of histological sample of plaque（right）
Large necrotic core tissue was identified as the attenuation area in the corresponding intravascular ultrasound image.

fibrofatty and necrotic components tend to attenuate the ultrasonic signal by scattering.3）
In abdominal ultrasonic imaging, pure cholesterol gallstone shows a strong acoustic shadow.6）
Echogenic coronary plaque without acoustic shadow by IVUS has been recognized as fibrousplaque. 1,2）However, several studies have shownthat such echogenic fibrous plaque with ultrasonicattenuation may have higher risk for distal flow disturbance（slow flow/no-reflow phenomenon）during
PCI.3,4）Furthermore, ultrasonic attenuation has notbeen nominated for a plaque qualitative classification in the existing IVUS guidelines.1,2）
Histopathologic examination of plaque withultrasonic attenuation obtained by directional
atherectomy（DCA）has been reported.3）The histological examination suggested that microcalcification with expansive positive arterial remodeling
was associated with attenuation. However, in ourpresent study, fatty tissue plaque area in the attenuation group was significantly larger than in the nonattenuation group. The calcification area in theattenuation group was not significantly different.
Furthermore, the percentage necrotic core plaquearea in the attenuation ＞ 120 ° group was significantly larger than that in the attenuation ＜− 120 °
group.
The limitation of the previous study is that the
specimen obtained by DCA from acute coronary
syndrome and stable angina pectoris patients does

not necessarily reflect the segments with attenuation. In that case, plaque with attenuation is detected in both acute coronary syndrome and stableangina pectoris. More importantly, the entire vessel
structure could not be evaluated by this method.
Thus, the methodological difference（DCA specimen vs postmortem examination）and possibly thesmall sample size（ n ＝ 6）of the previous study
might explain the discordant findings.3）
In general, necrotic core or fatty tissue（ lipid
pool/lipid core）protruding into the vessel lumen
may cause intraluminal thrombus formation inpatients with acute coronary syndrome.7）The presence of such lipid compounds is a predictor of theslow flow/no-reflow during PCI.8 − 11）In addition,
atherosclerotic plaque with ultrasonic attenuation
might cause transient coronary flow deterioration
during PCI in patients with acute coronary syndrome.4）Thus, these lipid components might cause
distal microembolization, resulting in the slow
flow/no-reflow phenomenon during PCI.
Pre-procedural detection of plaque with ultrasound attenuation might be feasible for risk stratifying the regions with higher risk for slow flow during PCI. In addition, selective use of distal protection devices may be mandated for preventing serious complications during PCI.12,13）
Study limitationsFirst, the histological examination might be
J Cardiol 2007 Oct; 50（4）: 223 – 228

Histological Analysis of Attenuated Plaqueinfluenced by the shrinkage artifact in the processing of histological specimens, in spite of the use ofpressure fixation.
Second, recent spectral analysis of IVUS
radiofrequency（ Virtual Histology）data has thepotential to provide detailed quantitative information on plaque components. By using such IVUS
Virtual Histology, it might be possible to evaluate
lipid components with ultrasonic attenuation in
vivo.
Third, because this study was a postmortem
IVUS and histological examination, it is impossible

要227

to elucidate the direct relationship between theplaque component with ultrasound attenuation anddistal flow disturbance during PCI in vivo. Further
study will be needed to investigate the relationshipbetween the degree of coronary arterial stenosis
assessed by coronary angiography and plaqueattenuation assessed by IVUS.
Finally, this is a retrospective analysis of a small
number of specimens from a single center. Thus theresults need to be confirmed by a larger multicenter
study.

約

超音波減衰を有するプラークの組織性状: 血管内エコーと組織の比較検討

目

山田亮太郎

大倉

宏之

久米

輝善

根石

陽二

川元

渡 邉

望

豊田

英嗣

吉 田

清

隆弘

的 : 血管内エコー上で音響陰影を伴わない中等度のエコー輝度のプラークは fibrous プラークと認識されている．Fibrous と思われるプラークで石灰化を疑わせる高輝度領域を伴わないにもか
かわらず，病変後方のエコーが著しく減衰している像（attenuation）を有した例では，経皮的冠動脈形成術後に slow flow/no-reflow をきたしやすいとの報告が散見される．しかし，attenuation を有した
プラークの組織性状を比較検討した報告は少ない．今回我々は，高周波血管内エコーで attenuationを有するプラークの組織性状を検討した．
方

法 : 剖検 36 例から得られた冠動脈を対象に血管内エコー（Atlantis SR Pro-, 40-MHz）により観察を行った．プラーク面積率＞ 40% 以上でカルシウム病変を伴わない中等度のエコー輝度プラー
クを，attenuation を有するプラーク（attenuation 群），attenuation を有さないプラーク（non-attenuation
群）の 2 群に分類した．血管内エコ ー後に attenuation を認めた箇所の組織性状を検討し fibrous，
fibrofatty，calcium，necrotic core の 4 つに分類した．
結果 : 組織性状の検討で fibrous は non-attenuation 群の 91.7% に存在したが，attenuation 群では68.0% のみであった（p ＜ 0.01）．一方で fibrofatty と necrotic core は attenuation 群で有意に高値であった（fibrofatty : 16.3 ± 13.8% vs 2.7 ± 3.1%, p ＜ 0.01 ; necrotic core : 13.0 ± 19.4% vs 3.9 ± 8.0%, p ＝0.03）．
結論 : 血管内エコー上で attenuation を有するプラークでは fibrofatty および necrotic core を有意に多く検出した．

J Cardiol 2007 Oct; 50（4）: 223−228

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