Download Imaging Human Atherosclerosis with ""To

Imaging Human Atherosclerosis with ""To-labeled
Low Density Lipoproteins
Ann M. Lees, Robert S. Lees, Frederick J. Schoen, Jonathan L. Isaacsohn,
Alan J. Fischman, Kenneth A. McKusick, and H. William Strauss
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The feasibility of localizing human atherosclerotic plaques by gamma scintillation
camera external Imaging with technetlum-99m-labeled low density lipoproteins
( T c - L D L ) was tested In 17 patients who had atherosclerosis. Imaging demonstrated focal accumulation of radlolabel consistent with T c - L D L sequestration by
plaques In the carotid, lilac, or femoral vessels of four patients 8 to 21 hours after
Intravenous Injection of the radlopharmaceutlcal. Focal accumulation of T c - L D L
also appeared In the location of coronary lesions In four patients, but this accumulation could not be distinguished with certainty from residual blood pool radioactivity. When carotid endarterectomy specimens from six patients who received T c LDL 1 day before endarterectomy were examined, the specimens had focal
accumulations of radlolabel, with two to four times greater radioactivity In some
regions of each specimen than In others; this occurred whether or not the lesions
were detected on the gamma camera Images. Lesion composition may have determined whether accumulation was quantitatively sufficient to produce an external
Image. Hlstologlcally, the Imaged carotid specimen had abundant foam cells and
macrophages and poorly organized Intramural blood consistent with a plaque
hemorrhage; in contrast, nonlmaged endarterectomy specimens were mature, fibrocalclfic plaques. We conclude that: 1) T c - L D L did accumulate In human atherosclerotic plaques; 2) In some patients, the accumulation of T c - L D L was sufficient
for detection by gamma camera Imaging; 3) the amount of LDL that accumulated
appeared to depend on lesion composition; and 4) the design of new radlopharmaceutlcals with reduced residual blood pool activity relative to plaque accumulation
should lead to Improved external Imaging of atherosclerosis.
(Arteriosclerosis 8:461-470, September/October 1988)
D
Dilation camera has been investigated, with the hope that
it will be useful for localizing lesions, including those likely
to lead to clinical complications as a result of acute plaque
changes,7 and for evaluating the efficacy of medical
treatment of hypertipidemia and hypertension at periodic
intervals. In contrast to current methods, the new technique depends on locating atherosclerotic plaques by
their intramural accumulation of radiolabel after injection
of a suitable radiopharmaceutical.
etection of atherosclerotic plaques by means other
than their space-occupying characteristics might permit localization of lesions at an earlier stage than is now
possible. Although vascular surgery,1-2 and, more recently,
balloon angioplasty and thrombolytic therapy 34 have
allowed tissue salvage in many patients with advanced
atherosclerosis, prevention or arrest of the disease before
critical stenosis develops would be preferable.58 Furthermore, identification of patients at high risk for acute
complications from asymptomatic plaques might permit
beneficial changes in patient management. As an approach
to these goals, a new noninvasive method to detect and
characterize atherosclerotic plaques with the gamma scin-
Since low density lipoproteins (LDL) had previously
been shown to be present in atheromata,8-11 we used
radiolabeled LDL as a probe for arterial lesions. Noninvasive external imaging of atherosclerosis appeared feasible when it was found that 12sl-labeled-LDL accumulated
focally in experimental arterial lesions In normocholesterolemic rabbits.12
In a subsequent pilot study with human subjects, we
showed that carotid atherosclerosis could be imaged with
12S
l-labeled LDL.13 Since this isotope is not optimal for
external imaging with the gamma camera, a method was
developed14 for labeling LDL with T c ( T c - L D L ) . The
new radiopharmaceutical not only provided clear external
images of experimental arterial lesions in animals, but also,
based on urinary excretion, appeared to be partially trapped
in tissues.14 In the present study, T c - L D L was administered to 17 human subjects to evaluate the possibility that
human atherosclerotic lesions could accumulate enough
From the Arteriosclerosis Center, Department cf Medicine,
New England Deaconess Hospital, Department of Applied Biological Sciences, Massachusetts Institute of Technology, Departments of Medicine and Radiology, Massachusetts General Hospital, and Department of Pathology, Brigham and Women's
Hospital, Boston, Massachusetts.
This research was supported in part by Grants HL32975 and
HL07416 from the National Heart, Lung, and Blood Institute,
National Institutes of Health.
This work was presented in part at the 58th Scientific Sessions
of the American Heart Association, Washington, DC, November
1985, and has previously appeared in abstract form (Circulation
1985 72; part 11:111-198).
Address for correspondence: Dr. Robert S. Lees, 110 Francis
Street, Suite 7F, Boston, MA 02215.
Received December 29,1987; revision accepted April 5,1988.
461
462
ARTERIOSCLEROSIS
VOL 8, No 5, SEPTEMBER/OCTOBER 1988
radiolabel to permit external imaging with the gamma
camera, and in carotid endarterectomy specimens from six
of the subjects, to determine the distribution of radiolabel
within atherosclerotic lesions and correlate tissue composition with the ability to obtain an external image.
Methods
Patients
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Seventeen patients were recruited from the Arteriosclerosis Center's outpatient clinic and from among the patients
of physicians at the Massachusetts General and New
England Deaconess Hospitals. Signed statements of consent were obtained from each patient after the nature of
the study had been fully explained. Pertinent data for each
patient are outlined in Table 1. Six patients (Nos. 3,4,7,8,
10, and 14) underwent carotid endarterectomy after imaging, and their surgical specimens were available for measurement of radioactivity and histologic study. Seven
patients (Nos. 1, 2, 3, 7, 10, 11, and 12) were on lipidlowering medications, with varying degrees of success.
Preparation of Radlolabeled Low Density
Llpoproteln
On Day 1, 100 ml of blood was drawn from an antecubital vein under sterile precautions and the patient's LDL
was isolated on Day 1 and Day 2 by sequential ultracentrifugation as described previously.1315 On Day 3, the LDL
was radiolabeled with " T c under sterile conditions, as
follows. Two to 6 mg of LDL (at a density of 1.050 g/ml)
were placed in an empty sterile vial. To the vial was added
70 to 100 mCi of sodium ""pertechnetate (E.I. du Pont de
Nemours & Company, North Billerica, MA). The reactants
were mixed, and to them was added 10 mg of sodium
dithionite dissolved immediately before use in 0.1 ml of
0.5 M glycine buffer (pH 9.8). The total volume of the
reaction mixture was about 1.1 ml. This mixture was
allowed to stand at room temperature for 30 minutes (20
minutes for Patients 1 and 7); then it was chromatcgraphed on a sterile 2.5 x 10 cm disposable Sephadex
G-50 column (Pharmacia Incorporated, Piscataway, NJ)
which was pre-equilibrated with sterile 0.15 M sodium
chloride brought to pH 8 with sterile sodium bicarbonate
buffer. After a volume equal to that of the reaction mixture
was allowed to flow through the column, 7.5 ml of the
saline/bicarbonate buffer was used to elirte the column.
The first 4 ml of eluate was discarded. The next 3.5 ml,
which contained the " T c - L D L , was collected in a sterile
30 ml technetium elution vial (E.I. du Pont de Nemours &
Company). The term """Tc-LDL", as used throughout this
report, refers to the 3.5 ml of eluate obtained from the
Sephadex G-50 column. It had been previously shown by
ultracentrifugation and paper electrophoresis that this
preparation consisted predominantly of 96nTc tightly bound
to LDL along with a much smaller, variable amount of
macromolecular inorganic technetium species.14
In the present study, each preparation was filtered
through a 0.22/tm filter just before use, and a small aliquot
was analyzed by paper electrophoresis as described
previously.14 In each case, the major peak was that of
" T c - L D L , similar to the published example, with a vari-
able amount of radioactivity which, as reported previously,
did not stain with oil red O after paper electrophoresis.
Imaging
The patients were immediately imaged with either a
standard or a large field-of-view gamma camera equipped
with an all-purpose parallel-hole collimator, and then
imaged again one to three times up to 25 hours after
injection of 10 to 20 mCi of T c - L D L . Anterior cervical
and thoracic views, anterior and posterior abdominal
views, and anterior views of the pelvis and lower extremities were recorded in all patients; additional views
were recorded when appropriate. Images were obtained
for up to 10B counts for each view, or up to 15 minutes at
the later imaging times. All images were reviewed by
three nuclear medicine physicians. Only regions of
arterial accumulation identified as positive by all three
reviewers were called positive.
Specimen Analysis
Six carotid endarterectomy specimens were available
for direct examination; specimens of other arteries were
not available for analysis.
After gross photography, the carotid endarterectomy
specimens were rinsed in normal saline until the rinses
were free of blood, and then divided into three to four
sections of differing atherosclerotic involvement based on
the degree of mural thickening and stenosis. The tissues
were blotted lightly before being weighed and were then
counted in a gamma counter (Packard 5630, Packard
Instrument Corporation, Des Plaines, IL).
After measurement of radioactivity, the sections of each
carotid endarterectomy specimen were fixed in 10% formalin, embedded in paraffin, cut in cross-section for light
microscopy, and stained with hematoxylin and eosin. A
maximum of 2 hours elapsed between the time carotid
specimens were removed surgically and the time they
were placed in fixative.
To determine the percent of injected T c that was
excreted in urine, 24-hour urine collections were obtained,
measured, and counted in three patients.
Definitions
A positive gamma scintillation camera image was
defined as the appearance of focal radiolabel accumulation in the region of a vessel visible above the blood pool
that increased in intensity over time relative to blood pool
radioactivity and was asymmetric relative to the contralateral vessel.
In carotid endarterectomy specimens, focal accumulation was defined as higher levels of radioactivity in some
regions of the specimen and lower levels elsewhere.
The histologic sections of carotid specimens were
classified by a semiquantitative assessment of the fraction
of each section that had abundant macrophages and
foam cells, and/or hematoma or necrosis (Category A)
relative to the fraction of the section that was mature
fibrous connective tissue, with or without areas of calcification (Category B). The primary distinction was
between tissue in which the functional metabolic status
and/or the structural organization appeared to be evolving
IMAGING HUMAN ATHEROSCLEROSIS
463
Lees et al.
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&-/C
small
bowel
large-^
bowel ;
cecum
Figure 1. Anterior sdntigram of Patient 13 at: A. 10 minutes, B. 1 hour, C. 4 hours, and D. 15 hours after Intravenous Injection of 20.4
mCi of T c - L D L . In A and B, organs with a large blood pool (including the heart, liver, and lungs) were prominent. With increasing time
after injection, the cardiac radioactivity (a blood pool marker) decreased, while radioactivity In the other organs (liver, intestine, and
kidneys) increased relative to the heart, suggesting a shift of radiolabel from blood into tissue. This patient had no focal vascular
accumulation of radiolabel.
and tissue which appeared to be quiescent. Since the
regions within the plaques were not sharply delineated,
exact morphometric measurements of the specimens
could not be made. Tissue in Category A had more than
50% dense inflammatory infiltrate with or without abundant macrophages and foam cells, and/or unorganized
or incompletely organized intramural blood or focal
necrotic debris. Tissue in Category B was mature fibrous
connective tissue, which contained smooth muscle cells
and/or fibroblasts, with or without dystrophic calcification. "Intermediate tissue" was sparseiy cellular and
densely fibrocalcific, with only small regions of macrophages, foam cells, hemorrhage, or necrosis.
Results
Sclntlgraphy
The typical appearance of sequential gamma camera
images of a patient given ""Tc-LDL is illustrated in Figure
1. Immediately after injection of the radiolabel, blood pool
radioactivity was evident in several organs. Over a 15hour period, the cardiac radioactivity decreased, while
radioactivity in the liver, intestine, and kidney increased
relative to the heart. The shift in relative radioactivity from
the heart, which is a blood pool marker, to organs known
to metabolize LDL 1617 was consistent with the transfer of
radioactivity from blood to tissue and indicated that the
radiopharmaceutical had the biological characteristics of
native LDL 1 8 1 7
Four patients (Table 1, Nos. 1 to 4) accumulated
sufficient ""Tc-LDL to produce images with the gamma
camera in locations where atherosclerotic arterial lesions
were known or likely to be present; four patients (Nos. 2,
5, 6, and 9) accumulated radloiabel in areas of the heart
anatomically consistent with the location of coronary
arteries known or likely to be atherosclerotic, but these
areas could not be distinguished with certainty from blood
pool radioactivity; ten patients (Nos. 7, 8, and 10 to 17)
had no focal arterial accumulation of radiolabel. The
doses of T c - L D L for each patient and the time parameters of imaging are listed in Table 2.
Arteries with focal accumulation of radiolabel that was
consistent with a positive image included the iliac, femoral, and carotid vessels. Patients 1, 2, and 4 had images
of the iliac and/or femoral arteries. Patient 1, who had a
464
ARTERIOSCLEROSIS
VOL 8, No 5, SEPTEMBER/OCTOBER 1988
Table 1. Patients Studied: Clinical Data and Results
Patient
no.
1
Age
(yrs)
Sex
TC*
TGt
44
M
224
66
Vascular
events or
symptoms^
TIA
5 yre
R carotid
endarterectomy
None
5 yre
Angina
Ml
Ml
M
199
248
Claudication
Angina
4
72
M
201
95
Carotid bruit
Ml
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M
132
60
7
8
55
56
F
M
533
174
172
91
9
58
M
296
132
10
59
M
176
84
11
12
13
14
15
16
60
60
62
65
65
66
M
M
M
F
M
F
145
306
218
224
223
294
125
105
84
125
190
503
Iliofemoral
6 yrs
49
48
+
L carotid
endarterectomy
3
6
4.5 yre
6 yre
105
149
Region with
positive
image
TIA
209
146
Current
symptoms
R femoral
embolectomy
M
M
Time
before
Imaging
10 yre
48
42
Related vascular
procedure^
Claudication
2
5
Time
before
imaging
Femoral bruits
TIA
Ml
Ml
Angina
3 yre
10.5 yre
7 yre
10 yre
None
None
None
10 yre
1 wk
Coronary bypass
Carotid angiogram
None
17 yre
11 yre
4 wks
2 yre
1 wk
4.3 yre
Ml
1 mo
Angina
L carotid bruit
6 yre
6 yre
R carotid bruit
TIA
TIA
8 mos
1 wk
13 mos
TIA
None
Carotid angiogram
Coronary angiogram
Coronary t-PA
Coronary
angioplasty
Coronary
angioplasty
None
L carotid
endarterectomy
None
Carotid angiogram
—
+
—
—
—
+
Femoral
2.5 yre
5 days
—
—
-
Carotid
-
Femoral
5 days
+
2 yre
1 wk
+
4.5 yre
-
1 mo
+
+
8 mos
L carotid
endarterectomy
Carotid artertogram
13 days
+
15 days
+
Angina
6 mos
6 wks
+
TIA
Ml
TIA
9 yre
6 yre
3 wks
Coronary
angioplasty
None
Coronary bypass
Carotid angiogram
Claudication
Claudication
19 yre
20 yre
6 yre
5 days
19 yre
20 yre
TIA
9 yre
5 mos
Positive
exercise test
Ml
3.5 wks
Angina
L carotid bruit
7 yre
7 yre
R carotid bruit
Angina
Ml
Ml
Angina
Ml
Stroke
Claudication
7 yre
15 yre
5 yre
2 wks
16 yre
12 yre
4 yre
2 yre
Coronary angiogram
None
Carotid angiogram
L carotid
endarterectomy
Carotid angiogram
None
Coronary angiogram
Coronary angiogram
None
None
None
Aortoiliac graft
+
—
5 days
13 mos
Aortoiliac
endarterectomies
None
Coronary angiogram
+
—
20 mos
L carotid
endarterectomy
Femoral grafts
Carotid
spectmen§
+
+
—
+
+
+
—
5 mos
1 day
+
—
+
12 days
7 days
+
12 days
—
5 days
4 days
+
—
—
—
6 mos
-
IMAGING HUMAN ATHEROSCLEROSIS
Table 1.
Patient
Lees et al.
465
(Continued)
no.
Age
(yrs)
Sex
TC*
TGf
17
75
M
379
196
Vascular
events or
symptoms*
Stroke
L carotid
stenosis
R carotid
stenosis
Coronary
insufficiency
Coronary
insufficiency
Time
before
Imaging
Related vascular
procedure*
Time
before
imaging
Current
symptoms
—
7yrs
—
5yrs
—
16 yrs
7 yrs
None
L carotid
endarterectomy
5 yrs
R. Carotid
endarterectomy
9 yrs
None
—
-
5 yrs
None
—
—
Region with
positive
image
Carotid
specimen§
*TC = total plasma cholesterol (mg/dl); fTC = plasma triglycerides (mg/dl); $L = left, R = right, Ml = myocardial infarction, TIA = transient
ischemic attack, t-PA = tissue plasminogen activator; §Carotid specimen = carotid endarterectomy specimen obtained for counting of
radioactivity and histological examination.
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10-year history of bilateral claudication, had asymmetrical
accumulation of radiolabel at several points in the iliofemoral system on the 21-hour images (Figure 2). Patient 2
had a 10-year history of left calf claudication and had left
calf claudication at the time of imaging; his 8-hour and
21-hour images showed focal, asymmetric, accumulation
of radiolabel in the region of the left femoral artery. Patient
4 had a history of generalized atherosclerosis including
recent carotid artery symptoms; although his carotid circulation did not image, his right femoral artery showed
focal accumulation of radiolabel at 16 hours.
In addition to the femoral disease described above,
Patient 2 also had had a myocardial infarction 10 years
before his imaging study. At the time of imaging, he
denied cardiac symptoms, but his physical examination
and electrocardiogram were consistent with severe coronary artery disease. The 8-hour image showed focal,
linear accumulation of radiolabel in an area of the heart
that was anatomically consistent with the left anterior
descending (LAD) coronary artery; however, the accumulation could not be unequivocally distinguished from blood
pool radioactivity. The same problem of interpretation
occurred in patients 6 and 9. In Patient 6, focal accumulation in the area of the LAD was evident in the 14-hour
and the 22-hour images; angioplasty of the patients LAD
artery had been performed 4.5 years before the imaging
study. One month before the study, an angiogram showed
diffuse atherosclerosis of the LAD coronary artery and
tight stenosis of the circumflex artery on which angioplasty
was performed; no focal accumulation of radiolabel was
evident in the area of the circumflex artery in the imaging
study. Patient 9 developed exerrjonal chest pain 6 months
before his imaging study and underwent coronary angioplasty of the LAD artery 6 weeks before imaging. In the
2-hour and 16-hour images, there was focal accumulation
of radiolabel in the area of the LAD artery, which intensified over time, but could not clearly be distinguished from
blood pool radioactivity. Uncertainty about a coronary
image also occurred with Patient 5, who had an occluded
right coronary artery recanalized by intracoronary injection of tissue plasminogen activator 1 week before imaging. Focal accumulation of radiolabel anatomically consistent with the location of the proximal right coronary
artery was visible on the 16-hour image but could not be
distinguished with certainty from blood pool radioactivity.
Patient 3 had a 9-hour image (Figures 3A and 3B)
which showed asymmetrical focal accumulation of radiolabel in the right internal carotid artery. An angiogram
performed before right carotid endarterectomy (Figure
3C) showed tight stenosis of the right internal, and moderate stenosis of the left internal, carotid arteries. These
lesions were associated with the recent onset of a prominent high-pitched right carotid bruit and no bruit on the
left. At surgery, a brown-stained tightly stenotic right
carotid lesion was removed, which appeared on gross
examination to have an organized, relatively recent intramural hemorrhage (Figure 3D).
Histology
Histologic examination of arterial specimens from the
six carotid endarterectomy patients made possible a
Table 2.
Patient
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Dose and Time Parameters of Imaging
Tc-LDL
injected
(mCl)
Time of
positive images
(hrs)
Duration of
imaging
study (hrs)
18.6
21
21
21.0
13.3
16.4
8,21
21
21.3
13.0
18.9
21.6
14.7
9
9
16
—
—
—
—
—
16
16
22
20
9
16
14
—
—
—
21
21
12.1
20.5
18.2
—
—
—
—
25
5*
16
20
10.0
—
23
19.8
19.5
19.3
20.4
'Later images could not be obtained because of surgical
scheduling.
466
ARTERIOSCLEROSIS
VOL 8, No 5, SEPTEMBER/OCTOBER 1988
I
transverse colon
:
•
%
.
'
'
%
'
- . - • - - '
.iliac:^
vv.V:^^-:|
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: bladder/
VT #
*•;•'
•
•vt
•S external
%^ Iliac
' . \ ' - - • - • • ' . .
:•'. -
j.Xfemoral '•;. •/
7 / artery f geiiitalia
Figure 2. Anterior sdntJgram of Patient 1 at 21 hours after Intravenous Injection of 18.6 mCi of T c - L D L . The field of view Included the
area between the umbilicus and mid-thigh. In addition to accumulation in bowel and genitalia, the distribution of radiolabel in the iliac and
femoral vessels was asymmetrically Irregular (arrows).
correlation between lesion composition as assessed by the
light microscopic appearance and the ability to image. The
area of tight stenosis in the imageable lesion of Patient 3
was largely composed of poorly organized intramural hematoma with abundant foam cells and macrophages (Figure.
3E). In contrast, the other five carotid specimens, which did
not image, were more fibrocalcific than cellular, hemorrhagic, or necrotjc. For example, the right carotid artery of
Patient 7, which did not image, appeared tightly stenosed
on angkxjram (Figure 4A). At surgery, a small, stenotic
lesion was removed (Figure 4B) which was too small and
hard to be cut into sections. The specimen was almost
entirely fibrous tissue, with focal calcification and little
cellularity (Figure 4C). The lesions in Patients 4, 8, and 14
were also predominantly fibrous, while the lesion in Patient
10 was intermediate with some cellular-hemorrhagic areas
and other areas which were fibrotjc.
Focal Radioactivity of Carotid Specimens
Although the carotid lesions of five carotid endarterectomy patients (Nos. 4,7, 8,10, and 14) did not accumulate
enough radiolabel to be visible above the blood pool on
external imaging, the five carotid specimens that could be
sectioned (including that of Patient 3, which did image)
showed focal accumulation of radioactivity, with up to two
to four times more radioactivity in the centers of plaques
than elsewhere in the specimens (Table 3). Because of
the complexity of the studies, it was not feasible to obtain
serial blood samples to measure the average plasma
radioactivity, which is necessary to make quantitative
comparisons of LDL accumulation among the specimens.
In addition, surgical scheduling precluded the possibility of
obtaining all the carotid specimens at the same time after
injection of ""Tc-LDL. However, even with these limitations in mind, it was clear that the difference in radiolabel
accumulation by Patient 3's specimen, which imaged, and
the other carotid specimens was marked. The anterior
and posterior portions from the center of Patient 3's
plaque accumulated 20 x i O " ^ and 18x10"*%, respectively, of the injected dose per gram. In contrast, the two
center portions of the fibrotic lesion of Patient 4 accumulated less than 1 x 10"*% of the injected dose per gram.
Radiolabel accumulation by other fibrotic lesions was
similar. The historically intermediate lesion of Patient 10
accumulated no more than 2 x 10~*% of the injected dose
per gram in any portion of the specimen.
Urine Radioactivity
Cumulative urinary excretion of T c - L D L was measured in three patients. Over a 24-hour period from the
IMAGING HUMAN ATHEROSCLEROSIS
Lees et al.
internal
.carotid
external
jugulars
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B
Figure 3. Imaging studies and pathology of carotid stenosis in Patient 3. A. External image 9 hours after injection
of 13.1 mCi of T c - L D L showing focal, asymmetrical
accumulation of radiolabel in the right distal common
carotid artery at the carotid bifurcation and the proximal
internal carotid artery (arrows). B. Sketch of A. C. Right
carotid angiogram showing extensive stenosis (large arrow)
and mural irregularities (small arrows) involving the distal
common and proximal internal carotid arteries. This corresponded to the region of accumulation of T c - L D L in the
gamma camera image. The left carotid angiogram showed
only a minor degree of internal carotid stenosis. D. Photograph of bisected right carotid endarterectomy specimen
showing the tight stenosis (arrow) and the long underlying,
poorly organized deep hematoma covered by a fibrous cap.
E. Photomicrograph of lesion showing numerous foam cells
and macrophage giant cells adjacent to a poorly organized
intramural hematoma. Hematoxylin and eosin stain, x 200
467
468
ARTERIOSCLEROSIS
V O L 8, No 5, SEPTEMBER/OCTOBER 1988
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B
Figure 4. Angiogram and pathology of carotid stenosis In
Patient 7. This patient had no focal vascular accumulation of
radblabel. A. Right carotid angiogram showing a tight stenosis at the origin of the Internal carotid artery (arrow).
B. Photograph of bisected right carotid endarterectomy specimen. The stenosis (arrow) was apparent, although the
calcific composition of the lesion was not visible in the
photograph. C. Photomicrograph of lesion. There is a mature,
fibrous plaque, with rare macrophages and foam cells. Hematoxyiln and eosln stain. x200
time of injection, the results, expressed as the percent of
injected dose excreted, were 4%, 11%, and 12% for
Patients 4, 5, and 8, respectively.
Discussion
The goal of this study was to demonstrate the feasibility
of detecting atherosclerotic plaques by their intramural
accumulation of a radiopharmaceutical, rather than by
their space-occupying characteristics, as is current practice. The approach is based on the noninvasive technique
of gamma camera sdntigraphy and takes advantage of
previous observations12'13 that radiolabeled LDL is rapidly
focally sequestered in abnormal arterial wall. The earlier
observations were confirmed and extended in the present
study, which showed that some human atherosclerotic
lesions in the carotid, iliac, and femoral circulations could
be imaged externally with the gamma scintillation camera
after injection of T c - L D L . Based on the data obtained
from carotid endarterectomy specimens, lesion composition may have determined whether " T c - L D L accumulation was quantitatively sufficient to produce an external
image. The carotid plaque that did image was made up of
tissue that could be described as being in a state of active
evolution; it had abundant foam cells and macrophages
as well as an area of poorly organized intramural hemorrhage. The total accumulation of radiolabel by this specimen was far greater than that of any other specimen.
Conversely, the five fibrocalcific carotid plaques that did
not image were made up of tissue which could be described
as quiescent, or "burned-out." However, regardless of
whether they imaged, all five specimens that could be
subdivided had greater radioactivity In the centers of
plaques and less radioactivity elsewhere. These findings
support the concept that the atherosclerotic process pro-
IMAGING HUMAN ATHEROSCLEROSIS
Table 3. Relative Radioactivity and Histology of
Carotid Endarterectomy
Patient
no.
3
4
7
8
10
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14
Specimen
Center of plaque, anterior
Center of plaque, posterior
Edge of plaque
Center of plaque, distal
Center of plaque, proximal
External carotid
Entire specimen
Plaque, proximal
Center of plaque, posterior
Center of plaque, anterior
Plaque, distal
External carotid
Center of plaque, anterior
Center of plaque, posterior
Edge of plaque, anterior
Edge of plaque, posterior
Center of plaque
Edge of plaque
External carotid
Relative
radioactivity*
1.00
0.92
0.27
1.00
0.89
0.55
—
1.00
1.00
0.68
0.63
0.47
1.00
0.53
0.57
0.39
1.00
0.55
0.47
Histologyt
Category A
Category A
Intermediate
Category B
Category B
Category B
Category B
Intermediate
Category B
Intermediate
Category B
Category B
Category A
Category A
Category B
Intermediate
Category B
Category B
Intermediate
T h e relative radioactivity was calculated for each patient's
specimen by dividing the cpm/gm of each piece of specimen by
the cpm/gm of the most radioactive piece from that patient.
tCategory A. Abundant macrophages and foam cells and
hematoma or necrosis. Category B. Mature fibrous connective
tissue with or without areas of calcification. Intermediate, sparsely
cellular, fibrocalcrfic, only small regions similar to Category A.
duces specific local tissue changes that might be used to
locate plaques by external imaging if suitable radiopharmaceuticals were available.
In this study, which employed T c - L D L as the radiopharmaceutical, extracellular matrix produced by cells
present in plaques may have mediated, at least in part,
the focal accumulation of radiolabeled LDL. For example,
the quantity and type of glycosaminoglycans,18 which are
known to bind LDL, may be important. Whatever the
specific tissue components that may be involved in radiolabel accumulation, the results of this study suggest that
the ability to image atherosclerotic plaque is dependent in
some way on tissue composition. Thus, the composition
of lesions may be at least partially elucidated by external
imaging. Based on the evidence from earlier studies 141920
and from the data presented here, we hypothesize that if
a lesion images with " T c - L D L , it is likely to be actively
evolving.
In recent years, emphasis has been placed on the
importance of acute rupture or fissure in chronic coronary
arterial atherosclerotic plaques as the critical event precipitating varying degrees of occlusion by thrombus, which,
in turn, results in cardiac symptoms ranging from unstable
angina to infarction to sudden death.7 It seems likely that
quiescent, mature fibrocalcific plaques would be least
likely to rupture, while softer lesions with a cellular,
hemorrhagic, or necrotic core would be more likely to
undergo acute plaque change. Further studies will be
needed to confirm the possibility that an appropriate
radiopharmaceutical could be used to detect such evolving plaques by external imaging.
Lees et al.
469
In addition to the specific tissue characteristics of
plaques, a major determinant of the ability to image with
the gamma camera is the relationship between the radioactivity in the lesion and that in the blood pool, known as
the target-to-background ratio. In this study, the target-tobackground ratio obtained with T c - L D L was clearly not
optimal, perhaps because the biological half-life of LDL in
the plasma ranges from 2 to 6 days, while the physical
half-life of T c is only 6 hours. This led to uncertainty in
the interpretation of focal cardiac radiolabel accumulation
in four patients, and, in light of the focal accumulation of
radiolabel measured ex vivo in the carotid endarterectomy
specimens, could well have led to negative imaging
studies in the presence of active disease.
We are now investigating a variety of ways to improve
the target-to-background ratio of the imaging radiopharmaceutical. Although a method of attaching to LDL an
imaging isotope with a half-life longer than the 6 hours of
T c is not yet available, this is one possible approach.
However, " " T c does have the advantage of being readily
available and inexpensive, and it appears to be partially
trapped intracellularly, as indicated by the 24-hour urinary
excretion of radiolabel, which ranged only from 4% to 12%;
this finding, in contrast to results obtained with 12SI-LDL21
where 20% to 50% of the isotope appeared in the urine in
24 hours, was consistent with animal data obtained
previously,1419-20 and suggested that the " T c that enters
cells bound to protein may be trapped to a considerable
extent in tissue, as has been demonstrated for certain
radiolabeled conjugates of LDL.22 Another approach would
be to develop an agent that would be cleared more rapidly
from plasma while being retained in atherosclerotic plaques.
Whatever radiopharmaceuticals prove to be optimal, the
present study demonstrates that external imaging of human
atherosclerosis is feasible and may aid in differentiating
quiescent from actively evolving plaques.
Acknowledgments
The authors thank Martha Barlai-Kovach, Frances Keech,
Helen Shing, and Sara Murray for their expert technical assistance and Samuel W. Stein for helpful discussions. This work
would not have been possible without the collaboration of the
physicians and surgeons of the Massachusetts General and New
England Deaconess Hospitals, particularly Herman K. Gold,
Robert C. Leinbach, Robert G. Ojemann, and Gary W. Gibbons.
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A M Lees, R S Lees, F J Schoen, J L Isaacsohn, A J Fischman, K A McKusick and H W Strauss
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Arterioscler Thromb Vasc Biol. 1988;8:461-470
doi: 10.1161/01.ATV.8.5.461
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