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Atherosclerosis and Lipoproteins |
From the Harvard-Thorndike Laboratory of the Department of Medicine, Cardiovascular Division (F.A.J., S.K.C., K.V.K., C.S., R.M.B., D.L., W.J.M.), and the Department of Radiology (W.J.M.), Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass; the National Heart, Lung, and Blood Institutes Framingham Heart Study (C.J.O., M.G.L., M.J.K., D.L.), Framingham, Mass; and the Cardiology Division (F.A.J., C.J.O.), Department of Internal Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, Mass.
Correspondence to Warren J. Manning, MD, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215. E-mail wmanning@ caregroup.harvard.edu
| Abstract |
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Key Words: magnetic resonance imaging aortas atherosclerosis imaging subclinical disease
| Introduction |
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Clinical risk factors and summary measures such as the Framingham Coronary Risk Score (FCRS) can predict the probability of future CVD events.2 Although these models perform well on large populations, not all individuals with a heavy risk factor burden develop overt CVD, whereas other subjects with few risk factors develop symptomatic CVD. Common noninvasive methods (eg, exercise testing and nuclear imaging) for diagnosing atherosclerosis are dependent on the presence of flow-limiting lesions, but non-flow-limiting subclinical disease may predate overt disease by decades.3 In addition, these other methods are unable to directly image or quantify atherosclerotic burden.
Cardiovascular magnetic resonance (CMR) offers unique advantages over other imaging modalities, including the ability to quantify atherosclerotic plaque burden4 6 and lack of ionizing radiation, and provides highly reproducible measures of aortic anatomy and atherosclerosis.7 However, CMR has not been applied to a longitudinally followed population-based cohort free of clinical heart disease to establish age- and sex-specific prevalence of subclinical atherosclerosis in the general population.
| Methods |
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Cardiovascular Magnetic Resonance
Subjects underwent thoracoabdominal aortic CMR with the use of a commercial 1.5-T whole-body CMR system (Gyroscan ACS-NT, Philips Medical Systems) with a PowerTrak 6000 gradient system (peak gradient 23 mT/m, rise time 219 ms).
Twenty-four transverse slices spanning the aortic arch to the aortoiliac bifurcation were obtained by using a free-breathing, ECG-gated, T2-weighted turbo spin-echo sequence. Black-blood aortic images were obtained by using time delays of 75 ms (thoracic) and 125 ms (abdominal) after the R wave. Spectral presaturation of the lipid signal was used to minimize chemical shift artifact from periadventitial fat. Other imaging parameters included the following: repetition time 3 heart beats, echo time 45 ms, turbo spin-echo factor 14, field of view 264 mmx330 mm, matrix size 256x512 (in-plane spatial resolution of 1.03 mmx0.64 mm), and slice thickness 5 mm, with a 10-mm slice gap. Thoracic aortic images were obtained with a commercial 5-element cardiac phased-array receiver coil with 4 signal averages. Abdominal aortic images were obtained with the body coil as a receiver with 6 averages. Total aortic imaging time was <20 minutes.
Aortic and Atherosclerotic Plaque Analysis
CMR data were transferred to a commercial EasyVision Work Station (version 3.0, Philips Medical Systems) and analyzed by reviewers blinded to all clinical data. Images were analyzed if they (1) were contained within the descending thoracic aorta and above the origin of the common iliac arteries and (2) had sufficient signal to noise and contrast to noise to allow clear visual definition of >50% of the inner circumference of the aortic wall. All data from a subject were excluded if
50% of the slices did not meet these criteria.
Aortic and plaque measurements were classified as thoracic or abdominal according to their location above or below the diaphragm, respectively. With the use of an interactive drawing tool, the inner aortic perimeter (a bright medial layer on T2-weighted images,4 Figure 1) and cross-sectional area were recorded.7 For aortic contour tracing, the semiautomatic interactive tool used an elliptical framework that was elongated to intersect the inner circumferential border of the aorta. For the plaque contour tracing, an interactive free-hand manual drawing tool was used. Intraobserver variability of aortic and plaque planimetry in our laboratory has been shown to be excellent.7
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Atherosclerotic plaque was defined as characteristic luminal protrusions4 of
1 mm in radial thickness7 that could be visually distinguished from the minimal residual blood signal. For each plaque, intimal perimeter, cross-sectional area, and maximal radial thickness were measured (Figure 1). Plaque perimeter was defined as the outer circumference of the plaque in contact with the inner border of the aorta (Figure 1), an index analogous to plaque surface area determined in autopsy studies.912 Atherosclerotic lesions were further classified as type A (maximal radial thickness
2.5 mm) or type B (thickness >2.5 mm). This threshold was chosen to allow
4 pixels in the readout direction across a given plaque. Plaque prevalence (percentage of subjects with plaque), type and number of plaques, intimal plaque perimeter, and total cross-sectional area were assessed.
To quantify the magnitude of atherosclerosis in each subject, 3 measures of atherosclerotic plaque burden (slice plaque burden [SPB], perimeter plaque burden [PPB], and area plaque burden [APB]) were determined as follows: SPB (%)=100 · (number of aortic slices with plaque/number of total aortic slices); PPB (%)=100 · (
intimal plaque perimeter/
intimal aortic circumference); and APB (%)=100 · (
plaque cross-sectional area/
aortic cross-sectional area).
Statistical Analysis
To account for double sampling of the highest risk group, all analyses used a weighting variable of 0.5 for subjects sampled from the highest risk category and 1.0 for subjects from other categories. Plaque prevalence is reported as a percentage, whereas plaque burden (SBP, PPB, and APB) is reported as mean and SD. Age-related trends were tested for plaque prevalence and plaque burden measures. Age-adjusted analyses were used when sex differences were assessed. Comparisons within subjects between abdominal and thoracic aortic plaque were made by using methods appropriate for paired data. Spearman correlations were calculated between FCRS and plaque prevalence and burden. All statistical analyses were conducted with the use of SAS/STAT (Release 6.12, SAS Institute Inc). A value of P<0.05 was considered significant.
| Results |
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We determined the interobserver variability of aortic and plaque planimetry by using the interactive drawing tool. Two trained observers (F.A.J., W.J.M.) independently analyzed 10 subjects for aortic and plaque cross-sectional areas. Interobserver variability was very high for aortic cross-sectional area (r=0.98) and plaque cross-sectional area (r= 0.94).
Aortic plaque of
1-mm radial thickness was identified in 38% of the women and 41% of the men (Figure 2). After adjustments were made for age, the prevalence of aortic plaque did not differ between men and women (P=0.38). In both sexes, there was an increasing trend in aortic atherosclerosis across age groups (P<0.01 for both sexes). For the entire group, aortic plaque was more prevalent in the abdomen than in the thorax (38.8% versus 6.3%, respectively; P<0.01). This difference was present for both sexes (both P<0.01, Figure 2a and Table 1). There was an increase in abdominal and thoracic aortic plaque across age groups in men (P=0.02 and P=0.01, respectively) and in women (both P<0.01). After adjustments were made for age, the prevalence of aortic atherosclerosis in the abdomen and in the thorax was similar (P=0.49 and P=0.22, respectively) among men and women.
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All quantitative measures (SBP, PPB, and APB) of atherosclerosis burden (Table 2) increased with age group adjusted for sex (P<0.01) and were greater in the abdomen (versus thorax) in both sexes (Figure 2b and Table 3). The average FCRS for the population was 2.25±3.82. Age-adjusted partial correlations of the FCRS with plaque prevalence (by magnetic resonance imaging) and burden measures were significant for men and for combined men and women (Table 4).
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| Discussion |
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Previous population-based atherosclerotic imaging studies have been largely limited to carotid ultrasonography for intimal-medial thickness1315 and calcium-based imaging techniques, such as chest x-ray of the thoracic aorta16 and electron-beam computed tomography of the epicardial coronary arteries.1719 As with our CMR data, carotid intimal-medial thickness and electron-beam computed tomographic atherosclerotic measures increase with age.20,21 At present, the optimal imaging method to assess for subclinical atherosclerosis has not been determined.22
In previous large autopsy studies of young (aged <35 years) trauma victims, aortic fibrous or complicated plaques (raised lesions) ranged from 0% to 12% by surface area and increased with age.912 Consistent with our findings, the 2 studies that distinguished between thoracic and abdominal aortic plaque found substantially greater plaque in the abdominal aorta.10,12 One study examined sex differences in aortic atherosclerosis and found that women in the highest age group (30 to 34 years) had greater surface involvement of raised lesions than did the corresponding men.12 Although not directly comparable because of age differences, our results are in accord with these autopsy data. Interestingly, all plaque burden measures in the oldest women were greater than those in the corresponding men, and the rate of increase of plaque burden was greater in women. These data may explain the postulated protective benefit of estrogen that lessens in older (postmenopausal) women.23
Autopsy data from a more analogous older population were reported as part of the International Atherosclerosis Project.24 Among white individuals from New Orleans and Oslo, advanced atherosclerotic lesions were less common in young women than in young men but were similar in older men and women. Although these data are consistent with our findings, raised atherosclerotic lesions were identified in a larger percentage of subjects than in the present report, suggesting a potential reduced sensitivity of this CMR method for minor lesions.
Study Limitations
The present study was not designed to characterize atherosclerotic plaque components. Many CMR-detected aortic plaques demonstrated a dark central core underneath a bright rim of tissue, consistent (but not diagnostic) of a lipid pool or calcium underlying a fibrous cap (Figure 1). Although plaque characterization and detailed vessel wall measurements may be desirable, 6 this would require additional CMR imaging using T1-weighted and proton density-weighted sequences.5 This was not possible because of external time constraints. Also, given the time constraints, we chose to obtain 24 slices with a 10-mm gap rather than to perform a more dense sampling of the aorta. In a prior study using similar indices,7 we confirmed that this approach had excellent intraobserver reproducibility for aortic anatomy and total subject plaque burden. Improvements in image quality may be obtained by newer pulse sequences, such as dual-inversion black-blood methods,25 or by respiratory motion correction. From an epidemiological perspective, the FHS offspring cohort is predominantly white. These results may not be applicable to other racial subgroups. Finally, the present study did not enroll adults aged <35 or > 90 years.
Conclusions
The present study reports the first CMR population-based study of subclinical aortic atherosclerosis in a free-living population. Among FHS offspring subjects free of clinical coronary disease, subclinical aortic atherosclerosis was seen in 40%. In both sexes, aortic plaque prevalence and all plaque burden measures were greater in the abdomen than in the thorax. Plaque prevalence and plaque burden increased with age and were correlated with the FCRS for men and women.
These CMR data add to prior autopsy-based prevalence estimates and previous CMR studies26,27 and help lay the groundwork for future studies regarding diagnosis, risk stratification, and the treatment of individuals with subclinical atherosclerosis.
| Acknowledgments |
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Received December 26, 2001; accepted January 28, 2002.
| References |
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