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Clinical Investigation |
1 Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea; and 2 Division of Cardiology, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
Correspondence: For correspondence or reprints contact: Kyung-Han Lee, Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, 50 Ilwondong, Kangnamgu, Seoul, Korea. E-mail: khnm.lee{at}samsung.com
| ABSTRACT |
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Key Words: atherosclerosis 18F-FDG PET atherogenic risk factor risk modification
| INTRODUCTION |
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PET with 18F-FDG is a valuable diagnostic tool that is widely used to survey the whole body of patients for malignant and inflammatory disease (6), including inflammatory lesions of the vasculature (7). The cellular responses of atherosclerosis are also best described as an inflammatory disease (8), and correspondingly, 18F-FDG uptake has been shown to be increased in arteries with active atherosclerosis (9–11). Furthermore, the presence of vessels with 18F-FDG uptake has been associated with greater atherogenic risk (12,13), and the cellular component that accumulates high levels of 18F-FDG has been found to be infiltrating macrophages within the inflamed plaques (14,15). 18F-FDG uptake of atherosclerotic lesions is a transient process that declines as the active inflammatory component recedes. If this were to occur in response to risk modification and were detectable by 18F-FDG PET, it would provide an opportunity to noninvasively monitor the response of atherosclerotic lesions to intervention therapy (4). In this study, we investigated the change in 18F-FDG uptake in major arteries on serial PET/CT scans of healthy adults that occurs in relation to atherogenic risk reduction induced by lifestyle modification.
| MATERIALS AND METHODS |
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Lifestyle modification was advocated for each subject at the end of the tests. Modification included individual dietary counseling by a registered dietitian and recommendations for physical exercise and weight reduction as indicated by a physician who informed the subjects of the test results.
18F-FDG PET/CT
PET/CT was performed on a scanner (Discovery LS; GE Healthcare) after at least 6 h of fasting at 45 min after intravenous injection of 370 MBq of 18F-FDG. Non–contrast-enhanced whole-body CT images were first acquired using an 8-slice helical CT scanner with a gantry rotation speed of 0.8 s. The following parameters were used to collect data: 80 mAs, 140 keV, a section width of 5 mm, and a table feed of 5 mm/rotation. PET emission images were then acquired from the thigh to the head for 5 min/frame. CT-based attenuation-corrected PET images were reconstructed using an ordered-subset expectation maximization algorithm (28 subsets, 2 iterations) and displayed in a 128 x 128 matrix (pixel size, 4.29 x 4.29 mm; slice thickness, 4.25 mm). Accurate coregistration of the CT and PET images was performed with commercially available software (Advantage Workstation; GE Healthcare).
Assessment of Vascular 18F-FDG Uptake
18F-FDG standardized uptake value (SUV) images were constructed with attenuation-corrected images, using injection dose, patient body weight, and a cross-calibration factor between the PET scanner and a dose calibrator. The images were visually evaluated for the presence of focal 18F-FDG uptake on the vascular walls of the aorta (proximal, descending thoracic, and abdominal) and the common carotid, subclavian, and common iliac arteries on the basis of agreement between 2 nuclear physicians unaware of the results of the subject's laboratory tests. Lesions located from the aortic arch to the diaphragm were treated as multiple sites. Vascular peak SUV (pSUV) was measured from axial views of the 18F-FDG PET images from regions of interest drawn over vessel walls as delineated by the CT images. All positive lesions, compared with blood-pool activity of the aortic arch, were confirmed to have 18F-FDG pSUV ratios of greater than 1. 18F-FDG–positive rates were defined as the proportion of subjects who had 1 or more positive lesions in a certain vascular region. The whole-body 18F-FDG index was calculated as the sum of (vessel–to–blood-pool pSUV ratio – 1) of all 18F-FDG–positive lesions in a subject. The carotid 18F-FDG index was similarly calculated from all 18F-FDG–positive carotid artery lesions.
Assessment of Atherogenic Risk Factors and Carotid Artery Ultrasonography
Atherogenic risk factors were assessed on the day of the PET/CT scan. Interviews provided subject age, sex, smoking habit, and presence of hypertension, diabetes mellitus, and statin medication. Physical examination evaluated systolic blood pressure (SBP), diastolic blood pressure (DBP), and body mass index (BMI). Laboratory tests included plasma levels of low-density lipoprotein (LDL), high-density lipoprotein (HDL), total cholesterol, triglyceride, immune reactive insulin, fasting blood sugar (FBS), glycosylated hemoglobin A1c, and high-sensitivity C-reactive protein (hsCRP).
Carotid artery IMT was measured in the posterior wall 10 mm proximal to the carotid bifurcation by high-resolution real-time B-mode ultrasonography (LOGIQ 7; GE Healthcare) in 29 and 52 subjects initially and at follow-up, respectively.
Statistical Analysis
All data are expressed as mean ± SD. The significance of differences in atherogenic risk factors between initial and follow-up studies was assessed by paired t tests with Bonferroni correction for multiple comparisons for continuous variables and
2 tests for rates of presence of risk and 18F-FDG–positive vessels. Correlation between whole-body or carotid 18F-FDG indices and atherogenic risk factors and the magnitudes of their changes were assessed by linear regression analysis. P values of less than 0.05 were considered statistically significant.
| RESULTS |
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90 mm Hg) was disclosed in 8 and 5 subjects at initial and follow-up physical examinations, respectively. Similarly, whereas 5 patients (8%) were reported to have diabetes, 4 and 2 subjects showed high FBS levels (>110 mg/dL) at initial and follow-up blood tests, respectively. Statin was being used in 3 and 8 subjects at the time of initial and follow-up examinations, respectively.
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18F-FDG PET/CT Findings at Initial and Follow-up Examinations
On the initial PET/CT scan, 50 of 60 subjects (83.3%) showed 1 or more 18F-FDG–positive lesions (average, 5.9 ± 5.0 lesions), leading to a total of 352 vascular sites. These were most frequent in the proximal aorta (n = 111, 31.5% of all lesions), followed by the abdominal (n = 100, 28.4%) and descending thoracic aorta (n = 88, 25.0%). The carotid, subclavian, and iliac arteries comprised 8.2%, 2.9%, and 4.0% of the lesions, respectively.
On the follow-up PET/CT scan, 18F-FDG–positive lesions were significantly reduced to an average of 2.1 ± 2.2 sites per subject (P < 0.0001) and a total of 124 sites (64.8% reduction). Of these, 111 were new lesions, whereas only 13 were lesions from the initial PET/CT scan (reversal rate, 96.3%). The relative distribution of 18F-FDG–positive sites was not different, with 37, 31, 39, 11, 5, and 1 lesions shown in the proximal, descending thoracic, and abdominal aortas and the carotid, subclavian, and iliac arteries, respectively. Figure 1 demonstrates an example in which an 18F-FDG–positive lesion in the right common carotid artery found on the initial PET/CT scan had disappeared 1 y later on the follow-up PET/CT scan.
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We also evaluated the effect of excluding from analysis 21 subjects who showed no change in major atherogenic risk factors, because they could represent a subgroup with less successful lifestyle modification. After exclusion, the summed whole-body 18F-FDG index still significantly decreased from 1.60 ± 1.34 at initial PET to 0.60 ± 0.75 at follow-up (P < 0.00001). In comparison, the respective values were 0.98 ± 1.07 and 0.41 ± 0.37 for the exclusion group. Also after exclusion, cholesterol and LDL were decreased and HDL increased at follow-up, and the follow-up whole-body 18F-FDG index correlated with age and BMI and inversely correlated with HDL.
| DISCUSSION |
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On the initial PET/CT scan, our healthy subjects showed an 83.3% incidence of 18F-FDG–positive arteries, a finding that is fairly consistent with previous observations made in patients undergoing cancer evaluation (11,12). As vascular 18F-FDG uptake corresponds to atherosclerotic plaques with inflammatory macrophages and normal vessels do not show measurable uptake on PET (14,15), our findings appear to illustrate the high prevalence of clinically silent atherosclerotic vessels in otherwise healthy adults (16). In our study, the magnitude of vascular 18F-FDG uptake was shown to correlate with atherosclerotic risk factors including low HDL, high LDL, high total cholesterol levels, and high systemic blood pressure. Similar relationships have been observed between large-vessel 18F-FDG uptake and the male sex (10), older age (12), hypertension (10,13), hyperlipidemia (9,12), low HDL (13), and elevated hsCRP (13).
Although most previous studies of vascular 18F-FDG have been performed on patients with cancer, we made this observation in otherwise healthy subjects, because direct metabolic effects from malignancy or from anticancer therapy may confound the results. Another unique feature of our study is that the pattern of vascular 18F-FDG uptake was serially monitored over 1 or 2 y as subjects underwent lifestyle modifications aimed at reducing their cardiovascular risk. It has been shown that vascular 18F-FDG uptake measurements are highly reproducible when PET is repeated at 2 wk (17). However, attempts to evaluate interscan reproducibility after significantly longer durations confront the difficulty of discerning observer variability from true vascular changes. We therefore looked at the subset of 21 study subjects who showed no change in major atherogenic risk factors during the observation period. As a result, the difference in the summed whole-body 18F-FDG index between the 2 PET studies tended to change less between studies when atherogenic risk remained constant. Furthermore, in subjects without a change of risk factors, a significant correlation in vascular region 18F-FDG index on initial and follow-up PET studies (Spearman r = 0.41, P < 0.0001) was seen. These findings indicate that the changes in 18F-FDG uptake measured in our study more likely reflect divergent vascular states associated with atherosclerosis rather than interscan observer variability. However, we cannot completely exclude the possibility that interscan observer variability contributed to at least part of the changes in 18F-FDG uptake measured in our study.
Much of the protective effect of risk-modifying interventions is mediated through improved blood pressure and lipoprotein levels, particularly by increasing HDL levels (18), and such improvements were indeed seen in our subjects. At follow-up, 96.3% of the initially positive vessels showed reversal of 18F-FDG uptake, whereas substantially fewer new sites appeared, and semiquantified 18F-FDG indices also significantly decreased. The magnitude of vascular 18F-FDG uptake measured by PET appears to mirror the inflammatory activity and macrophage content of the lesions (19). Although virtually all atherosclerotic lesions contain at least some macrophages, 18F-FDG uptake is likely no longer visible on PET when the number of infiltrating macrophages falls below a certain sensitivity threshold.
We observed that the magnitude of reduction in 18F-FDG uptake on follow-up PET/CT correlated closely with the amount of increase in HDL during the same period. This finding relates to a recent report by Tahara et al. in which carotid artery 18F-FDG uptake levels decreased with simvastatin treatment with an accompanying increase in plasma HDL (20). It is established that HDL levels inversely correlate to cardiovascular risk in humans, which has been attributed to the antiatherogenic properties of HDL that mediate reverse cholesterol transport from peripheral tissues and macrophages to the liver for excretion (21).
CONCLUSION
The results of this study demonstrate that vascular 18F-FDG uptake that correlates with atherogenic risk factors is significantly reversed with risk reduction through lifestyle modification, and the magnitude of reversal closely correlates with the amount of increase in plasma HDL. Therefore, serial 18F-FDG PET/CT may have a role as a noninvasive method to monitor the response of inflammatory change in atherosclerotic lesions to risk modification therapy.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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| References |
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