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Basic Science Investigations |
1 Department of Investigative Radiology, Research Institute, National Cardiovascular Center, Suita, Japan
2 Laboratory of Genome Bio-Photonics, Photon Medical Research Center, Hamamatsu University School of Medicine, Hamamatsu, Japan
3 Department of Pathofunctional Bioanalysis, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto, Japan
4 Department of Nuclear Medicine and Diagnostic Imaging, Graduate School of Medicine, Kyoto University, Kyoto, Japan
5 Institute for Experimental Animals, Kobe University School of Medicine, Kobe, Japan
| ABSTRACT |
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Key Words: 18F-FDG atherosclerosis vulnerable plaque macrophage Watanabe heritable hyperlipidemic rabbit
| INTRODUCTION |
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Plaque vulnerability is characterized by a large lipid-rich atheromatous core, a thin fibrous cap, and infiltration by inflammatory cells, such as macrophages (25). Several imaging approaches have been adapted to detect vulnerable atherosclerotic plaques (6). Conventional x-ray contrast angiography detects morphologic characteristics, such as stenosis. However, because both types of plaques cause stenosis (7), it is difficult to determine whether a plaque is stable or vulnerable by x-ray contrast angiography. MRI approaches, including magnetic resonance angiography, have also been developed by use of gradient-echo techniques and have been successful in discriminating lipid cores, fibrous caps, and hemorrhaged lesions (8,9). Moreover, an intravascular ultrasonography technique that can directly image an atheroma recently was reported (10). These new approaches provide images based on the morphologic characteristics of the atheroma but do not provide biologic information, such as the presence of inflammation, which is an important characteristic of vulnerable plaque.
Meanwhile, nuclear medical techniques are able to detect atherosclerotic lesions along with biopathologic functions and therefore offer a great advantage for the discrimination of vulnerable plaques on the basis of biologic characteristics (11). In fact, a clinical trial to detect atherosclerotic lesions by nuclear medical imaging with 18F-FDG recently was reported (12). However, there is the possibility that 18F-FDG is taken up by surrounding cells, including smooth muscle cells, as well as macrophages that have infiltrated vulnerable plaques, because glucose is an essential substrate for energy production in various cells. The uptake of radioactivity by the tissue surrounding vulnerable plaques, including the blood, prevents the visualization of the plaques. Thus, to evaluate the usefulness of 18F-FDG for imaging vulnerable plaques, it is necessary to clarify causative biologic factors in the uptake of 18F-FDG by atherosclerotic lesions.
Watanabe heritable hyperlipidemic (WHHL) rabbits have been widely used as an animal model of atherosclerosis (13,14). In WHHL rabbits, the atherosclerotic plaque does not rupture, but its pathologic characteristics have been reported to resemble those of the human lesion (15,16,17). Thus, although the aortic plaque of WHHL rabbits does not reflect the plaque rupture process absolutely, the use of aortic segments of WHHL rabbits could be effective for investigating the biologic factors responsible for the accumulation of 18F-FDG.
Therefore, in the present study, by using the WHHL rabbit aorta, we investigated the relationship between the accumulation of 18F-FDG and the biopathologic characteristics of atherosclerotic lesions (macrophage density and intimal thickness) and performed PET imaging of vulnerable plaques.
| MATERIALS AND METHODS |
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18F-FDG Distribution
The rabbits were initially anesthetized with a bolus injection of sodium pentobarbital (30 mg/kg, intravenous), followed by a continuous injection of propofol (10 mg/kg/h, intravenous). 18F-FDG (260555 MBq) was injected into a marginal ear vein. Arterial blood samples were collected from an auricular artery at 60, 120, 180, and 240 min after injection of the radiotracer. At 4 h after the 18F-FDG injection, the animals were sacrificed with an overdose of sodium pentobarbital. The thoracic and abdominal aortas were removed, and adjacent fat and connective tissue were detached. The aortas were cut into 6-mm segments and weighed. The segments were immediately fixed in a solution containing L-(+)-lysine hydrochloride (75 mmol/L) and 4% paraformaldehyde in phosphate buffer (37.5 mmol/L; pH 7.4), and radioactivity was measured with a well-type
-counter (PS-201; Aloka Co., Ltd.). Blood samples were also weighed, and radioactivity was measured. The results were expressed as the differential uptake ratio (DUR), which was calculated as (tissue activity/tissue weight)/(injected radiotracer activity/animal body weight), with activities given in becquerels and weights given in kilograms.
Histologic Analysis
Each arterial segment was embedded in paraffin. Consecutive 5-µm-thick slices were prepared 1, 3, and 5 mm from the end of the 6-mm segments used to measure radioactivity. The slices were subjected to Azan-Mallory staining or immunohistochemical staining. Immunohistochemical analysis was performed by the method of Tsukada et al. (18) with rabbit macrophage-specific monoclonal antibody RAM-11 (Dako Corp.). These slices were also costained with hematoxylin for identification of the nucleus. The number of macrophages was determined by counting the nuclei of RAM-11positive cells in each slice. The Azan-Mallorystained slice image was read with an optical scanner, and the intimal and whole areas of the vessels were measured. The density of macrophages and the ratio of the area of the intima to the area of the whole cross section in each 6-mm segment were obtained by averaging the data at the 3 positions, 1, 3, and 5 mm. The data for each 6-mm segment were used for comparison with 18F-FDG uptake.
CT and PET Imaging
CT and 18F-FDG PET experiments were performed on each normal and WHHL rabbit before sacrifice. The rabbits were fixed on a board and moved carefully from a CT scanner (Auklet TSX-003A; Toshiba Co., Ltd.) to a PET scanner (ECAT EXACT HR; Siemens AG) to maintain the stereotactic position on the board. Iohexol, a contrast agent, was injected through a marginal ear vein. A helical CT angiogram was acquired at 180 min after injection of 18F-FDG, and PET scanning was performed for 15 min starting at 210 min after injection of 18F-FDG. PET images and CT images were automatically coregistered by use of automatic image registration software (19). PET images were calibrated to the injected dose of 18F-FDG. For measurement of radioactivity in the thoracic aorta, 6 circular regions of interest (4.1 mm2 for each region of interest) were drawn on the successive cross sections (1 mm apart) of the aorta.
Statistical Analysis
Data are presented as the mean ± SD. Statistical analysis was performed with the MannWhitney U test for comparing the aortic segments of the control and WHHL rabbits. Correlation coefficients were assessed with Spearman rank correlation coefficients. Statistical significance was established at a P value of <0.05.
| RESULTS |
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-counter, were 209.3 ± 34.2 and 184.3 ± 17.0 kBq/g, respectively, and the DURs corrected for the body weight and the injected dose were 1.03 ± 0.16 and 0.71 ± 0.07, respectively; the DUR in the WHHL rabbits was significantly higher than that in the control rabbits (P < 0.05).
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| DISCUSSION |
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In the aortas of the control rabbits, the intima was thin, and no macrophages were seen. However, in those of the WHHL rabbits, a thick intima and macrophage infiltration were observed. These pathologic findings in the WHHL rabbits are consistent with results reported previously (16,20).
The uptake of 18F-FDG in the WHHL rabbits was studied comparatively by using a control group. As shown in Table 1, uptake was significantly higher in the aortas of the WHHL rabbits than in those of the control rabbits. Furthermore, in the WHHL rabbits, higher 18F-FDG accumulation was observed in the thoracic aorta than in the abdominal aorta. Because macrophage density was greater in the thoracic aorta than in the abdominal aorta, the increased 18F-FDG accumulation in the thoracic aorta may have been attributable to the macrophage density (Table 1). Therefore, in the WHHL rabbits, the relationship between the accumulation of 18F-FDG and the biopathologic characteristics of the atherosclerotic lesions, including the number of macrophages, was quantitatively examined.
In general, the intima is inflamed and extensive macrophage infiltration is observed in vulnerable plaques. In contrast, in stable plaques, the intima is not inflamed and macrophages have not infiltrated (25). Therefore, the relationship between 18F-FDG accumulation and the number of macrophages in atherosclerotic lesions was investigated. The accumulation of 18F-FDG was well correlated with the number of macrophages (Fig. 2A). Furthermore, the relationship between 18F-FDG accumulation and intimal thickness was examined. In a comparison of vulnerable plaques and stable plaques, the intimal thicknesses are similar, but the components of the intima are different; macrophage density is greater in vulnerable plaques than in stable plaques, and smooth muscle cells are the major cellular components of stable plaques (21). If cells other than macrophages, such as smooth muscle cells, contribute to accelerated 18F-FDG uptake, then the accumulation of 18F-FDG should be correlated with intimal thickness. However, the correlation between 18F-FDG uptake and the ratio of the area of the intima to the area of the whole cross section was poor (Fig. 2B). Also, there was no relationship between the number of macrophages and intimal thickness. These findings suggest that macrophage density is mainly responsible for 18F-FDG accumulation in plaques. In fact, both Rudd et al. and Lederman et al. showed that tritium-labeled glucose analogs accumulate in macrophage-rich atherosclerotic plaques in vitro (12,22). These reports are consistent with our in vivo findings.
Furthermore, it was reported that macrophages contribute extensively to the development of inflammation in plaques (23,24), and ruptured plaques have large numbers of macrophages (25). Thus, macrophage density is considered an important determinant of plaque vulnerability. Therefore, the finding that 18F-FDG uptake is directly proportional to macrophage density suggests the usefulness of 18F-FDG for the assessment of plaque vulnerability.
PET imaging with 18F-FDG detected intense radioactivity in the WHHL rabbit aortas, whereas the control rabbit aortas could not be imaged (Fig. 3). Because the blood-to-atherosclerotic lesion ratio in the WHHL rabbits was higher than that in the control rabbits and no difference was detected in blood radioactivities between the control and the WHHL rabbits, these results indicate that the strong radioactivity detected by PET in the WHHL rabbit aortas was attributable to vulnerable plaques and not to blood. In fact, the radioactivities in both the thoracic aortic lesions measured by PET and the removed thoracic aortic segments measured by the well-type
-counter were higher in the WHHL rabbits than in the control rabbits. However, the radioactivity measured by PET was lower than that measured by the well-type
-counter. This underestimation could have been attributable to the low spatial resolution of the PET scanner (full width at half maximum, 6 mm). Because the diameter of the aorta is smaller than the spatial resolution, 5 mm for the WHHL rabbits and 2.5 mm for the control rabbits, a partial-volume effect can be observed. The radioactivity was underestimated more in the control rabbits because the aorta is thinner in these rabbits than in the WHHL rabbits. The radioactivity obtained by the well-type
-counter was higher in the WHHL rabbits than in the control rabbits, but the difference was not significant. However, the DUR in the WHHL rabbits was significantly higher than that in the control rabbits because of differences in body weight and injected dose.
Because atherosclerotic plaques are small and the spatial resolution of PET is low, the use of new imaging devices will be required to image vulnerable atherosclerotic plaques accurately. Devices combining PET and CT or MRI have been developed to overcome the lack of anatomic information on PET images (26); in fact, the routine clinical use of PET/CT scanners is now under way (27). These techniques should make it possible to detect 18F-FDG accumulation in vulnerable plaques in atherosclerosis. However, the detection of carotid plaques with 18F-FDG PET would be difficult because the carotid artery is too narrow. The detection of coronary plaques would also be difficult because of high myocardial background radioactivity and cardiac motion. These difficulties might be overcome by use of a small positron-sensitive detector that can be inserted into the coronary artery and placed in close contact with lesions (22,28,29).
Determination of the 18F-FDG uptake for discriminating stable and vulnerable plaques is clinically important. It is applicable in assessing the risk of rupture among atherosclerotic plaques and in judging the therapeutic effect of some pharmaceutical agents for atheromas, such as statins. However, for these purposes, there is a limitation in studies with WHHL rabbits. Therefore, we did not evaluate the accuracy of 18F-FDG for distinguishing vulnerable from stable plaques. In WHHL rabbits, the rupture of aortic atherosclerotic plaques was not observed; moreover, there was considerable overlap, with some segments having large numbers of macrophages but little or no increase in 18F-FDG uptake, although there was a good correlation between the accumulation of 18F-FDG and the number of macrophages (Fig. 2A). It has been reported that the formation of aortic atherosclerotic plaques is related to the age of WHHL rabbits (20), although that relationship was not clear from the present study. In addition, vulnerable plaques are characterized by several features, such as a thin cap, in addition to the number of macrophages. Thus, further study of the relationship between the accumulation of 18F-FDG and factors such as age and cap thickness is required.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Hideo Saji, PhD, Department of Pathofunctional Bioanalysis, Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan.
E-mail: hsaji{at}pharm.kyoto-u.ac.jp
| REFERENCES |
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