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Distribution of Abdominal Aortic Calcium by Computed Tomography

Posted Oct 31, 2013 Updated Apr 10, 2023
By Michael L. Chuang MD , Richard W. Leslie BA , Joseph M. Massaro PhD , Emily S. Mers BS , Caroline S. Fox MD MPH , Udo Hoffmann MD MPH , Christopher J. O'Donnell MD MPH
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Rationale and Objectives

Abdominal aortic calcification (AAC) can be quantified using computed tomography (CT), but imaging planes are prescribed based on bony landmarks, so that individual variation between the landmark and the aortoiliac junction can result in variable aortic coverage. In the Framingham CT substudy, we scanned a 15-cm (Z-direction) abdominal segment cranial to the S1 vertebral body. We sought to determine the range and distribution of length of aorta scanned and the distribution of AAC within the abdominal aorta and to compare burden of AAC measured from fixed-length segments versus AAC from all slices cranial to the aortoiliac bifurcation.

Materials and Methods

AAC was quantified by modified Agatston score (AS) in 100 Framingham Heart Study participants (60 ± 13 years old, 51 men). We compared the AS measured from 5-cm and 8-cm segments with the AS ALL (total visualized aorta).

Results

Of 100, 73 participants had AAC >0. The total length of aorta imaged was ≥8 cm in 84% of participants. Qualitatively, 5-cm and 8-cm segments correctly identified 96% and 99%, respectively, of participants as having or not having AAC. Quantitatively, AS 8cm was within 20% of AS ALL in four-fifths and within 30% of AS ALL in nine-tenths of participants. AS 5cm more severely underestimated AS ALL .

Conclusion

The use of S1 as the caudal imaging landmark in a 15-cm slab yields ≥8 cm aortic coverage in most adults. Both 5-cm and 8-cm analysis strategies are comparable to analyzing the total visualized abdominal aorta for prevalent AAC, but only 8-cm segment analysis yields quantitatively similar measures of AAC.

Abdominal aortic calcification (AAC) is associated with increased burden of cardiovascular morbidity and mortality and may be a predictor of future adverse cardiovascular disease events . Autopsy and imaging studies suggest that the prevalence of abdominal aortic atherosclerosis, including calcified aortic plaques, generally is greatest at the bifurcation of the abdominal aorta, with decreasing burden of atherosclerosis at a greater distance from the aortic bifurcation. Computed tomography (CT) allows quantitative measurement of AAC, an index of atherosclerosis burden, but there is no standard method for CT-based assessment of AAC. Although it clearly is desirable to scan comparable segments of abdominal aorta across individuals for purpose of comparisons, in practice this can be challenging since the aorta is not well visualized on CT scout images. CT imaging planes for the abdominal aorta are most commonly prescribed based on bony (vertebral) landmarks, but there can be individual variation between the level of the aortoiliac bifurcation and any given vertebral landmark, thus resulting in variable-length coverage of the abdominal aorta. The need to minimize radiation exposure creates an “overcoverage” strategy (ie, imaging a longer segment of the abdomen than likely to be needed) impractical.

In the Framingham CT substudy, a 15-cm extent in the Z (cranial-caudal) direction was prespecified for abdominal imaging. The slab was prescribed so that the S1 vertebral body delineated the caudalmost extent of imaging. S1 was selected as the lower end point to avoid irradiation of pelvic organs, as some study participants were women of childbearing age. This protocol provided views of the lower abdominal aorta and the upper portions of the iliac arteries. The purposes of this study were to determine the lengths of abdominal aorta imaged and to assess the effect of various analysis schemes (fixed and variable-length segments of abdominal aorta) on quantitative calcium burden. We hypothesized that AAC within the imaged portion of abdominal aorta tends to be concentrated caudally, based on autopsy and imaging literature, and that fixed and variable-length segment measurement methods may yield comparable quantitative results.

Materials and methods

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Study Sample

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CT Imaging Protocol

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Image Analysis Protocol

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Data and Statistical Analyses

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Results

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Table 1

Baseline Characteristics of the Study Sample

Men Women Men (All) Women (All) No. 51 49 1625 1914 Age, y 60 ± 13 61 ± 13 62 ± 10 61 ± 10 Body mass index, kg/m 2 28.7 ± 4.3 26.7 ± 4.7 28.8 ± 4.6 27.7 ± 5.9 Hypertension 42% 35% 49% 44% Dyslipidemia 22% 8.3% 25% 18% Current smoking 12% 12.5% 13% 14% Diabetes mellitus 16% 4.2% 14% 9%

ALL refers to all Framingham Offspring cohort members participating in the cycle 7 examination.

Figure 1, Distribution of study participants by length (in cm) of abdominal aorta scanned.

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Figure 2, Effect of analyzed-segment length on abdominal aortic calcification (AAC) in a single study participant. A total of 9.75 cm was scanned in this 81-year old man. (a) The arrow points to the abdominal aorta, and the square shows the crop used for B. (b) Imaged abdominal aorta slice-by-slice with the most cranial slice at the top left , proceeding to the most caudal slice at the bottom right . The bottom four rows ( dashed line at right ) show the slices analyzed under the Agatston score (AS) 5cm protocol (AS 5cm = 937), and the solid line shows the 32 slices analyzed under the AS 8cm protocol (AS 8cm = 1189). All of the slices shown were analyzed to obtain AS ALL = 1448.

Figure 3, Comparison of quantitative abdominal aortic calcification (AAC) score by fixed-length segments versus entire visualized portion of abdominal aorta in (a) the overall study group with AAC >0 ( n = 61) and (b) among study participants with AAC >400 Agatston score ( n = 41).

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Discussion

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Comparison with the Current Literature

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Implications for Research

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Clinical Implications

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Limitations

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Conclusions

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Academic Radiology, Volume 20, Issue 11
Journals General Radiology
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