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Vascular Calcification in ex Vivo Carotid Specimens: Precision and Accuracy of Measurements with Multi–Detector Row CT¹

¹ From the Departments of Radiology (U.H., D.C.K., J.H., R.C., T.J.B.) and Vascular Surgery (G.L.), Massachusetts General Hospital and Harvard Medical School, 100 Charles River Plaza, Suite 400, Boston, MA 02114. Received August 21, 2002; revision requested October 24; final revision received March 26, 2003; accepted April 14. Funded in part by the Center for the Integration of Medicine and Innovative Technology (CIMIT), Boston, Mass, and the New York Cardiac Center. Address correspondence to U.H. (e-mail: uhoffman@partners.org).

View larger version (46K): [in a new window]   Figure 1. Transverse CT scan (sequential scanning mode, 1.25-mm section thickness, 140 kVp, 140 mA) demonstrates the experimental setup. CEA specimens are embedded in plastic tubes filled with saline. The bone window shows the calcified plaques (bright structures) in excised vessels.

View larger version (12K): [in a new window]   Figure 2. Graph illustrates tube energy setting conversion factors used to calculate modified Agatston scores. The attenuation of phantom structures—that is, hydroxyapatite cylinders—was measured with different tube energies and converted to a 40-kVp standard setting. Graph shows an example of attenuation values (in Hounsfield units) measured at 120-140 kVp. Similar plots were generated for 80- and 100-kVp settings.

View larger version (22K): [in a new window]   Figure 3a. Graphs illustrate comparison of multi-detector row CT- and ashing-derived mineral mass and volume values for 16 ex vivo CEA specimens performed with a standard multi-detector row CT protocol (140 kVp, 140 mA, 1.25-mm section thickness). (a) Correlation between multi-detector row CT-derived mineral mass and ashing-derived mineral mass (r = 0.99, P < .001). (b) Correlation between multi-detector row CT-derived calcified plaque volume and ashing-derived mineral volume (r = 0.95, P < .001). The volume of ashing remnants was calculated as the density of the major component of calcified plaque minus the density of hydroxyapatite (3.153 g/cm3).

View larger version (26K): [in a new window]   Figure 3b. Graphs illustrate comparison of multi-detector row CT- and ashing-derived mineral mass and volume values for 16 ex vivo CEA specimens performed with a standard multi-detector row CT protocol (140 kVp, 140 mA, 1.25-mm section thickness). (a) Correlation between multi-detector row CT-derived mineral mass and ashing-derived mineral mass (r = 0.99, P < .001). (b) Correlation between multi-detector row CT-derived calcified plaque volume and ashing-derived mineral volume (r = 0.95, P < .001). The volume of ashing remnants was calculated as the density of the major component of calcified plaque minus the density of hydroxyapatite (3.153 g/cm3).

View larger version (21K): [in a new window]   Figure 4a. (a) Conventional and (b) normalized Bland-Altman scatterplots for multi-detector row CT-derived mineral mass (MMCT) as compared with the mineral mass of ashing remnants (MMA), the independent reference standard. The difference between the two mass values is calculated as follows: MMCT - MMA. The mean mineral mass is calculated as follows: (MMCT + MMA)/2. The normalized difference between the two mass values is calculated as follows: (MMCT - MMA)/MMA. (c) Conventional and (d) normalized Bland-Altman scatterplots for multi-detector row CT-derived calcified volume (VCT) as compared with the volume of ashing remnants (VA). The difference between the two volume values is calculated as follows: VCT - VA. The mean calcified volume is calculated as follows: (VCT + VA)/2. The normalized difference between the two volume values is calculated as follows: (VCT - VA)/VA. The multi-detector row CT-derived mineral mass is a less biased and more precise measurement of the mineral content of nonmoving ex vivo CEA specimens as compared with the conventional volume score. Volume measurements systematically are overestimations of the mineral content.

View larger version (21K): [in a new window]   Figure 4b. (a) Conventional and (b) normalized Bland-Altman scatterplots for multi-detector row CT-derived mineral mass (MMCT) as compared with the mineral mass of ashing remnants (MMA), the independent reference standard. The difference between the two mass values is calculated as follows: MMCT - MMA. The mean mineral mass is calculated as follows: (MMCT + MMA)/2. The normalized difference between the two mass values is calculated as follows: (MMCT - MMA)/MMA. (c) Conventional and (d) normalized Bland-Altman scatterplots for multi-detector row CT-derived calcified volume (VCT) as compared with the volume of ashing remnants (VA). The difference between the two volume values is calculated as follows: VCT - VA. The mean calcified volume is calculated as follows: (VCT + VA)/2. The normalized difference between the two volume values is calculated as follows: (VCT - VA)/VA. The multi-detector row CT-derived mineral mass is a less biased and more precise measurement of the mineral content of nonmoving ex vivo CEA specimens as compared with the conventional volume score. Volume measurements systematically are overestimations of the mineral content.

View larger version (21K): [in a new window]   Figure 4c. (a) Conventional and (b) normalized Bland-Altman scatterplots for multi-detector row CT-derived mineral mass (MMCT) as compared with the mineral mass of ashing remnants (MMA), the independent reference standard. The difference between the two mass values is calculated as follows: MMCT - MMA. The mean mineral mass is calculated as follows: (MMCT + MMA)/2. The normalized difference between the two mass values is calculated as follows: (MMCT - MMA)/MMA. (c) Conventional and (d) normalized Bland-Altman scatterplots for multi-detector row CT-derived calcified volume (VCT) as compared with the volume of ashing remnants (VA). The difference between the two volume values is calculated as follows: VCT - VA. The mean calcified volume is calculated as follows: (VCT + VA)/2. The normalized difference between the two volume values is calculated as follows: (VCT - VA)/VA. The multi-detector row CT-derived mineral mass is a less biased and more precise measurement of the mineral content of nonmoving ex vivo CEA specimens as compared with the conventional volume score. Volume measurements systematically are overestimations of the mineral content.

View larger version (23K): [in a new window]   Figure 4d. (a) Conventional and (b) normalized Bland-Altman scatterplots for multi-detector row CT-derived mineral mass (MMCT) as compared with the mineral mass of ashing remnants (MMA), the independent reference standard. The difference between the two mass values is calculated as follows: MMCT - MMA. The mean mineral mass is calculated as follows: (MMCT + MMA)/2. The normalized difference between the two mass values is calculated as follows: (MMCT - MMA)/MMA. (c) Conventional and (d) normalized Bland-Altman scatterplots for multi-detector row CT-derived calcified volume (VCT) as compared with the volume of ashing remnants (VA). The difference between the two volume values is calculated as follows: VCT - VA. The mean calcified volume is calculated as follows: (VCT + VA)/2. The normalized difference between the two volume values is calculated as follows: (VCT - VA)/VA. The multi-detector row CT-derived mineral mass is a less biased and more precise measurement of the mineral content of nonmoving ex vivo CEA specimens as compared with the conventional volume score. Volume measurements systematically are overestimations of the mineral content.

View larger version (16K): [in a new window]   Figure 5. Bar graph illustrates measurement variability of scoring algorithms for quantification of ex vivo vascular calcifications. Mineral mass (MM) and modified Agatston score (ASM) measurements are more reproducible than conventional Agatston score (AS) and volume score (VS) measurements. Mean coefficients of variation (±SD) for 16 CEA specimens are depicted. Black bars: overall measurement variability among all multi-detector row CT protocols, including those involving variations in tube current, tube voltage, and section thickness (described in Materials and Methods). Gray bars: measurement variation with 0.60-, 1.25-, 2.50-, 3.75-, and 5.00-mm section thicknesses; measurements were obtained at 120 kVp and 50 mA. White bars: measurement variation with 80, 100, 120, and 140 kVp; measurements were obtained with a 1.25-mm section thickness and 150 mA.

View larger version (15K): [in a new window]   Figure 6. Graph illustrates variations in section thickness and resulting changes in calcium scoring measurements. Data calculated at 1.25-mm (black), 2.50-mm (dark gray), 3.75-mm (light gray), and 5.00-mm (white) section thicknesses are compared with data obtained at a 0.6-mm section thickness with 120 kVp and 50 mA. AS = conventional Agatston score, ASM = modified Agatston score, MM = calibrated mineral mass, VS = volume score.

View larger version (16K): [in a new window]   Figure 7. Graph illustrates variations in tube voltage (80, 100, 120, and 140 kVp) and tube current (50 mA [dashed line] and 140 mA [solid line]) and resulting changes in multi-detector row CT volume score measurements in 16 ex vivo CEA specimens. Measurements were performed at a 1.25-mm section thickness. Calcified plaque volume was calculated as the sum of measurements in the 16 CEA specimens. The calcified plaque volume is inversely related to tube voltage (y = -395.4x = 6,703, r = -0.95, P < .001) but unaffected by changes in tube current (50-140 mA, P = 0.45) owing to the experimental setup (ie, with small embedding volume and low image noise).