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DOI:10.2214/AJR.05.1369
AJR 2007; 188:1063-1068
© American Roentgen Ray Society

Original Research

Coronary Artery Calcium Scoring: Influence of Reconstruction Interval and Reconstruction Increment Using 64-MDCT

Thomas Schlosser1,2, Peter Hunold1, Thomas Voigtländer2, Axel Schmermund2 and Jörg Barkhausen1

1 Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Hufelandstrasse 55, Essen 45122, Germany.
2 Cardiovascular Center Bethanien, Frankfurt 60389, Germany.

Received August 8, 2005; accepted after revision August 18, 2006.

 
Address correspondence to T. Schlosser (thomas.schlosser{at}uni-essen.de).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. Assessment of coronary artery calcification is increasingly used for cardiovascular risk stratification. However, a scanning protocol for modern MDCT has not been established. In this study, we evaluated the impact of the reconstruction interval within diastole and the reconstruction increment on the coronary calcium score.

MATERIALS AND METHODS. In 40 consecutive patients Agatston scores and volumetric scores were assessed using a 64-MDCT scanner. The patients were assigned to two groups at random with 20 patients each: in group A, collimation was 64 x 0.6 mm; in group B, it was 20 x 1.2 mm. All CT examinations were performed with retrospective ECG gating. For each patient, five data sets were created throughout diastole (50%, 55%, 60%, 65%, and 70% of the R-R interval). For each reconstruction, two data sets were calculated with a reconstruction increment of 3.0 and 1.5 mm, respectively. For all reconstructions, the mean Agatston scores and volumetric scores ± SD and the coefficient of variance were assessed. Furthermore, for each reconstruction, patients were assigned a percentile rank that described the level of cardiovascular risk.

RESULTS. Four patients had to be excluded from the study because no coronary calcium was detected on any of the reconstructions. In both groups, the mean Agatston score was not significantly different between reconstruction increment 3.0 mm and reconstruction increment 1.5 mm (group A, 112.1 ± 92.5 and 114.3 ± 93.6, p = 0.28; group B, 164.8 ± 203.0 and 169.4 ± 207.9, p = 0.29, respectively). However, in two cases, very small calcified lesions in the circumflex coronary artery were only detected using a reconstruction increment of 1.5 mm. In both groups, the mean coefficient of variation was not significantly different at reconstruction increment 1.5 mm (group A, 11.4 ± 8.2; group B, 12.5 ± 7.6) and reconstruction increment 3.0 mm (group A, 14.8 ± 9.3; group B, 14.2 ± 9.1; group A, p = 0.18; group B, p = 0.48). Based on the reconstruction increment and reconstruction interval, 77% of the patients (n = 14) in group A were assigned to one risk group and 23% (n = 4) to two different risk groups according to percentile strata. In group B, 83% of the patients (n = 15) were assigned to one risk group and 17% (n = 3) to two different risk groups. In contrast to the Agatston score, the volumetric score was significantly higher in both groups at reconstruction increment 1.5 mm (group A, 105.4 ± 78.5 mm3; group B, 153.8 ± 182.5 mm3) compared with reconstruction increment 3.0 mm (group A, 90.0 ± 73.11 mm3; group B, 138.2 ± 166.8 mm3; p < 0.05).

CONCLUSION. Using a 64-MDCT scanner, the calcium score calculated from different reconstructions within early diastole is variable, but the difference can be minimized using overlapping slice reconstructions. The variation does not lead to a different risk estimation in most patients. In patients with mild coronary calcifications, the use of overlapping slices may help to detect small calcified plaques. Furthermore, we recommend the use of ECG-controlled tube current modulation to reduce the radiation exposure.

Keywords: cardiac imaging • cardiovascular disease • coronary calcium • CT


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Several studies have shown that the extent of coronary artery calcification (CAC) is associated with the probability of significant coronary artery stenoses and is related to the risk of myocardial infarction and sudden cardiac death [1-5]. Moreover, the total amount of CAC correlates with the overall coronary plaque burden [6-8], and the progression of coronary artery disease is associated with increasing CAC [9, 10].

Initially, electron beam tomography (EBT) and subsequent MDCT were introduced for the quantification of CAC. For CAC quantification, a good correlation was found between both technologies [11]. However, in a recently published phantom study, it has been shown that thin-slice and overlapping helical CT is more sensitive for detecting small amounts of calcium in comparison with EBT [12]. This has an important impact in the clinical setting because the presence or absence of coronary calcium predicts the risk of new cardiac events. In addition, it has been shown that the calcium scores assessed on MDCT were less influenced by heart rate than those assessed on EBT [13].

However, in a recent study performed using a 16-MDCT scanner, it was shown that the Agatston and volumetric scores both proved to be highly dependent on the reconstruction interval used [14]. Moreover, the variable Agatston scores resulted in a completely different estimation of the level of coronary risk according to percentile strata.

Recently, a new generation of MDCT scanners has been introduced, equipped with more and thinner detector rows and a decreased gantry rotation time [15]. We hypothesized that despite improved overall variability using a 64-MDCT scanner, the reconstruction increment still has a significant influence on the extent of the Agatston and volumetric scores and that overlapping slice reconstruction can minimize the variability of the calcium score. Moreover, we hypothesized that the reconstruction interval in early diastole influences the calcium score and results in a different risk estimation depending on the reconstruction window.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
The study was performed in accordance with all regulations of the local institutional review board and all patients gave written informed consent.

Forty consecutive patients (23 men, 17 women; total age range, 43-84 years; total mean age, 58.3 ± 9.3 years; men, age range, 43-84 years; mean age, 63.9 ± 8.4 years; women, age range, 52-82 years; mean age, 67.2 ± 7.6 years) with known or suspected coronary artery disease who underwent CAC scanning for clinical indications were included in the study. After coronary calcium scoring, CT coronary angiography was performed in all patients. To optimize image quality, ß-blockers were administered. Patients whose heart rates exceeded 65 beats per minute (bpm) received 5 mg of bisoprolol orally 1 hour before the MDCT examination. In case of insufficient heart rate reduction, up to four vials (20 mg) of metoprolol were injected IV. The age between both groups was not significantly different (Mann-Whitney U test, p > 0.05) [16]. Exclusion criteria were heart rate greater than 65 bpm after application of ß-blockers and patients with coronary stents or coronary bypass grafts.

Imaging
The MDCT examinations were performed using a commercially available, 64-MDCT scanner (SOMATOM Sensation Cardiac 64, Siemens Medical Solutions) with a gantry rotation time of 330 milliseconds. The patients were assigned to two groups at random, each with 20 patients: in group A, the collimation was set to 64 x 0.6 mm, whereas in group B, the collimation was 20 x 1.2 mm. The reconstructed slice thickness for both groups was 3 mm (field of view, 26 x 26 cm; matrix, 512 x 512; pixel size, 0.5 x 0.5 mm). All CT examinations were performed with retrospective ECG gating and dose reduction by using ECG-controlled tube current modulation (ECG pulsing). Using ECG pulsing within every cardiac cycle, the tube current is raised to the nominal level during a limited interval in the diastolic phase in which data are reconstructed (190 mAs, 120 kV). During the remaining part of the cardiac cycle, the tube output can be reduced approximately 80% by a corresponding decrease in the tube current [17, 18]. The width of the time interval with nominal tube output was selected from 50% to 70% of the R-R interval. The volume CT dose index (CTDIvol) was assessed for each patient.

Data Reconstruction and Analysis
After scanning was completed, for each patient, five data sets were created throughout diastole (50%, 55%, 60%, 65%, and 70% of the R-R interval). For each reconstruction, two data sets were calculated with reconstruction increments of 3.0 and 1.5 mm, respectively. For quantification of CAC, all reconstructions were transferred to a PCbased workstation (Syngo CaScoring Wizard, Siemens Medical Solutions). CAC was defined as the presence of more than two contiguous pixels with Hounsfield units greater than 130. These lesions were automatically identified and marked in color by the workstation. In a second step, a radiologist with 4 years of experience with coronary artery CT and a cardiologist with 11 years of experience with coronary artery CT differentiated between calcium and noise by placing regions of interest around the CAC. All values of the left main coronary artery (LM), left anterior descending coronary artery (LAD), circumflex coronary artery (LCX), and right coronary artery (RCA) were added to calculate the total Agatston and volumetric scores. Both scores were determined for all reconstructions in all patients.

Statistical Analysis
The total ranges of Agatston scores, mean scores, SD, and coefficients of variation of the five different time points at early diastole were calculated for all patients and for all reconstructions using a descriptive statistics tool (version 10.0.7, SPSS). The mean Agatston score of reconstruction increment 1.5 mm was compared with the mean Agatston score of reconstruction increment 3.0 mm using a sign test (p ≤ 0.05 needed to indicate a significant difference) [16]. The corresponding analyses were performed using the volumetric scores.

Moreover, depending on total Agatston score, age, and sex, patients were assigned to a cardiovascular risk group (percentile ≤ 25, low risk; 26-50, moderate risk; 51-75, increased risk; 76-90, high risk; and > 90, very high risk) based on the different reconstructions. The percentile values were derived from a European general, unselected population as described by Schmermund et al. [19] on the basis of the data from the Heinz Nixdorf Recall Study.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Four patients had to be excluded from the study because no CAC was detected in any of the reconstructions. The mean heart rate of the remaining patients (n = 36) was 57 ± 4 bpm (range, 51-64 bpm). The mean CTDIvol for group A (n = 18) was 6.2 ± 1.2 mGy. For group B (n = 18), the CTDIvol was 5.9 ± 1.5 mGy; this difference was not statistically significant (p =0.67).

Agatston Score
The mean Agatston score of all patients was 136.9 ± 151.8. For group A, the mean Agatston score was 112.1 ± 92.5 (reconstruction increment, 3.0 mm; range, 10-270 mm). After data reconstruction using a reconstruction increment of 1.5 mm, the mean Agatston score of the same patient group was not statistically significantly different (114.3 ± 93.6; range, 11-279; p = 0.28) (Table 1). However, in two cases, very small calcified lesions in the LCX were only detected using a reconstruction increment of 1.5 mm.


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  		TABLE 1: Mean Agatston and Volumetric Scores for Group A and Group B

 

The mean Agatston score for group B was 164.8 ± 203.0 (reconstruction increment, 3.0 mm; range, 3-445 mm). Corresponding to group A, the mean Agatston score for group B after data reconstruction using a reconstruction increment of 1.5 mm was not statistically significantly different (169.4 ± 207.9; range, 4-454; p = 0.29).

The coefficient of variation was assessed to measure the variation between the reconstruction intervals during early diastole. The mean coefficient of variation for group A using a reconstruction increment of 3.0 mm was 14.8% ± 9.3% (range, 4-27%). Using a reconstruction increment of 1.5 mm, the mean coefficient of variation was not statistically significantly different (11.4% ± 8.2%; range, 3-24%; p = 0.18).

The mean coefficient of variation for group B was 14.2% ± 9.1% (reconstruction increment, 3.0 mm; range, 3-30%). After data reconstruction using a reconstruction increment of 1.5 mm, the mean coefficient of variation was not statistically significantly different (12.5% ± 7.6%; range, 3-26%; p = 0.48).


Figure 1
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  		Fig. 1 —Slight but significant negative correlation was found between the total Agatston score and coefficient of variation in all patients (R =-0.55; p <0.01).

 
A slight but significant negative correlation was found between the total Agatston score and the coefficient of variation (R = -0.55; p < 0.01) (Fig. 1).

For group A, the maximum Agatston scores were determined at the 60% and 65% reconstruction intervals in 77% of all cases. This was also true for group B, in which for 72% of cases the maximum Agatston score was found at the 60% and 65% reconstruction intervals.

Depending on the reconstruction increment and reconstruction interval, 77% of the 18 patients (n = 14) in group A were assigned to one risk group and 23% of the patients (n = 4) were assigned to two risk groups according to percentile strata. In group B, 83% of the 18 patients (n = 15) were assigned to one risk group and 17% of the patients (n =3) were assigned to two risk groups.

Artifacts based on the reconstruction interval that led to over- and underestimation of the calcium score are shown in Figures 2 and 3.


Figure 2
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  		Fig. 2 —Transverse CT scans in 63-year-old man. Upper row shows CT images (collimation, 0.6 mm; slice thickness, 3 mm; reconstruction increment, 3 mm) of same slice position at different reconstruction intervals, from 50% on left to 70% on right, of the R-R interval in 5% steps. Lower row shows CT images of consecutive slice position at same reconstruction intervals. Two calcified plaques located in proximal left anterior descending artery are clearly detectable using 60%, 65%, and 70% reconstruction intervals, resulting in total Agatston score of 53, 50, and 50, respectively. Mild smearing artifacts are seen in 55% reconstruction (total Agatston score, 42), whereas 50% reconstruction results in a distinct underestimation of Agatston score (19) due to severe smearing artifacts.

 

Figure 3
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  		Fig. 3 —Transverse CT scans in 81-year-old man. Upper row shows CT images (collimation, 0.6 mm; slice thickness, 3 mm; reconstruction increment, 3 mm) of same slice position at different reconstruction intervals, from 50% on left to 70% on right, of the R-R interval in 5% steps. Lower row displays same CT images; calcified plaques are marked in red. Two calcified plaques located in left mainstem and proximal left anterior descending arteries are clearly visualized using 50%, 55%, 60%, and 65% reconstruction intervals, resulting in total Agatston score of 78, 87, 81, and 88, respectively. In 70% reconstruction interval, calcium is deformed and blurred, resulting in overestimation of total Agatston score of 122.

 
Volumetric Score
The mean volumetric score of all patients was 112.7 ± 124.4 mm3. For group A, the mean volumetric score was 90.0 ± 73.11 mm3 (reconstruction increment, 3.0 mm; range, 10-225 mm3). After data reconstruction using a reconstruction increment of 1.5 mm, the mean volumetric score of the same patient group was higher (105.4 ± 78.5 mm3; range, 11-279 mm3) (Table 1); this difference was statistically significant (p < 0.05) (Fig. 4A, 4B). The mean volumetric score for group B was 138.2 ± 166.8 mm3 (reconstruction increment, 3.0 mm; range, 3-362 mm3). The mean volumetric score for group B after data reconstruction using a reconstruction increment of 1.5 mm was slightly higher (153.8 ± 182.5 mm3; range, 4-402 mm3); again, this difference was statistically significant (p < 0.05) (Fig. 4A, 4B).


Figure 4
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  		Fig. 4A —Graphs of volumetric scores. Graphs show individual volumetric scores of all patients of group A (A) and group B (B) at reconstruction increments of 1.5 and 3.0 mm.

 

Figure 5
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  		Fig. 4B —Graphs of volumetric scores. Graphs show individual volumetric scores of all patients of group A (A) and group B (B) at reconstruction increments of 1.5 and 3.0 mm.

 
The mean coefficient of variation for group A using a reconstruction increment of 3.0 mm was 11.6% ± 7.6% (range, 2-23%). Using a reconstruction increment of 1.5 mm, the coefficient of variation was not statistically significantly different (8.1% ± 6.7%; range, 2-23%; p = 0.13).

The mean coefficient of variation for group B was 10.2% ± 6.5% (reconstruction increment, 1.5 mm; range, 3-23%). After data reconstruction using a reconstruction increment of 3.0 mm, the mean coefficient of variation was not statistically significantly different (10.9% ± 7.3%; range, 3-26%; p = 0.40).


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In a patient study performed using a 4-MDCT scanner, Gerber et al. [20] showed that a reconstruction window beginning at 60% to 70% of the R-R interval seems to be most advantageous for retrospective gating of MDCT studies performed to quantify coronary calcium. Mahnken et al. [21] showed that the median of the best visibility with the least motion artifacts was given for image reconstruction at 60% (LM, LAD, LCX) and 50% (RCA) of the R-R-interval. In the present study, we investigated the influence of the reconstruction interval in early diastole and the reconstruction increment on the Agatston CAC score and the volumetric CAC score using a 64-MDCT scanner. Furthermore, we studied the influence of variable CAC scores on the estimation of the coronary risk. Our data show that the Agatston and the volumetric scores depend on the reconstruction interval, but the variation of the total score has only a minor effect on the risk estimation of the patients. The reconstruction increment had no significant influence on the Agatston score. However, in contrast to the Agatston score, the volumetric score was significantly higher in overlapping image reconstructions. Moreover, even using the latest MDCT scanner, thin-slice overlapping CT is considered to have an advantage in detecting small amounts of CAC.

MDCT is a highly sensitive method for detecting CAC and thus allows for the identification of subclinical coronary artery atherosclerosis. Accordingly, cardiovascular risk can be assessed. However, different studies performed with 4- and 16-MDCT have shown that the total calcium score highly depends on the image reconstruction interval [14, 21]. Motion of the heart and the coronary arteries may result in smearing artifacts that lead to an underestimation of the Agatston score. This is true particularly for small and moderate calcified plaques. Another mechanism is related to more severe coronary calcifications. Especially in the systolic phase, the severely calcified lesions are deformed and blurred, resulting in increased calcium scores. These findings have already been shown in phantom studies [22].

In contrast to previous published studies, we used an MDCT scanner with a higher temporal and spatial resolution. Furthermore, we used ECG-controlled tube current modulation that results in a significant reduction of the radiation dose. One limitation of the MDCT scanning protocol is the higher radiation exposure compared with the sequential MDCT scanning protocol and EBT. In a phantom study Hunold et al. [23] showed that the effective radiation doses at unenhanced EBT calcium scoring were 1.0 and 1.3 mSv for male and female patients, respectively. With use of prospective triggering and a sequential MDCT protocol, the effective doses were slightly higher than those at EBT. The radiation exposure at MDCT with retrospective ECG gating was at least threefold higher than at EBT. However, these examinations were not performed with tube current modulation [23]. With this technique the effective dose is further reduced about 40%. However, this technology allows only analysis of a limited number of reconstructions within the cardiac cycle. According to the recommendation of Mahnken et al. [21], we selected a diastolic window from 50% to 70% of the R-R interval for data reconstruction.

A recently published study performed using a 16-MDCT scanner showed that variation of the Agatston scores of diastolic reconstructions in patients with mild CAC can result in completely different estimations of coronary risk, ranging from low to increased [14]. In our study, 77% of the patients in group A and 83% in group B were assigned to one risk group and only 23% in group A and 17% in group B were assigned to two risk groups. The low variation might be caused by fewer motion artifacts and partial volume effects because of the better temporal and spatial resolution that can be achieved by using 64-MDCT scanners. The degree of variation depended on the extent of the CAC. The coefficient of variation decreased with increasing total scores because the coefficient of variation depends on the SD and the mean. Thus, at low total Agatston scores, even low absolute deviations can result in high relative deviations and, therefore, a high coefficient of variation.

Furthermore, the assessment of the Agatston score using reconstruction increments of 1.5 and 3.0 mm resulted only in marginal differences that were not statistically significant. This was true for both groups (collimation of 0.6 and 1.2, respectively). In most of our patients the differences had no influence on the risk estimation. However, in two cases, very small coronary calcifications were only detected with overlapping slice reconstruction. This might be explained by the reduced partial volume effect in overlapping slice reconstructions. Especially in younger patients, the detection of small coronary calcifications has an influence on the estimation of the cardiovascular risk. In this population, it may be advisable to use overlapping slices. If no calcium was detected in nonoverlapping slices, additional overlapping slices might be reconstructed.

In contrast to the Agatston score, the reconstruction increment had a significant influence on the volumetric score in both groups. This might be caused by a higher susceptibility of the volumetric score to partial volume effects. Similar results were reported by Vliegenthart et al. [24] who showed that EBT scans with section thickness of 3.0 mm yielded less accurate calcified volume estimates than 1.5-mm scans.

In conclusion, on 64-MDCT, the calcium score calculated from different reconstructions within early diastole is variable. The use of overlapping slice reconstructions does not result in a significant reduction of the coefficient of variation, and the variation does not lead to a different risk estimation in most patients. In patients with mild coronary calcifications, the use of overlapping slices may help to detect small calcified plaques. Furthermore, we recommend the use of ECG-controlled tube current modulation to reduce the radiation exposure.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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