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Electron Beam CT Versus 16-MDCT on the Variability of Repeated Coronary Artery Calcium Measurements in a Variable Heart Rate Phantom

¹ Department of Clinical Radiology, Hiroshima University Hospital, 1-2-3, Kasumi-cho, Minami-ku, Hiroshima 734-8551, Japan.
² Imaging Application Tech Center, GE Yokogawa Medical Systems, Tokyo, Japan.
³ Department of Radiology, Mazda Hospital, Mazda Motor Corporation, Hiroshima, Japan.
⁴ Department of Radiology, Division of Medical Intelligence and Informatics, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.

Received July 3, 2004; accepted after revision November 12, 2004.

 
Address correspondence to J. Horiguchi (horiguch{at}hiroshima-u.ac.jp).

Abstract

Top Abstract Introduction Materials and Methods Results Discussion APPENDIX 1: Summary of... References

 
OBJECTIVE. High reproducibility of coronary artery calcium (CAC) scoring is a key requirement for monitoring the progression of coronary atherosclerosis. The purposes of this study were to compare electron beam CT and 16-MDCT scanners in the variability of repeated CAC measurements and to assess the factors influencing this variability.

MATERIALS AND METHODS. CAC models of different sizes attached to a cardiac phantom with a programmable variable heart rate were scanned three times, and interscan variability of the CAC measurement was calculated each time. For helical CT, different slice-thickness images of either retrospective ECG-gated or prospective ECG-triggering reconstruction were obtained. The detection of small amounts of calcium, variability of the Agatston score, and CAC measurement algorithms (Agatston, volume, and mass scores) were compared between CT scanners and protocols.

RESULTS. All 1-mm-sized calcium models were detected on 0.625- and 1.25-mm helical CT, whereas some were missed on electron beam CT and 2.5-mm helical CT. Retrospective ECG-gated thin-slice helical CT showed the lowest variability. Reduction of variability by volume and mass scoring algorithms was less effective on 0.625- and 1.25-mm-thickness CT.

CONCLUSION. Retrospective ECG-gated thin-slice helical CT has the potential to be a useful tool for monitoring coronary atherosclerosis.

Introduction

Top Abstract Introduction Materials and Methods Results Discussion APPENDIX 1: Summary of... References

 
Electron beam CT is an accepted standard for the detection and quantification of coronary artery calcium (CAC) [1]. The amount of CAC is related to the risk of myocardial infarction and sudden cardiac death [2, 3]. Monitoring CAC is suggested to assess the progression and regression of coronary atherosclerosis, thereby documenting risk factors and the need for medical intervention [4, 5]. For this purpose, low interscan variability of CAC measurements is mandatory. The normal progression of the CAC score per year is reported to be 14-27% (average, 24%) [6] and can climb to 33-48% in patients with significant coronary disease [7, 8]. However, in previous studies, the variability using electron beam CT yields 20-37% [9-12], which jeopardizes the detection of any changes in this range.

Recently, new-generation 16-MDCT scanners have become available that allow either prospective ECG-triggering or retrospective ECG-gated reconstruction for cardiac study. The main purposes of this study were to compare electron beam CT and 16-MDCT in the variability of repeated CAC measurement and to assess the factors influencing the variability to optimize the scanning protocol.

Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion APPENDIX 1: Summary of... References

 
An originally designed cardiac phantom with CAC models was used (Fig. 1). For the calcium models, three materials were used: silicon (305 H), putty (501 H), and polytetrafluoroethylene (929 H) with four different sizes and volumes (size/volume): 1 mm/10 mm³, 3 mm/30 mm³, 5 mm/50 mm³, and 10 mm/100 mm³ (total of 12 models).

View larger version (77K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1 —Photograph (left) shows cardiac phantom, and graphic (right) shows balloon. Phantom consisted of five components: driver, control, support, rubber balloon, and ECG. A controller with an ECG-synchronizer drove the balloon. The motion was achieved by setting four driver sequences—that is, two speeds of fast emptying for the systolic phase and fast and slow filling for the diastolic phase. The balloon was filled with a mixture of water and contrast medium (58 H) to simulate noncontrast blood and was submerged in corn oil (-118 H), simulating epicardial and pericardial fat. Coronary artery calcium models were packed inside rubber tubes (mimicking coronary arteries) attached to the balloon surface. The ends of the balloon were stabilized to a fixed support at a distance of 10 cm. There was therefore neither through-plane motion (along z-axis) nor twist motion of the balloon. The volumes of the balloon phantom were approximately 100 and 200 mL at the systolic and diastolic phases, respectively. The time-balloon volume curve was similar to sinusoidal in heart rate shift sequences. The balloon was barrel-shaped at the diastolic phase and nearly, but not exactly, cylindrical at the systolic phase. Deformity of the balloon was seen in some images in high heart rate and arrhythmia sequences. This movement of the balloon resulted in some through-plane motion of calcium models.

View larger version (11K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2 —Graph shows three types of variable heart rate sequences: low heart rate (60 beats per minute [bpm]) with shifting (•), high heart rate (85 bpm) with shifting (), and arrhythmia ().

Electron Beam CT Protocol
The standard electron beam CT protocol used was as follows: 100-msec acquisition time, 35-40 continuous gapless slices of 3 mm in thickness, 130 kV, and 625 mA. The single-section mode images were obtained by ECG-triggering to 80% of the R-R interval. Image reconstruction was performed with a 512 x 512 pixel matrix using a sharp reconstruction kernel. The display field was 18 cm.

Retrospective ECG-Gated Helical CT Protocol
Volumetric data of the phantom were obtained using the helical mode with scanning parameters of 0.625-mm collimation width x 16 detectors, gantry rotation speeds of 0.5 sec/rotation (sequences 1 and 3) and 0.6 sec/rotation (sequence 2), 120 kV, and 100 mA. The pitch was set to 0.275 to enable multisector reconstruction, where pitch was defined as follows: [table feed x rotation time/total nominal section width]. Multisector reconstruction means that images are retrospectively reconstructed to improve temporal resolution by combining some (n = 2-4) adjacent cardiac cycles [13]. Four kinds of reconstruction were created: first, 0.625-mm-thickness images with 0.625-mm increment; second, 1.25 mm/1.25 mm; third, 2.5 mm/1.25 mm—that is, an overlapping reconstruction; and, fourth, 2.5 mm/2.5 mm. The center of the temporal window was set to 80% of the R-R interval. The matrix size and field of view were the same as for the electron beam CT protocol, and the reconstruction kernel was standard. Almost all helical CT images had a temporal resolution of 100-250 msec, although the temporal resolution achieved by multisector reconstruction differed according to heart rate and the number of cardiac cycles used for image reconstruction.

Prospective ECG-Triggering Axial CT Protocol
Axial imaging with a thickness of 0.625, 1.25, and 2.5 mm was performed using prospective triggering so that the center of the temporal window corresponded to 80% of the R-R interval. The scanning parameters were a gantry rotation speed of 0.5 sec/rotation, 120 kV, and 100 mA. The matrix size and field of view were the same as for the electron beam CT protocol, and the reconstruction kernel was standard. The temporal resolution was 250 msec.

The phantom scan and reconstruction protocols for electron beam CT and 16-MDCT are summarized in Appendix 1.

CAC Measurement
Calcium score, volume, and mass were determined on a commercially available external workstation (Advantage Windows [version 4.1], GE Healthcare) using CAC-scoring software (Smartscore [version 3.5], GE Healthcare) with both electron beam CT and helical CT. According to the Agatston method [1], we defined the regions of interest by vessel and slice with the threshold option for pixels greater than 130 H to measure the area and peak density of plaques. Depending on the peak density of the plaque, an area of at least 0.52 mm² (2 pixels) was multiplied by one of the following cofactors: a factor of 1 for 130-199 H, a factor of 2 for 200-299 H, a factor of 3 for 300-399 H, and a factor of 4 for densities 400 H or greater. The total calcium score was calculated as the sum of the individual lesion scores in all coronary arteries. The Agatston score was calculated using the following equation:

The different quantification algorithms, calcium volume, and mass were calculated using the following equations:

and

Signal-to-Noise Ratio (SNR)
The mean and SD of the CT value inside the balloon that represented noncontrast blood were measured, and the SNR was calculated by the mean value divided by the SD. These values for the electron beam CT and 16-MDCT protocols were compared. Nine scans were used for the calculation (one balloon phantom, three heart rate sequences, and three sequential scans).

Detection of Small Amounts of Calcium
The detectability of calcium models sized 1 mm on electron beam CT and 16-MDCT protocols was compared. Twenty-seven scans were used for the analysis (three CAC materials, three heart rate sequences, and three sequential scans).

Agatston Score and Variability
The Agatston score of the first scan and the variability of the Agatston score in three sequential scans were compared between CT scanners and protocols.

The percentage of variability was determined by calculating the mean numeric difference between each of the three score values and dividing this by the mean score as follows:

where abs is absolute value, S1 is CAC score on the first scan, and S2 and S3 are the CAC scores on the second and third scans, respectively. Eighty-one scans were used for the calculation (nine CAC materials, three heart rate sequences, three sequential scans), and 27 sets of variability data were obtained.

Heart Rate Sequences
The mean variability of the Agatston score on each of the three sequential scans was compared among heart rate sequences (low, high, and arrhythmia) on each CT protocol.

CAC Measurement Algorithm
Whether calcium volume or mass measurements reduce the variability of CAC measurement more than the Agatston method was assessed.

Statistical Analysis
For statistical analysis, t tests were used to determine differences in variability among CT protocols and scoring algorithms. A p value of less than 0.05 was considered as identifying significant differences.

Results

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SNR
The mean and SD of the CT value and the SNR are shown in Table 1. The SNR depended on slice thickness. The SNR on 0.625-mm-thickness CT images was comparable to that on electron beam CT.

Detection of Small Amounts of Calcium
Calcium models of 1 mm were detected on 0.625- and 1.25-mm thickness CT, irrespective of helical or axial scanning. In contrast, detectability of 1-mm-calcium was 59% (16/27), 96% (26/27), 93% (25/27), and 89% (24/27) on electron beam CT, 2.5 mm/1.25 mm helical CT, 2.5 mm/2.5 mm helical CT, and 2.5 mm axial CT, respectively. Both positive and negative scans were observed in all 1-mm-calcium scans on electron beam CT and helical CT. The concurrence of detection and nondetection of the CAC in three serial scans results in a considerable increase in the variability calculation as defined earlier. For example, S1 = 0, S2 = 0, and S3 = 3 yields a variability of 6. The variability of the CAC is therefore obtained by calculating the results for detection of calcium sized 3, 5, and 10 mm.

Agatston Score and Variability
The absolute values of the Agatston score on electron beam CT and 16-MDCT protocols are shown in Table 2. The scores were comparable among the CT protocols. The Agatston scores on 0.625-mm axial CT, however, showed higher values than the other CT protocols. The mean, mean - SD, mean + SD, and median variability of electron beam CT and 16-MDCT are shown in Figure 3. Electron beam CT showed higher variability than helical CT, and statistical differences were observed versus 0.625 mm (p = 0.03) and versus 1.25 mm (p = 0.04). The use of thin-slice imaging (0.625 and 1.25 mm instead of 2.5 mm) reduced the variability on both helical and axial scanning. On helical CT alone, 0.625 mm showed lower variability than 1.25 mm (p = 0.04). Overlapping reconstruction also reduced the variability (p = 0.04: 2.5 mm/1.25 mm helical vs 2.5 mm/2.5 mm helical). Helical CT showed lower variability than axial CT, and statistical significances were observed on 0.625-mm (p < 0.01) and 1.25-mm (p < 0.01) scans.

View larger version (94K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3 —Graph shows medians (z) and means (bars) of variability of Agatston score. Vertical lines show range: mean + SD (top) and mean -SD (bottom). Helical CT shows lower variability than axial CT; 0.625-mm helical CT showed lowest variability. Use of thin-slice images and overlapping reconstruction improves reproducibility. EBCT = electron beam CT.

Heart Rate Sequences
The mean variability of the Agatston score on the three heart rate sequences is shown in Figure 4. No statistical difference was observed between low and high heart rates on either electron beam CT or 16-MDCT.

View larger version (104K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4 —Graph shows mean variability of Agatston score in three heart rate sequences: 60 beats per minute (bpm) (white bars), 85 bpm (gray bars), and arrhythmia (black bars). No difference is seen between heart rates of 60 and 85 bpm on either electron beam CT or 16-MDCT. EBCT = electron beam CT.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5 —CAC measurement algorithm. Graph shows mean variability by Agatston score (white bars), volume score (gray bars), and mass score (black bars). Reduction of variability by volume scoring algorithm was effective only on electron beam CT (p = 0.05). Mass scoring algorithm was effective on electron beam CT (p < 0.01), 2.5-mm helical CT (p < 0.01), and 2.5-mm axial CT (p = 0.02). Variability in volume and mass measurements for EBCT was almost the same as for helical CT protocols.

Top Abstract Introduction Materials and Methods Results Discussion APPENDIX 1: Summary of... References

Factors influencing interscan variability are partial volume effect [15], the use of the step function in the Agatston method to quantitate calcium [10], coronary artery motion [16], image noise [17], field inhomogeneity [18], lack of calibration [19], total amount of CAC [12], and so on. To reduce partial volume effect, researchers have suggested the use of thin-slice images [7, 12, 20] and overlapping image reconstruction [21-24].

This study also supports the theory that both thin-slice images and overlapping image reconstruction contribute to reducing variability. Prospective ECG-triggering axial scanning, however, even if obtaining thin-slice images, yields higher variability than retrospective ECG-gated helical scanning. For the reduction or minimization of motion artifacts of the heart, high temporal resolution levels of 19 [25] and 41.8 [26] msec are required. The temporal resolution of prospective ECG-triggering (250 msec) is not considered enough to suppress coronary artery motion artifacts.

Low or high heart rate did not have a considerable effect on variability. This tendency has also been confirmed in clinical studies using electron beam CT [12]. This suggests that although retrospective helical CT is inferior to electron beam CT in temporal resolution, it can be used to measure CAC in a wide range of heart rates.

The volumetric approach proposed by Callister et al. [9] has been shown to improve the reproducibility of CAC measurement on both electron beam CT [9, 10, 12] and helical CT [22, 23]. Interestingly, on helical CT, which differed from the results on electron beam CT, there was not a significant difference of variability among CAC algorithms, although the variability using volume or mass score tended to be lower than that using the Agatston score. This difference is considered to be due to the fact that partial volume effect is reduced on these helical CT protocols themselves; thus, the volumetric quantification algorithm does not give much additional effect. The volumetric approach is accepted as having high reproducibility in CAC measurement. However, patient management based on volume score is difficult because of the paucity of representative data on CAC distribution [27]. Thin-slice or overlapping helical CT, showing high reproducibility using Agatston, volume, or mass scoring, is considered advantageous regardless of which scoring algorithm becomes mainstream in the future.

One limitation of our study is that we did not assess tube current. For an imaging technique to become a tool for the assessment of CAC, reducing radiation exposure is an important concern, especially for a retrospective ECG-gated helical CT protocol. High-density noise, which is difficult to distinguish from calcification [17], is shown to have a considerable effect on variability [12]. Taking these factors into consideration, we believe that 1.25- or 2.5-mm overlapping helical CT may be an option, although the best result in variability was obtained from 0.625-mm helical CT. We, however, consider the phantom study not suited for answering the question of how much tube current can be reduced.

The other limitation of our study is the calcium model setting. We used smooth calcium models with high and homogeneous CT values. To better reflect the actual calcium plaques, however, we should have used a model that was irregular and included inhomogeneous and low-density material.

In conclusion, 16-MDCT with retrospective reconstruction yields low variability in CAC measurements and has the potential to be a useful tool in monitoring the progression of coronary atherosclerosis.

APPENDIX 1: Summary of Phantom Scanning and Reconstruction Protocol on Electron Beam CT and 16-MDCT

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• No. of phantoms: 1
  
• No. of heart rate curves per scan (in beats per minute [bpm]): 60, 85, and arrhythmia
  
• No. of scans per protocol: three (home position = -1, 0, and 1 mm)
  
• No. of protocols: 8
  
  • 1 on EBCT: 3/3 mm
    
  • 4 on retrospective MDCT (slice thickness/increment): 0.625 mm/0.625 mm, 1.25 mm/1.25 mm, 2.5 mm/1.25 mm, 2.5 mm/2.5 mm
    
  • 3 on prospective MDCT (slice thickness): 0.625, 1.25, and 2.5 mm
    
  
  

References

Top Abstract Introduction Materials and Methods Results Discussion APPENDIX 1: Summary of... References

 

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