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DOI:10.2214/AJR.07.2296
AJR 2007; 189:981-988
© American Roentgen Ray Society

Original Research

Soft and Intermediate Plaques in Coronary Arteries: How Accurately Can We Measure CT Attenuation Using 64-MDCT?

Jun Horiguchi1, Chikako Fujioka1, Masao Kiguchi1, Yun Shen2, Christian E. Althoff3,4, Hideya Yamamoto5 and Katsuhide Ito3

1 Department of Clinical Radiology, Hiroshima University Hospital, 1-2-3, Kasumi-cho, Minami-ku, Hiroshima, 734-8551, Japan.
2 CT Lab of Great China, GE Healthcare, Mongkok Kowloon, Hong Kong.
3 Department of Radiology, Division of Medical Intelligence and Informatics, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
4 Present address: Institute of Radiology, Universitaetsmedizin-Charité -Berlin, Berlin, Germany.
5 Department of Molecular and Internal Medicine, Division of Clinical Medical Science, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.

Received November 15, 2006; accepted after revision May 18, 2007.

 
Y. Shen is an employee of GE Healthcare.

Address correspondence to J. Horiguchi.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The objective of this study was to validate the accuracy of 64-MDCT densitometry of soft and intermediate plaques.

MATERIALS AND METHODS. Acrylonitrile–butadiene–styrene resin (47 H) and acrylic (110 H) were used to simulate soft and intermediate plaques, respectively, in coronary artery models (diameters of 3 and 4 mm). The variable parameters were heart rate (50, 65, 80, and 95 beats per minute), reconstruction algorithm (half and segmentation), coronary artery enhancement (150, 250, 350, and 450 H), CT densitometry site (arterial lumen or center), shape of plaque (D-shaped, centric, and eccentric), and level of stenosis due to plaque (25%, 50%, and 75% of arterial diameter). Measured CT attenuation values of soft and intermediate plaques were compared for different combinations of parameters. Repeated measures analysis of variance, Wilcoxon's signed rank, Mann-Whitney U, and Kruskal-Wallis tests were used for statistical analyses.

RESULTS. For measuring soft plaque, CT densitometry was accurate at low heart rates with the use of a half reconstruction algorithm (p < 0.01) on intracoronary artery enhancement of 250 H (p < 0.01). For both soft and intermediate plaques, the densitometry measurements near the arterial lumen were overestimated and higher than those at the center (p < 0.01). For plaques that were 50% or more of the arterial diameter, accurate CT densitometry was possible.

CONCLUSION. Coronary artery enhancement has a significant impact on 64-MDCT densitometry measurements of coronary artery plaques, especially of soft plaques. A large plaque size, densitometry performed not near the arterial lumen but at the center of the plaque, intracoronary enhancement of 250 H, and a low heart rate increase the accuracy of plaque densitometry.

Keywords: cardiac CT • coronary artery • plaque


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Most patients with acute coronary syndromes present with unstable angina, acute myocardial infarction, and sudden coronary death. The most common cause of coronary thrombosis is plaque rupture followed by plaque erosion, whereas calcified nodules are infrequently associated with erosion [1]. The term "vulnerable plaque" was introduced to define plaques susceptible to such ischemic complications [2]. The detection and characterization of vulnerable plaques remain difficult tasks even using invasive techniques such as intravascular sonography [3], optical coherence tomography [4], plaque thermography [5], and angioscopy [6].

Kopp et al. [7] opened the way to plaque characterization using MDCT by calculating CT attenuation. Schroeder et al. [8], in their analysis of the composition of 34 plaques on 4-MDCT and intracoronary sonography, found that soft plaques had a mean attenuation (± SD) of 14 ± 26 H (range, –42 to 47 H) and intermediate plaques, of 91 ± 21 H (range, 61–112 H). Leber et al. [9], in their analysis of 58 plaques using 16-MDCT, showed that soft plaques had a mean attenuation of 49 ± 22 H (range, 14–82 H) and intermediate plaques, of 91 ± 22 H (range, 34–125 H), although the values for soft and intermediate plaques overlapped. In the most recent study to date of 16-MDCT involving 252 plaques, Pohle et al. [10] found that the attenuation values of soft (58 ± 43 H) and intermediate (121 ± 34 H) plaques were statistically different; however, there was substantial overlap. They therefore concluded that the differentiation of "vulnerable" from "stable" plaques based on their CT attenuation is doubtful.

On the other hand, the results of some ex vivo studies indicate that plaque differentiation may be possible [11–13]. After their analysis of the composition of 34 plaques on 4-MDCT and intracoronary sonography [8], Schroeder et al. [13] suggested the following MDCT attenuation criteria for differentiation of plaques: £ 60 H, predominantly lipid-rich plaques; 61–119 H, intermediate plaques; and ≥ 120 H, predominantly calcific plaques.


Figure 1
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  		Fig. 1 —Cardiac phantom (ALPHA 2, Fuyo Corporation). Photograph shows balloon and support components of cardiac phantom. Balloon attached with coronary artery models was surrounded by water.

 


Figure 2
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  		Fig. 2 —Graph shows eight heart rate sequences that were programmed in phantom: sequence 1, 50 beats per minute (bpm); sequence 2, 65 bpm; sequence 3, 80 bpm; sequence 4, 95 bpm; sequence 5, 50 bpm with shifting; sequence 6, 65 bpm with shifting; sequence 7, 80 bpm with shifting; and sequence 8, 95 bpm with shifting.

 
Using static cardiac phantoms for their study, Schroeder et al. [14] showed that plaque densitometry was accurate on thin-slice images (1 vs 2.5 mm), which are highly dependent on surrounding contrast enhancement. Cademartiri et al. [15], showing that CT attenuation of plaque significantly varied (22 ± 22, 50 ± 26, 107 ± 36, 152 ± 67 H) according to luminal enhancement (35 ± 10, 91 ± 7, 246 ± 18, 511 ± 89 H, respectively), concluded that it is difficult to identify absolute ranges of attenuation that relate to specific plaque characteristics.

We thought about the need for an experiment using 64-MDCT with a thin collimation and improved temporal and spatial resolution, as pointed out by Nikolaou et al. [16]. For such an experiment, the CT attenuations of plaque models should mimic those of real plaques measured on cadavers' arteries [11, 12], and the range of coronary enhancement should simulate that seen in clinical situations. In addition, most important but never reported, to our knowledge, a pulsating cardiac phantom with variable heart rates was validated with different temporal resolutions. The purpose of this study was to validate the accuracy of 64-MDCT densitometry of soft and intermediate plaques using coronary artery plaque models on a pulsating cardiac phantom.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Intravascular Enhancement of Clinical Studies: Coronary CT Angiography on 64-MDCT
This part of the study was to investigate patients' Hounsfield attenuation values for blood enhancement in clinical 64-MDCT coronary angiography and to adjust the attenuation values for the coronary artery fluid used for the phantom study. This study was approved by our institutional review committee. After seven patients with heavy calcifications or a stent or stents in the main coronary branches had been excluded, 30 consecutive patients (19 men and 11 women; mean age ± SD, 64 ± 11 years; age range, 42–85 years) undergoing cardiac MDCT investigation were enrolled. Both this clinical study and the following phantom experiments of retrospective ECG-gated 64-MDCT coronary angiography were performed using the same unit (LightSpeed VCT, GE Healthcare) with the same scanning parameters except tube current and pitch. No ß-blockers were administered in the series.

After a test injection of 15 mL of nonionic contrast medium (iopamidol [Iopamiron 370, Schering]) was administered, a timing bolus scan was obtained with contrast medium (amount of contrast medium = 0.7 x body weight) at an injection time of 10 seconds, followed by a 25-mL saline chaser administered at the same rate. The scan covered the entire volume of the heart during a single breath-hold, consisting of 5 or 6 heartbeats, with simultaneous recording of the ECG trace. Detector collimation was 64 x 0.625 mm, gantry rotation speed was 350 milliseconds per rotation, and tube voltage was 120 kV at a tube current of 550–750 mAs (depending on patient size, with a dose-reduction model using the ECG modulation technique).

CT pitch factors ranged from 0.18 to 0.24 by heart rate according to the manufacturer's recommendations for a coronary CT angiography protocol. For heart rates < 75 beats per minute (bpm), a half-scan algorithm was applied; for heart rates ≥ 75 bpm, a two-segment reconstruction algorithm, offering improved temporal resolution, was used. For image reconstruction, cardiac phase imaging with the center of the temporal window corresponding to 65% of the R-R interval was used. Other cardiac phase images were reconstructed in cases in which the image quality of the phase images was better than that at 65% of the R-R interval. CT attenuation values in the regions of interest (ROIs) were 316 ± 67 H (range, 167–415 H), 309 ± 65 H (range, 173–429 H), 297 ± 66 H (range, 173–420 H), and 310 ± 60 H (range, 168–418 H) on the proximal left main, left anterior descending, left circumflex, and right coronary arteries, respectively. Thereafter, for coronary artery enhancement models, four concentrations of contrast medium and water with CT attenuation values of 150, 250, 350, and 450 H were prepared.

Pulsating Cardiac Phantom
A prototype cardiac phantom was used (ALPHA 2, Fuyo Corporation). The phantom consists of five components: driver, control, support, rubber balloon, and ECG (Fig. 1). A controller with an ECG-synchronizer drives the balloon. The construction of the phantom is described in detail elsewhere [17, 18]. The main characteristic features of this phantom are programmable variable heart rate sequences and programmable heart movements that mimic natural heart movements. In this study, eight heart rate sequences were programmed (Fig. 2).

Coronary Artery Plaque Models
Acrylonitrile–butadiene–styrene resin (47 H) and acrylic (110 H) were used to represent soft and intermediate plaques, respectively. In addition, three plaque shapes (D-shaped, centric, and eccentric) and two coronary diameters (3 and 4 mm) were prepared. Using different combinations of plaque shape and artery diameter, the manufacturer of the phantom made seven plaque models. All plaque models had three levels of stenosis (Fig. 3). These seven plaque models were attached to the balloon phantom (mimicking the heart), with the long axis of the model corresponding to the z-axis, and were surrounded by water (Fig. 4).


Figure 3
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  		Fig. 3 —Coronary artery plaque models. This drawing shows plaque size, plaque shape, and stenosis levels of coronary artery plaque models. Seven plaques were prepared with different combinations of plaque shape, plaque CT attenuation, and coronary artery diameter, respectively: D-shaped, 40 H, 4 mm; D-shaped, 40 H, 3 mm; centric, 40 H, 4 mm; centric, 40 H, 3 mm; eccentric, 40 H, 4 mm; D-shaped, 100 H, 4 mm; and centric, 100 H, 4 mm.

 

Figure 4
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  		Fig. 4 —Balloon phantom. Seven plaque models were attached to balloon phantom (mimicking heart) and were surrounded by water. Balloon was filled with mixture of water and contrast medium (CT attenuation = 40 H).

 

Cardiac Phantom Scan
Volumetric data of the phantom were obtained. The tube current used was 650 mA. The pitch was set to 0.175 to allow improved temporal resolution by two- and four-segment reconstruction algorithms. Other scanning parameters were the same as those used for the clinical study, as described earlier. A half-scan algorithm provided a temporal resolution of 175 milliseconds; a two-segment, 97–137 milliseconds; and a four-segment, 52–95 milliseconds. The temporal resolution of a two- or four-segment reconstruction algorithm varied on sequences performed with heart rate shifting.

Theme 1: Heart Rate and Temporal Resolution for the Imaging of Coronary Artery Plaque
Degradation of image quality caused by motion artifacts decreased the detection of coronary artery plaque and the accuracy of CT densitometry. Image quality at a static state and image quality at eight heart rates were compared. A two-segment reconstruction algorithm was applied for heart rates of ≥ 65 bpm and a four-segment algorithm was used for heart rates of ≥ 80 bpm. The segmentation algorithm was not used at static state or at 50 bpm because of its lack of effectiveness in improving temporal resolution. For intracoronary artery enhancement, 250 and 350 H, which are considered representative levels judging from our clinical study results, were chosen. Details of the settings are summarized in Appendix 1. The plaque used for this theme was a D-shaped model (semicircular shape) with 50% stenosis.

Image quality was subjectively evaluated by three reviewers (experience interpreting cardiac CT: 7, 3, and 3 years) who were unaware of the kinds of plaque models and who graded image quality using a 3-point scale: 2, no or minor motion artifacts, recognized as semicircular shape; 1, mild motion artifacts, however recognized as almost semicircular shape; 0, significant motion artifacts, not recognized as semicircular shape. When the grades assigned for image quality differed among reviewers, the consensus of two reviewers was defined as the grade. Images were displayed with fixed window settings (window width, 700 H; window level, 200 H).

Theme 2: Does CT Attenuation of Coronary Plaque Change by Coronary Artery Enhancement?
CT attenuation values of soft and intermediate plaques were compared at four different intracoronary artery enhancement values (150, 250, 350, and 450 H). ROIs that were 1 mm2 were set at the center of the plaques (Fig. 5A). Static and low-heart-rate sequences were chosen to avoid or minimize the effect of motion artifacts. Details of the settings are summarized in Appendix 2. CT densitometry was performed in five slices per plaque in each circumstance by one reviewer (3 years of experience).


Figure 5
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  		Fig. 5A —Region-of-interest (ROI) setting in coronary artery plaque. Drawing shows typical ROI placement (dotted lines) in coronary artery plaques (gray). All ROIs were ovoid and 1 mm2. ROIs were set at center of plaques in theme 2 (A), near arterial lumen and at center of plaque in theme 3 (B), at center of plaques in three stenotic levels in theme 4 (C), and at center of plaques in various types of plaque shape and coronary artery diameters in theme 5 (D).

 
Theme 3: Does Placing the ROI near the Arterial Lumen Affect CT Attenuation?
ROIs were placed near the arterial lumen and at the center of the plaque (Fig. 5B), and the CT attenuation values of soft and intermediate plaques were compared between measurement sites. Two different CT attenuation values of intracoronary artery enhancement (250 and 350 H), which were representative in our clinical study, were used. Details of the settings are summarized in Appendix 3.


Figure 6
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  		Fig. 5B —Region-of-interest (ROI) setting in coronary artery plaque. Drawing shows typical ROI placement (dotted lines) in coronary artery plaques (gray). All ROIs were ovoid and 1 mm2. ROIs were set at center of plaques in theme 2 (A), near arterial lumen and at center of plaque in theme 3 (B), at center of plaques in three stenotic levels in theme 4 (C), and at center of plaques in various types of plaque shape and coronary artery diameters in theme 5 (D).

 
Theme 4: Is CT Attenuation of Coronary Plaque Affected by the Level of Arterial Stenosis?
CT attenuation values of soft and intermediate plaques were compared at three levels of stenosis: 25%, 50%, and 75% of the arterial diameter (Fig. 5C). Details of the settings are summarized in Appendix 4.


Figure 7
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  		Fig. 5C —Region-of-interest (ROI) setting in coronary artery plaque. Drawing shows typical ROI placement (dotted lines) in coronary artery plaques (gray). All ROIs were ovoid and 1 mm2. ROIs were set at center of plaques in theme 2 (A), near arterial lumen and at center of plaque in theme 3 (B), at center of plaques in three stenotic levels in theme 4 (C), and at center of plaques in various types of plaque shape and coronary artery diameters in theme 5 (D).

 

Theme 5: Is CT Attenuation of Coronary Plaque Affected by the Coronary Artery Diameter or the Stenosis Shape?
CT attenuation values of soft plaque were compared for the following five combinations of plaque shape and coronary artery diameter: D-shaped and 3 mm, D-shaped and 4 mm, centric and 3 mm, centric and 4 mm, and eccentric and 4 mm. The stenotic ratio was 50% in area. ROIs were set at the center of the plaques (Fig. 5D). Details of the settings are summarized in Appendix 5.


Figure 8
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  		Fig. 5D —Region-of-interest (ROI) setting in coronary artery plaque. Drawing shows typical ROI placement (dotted lines) in coronary artery plaques (gray). All ROIs were ovoid and 1 mm2. ROIs were set at center of plaques in theme 2 (A), near arterial lumen and at center of plaque in theme 3 (B), at center of plaques in three stenotic levels in theme 4 (C), and at center of plaques in various types of plaque shape and coronary artery diameters in theme 5 (D).

 
Statistical Analysis
All statistical analyses were performed using a commercially available software package (Statcel2, OMS Publishing). For statistical analyses, Mann-Whitney U, repeated measures analysis-of-variance, Wilcoxon's signed rank, and Kruskal-Wallis tests were used to determine differences. When statistical significance was observed by repeated measures analysis of variance, the results were made post hoc using the Scheffé test for multiple pairwise comparisons. A p value of < 0.05 was considered to identify significant differences.


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Theme 1
Subjective evaluation of image quality of plaques for different combinations of heart rate sequence and reconstruction algorithm is summarized in Table 1. Assessable image quality of soft plaque was obtained with all combinations of heart rates and reconstruction algorithms tested, whereas images of intermediate plaque were of poor quality on a coronary artery enhancement level of 250 H at high-heart-rate sequences. The image quality of intermediate plaques at 350 H was better than that at 250 H (Mann-Whitney U test, p < 0.01). When reconstruction algorithms were compared, the segmentation algorithm with improved temporal resolution tended to yield better image quality.


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  		TABLE 1: Subjective Evaluation of Image Quality with Different Combinations of Heart Rate Sequences and Reconstruction Algorithms

 

Theme 2
CT attenuation values of soft and intermediate plaques on four levels of intracoronary artery enhancement for four combinations of heart rate and reconstruction algorithm are shown in Figure 6A, 6B. CT attenuation values of soft plaque were overestimated on intracoronary artery enhancement levels of 350 and 450 H (p < 0.01). The CT densitometry measurements were accurate and lower for the static model and at 50 bpm (p < 0.01). Intermediate plaques were not detectable on intracoronary artery enhancement of 150 H.


Figure 9
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  		Fig. 6A —Relationship of CT attenuation values between plaque and intracoronary enhancement. Graphs show results for soft (A) and intermediate (B) plaques. Repeated measures analysis of variance revealed that CT attenuation values of soft plaque were different among intracoronary artery enhancement levels (p < 0.01) and combinations of heart rates and reconstruction algorithms (p < 0.01). Scheffé test for multiple pairwise comparisons revealed that CT attenuation values were significantly different between intracoronary enhancement of 150 and 350 H (p < 0.01), 150 and 450 H (p < 0.01), and 250 and 450 H (p < 0.01) on static cardiac phantom using half reconstruction and between intracoronary enhancement of 150 and 350 H (p < 0.01), 150 and 450 H (p < 0.01), 250 and 350 H (p < 0.01), and 250 and 450 H (p < 0.01) on cardiac phantom at 50 beats per minute (bpm) using half reconstruction algorithm. Scheffé test revealed that CT attenuation values of plaque on intracoronary artery enhancement of 150 H were significantly different between static cardiac phantom with half reconstruction algorithm and 65 bpm with half reconstruction (p < 0.01) and between 50 bpm with half reconstruction and 65 bpm with half reconstruction (p < 0.01). Scheffé test also revealed that CT attenuation values of plaque on intracoronary artery enhancement of 250 H were significantly different between static phantom with half reconstruction and 65 bpm with half reconstruction (p < 0.01) and between 50 bpm with half reconstruction and 65 bpm with half reconstruction (p < 0.01). In contrast, CT attenuation values of intermediate plaque were not statistically different based on intracoronary artery enhancement level (p = 0.09) or combinations of heart rate and reconstruction algorithm (p = 0.10).

 

Figure 10
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  		Fig. 6B —Relationship of CT attenuation values between plaque and intracoronary enhancement. Graphs show results for soft (A) and intermediate (B) plaques. Repeated measures analysis of variance revealed that CT attenuation values of soft plaque were different among intracoronary artery enhancement levels (p < 0.01) and combinations of heart rates and reconstruction algorithms (p < 0.01). Scheffé test for multiple pairwise comparisons revealed that CT attenuation values were significantly different between intracoronary enhancement of 150 and 350 H (p < 0.01), 150 and 450 H (p < 0.01), and 250 and 450 H (p < 0.01) on static cardiac phantom using half reconstruction and between intracoronary enhancement of 150 and 350 H (p < 0.01), 150 and 450 H (p < 0.01), 250 and 350 H (p < 0.01), and 250 and 450 H (p < 0.01) on cardiac phantom at 50 beats per minute (bpm) using half reconstruction algorithm. Scheffé test revealed that CT attenuation values of plaque on intracoronary artery enhancement of 150 H were significantly different between static cardiac phantom with half reconstruction algorithm and 65 bpm with half reconstruction (p < 0.01) and between 50 bpm with half reconstruction and 65 bpm with half reconstruction (p < 0.01). Scheffé test also revealed that CT attenuation values of plaque on intracoronary artery enhancement of 250 H were significantly different between static phantom with half reconstruction and 65 bpm with half reconstruction (p < 0.01) and between 50 bpm with half reconstruction and 65 bpm with half reconstruction (p < 0.01). In contrast, CT attenuation values of intermediate plaque were not statistically different based on intracoronary artery enhancement level (p = 0.09) or combinations of heart rate and reconstruction algorithm (p = 0.10).

 
Theme 3
Plaque CT attenuation values near the arterial lumen and at the center of plaque are shown in Figure 7. In all combinations of plaque (soft and intermediate) and intracoronary enhancement (250 and 350 H), plaque CT attenuation values near the lumen were overestimated and higher than those at the center (p < 0.01).


Figure 11
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  		Fig. 7 —Differences in CT attenuation values between region-of-interest placement inside plaque. CT attenuation values of 250 and 350 H were chosen as representative levels from our clinical study results. Wilcoxon's signed rank test revealed that CT attenuation values of plaque near lumen (dark gray) were overestimated and higher than those at center (light gray): soft plaque and intracoronary enhancement of 250 H, soft plaque and 350 H, intermediate plaque and 250 H, and intermediate plaque and 350 H (p < 0.01).

 
Theme 4
Plaque CT attenuation values in three stenotic levels are shown in Figure 8. For all combinations of plaque (soft and intermediate) and intracoronary artery enhancement (250 and 350 H), CT attenuation values with 25% stenosis were overestimated and higher than those with 50% or 75% stenosis (p < 0.01).


Figure 12
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  		Fig. 8 —Relationship between plaque CT attenuation values and stenosis level. CT attenuation values of plaque between three stenotic levels—that is, 25% (dark gray), 50% (light gray), and 75% (black) (in diameter). Kruskal-Wallis test revealed that CT attenuation values of plaque were different between stenosis levels: soft plaque and intracoronary enhancement of 250 H, soft plaque and 350 H, intermediate plaque and 250 H, and intermediate plaque and 350 H (p < 0.01).

 

Theme 5
Plaque CT attenuation values in different combinations of shape and coronary artery diameter are shown in Figure 9. Plaque CT attenuation values were different between the combinations (p < 0.01). D-shaped plaque in a coronary artery with a diameter of 4 mm showed lower CT attenuation values than the other combinations and was close to the real CT attenuation.


Figure 13
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  		Fig. 9 —Plaque CT attenuation values in combinations of plaque shape and coronary artery diameter. Kruskal-Wallis test revealed that CT attenuation values of plaque were statistically different between combinations of soft plaque and coronary artery enhancement of 250 H (gray) and soft plaque and coronary artery enhancement of 350 H (black) (p < 0.01).

 

Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Although the identification of calcified plaque is straightforward because of its higher CT attenuation, differentiation of noncalcified plaque from calcified plaque is challenging. A lipid core larger than 1 mm2 or a lipid-core-to-plaque ratio of greater than 20% and a fibrous cap thinner than 0.7 mm have been shown to correlate well with plaque rupture [19]. The usual location of a lipid core within an atherosclerotic plaque is just below the thin cap.

Factors and Circumstances Necessary for Accurate CT Densitometry of Coronary Plaque
There is a consensus that a low heart rate is an advantage for cardiac CT examination and that multisegment reconstruction, with its improved temporal resolution, is effective in high heart rates, especially when stable. In the current study, the two- or four-segment reconstruction was sometimes helpful for better delineation of the plaques. However, because of the finding that soft plaque showed higher CT attenuation values on half- and two-segment reconstructions at 65 bpm, plaque CT densitometry seems more accurate and can be performed better on a patient with a lower heart rate.

The results of the current study suggest that coronary artery enhancement has a significant impact on the CT attenuation of plaque, especially that near the coronary artery lumen—that is, just below the thin cap. This is considered due to partial volume averaging, which is theoretically dominant for low-attenuation plaque surrounded by the high-attenuation coronary artery lumen. The results suggest that it is difficult to predict the presence of soft plaque on the basis of a CT attenuation measured in an ROI set near the coronary lumen. High coronary artery enhancement (350 H) is advantageous for better delineation of both soft and intermediate plaques. However, as shown in this study, soft plaque (50% stenosis) was detected on all heart rate sequences on coronary artery enhancement of 250 H. This finding suggests that this level of enhancement might be better for characterization of soft plaque, as suggested by Schroeder et al. [13, 14].

From the results of themes 4 and 5, it seems that a minimal plaque size—for example, resulting in luminal stenosis of 50% or more— is needed to precisely measure the attenuation of plaque, especially of soft plaque. An appropriate size for an ROI remains unresolved. Because CT densitometry of D-shaped soft plaque causing 50% stenosis within a 3-mm coronary artery failed in yielding accurate measures, it follows that an ROI should be situated at the center of plaque to avoid the influence of coronary artery enhancement.

Technical matters for enhancement of the left ventricle remain to be mentioned. We filled a mixture of water and contrast medium (40 H) into a balloon (mimicking the heart) instead of filling a mixture with the same enhancement as the coronary arteries. In actual coronary CT angiography, the left ventricle is enhanced to the same level as the coronary arteries, whereas the enhancement of the right ventricle and atrium has already decreased. The wall of the left ventricle does not enhance much at the timing of coronary CT angiography. In addition to these technical matters, the epicardial coronary arteries are some distance from the heart. We therefore think that the CT attenuation of coronary plaque is not influenced much by the enhancement of the heart except in two instances: The plaque in the coronary orifice is influenced by the enhancement of the aorta, and the plaque in the myocardial bridge is influenced by the enhancement of the left ventricle.

Study Limitations
This study has several limitations. Atherosclerotic plaques have a complex composition—that is, lipid-rich, fibrous, and calcified areas coexist and are often intermixed. We prepared only one CT attenuation as a model for each of the soft (47 H) and the intermediate (110 H) plaques, although the previously reported CT attenuation values of the plaques had various ranges. We used plaques with a uniform CT attenuation because the purpose of this study was to investigate the effect of coronary artery enhancement on the CT attenuation of plaque. In addition, the shape of the coronary artery was always round; therefore, positive remodeling was not simulated. Next, we changed coronary artery enhancement levels on static and low-heart-rate sequences and did not investigate coronary artery enhancement on high-heart-rate sequences. This omission needs to be mentioned if the results are translated into an in vivo situation. Finally, we did not simulate large variations of heart rate, such as arrhythmia or premature heart beat, or changes in body posture.

In conclusion, coronary artery enhancement has a significant impact on 64-MDCT densitometry of coronary artery plaque, especially of soft plaque. Large plaque size, densitometry performed not near the lumen but at the center of the plaque, intracoronary artery enhancement of 250 H, and low heart rates increase the accuracy of plaque densitometry.

GoGoGoGoGo


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  		APPENDIX 1: Design for Theme 1: Heart Rate and Temporal Resolution for the Imaging of Coronary Artery Plaque

 

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  		APPENDIX 2: Design for Theme 2: Does CT Attenuation of Coronary Plaque Change by Coronary Artery Enhancement?

 

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  		APPENDIX 3: Design for Theme 3: Does Placing the Region of Interest (ROI) near the Arterial Lumen Affect CT Attenuation?

 

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  		APPENDIX 4: Design for Theme 4: Is CT Attenuation of Coronary Plaque Affected by the Level of Arterial Stenosis?

 

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  		APPENDIX 5: Design for Theme 5: Is CT Attenuation of Coronary Plaque Affected by the Coronary Artery Diameter or the Stenosis Shape?

 


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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