Rationale and Objectives
To investigate the stabilities of plaque attenuation and coronary lumen for different plaque types, stenotic degrees, lumen densities, and reconstruction methods using coronary vessel phantoms and the visualization of coronary plaques in clinical patients through coronary computed tomography (CT) angiography.
Materials and Methods
We performed 320-detector volume scanning of vessel tubes with stenosis and a tube without stenosis using three types of plaque CT numbers. The stenotic degrees were 50% and 75%. Images were reconstructed with filtered back projection (FBP) and two types of iterative reconstructions (AIDR3D and FIRST [forward-projected model-based iterative reconstruction solution]), with stenotic CT number of approximately 40, 80, and 150 HU (Hounsfield unit), respectively. In each case, the tubing of the coronary vessel was filled with diluted contrast material and distilled water to reach the target lumen CT numbers of approximately 350 HU and 450 HU, and 0 HU, respectively. Peak lumen and plaque CT numbers were measured to calculate the lumen–plaque contrast. In addition, we retrospectively evaluated the image quality with regard to coronary arterial lumen and the plaque in 10 clinical patients on a 4-point scale.
Results
At 50% stenosis, the plaque CT number with contrast enhancement increased for FBP and AIDR3D, and the difference in the plaque CT number with and without contrast enhancement was 15–44 HU for FBP and 10–31 HU for AIDR3D. However, the plaque CT number for FIRST had a smaller variation and the difference with and without contrast enhancement was −12 to 8 HU. The visual evaluation score for the vessel lumen was 2.8 ± 0.6, 3.5 ± 0.5, and 3.7 ± 0.5 for FBP, AIDR3D, and FIRST, respectively.
Conclusions
The FIRST method controls the increase in plaque density and the lumen–plaque contrast. Consequently, it improves the visualization of coronary plaques in coronary CT angiography.
Introduction
Coronary computed tomography angiography (CTA) with electrocardiogram (ECG) gating is an accurate noninvasive method to evaluate coronary artery disease . Potential applications of coronary CTA require high visualization of coronary arteries while maintaining radiation dose . In addition, coronary CTA raises concerns regarding evaluations of coronary stenosis and coronary plaque. Previous studies have associated high-risk plaque characteristics (e.g., positive remodeling, low CT number plaque, napkin ring, sign, and spotty calcium), as characterized by coronary CTA, with culprit lesions of the acute coronary syndrome . Therefore, diagnostic accuracy relies on knowledge of the plaque burden and high-risk plaque features. Regarding coronary plaque, CT number of coronary plaque varies with the increasing contrast enhancement of coronary lumen owing to partial volume effects, beam hardening, and plaque vascularity . Noncontrast CTA and dual-phase coronary CTA from noncontrast (first phase) and contrast enhancement (second phase) were previously applied to achieve accurate CT number of the coronary plaque .
Recently, an algorithm called “forward-projected model-based iterative reconstruction solution” (FIRST) was developed as an iterative method for image reconstructions . Unlike AIDR3D , FIRST is an iterative reconstruction algorithm that models system optics, such as the detector element aperture, and improves image quality by iteratively minimizing a penalty-based cost function. FIRST can potentially improve the spatial resolution and CT number because it employs a more accurate model of X-ray physics (considering partial volume effects, beam hardening, etc.) than the former iterative method does, as well as an improved filtered back projection (FBP) method.
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Materials and Methods
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Phantom Study
Phantom
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ECG-gated Single-heartbeat CTA
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Peak CT Number and Plaque CT Number
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Clinical Study
Patients
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Data Acquisition
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Quantitative Evaluation
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Qualitative Evaluation
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Statistical Analysis
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Results
Phantom Study
Lumen CT Number
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TABLE 1
Lumen CT Number with and without Stenosis for FBP, AIDR3D, and FIRST
Lumen CT Number without Stenosis (HU) Stenosis Plaque Type S-plaque (HU) I-plaque (HU) C-plaque (HU) FBP 370.5 331.8 325 338.8 451.7 386.1 401.3 383.7 AIDR3D 370.4 50% 328.2 362.5 335.2 453.6 377.2 384.5 377.2 FIRST 387.9 371.9 378.4 386.6 464.7 444.7 441 449.6 FBP 370.5 214.5 229.8 230 451.7 255.4 276.3 282.3 AIDR3D 370.4 75% 210 218.9 219.2 453.6 253.2 273.2 280.9 FIRST 387.9 266.6 270.6 273.4 464.7 314 322.1 343.3
CT, computed tomography; FBP, filtered back projection; FIRST, forward-projected model-based iterative reconstruction solution; HU, Hounsfield unit. C-plaque, calcified plaque; I-plaque, intermediate plaque; S-plaque, soft plaque.
![Figure 2, Difference in peak CT numbers with and without stenosis for FBP, AIDR3D, and FIRST at different plaque types and lumen densities, calculated from peak lumen CT number without stenosis–peak lumen CT number with stenosis [HU]. ( a ) and ( b ) represent the cases with 50% and 75% stenoses, respectively. CT, computed tomography; FBP, filtered back projection; FIRST, forward-projected model-based iterative reconstruction solution; HU, Hounsfield unit. C-plaque, calcified plaque; I-plaque, intermediate plaque; S-plaque, soft plaque.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/ImprovedEstimationofCoronaryPlaqueandLuminalAttenuationUsingaVendorspecificModelbasedIterativeReconstructionAlgorithminContrastenhancedCTCoronaryAngiography/1_1s20S1076633217301150.jpg)
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Plaque CT Number
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TABLE 2
Plaque CT Number with and without Contrast Enhancement at 50% and 75% Stenoses for FBP, AIDR3D, and FIRST
Stenosis Plaque Type Plaque CT Number without Contrast Enhancement (HU) Plaque CT Number with Contrast Enhancement (HU) Low Lumen Density High Lumen Density 50% FBP S-plaque 48.2 80.2 92.4 I-plaque 87.4 104.5 106.8 C-plaque 152.2 173 167.6 AIDR3D S-plaque 49.5 80 66 I-plaque 89.4 103.7 99.6 C-plaque 151.2 171.6 171.6 FIRST S-plaque 50.1 50.2 40.2 I-plaque 87.8 86.5 75.9 C-plaque 148.5 153 156.1 75% FBP S-plaque 50.1 47.7 57.7 I-plaque 90.2 92.3 103 C-plaque 158.2 169.6 167.6 AIDR3D S-plaque 50.6 45.4 61.3 I-plaque 91.6 96.3 102.1 C-plaque 149.7 167.3 148.9 FIRST S-plaque 49.8 34.5 41.9 I-plaque 86.8 82.2 84.3 C-plaque 150.3 148.5 154.2
CT, computed tomography; FBP, filtered back projection; FIRST, forward-projected model-based iterative reconstruction solution; HU, Hounsfield unit. C-plaque, calcified plaque; I-plaque, intermediate plaque; S-plaque, soft plaque.
![Figure 3, Difference in plaque CT numbers with and without contrast enhancement for FBP, AIDR3D, and FIRST at different plaque types and lumen densities, calculated from plaque CT number without stenosis–plaque CT number with stenosis [HU]. ( a ) and ( b ) represent the cases with 50% and 75% stenoses, respectively. CT, computed tomography; FBP, filtered back projection; FIRST, forward-projected model-based iterative reconstruction solution; HU, Hounsfield unit. C-plaque, calcified plaque; I-plaque, intermediate plaque; S-plaque, soft plaque.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/ImprovedEstimationofCoronaryPlaqueandLuminalAttenuationUsingaVendorspecificModelbasedIterativeReconstructionAlgorithminContrastenhancedCTCoronaryAngiography/2_1s20S1076633217301150.jpg)
TABLE 3
Ratio of Lumen-plaque Contrast in Each Method to That of FBP
Stenosis Plaque Type Low Lumen Density High Lumen Density FBP AIDR3D FIRST FBP AIDR3D FIRST 50% S-plaque 1.0 1.0 1.3 1.0 1.1 1.4 I-plaque 1.0 1.2 1.3 1.0 1.0 1.2 C-plaque 1.0 1.0 1.4 1.0 1.0 1.4 75% S-plaque 1.0 1.0 1.4 1.0 1.0 1.4 I-plaque 1.0 0.9 1.4 1.0 1.0 1.4 C-plaque 1.0 0.9 1.6 1.0 1.2 1.6
FBP, filtered back projection; FIRST, forward-projected model-based iterative reconstruction solution. C-plaque, calcified plaque; I-plaque, intermediate plaque; S-plaque, soft plaque.
Lumen-plaque contrast is calculated from subtraction of plaque CT (computed tomography) number from peak lumen CT number.
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Clinical Study
Quantitative Evaluation
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Qualitative Evaluation
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Discussion
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Acknowledgments
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