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Coronary Artery Calcium

Posted Aug 31, 2015 Updated Apr 10, 2023
By Zhen Qian PhD , Abinashi Dhungel PhD , Gustavo Vazquez MD , Michael Weeks PhD , Szilard Voros MD , Sarah Rinehart MD
12 min read

Rationale and Objectives

Coronary artery calcium (CAC) images can be reconstructed with thinner slice thickness on some modern multidetector-row computed tomography scanners without additional radiation. We hypothesized that the isotropic 0.5-mm CAC reconstruction outperforms the conventional 3.0-mm reconstruction in detecting and quantifying coronary calcium, and we proposed to compare them by validating against spatially registered intravascular ultrasound with radiofrequency backscatter-virtual histology (IVUS-VH).

Materials and Methods

Twenty-seven patients were enrolled, and 5976 mm of coronary arteries were analyzed. A semiautomatic software was developed to coregister CAC and IVUS-VH on a detailed slice-by-slice basis. Calcium detection and calcium volume quantification were evaluated and compared using varying calcium attenuation thresholds. Algorithms for deriving individualized optimal threshold and comparable Agatston score on the 0.5-mm reconstruction were developed.

Results

The isotropic 0.5-mm reconstruction achieved significantly higher area under receiver-operating curve than the conventional 3.0-mm reconstruction (0.9 vs. 0.74, P < .001). Using the optimal threshold, the 0.5-mm reconstruction had higher sensitivity (0.79 vs. 0.65), specificity (0.85 vs. 0.77), positive predictive value (0.42 vs. 0.29), and negative predictive value (0.97 vs. 0.94) than the 3.0 mm. Individualized optimal threshold was significantly correlated with the image noise (r = 0.66, P < .001) in the 0.5-mm reconstruction.

Conclusions

By optimizing the calcium threshold, the 0.5-mm reconstruction is superior to the conventional 3.0-mm in detecting and quantifying calcium, which may improve the clinical value of CAC without additional radiation.

Coronary artery calcium (CAC) scoring is a noninvasive, low-radiation computed tomography (CT)-based imaging technique that quantifies the calcium in the coronary vasculature . It provides a well-validated risk stratification scheme for future cardiovascular events . CAC studies were originally developed and validated on the electron-beam CT scanner (EBCT) , and were quantified by calculating the Agatston score and/or the volume score , in which a minimum attenuation value of 130 Hounsfield unit (HU) is typically used as the threshold to detect coronary calcium. Recent studies have shown that the fixed 130-HU threshold may not be optimal for detecting calcium , and a scanner-specific and individualized attenuation threshold is more desirable .

With the increasing popularity and accessibility of multidetector-row CT (MDCT), CAC studies are now typically performed on MDCT scanners. To make the CAC quantification on the MDCT comparable to the original EBCT, current MDCT-based CACs are reconstructed with a 3.0-mm slice thickness, which is the typical collimation thickness on the EBCT scanners. However, CAC volumes acquired on most MDCT scanners can be reconstructed with thinner slice thickness because of MDCT’s smaller detector size. For example, CAC images can be acquired volumetrically using a prospective single-rotation protocol on a 320-detector-row CT scanner. Because of the 0.5-mm collimation thickness, CAC images can be reconstructed with the 0.5-mm slice thickness to achieve an isotropic resolution without increasing the radiation dose. Compared to 3.0 mm, thinner slice thickness potentially reduces partial volume artifact and may improve the sensitivity/specificity and accuracy in quantifying calcium. Some studies reported that the thinner-slice reconstruction resulted in significantly higher Agatston and volume scores and was able to identify more subclinical calcification lesions. On the other hand, some researchers found that the image noise level was significantly higher in the thinner-slice reconstructions, and the conventional threshold of 130 HU leads to more false positives. Mühlenbruch et al. empirically used the threshold of 350 HU in the 1.0-mm reconstruction. However, little is known about the optimal calcium threshold in thinner-slice reconstructions.

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Materials and methods

General Study Design

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CAC Image Acquisition and Analysis

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Table 1

CT Imaging Parameters of the CAC Scan

Image Acquisition Parameters Values Detector width, mm 0.5 No. of detectors 320 Gantry rotation time, milliseconds 350 Scan mode Volumetric EKG synchronization Prospectively triggered Tube voltage, kVp 120 Tube current, mA 100–550 Reconstruction slice thickness, mm 3.0 and 0.5 Reconstruction kernel FC12

CAC, coronary artery calcium; CT, computed tomography; EKG, electrocardiogram.

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IVUS Image Acquisition and Analysis

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Figure 1, A visual comparison of a vessel segment coregistered in intravascular ultrasound with radiofrequency backscatter (IVUS) and coronary artery calcium (CAC). (a,b) the longitudinal views of the grayscale IVUS and IVUS-virtual histology (VH), respectively; (c, d) the maximum intensity projection views of the vessel in the noncontrast-enhanced CAC with 0.5-mm and 3.0-mm reconstructions, respectively. The red arrow pointed to the most stenotic location. (e,f,g,h) The cross section at the blue arrow of the grayscale IVUS, IVUS-VH, 0.5-mm CAC, and 3.0-mm CAC, respectively. (i,j,k,l) The cross section at the yellow arrow of the grayscale IVUS, IVUS-VH, 0.5-mm CAC, and 3.0-mm CAC, respectively.

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CAC and IVUS Registration

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Figure 2, (a) Two-dimensional images that were equally spaced and perpendicular to the centerline were extracted from the 3-dimensional coronary artery calcium, and stacked up to form the straight-vessel view in (b) , which was comparable to the intravascular ultrasound with radiofrequency backscatter-virtual histology in (c) . The green dash-dot lines indicated the manually selected landmarks.

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Figure 3, An example of calcium volume curves in (a) 0.5-mm–reconstructed coronary artery calcium (CAC), (b) 3.0-mm–reconstructed CAC, and (c) intravascular ultrasound with radiofrequency backscatter-virtual histology that had been aligned to each other. (Color version of figure is available online.)

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Statistical Analysis

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Results

Patient Demographics

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Table 2

The Demographic Information of the Enrolled Subjects

Patient Demography CAD Normal Patient number 14 13 Age, y 61.5 ± 7.9 48.6 ± 10.2 Sex 10 M, 4 F 8 M, 5 F

CAD, coronary artery disease; F, female; M, male.

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Using Threshold of 130 HU

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Figure 4, (a , b , c) Scatter diagrams and linear regression lines of the 0.5-mm–reconstructed coronary artery calcium (CAC) versus intravascular ultrasound with radiofrequency backscatter-virtual histology (IVUS-VH), the 3.0-mm–reconstructed CAC versus IVUS-VH, and the 0.5-mm versus 3.0-mm reconstructions, respectively, using the 130-HU threshold. (d , e , f) Bland–Altman plots of the 0.5-mm–reconstructed CAC versus IVUS-VH, the 3.0-mm–reconstructed CAC versus IVUS-VH, and the 0.5-mm versus 3.0-mm reconstructions, respectively, using the 130-HU threshold. (Color version of figure is available online.)

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Table 3

Summarized Calcium Detection and Quantification Performances of the 0.5-mm and 3.0-mm Reconstructions Using the 130-HU Threshold

Slice Thickness, mm Correlation Coefficient, r_P_ Value Sensitivity Specificity PPV NPV 0.5 0.83 <.0001 0.94 0.50 0.21 0.98 3.0 0.79 <.0001 0.56 0.93 0.54 0.94

NPV, negative predictive value; PPV, positive predictive value.

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Using Thresholds Between 50–400 HU

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Figure 5, (a) Receiver-operating characteristic (ROC) curves and (b) correlation coefficient values of the 0.5-mm and 3.0-mm–reconstructed coronary artery calcium (CAC) using the calcium attenuation thresholds in the range of 50–400 HU, compared to intravascular ultrasound with radiofrequency backscatter-virtual histology (IVUS-VH). (c,d) Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the 0.5-mm–reconstructed CAC and 3.0-mm–reconstructed CAC, respectively, using the calcium attenuation thresholds in the range of 50–400 HU. AUC, area under the curve.

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Table 4

Comparison Between 0.5-mm and 3.0-mm Reconstructions Using Optimal Thresholds

Reconstruction, mm Optimal Threshold Sens Spec PPV NPV Correlation Coefficient, r_P_ Value 0.5 171 HU 0.79 0.85 0.42 0.97 0.82 <.0001 3.0 107 HU 0.65 0.77 0.29 0.94 0.76 <.0001

NPV, negative predictive value; PPV, positive predictive value; Sens, sensitivity; Spec, specificity.

The sensitivity, specificity, PPV, NPV, correlation coefficient, and the corresponding P values were calculated at the respective optimal thresholds of the 0.5-mm and 3.0-mm CAC.

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Individualized Thresholds and the Relationship with Image Noise

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Tind=imagenoise×4.66+20.44HU T

ind

=

image

noise

×

4.66

+

20.44

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Figure 6, (a , b) Scatter diagram and regression line of the individual best threshold versus individual image noise in the 0.5-mm and 3.0-mm reconstructions, respectively. (c) A calcific lesion was detected in the 0.5-mm reconstruction using the threshold of 190 HU, which was calculated from Equation (1) based on image noise. However, in (d) , the lesion was missed by the 3.0 mm using the threshold of 130 HU.

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Deriving Comparable Agatston Score From 0.5-mm Reconstruction

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Weightedscore=⎛⎝⎜⎜⎜⎜1,Tind≤HUMAX<Tind×0.74+113HU2,Tind×0.74+113HU≤HUMAX<Tind×0.37+275HU3,Tind×0.37+275HU≤HUMAX<436HU4,HUMAX≥436HU Weighted

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Discussion

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Conclusion

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Acknowledgment

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Academic Radiology, Volume 22, Issue 9
Journals General Radiology
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