Optimization of HU threshold for coronary artery calcium scans reconstructed at 0.5-mm slice thickness using iterative reconstruction

Abstract

Purpose: This work investigated the simultaneous influence of tube voltage, tube current, body size, and HU threshold on calcium scoring reconstructed at 0.5-mm slice thickness using iterative reconstruction (IR) through multivariate analysis. Regression results were used to optimize the HU threshold to calibrate the resulting Agatston scores to be consistent with those obtained from the conventional protocol.

Methods: A thorax phantom set simulating three different body sizes was used in this study. A total of 14 coronary artery calcium (CAC) protocols were studied, including 1 conventional protocol reconstructed at 3-mm slice thickness, 1 FBP protocol, and 12 statistical IR protocols (3 kVp values*4 SD values) reconstructed at 0.5-mm slice thickness. Three HU thresholds were applied for calcium identification, including 130, 150, and 170 HU. A multiple linear regression method was used to analyze the impact of kVp, SD, body size, and HU threshold on the Agatston scores of three calcification densities for IR-reconstructed CAC scans acquired with 0.5-mm slice thickness.

Results: Each regression relationship has R² larger than 0.80, indicating a good fit to the data. Based on the regression models, the HU thresholds as a function of SD estimated to ensure the quantification accuracy of calcium scores for 120-, 100-, and 80-kVp CAC scans reconstructed at 0.5-mm slice thickness using IR for three different body sizes were proposed. Our results indicate that the HU threshold should be adjusted according to the imaging condition, whereas a 130-HU threshold is appropriate for 120-kVp CAC scans acquired with SD = 55 for body size of 24.5 cm.

Conclusion: The optimized HU thresholds were proposed for CAC scans reconstructed at 0.5-mm slice thickness using IR. Our study results may provide a potential strategy to improve the reliability of calcium scoring by reducing partial volume effect while keeping radiation dose as low as reasonably achievable.

Keywords: Agatston score; coronary artery calcification; iterative reconstruction; protocol optimization.

Figure 1

Illustration of the anthropomorphic chest phantom (HA: calcium hydroxyapatite; HU: Hounsfield unit).

Figure 2

CT images acquired with four different acquisition/reconstruction combinations for QRM_small (top row), QRM_medium (mid row), and QRM_large (bottom row): (a) FBP, 0.5 mm, 120 kVp, SD = 55; (b) AIDR, 0.5 mm, 120 kVp, SD = 55; (c) AIDR, 0.5 mm, 120 kVp, SD = 75; and (d) AIDR, 0.5 mm, 120 kVp, SD = 115

Figure 3

Histograms of calcifications with HA density of 200, 400, and 800 mg/cm³ (left to right) in CT scans acquired with three different acquisition/reconstruction combinations for QRM_medium: (a) FBP, 0.5 mm, 120 kVp, SD = 55; (b) AIDR, 0.5 mm, 120 kVp, SD = 75; and (C) AIDR, 0.5 mm, 120 kVp, SD = 115.

Figure 4

Comparison of MTF values from 120‐kVp CAC scans acquired with four different acquisition/reconstruction combinations for QRM_small (left), QRM_medium (mid), and QRM_large (right). ROI was placed over the boundaries (a) at soft tissue/lung interface and (b) between the large homogeneous insert with HA density of 200 mg/cm³ and the surrounding soft tissue.

Figure 5

Box and whisker diagrams for Agatston score of calcification with HA density of 200, 400, and 800 mg/cm³ with respective to (a) tube voltage, (b) SD, (c) body size, and (d) HU threshold.

Figure 6

The HU thresholds as a function of SD estimated based on the regression models for (a) 120‐kVp, (b) 100‐kVp, and (c) 80‐kVp CAC scans reconstructed at 0.5‐mm slice thickness using AIDR 3D for three phantom sizes to reach the same Agatston score obtained from routine CAC scans.