Computed Tomography Number Measurement Consistency Under Different Beam Hardening Conditions: Comparison Between Dual-Energy Spectral Computed Tomography and Conventional Computed Tomography Imaging in Phantom Experiment

Abstract

Purpose: To compare computed tomography (CT) number measurement consistency under different beam hardening conditions in phantom experiment between dual-energy spectral CT and conventional CT imaging.

Materials and methods: A phantom with 8 cells in periphery region and 1 cell in central region were used. The 8 conditioning tubes in the periphery region were filled with 1 of the 3 iodine solutions to simulate different beam hardening conditions: 0 for no beam hardening (NBH), 20 mg/mL for weak beam hardening (WBH) and 50 mg/mL for severe beam hardening (SBH) condition. Test tube filled with 0, 0.1, 0.5, 1, 2, 5, 10, 20, and 50 mg/mL iodine solution was placed in the central cell alternately. The phantom was scanned with conventional CT mode with 80, 100, 120, and 140 kVp and dual energy spectral CT mode. For spectral CT, 11 monochromatic image sets from 40 to 140 keV with interval of 10 keV were reconstructed. The CT number shift caused by beam hardening was evaluated by measuring the CT number difference (ΔCT) with and without beam hardening, with the following formulas: ΔCTWBH = |CTWBH - CTNBH| and ΔCTSBH = |CTSBH - CTNBH|. Data were compared with 1-way analysis of variance.

Results: Under both WBH and SBH conditions, the CT number shifts in all monochromatic image sets were less than those for polychromatic images (all P < 0.001). Under WBH condition, the maximum CT number shift was less than 6 Hounsfield units for monochromatic spectral CT images of all energy levels; under SBH condition, only monochromatic images at 70 keV and 80 keV had CT number shift less than 6 HU.

Conclusion: Dual energy spectral CT imaging provided more accurate CT number measurement than conventional CT under various beam hardening conditions. The optimal keV level for monochromatic spectral CT images with the most accurate CT number measurement depends on the severities of beam hardening condition.

FIGURE 1

Photograph of the standard phantom. Eight cells were arranged at intervals of 45° on the periphery region of the standard phantom and one at the center. In this test, 8 condition tubes on the periphery region were filled with 0, 20, and 50 mg/mL of iodine to simulate NBH, WBH, and SBH conditions, respectively, 1 test tube filled with different concentration iodine solutions was placed in central cell to measure CT number shift under WBH and SBH conditions.

FIGURE 2

Comparisons of 120 kVp polychromatic images and 70 keV monochromatic images of phantom under NBH (A and B), WBH, (C and S), and SBH (E and F) conditions. The central test tube was filled with water. On the 120-kVp polychromatic images, the CT numbers of the central test tube under NBH (A), WBH (C), and SBH (E) condition were 13.02 HU, −5.37 HU, and −78.07 HU, respectively. The ΔCT_WBH and ΔCT_SBH were 18.39 HU and 91.09 HU. On the 70 keV monochromatic images, the CT numbers of the central test tube under NBH (B), WBH (D) and SBH (F) condition were 1.54 HU, 3.31 HU, and −2.66 HU, respectively. The ΔCT_WBH and ΔCT_SBH were 1.77 HU and 4.20 HU, respectively.