Comparative study of the image quality of twin beam dual energy and single energy carotid CT angiography.

Purpose: The aim of this study was to compare the image quality of low virtual monochromatic image (VMI) from the twin-beam dual energy (TBDE) mode to that from the single energy (SE) mode using both standard and low tube voltage acquisition in carotid computed tomography angiography (CTA).

Methods: In this prospective study, 221 patients were imaged using TBDE mode with 120 kV or SE mode with 80/90/120 kV, patient numbers in all groups are equal(n = 55) except in the group using 80 kV SE mode(n = 56). VMIs ranging from 40 to 90 keV with a step of 1 keV were reconstructed from TBDE mode. In objective image quality assessment, regions of interest (ROIs) were placed in both carotid artery lumens and in both sternocleidomastoid muscles to calculate and compare the contrast-to-noise ratio (CNR) and the dose-normalized CNR (CNRD) among groups. In subjective assessment, a total of 13 arterial segments were assessed using a four-point Likert scale by two observers. Cohen's kappa test was used to quantify the level of agreement between the two observers.

Results: For ROI1 at the level of the carotid bifurcation, VMIs showed a higher CNR than the 120 kV SE group (p = 0.028)/ 90 kV SE group (p = 0.037) when their energy level were lower than 79 keV (56.92 ± 16.01)/56 keV (90.08 ± 22.14) respectively. The 90 kV SE (80.68 ± 24.47) showed the best CNR in all the SE groups. For CNRD, the 120/90/80 kV SE group was equivalent to 83/63/67 keV VMIs respectively. For ROI2 at the level of the origin of the common carotid artery, VMIs also showed a higher CNR than the 120 kV SE group (p = 0.015)/90 kV SE group (p = 0.034) when their energy level were lower than 83 keV (46.31 ± 14.47)/60 keV (72.23 ± 16.96) respectively. The 90 kV SE (64.98 ± 18.51) showed the best CNR in all the SE groups. For CNRD, the 120/90/80 kV SE group was equivalent to 88/72/75 keV VMIs respectively. The highest subjective rating score was 50 keV TBDE(3.70 ± 0.53). Cohen's Kappa values(0.79-0.85) suggest a substantial level of agreement between the two observers.

Conclusions: The novel TBDE technique with low keV VMI reconstruction provides better image quality of carotid CTA than the low tube voltage scan in the SE mode. VMIs with a keV level below 56 keV have a higher CNR than those from SE scans with 80/90/120 kV. Subjectively, the optimal keV energy level in TBDE for carotid CT angiography is 50 keV.