Optimizing carotid plaque visualization: the advantage of subtraction imaging from dual-layer spectral CT

Computed tomography angiography (CTA) has become the primary method for detecting carotid atherosclerosis. However, the visibility of the arterial wall on the CTA is poor because of the high attenuation lumen filled with contrast agent. To optimize the display of the carotid arterial walls, this study proposed a black blood technique by generating subtraction images (CTA_sub) based on dual-layer spectral CT, and explored its role in the assessment of carotid atherosclerotic plaques.

CTA_sub were generated by subtracting virtual monochrome images (VMI) from virtual non-contrast images (VNC) obtained from SDCT scans. Two senior radiologists and two junior radiologists read conventional CTA images (CTA_con) with and without CTA_sub respectively. The inter-reader agreement for plaque classification of each extracranial carotid artery and diagnostic confidence were compared. The extracranial carotid wall visibility, outer margin conspicuity of non-calcified components, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the non-calcified components as well as the stenosis rate at the narrowest level were compared between CTA_sub and CTA_con. Using high-resolution magnetic resonance imaging (hrMRI) as the standard, the accuracy in plaque burden estimation on CTA_con and CTA_sub were compared. The paired t-test and the Wilcoxon signed-rank test were used for the comparison between CTA_sub and CTA_con. The Cohen κ value and the intraclass correlation coefficient (ICC) were used to evaluate the consistency.

A total of 119 patients were included in the comparisons between CTA_sub and CTA_con and a total of 31 patients were enrolled in the plaque burden comparison between CTA and hrMRI. CTA_sub improved the consistency of plaque classification between radiologists and increased their diagnostic confidence. CTA_sub also provide better arterial wall visibility and clearer outer margin conspicuity of non-calcified components. The SNR and CNRs of non-calcified components were obviously elevated on CTA_sub (SNR: 12.86 ± 6.51 vs. 7.77 ± 4.30; CNR_{non-calcified/fat}: 33.13 ± 2.65 vs. 24.96 ± 7.90; CNR_{non-calcified/muscle}: 8.58 ± 4.27 vs. 6.64 ± 4.11, all p < 0.001). Moreover, CTA_sub displayed good to excellent consistency with CTA_con in stenosis rate measurement (NASCET: ICC = 0.919; ECST: ICC = 0.889, both p < 0.001) and a more closer estimation of plaque burden to hrMRI than CTA_con(ICC = 0.938 vs. 0.842, p < 0.001).

As a complement to CTA_con, CTA_sub not only improves visualization of the extracranial carotid wall and non-calcified plaque components but also enables accurate quantification of stenosis rate and plaque burden. Howerer, CTA_sub can not be used alone when calcified components are evaluated.