Introduction

High resolution MR vessel wall imaging (VWI) has demonstrated the potential to reveal the lesions of intracranial atherosclerotic disease (ICAD) and characterize their morphological and signal features which are intimately associated with recent clinical events 1,2. Conventional intracranial VWI techniques based on 3D turbo spin-echo (TSE), with a thin, oblique slab to specifically target a limited imaging volume, have been shown to be reliable in quantifying vessel wall morphology of ICAD3,4. However, the 3D targeted VWI approach only covers major intracranial arteries, which may miss the culprit lesions at some arterial branches. Recently, 3D whole-brain VWI was proposed and optimized offering large spatial coverage, improved cerebrospinal fluid (CSF) suppression, and enhanced T1 weighting5,6. This technique exhibits excellent reproducibility in quantification of intracranial vessel dimensions in healthy volunteers7. The aim of this study is to further evaluate the clinical reliability of 3D whole-brain VWI in patients with ICAD via a comparison with 3D targeted VWI and 2D TSE.

Materials and Methods

Fifteen patients (13 males, age 40-69 years) with recent (127.87±134.02 days) ischemic cerebrovascular events and clinically confirmed etiology as ICAD were recruited to undergo two repeated VWI sessions with an interval of at least one week on a 3T MR system (Siemens Trio). In each session, the following imaging protocol was performed: pre-contrast T1-weighted VWI (i.e. whole-brain VWI, targeted VWI, and 2D TSE), contrast-enhanced MRA, and post-contrast T1-weighted VWI (i.e. only one 3D VWI followed by 2D TSE). Relevant imaging parameters of the two 3D sequences were matched: isotropic 0.53mm spatial resolution without interpolation, GRAPPA factor = 2, scan time = 8min 10sec for whole-brain VWI and 8min 3sec for targeted VWI. For each patient, 3 contiguous short-axis slices of the most severe plaque reconstructed from 3D images were used for the single-slice 2D TSE scan (spatial resolution 0.53×0.53×2mm3). Lumen and vessel wall area, volume, plaque burden, percent stenosis, remodeling ratio, lesion-to-wall contrast ratio (CR), and enhancement ratio (ER) were measured at all plaques. Using VesselMass software, two readers (both with 8-year experience in vascular MR) independently performed above geometric and signal measurements on the images from the first imaging session. After two weeks, one reader performed a second-round measurement on the same data, and the other performed measurement on the images from the second imaging session. Following analyses were then performed: 1) the intra- and inter-observer agreement and scan-rescan reproducibility of each sequence; 2) the agreement in dimension quantification between each of the 3D sequences and 2D TSE; 3) the superiority in lesion contrast among the three sequences.

Results

Twenty-eight plaques were detected in the 15 patients. All 3D scans provided good delineation of plaques (arrows in Fig.1 and 2). However, some plaques were missed by targeted VWI due to the limited coverage (arrowheads in Fig. 1). Outer boundary of some plaques was difficult to discern on targeted VWI because of insufficient CSF suppression (Fig. 2). All morphologic measurements and intra-class correlation coefficients (ICC) with 95% confidence interval (CI) of each sequence summarized in Table 1 exhibited good to excellent reproducibility (all ICC≥0.72). The paired morphologic measurements of 15 plaques for 3D and 2D techniques and their agreement are summarized in Table 2. All ICCs were greater or equal to 0.79, indicating excellent agreement between either of the 3D VWI technique and 2D TSE. In addition, the lesion-to-wall CR of whole-brain VWI (1.97±0.31) was significantly higher than that of targeted VWI (1.46±0.19, p<0.001) and 2D TSE (1.67±0.18, p=0.02). The plaque ER of whole-brain VWI (1.94±0.61) was slightly higher than that of 2D TSE (1.86±0.33, p=0.374) but significantly higher than that of targeted VWI (1.58±0.46, p<0.001) (Fig. 3).

Discussion and Conclusion

In general, this study demonstrated excellent agreement between 3D and 2D techniques, and good to excellent reproducibility of inter-scan, intra-, and inter-observer. However, whole-brain VWI generally exhibited higher ICCs than targeted VWI and 2D TSE. This was more likely attributed to the improved CSF suppression and T1 weighting of whole-brain VWI. The remarkable attenuation of surrounding CSF signals improved the conspicuity of outer boundary of vessel wall, hence improved the accuracy of morphological measurements. Additionally, improved T1 contrast resulted in significantly higher lesion-to-wall CR and plaque ER in whole-brain VWI than in targeted VWI and 2D TSE. This might contribute to a better capacity to evaluate plaque features. With such high reproducibility for plaque geometric and burden measurement, the whole-brain VWI can potentially be used for quantifying longitudinal morphologic changes in ICAD, and hence monitoring therapy and disease progression. In conclusion, the proposed whole-brain VWI is a reproducible and more sensitive MR method for ICAD assessment.

Acknowledgements

This work was supported in part by National Natural Science Foundation of China (81830056) and National Key R&D Program of China (2016YFC0100100).

References

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