Qualitative and quantitative reproducibility of 3D MERGE and SNAP sequences for carotid vessel wall imaging across Siemens and Philips 3T scanners

Abstract

Background: Three-dimentional (3D) vessel wall magnetic resonance imaging (MRI) sequences have emerged as new imaging tools for evaluating carotid atherosclerosis. However, their reproducibility across different vendors has not yet been investigated, which not only restricts their use in multicenter studies but also hinders their broader application in clinical practice. In this study, we aim to assess the qualitative and quantitative reproducibility on the same subjects using matched 3D carotid vessel wall MRI sequences on both Siemens and Philips scanners, specifically, 3D motion-sensitized driven equilibrium prepared rapid gradient echo (MERGE) and simultaneous non-contrast angiography and plaque (SNAP) imaging which are two representative 3D vessel wall MRI sequences with superior delineation of vessel wall morphology and carotid plaque.

Methods: As a cross-sectional study, six volunteers (1 female and 5 males, age 22-67 years) were scanned at 3T MRI machines of both vendors. Image quality was evaluated by two experienced reviewers using a 4-point scale, and quantitative measurements, including mean/maximum wall thickness and normalized wall/lumen index, were calculated from segmentation masks generated by the 3D localization, analysis, and thickness and tissue evaluation (LATTE) framework and a novel 3D thickness measurement using Laplacian method.

Results: There was no significant difference in image quality scores between Siemens and Philips platforms, except in the external carotid artery region. High consistency [intra-class correlation coefficient (ICC) >0.75] was obtained between the two platforms for quantitative metrics. Images on one carotid patient on Siemens show good visualization of vessel wall and plaque morphology and detection of intraplaque hemorrhage.

Conclusions: 3D MERGE and SNAP images have sufficient image quality and consistent quantitative measurements on Siemens and Philips scanners, despite lower image quality in Siemens platforms, probably due to suboptimal coil configuration or image processing. This suggests the feasibility of evaluating carotid atherosclerosis using matched 3D carotid vessel wall MRI protocols across different MRI vendors.

Keywords: Vessel wall imaging; carotid atherosclerosis; multi-vendor reproducibility.

Figure 1

Phantom experiment to compare Siemens and Philips image quality. Two ACR phantoms (one in the Siemens site and the other in the Philips site) were used. Same coils and protocols as the human experiments were used here. Both 3D MERGE and SNAP have visually reasonable image qualities on both Philips and Siemens scanners, although they exhibit different uniformity patterns. SNR is measured in homogeneous ROIs on both left and right side for each image, defined by dividing the ROI-averaged signal intensity by the standard deviation. Results show that SNR (averaging left and right) is similar in both sequences (Philips vs. Siemens): 21.4 vs. 21.8 in 3D MERGE; 22.75 vs. 20.6 in SNAP. 3D, 3 dimensional; ACR, American College of Radiology; MERGE, motion-sensitized driven equilibrium prepared rapid gradient echo; ROI, region of interest; SNAP, simultaneous non-contrast angiography and plaque; SNR, signal to noise ratio.

Figure 2

Qualitative comparison of the 3D MERGE images between two vendors acquired on the same volunteer (A,B,E,F) and the corresponding wall/lumen segmentation using 3D LATTE after manual correction (C,D,G,H). (A-D) One slice in the original acquisition view (i.e., coronal); (E-H) one slice in the reformatted axial view. 3D, 3 dimensional; ECA, external carotid artery; ICA, internal carotid artery; LATTE, localization, analysis, and thickness and tissue evaluation; MERGE, motion-sensitized driven equilibrium prepared rapid gradient echo.

Figure 3

Qualitative comparison of the SNAP images between two vendors acquired on the same volunteer in the original coronal view (A,B) and reformatted axial view (C,D). SNAP, simultaneous non-contrast angiography and plaque.

Figure 4

Comparison of image quality scores for two vendors given by two reviewers. (A) MERGE, N=12, 6 subjects × 2 sides; (B) SNAP, N=8, 4 subjects × 2 sides; averaged on two reviewers. 3D MERGE, 3 dimensional motion-sensitized driven equilibrium prepared rapid gradient echo; CCA, common carotid artery; ECA, external carotid artery; ICA, internal carotid artery; SNAP, simultaneous non-contrast angiography and plaque.

Figure 5

Comparison of carotid arterial morphological measurements from 3D MERGE images on the two vendors. They were calculated based on the segmentation results with LATTE and 3D thickness measurement. Four location-specific metrics, mean wall thickness (A), maximum wall thickness (B), normalized wall index (C), and normalized lumen index (D), were computed at each distance of a coordinate system from CCA to ICA centered on the bifurcation. Then each metric was averaged across the distance within each of 11 locations (7 mm for each location, ranging from −38.5 to 38.5 mm). Bland-Altman analysis and ICC were used to quantify the consistency of measurements between the two vendors. Results show that all metrics have good consistency (ICC >0.75), with 1.96 standard deviations ranging from 15% to 22%. 3D, 3 dimensional; CCA, common carotid artery; ICA, internal carotid artery; ICC, intraclass correlation coefficient; LATTE, localization, analysis, and thickness and tissue evaluation; MERGE, motion-sensitized driven equilibrium prepared rapid gradient echo; RPC, reproducibility coefficient; SSE, sum of squared error; SD, standard deviation.

Figure 6

The cross-vendor percentage difference {i.e., (Philips − Siemens)/[(Philips + Siemens)/2]*100%} of quantitative metrics along the coordinate from CCA to ICA, with center being the bifurcation slice. In each plot, the average difference of all 6 volunteers is displayed as the curve, and the standard deviation as the error bar; and left and right carotid arteries are displayed separately. It is observed that in general the average differences for all metrics are below 15% in all regions. Mean wall thickness and maximum wall thickness tend to have highest difference around the bifurcation, likely due to the discrepancy in 3D thickness measurement caused by vessel complexity. Normalized wall index and lumen index tend to have highest difference in upper region of ICA, likely due to the smaller size of ICA compared with CCA. 3D, 3 dimensional; CCA, common carotid artery; ICA, internal carotid artery.

Figure 7

3D carotid vessel MR images were acquired at the Siemens scanner on one carotid atherosclerotic patient (male, 70 years old), with both coronal view (A,B) and re-formatted axial view (C,D) displayed. The vessel wall and plaque structures are clearly visualized in the 3D MERGE images (C), and IPH on both left and right carotid arteries can be depicted by the hyperintensity signals in the SNAP images (D, red arrows). These are consistent with the previous studies using Philips scanners. 3D, 3 dimensional; MERGE, motion-sensitized driven equilibrium prepared rapid gradient echo; MR, magnetic resonance; SNAP, simultaneous non-contrast angiography and plaque.