Synopsis

We sought to quantitatively investigate the performances on blood suppression efficiency, wall-lumen CNR and plaque burden measurements for the RECS-3D MERGE sequence in carotid vessel wall MR imaging.

Twelve patients with carotid artery stenosis were recruited in this study. Lumen SNR and wall-lumen CNR were calculated and plaque burden measurements (i.e., lumen area, wall area, mean wall thickness and maximum wall thickness) were measured.

The RECS-3D MERGE was quantitatively demonstrated to have the ability to provide the comparable blood suppression, black-blood image quality and the highly correlated morphometry with the current plaque imaging protocol. The RECS-3D MERGE could be a promising tool for plaque burden measurement in a short scan time.

Purpose

Among 3D black-blood techniques, 3D MERGE is a promising one since it is able to provide excellent blood-suppression performance¹ and cover large anatomical volume². However, it is limited by the lower SNR and CNR of vessel wall³. In a previous study, we proposed a technique termed RECS-3D MERGE to provide high image quality for 3D black-blood imaging⁴. However, the performances on blood suppression efficiency, wall-lumen CNR and plaque burden measure for RECS-3D MERGE has not been quantitatively investigated. In this study, we sought to quantitatively conducts comparisons for RECS-3D MERGE and 3D MERGE. 2D DIR-FSE was the imaging reference for the investigation.

Materials and Methods

The RECS-3D MERGE sequence consists of 3D MERGE sequence, a delay time (TD) for relaxation enhancement, and a pseudo-centric phase encoding order used for under-sampling acquisition (Fig. 1). Compressed sensing (CS) reconstruction was employed to reconstruct the under-sampled images⁵.

After institutional review board approval and written informed consent, 12 patients with carotid artery stenosis were recruited. MR images were acquired using a clinical 3T scanner (Signa TM; GE Medical Systems, Milwaukee, WI). The imaging parameters of 3D MERGE were: TR/TE 6.2/2.9 ms, flip angle 6°, FOV 180 × 180 mm², receiver bandwidth 244 Hz/pixel, matrix size 256 × 256, interpolated to 512 × 512, slice thickness 1.4 mm, number of slices 48, and signal averages 1. The scan time was 96 seconds.

The RECS-3D MERGE has the same imaging parameters as the 3D MERGE. And the TD and the acceleration factor in the RECS-3D MERGE were set to 800 ms and 3x. The scan time was 98 seconds.

The imaging parameters of 2D DIR-FSE were: TR/TE 1000/18 ms, FOV 180 × 180 mm², receiver bandwidth 488 Hz/pixel, matrix size 256 × 256, interpolated to 512 × 512, slice thickness 2 mm, number of slices 12, signal averages 2. The scan time was 444 seconds.

Native RECS-3D MERGE and 3D MERGE images were reformatted to the axial plane with a 2-mm slice thickness such that slices were registered to the 2D DIR-FSE images

The SNR in the vessel lumen and the CNR between the lumen and the vessel wall were used as the measure of flowing signal suppression and the image quality for black-blood imaging. Vessel lumen area (LA), vessel wall area (WA), mean wall thickness (MWT), and maximum wall (MaxWT) thickness were compared. The statistical significance was defined as P < 0.05.

Results

There were no statistically significant differences in lumen SNR and wall-lumen CNR between RECS-3D MERGE and 2D DIR-FSE, and no statistically significant difference was found in lumen SNR between 3D MERGE and 2D DIR-FSE. However, the CNR of 3D MERGE was significantly lower than that of 2D DIR-FSE. Representative axial images obtained with the three techniques in two volunteers are shown in Fig. 2. It is observed that the black-blood axial images at RECS-3D MERGE exhibit the comparable flow suppression of blood signal to the corresponding images at 2D DIR-FSE. And the depictions of outer vessel wall boundary, lumen-wall interface and plaque burden for both techniques were similar. However, the delineations of outer vessel wall boundary are negatively affected by artifacts in the images of 3D MERGE, resulting in the obscured delineations (see arrows in Fig. 2).

The intra-class correlation coefficients (ICCs) are 0.995, 0.990, 0.925 and 0.960 for LA, WA, MWT and MaxWT for RECS-3D MERGE and 2D DIR-FSE. The ICCs for 3D MERGE and 2D DIR-FSE are less than those between RECS-3D MERGE and 2D DIR-FSE. Scatter plots for LA, WA, MWT and MaxWT between RECS-3D MERGE and 2D DIR-FSE are shown in Fig. 3. The Bland-Altman plots for them are shown in Fig. 4. Bland-Altman plots for measurements between RECS-3D MERGE and 2D DIR-FSE demonstrate random error scattering patterns with no significant bias and no significant correlation between bias and mean.

Discussion

This study quantitatively demonstrated RECS-3D MERGE could considerably improve the image quality of MSDE prepared 3D black-blood MR imaging. This result is coincident with the findings in the previous study⁴, in which no significant differences in image quality of the area covering plaque were found between RECS-3D MERGE and 2D DIR-FSE.

The improved tissue signal intensity and image quality achieved by TD increase the probability of signal recovery because high SNR would be advantageous to obtain the solution m* more approaching to the original signal m when solving the L1-norm optimization problem⁶.

Acknowledgements

References

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