Accurate morphological measurements and classification of carotid atherosclerotic plaques require imaging with high spatial resolution, and may therefore benefit from increased signal-to-noise ratio (SNR) intrinsically available on ultra-high field (7T) systems^1-3. Several studies have already investigated carotid vessel wall imaging at 7T and compared it with state-of-the-art 3T protocols^1-3. These initial investigations have focused on 2D multi-slice imaging. Better than this approach, 3D vessel wall imaging allows characterizing extensive vascular territories while minimizing partial volume artifacts in plaque-prone regions, such as the carotid bulb and bifurcation^4-5. Here, we present our initial experience of 3D carotid vessel wall imaging on a whole body 7T clinical magnet using a custom made carotid coil.

MRI acquisition: 5 volunteers were imaged on a 7T whole body scanner (Siemens Magnetom) using a dedicated, custom designed, 8 channel carotid coil (Figure 1). The coil consists of two pads that lie on the left and right side of the neck. Each pad contains 2 transmit elements and 4 receive elements. After the acquisition of scout images, a 3D time-of-flight (TOF) non contrast-enhanced, bright blood sequence was acquired to identify the carotid arteries. Subsequently, black blood vessel wall imaging was performed using 3D weighted SPACE (Sampling Perfection with Application optimized Contrast using different flip angle Evolutions) with 5 different acquisition settings as detailed in Table 1. Other relevant imaging parameters common to all 3D SPACE acquisitions were: repetition time (TR) 1500 ms; echo time (TE) 100-106 ms; bandwidth 528-531 Hz/pixel; number of averages, 2; partial Fourier 6/8; echo train duration: 200ms. No parallel imaging was used.

MRI analysis: Inner and outer vessel wall contours were delineated on axial slices using Osirix software (http://www.osirix-viewer.com). Coronal acquisitions were reformatted in the axial plane before analysis. An additional noise region outside of the neck was also drawn. SNR in the vessel wall was calculated as the signal intensity in the wall divided by the standard deviation of the noise region in the same axial slice. Vessel wall to lumen contrast-to-noise ratio (CNR) was calculated as the difference between vessel wall and lumen SNR.

Statistical analysis: After checking for data normality (d’Agostino and Pearson omnibus test), SNR and CNR measurements were compared among different acquisitions using non-parametric tests. A Wilcoxon paired test was used to compare SNR and CNR between 0.8 mm³ and 0.6 mm³ coronal T2W SPACE with isotropic resolution, while a Friedman paired test was used to evaluate difference between the 3 axial acquisitions (0.6, 0.5 and 0.4 mm² with anisotropic voxels). p values less than 0.05 were considered significant.

Images from all subjects and all acquisitions were of sufficient image quality for further analysis. Figure 2 shows curved multi-planar reconstructions from the 0.8 and 0.6 mm³ coronal acquisitions with isotropic voxels, with zoomed in view of the carotid vessel wall at the bottom (orange star indicates the vessel lumen). Figure 3 shows representative images from axial acquisitions (A, 0.6 mm²; B, 0.5 mm²; C, 0.4 mm²), and depicts examples of vessel wall (orange circle), lumen (green circle) and noise (yellow circle) tracings. Average vessel wall SNR (0.8 mm³: 20.7 +/- 11.18; 0.6 mm³: 12.6 +/- 5.8) and vessel wall/lumen CNR (0.8 mm³: 14.6 +/- 8.1; 0.6 mm³: 9.0 +/- 4.5) measurements were significantly different between the two coronal acquisitions with isotropic voxels (Figure 4). Vessel wall SNR was significantly different between the 3 axial acquisitions (0.6 mm²: 36.9 +/- 21.0; 0.5 mm²: 25.3 +/- 11.5, 0.4 mm²: 21.6 +/- 10.9) while CNR measurements indicated no significant difference (0.6 mm²: 22.4 +/- 13.1; 0.5 mm²: 16.7 +/- 7.1, 0.4 mm²: 15.1 +/- 7.7) (Figure 4). In all cases SNR and CNR were comparable to values reported in the literature for similar acquisitions performed on 3T magnets, while achieving equivalent or higher spatial resolution. In conclusion, we demonstrate feasibility of 3D imaging of the carotid vessel wall at ultra-high field using 3D T2W SPACE. SNR and CNR measurements indicate good image quality and good vessel wall/lumen delineation even at high spatial resolution. These results warrant investigation of these techniques in patients with carotid artery disease, and the development of these protocols for multi-contrast and quantitative imaging of the carotid vessel wall at 7T.