Cervical Spinal Cord DTI Is Improved by Reduced FOV with Specific Balance between the Number of Diffusion Gradient Directions and Averages

Abstract

Background and purpose: Reduced-FOV DTI is promising for exploring the cervical spinal cord, but the optimal set of parameters needs to be clarified. We hypothesized that the number of excitations should be favored over the number of diffusion gradient directions regarding the strong orientation of the cord in a single rostrocaudal axis.

Materials and methods: Fifteen healthy individuals underwent cervical spinal cord MR imaging at 3T, including an anatomic 3D-Multi-Echo Recombined Gradient Echo, high-resolution full-FOV DTI with a NEX of 3 and 20 diffusion gradient directions and 5 sets of reduced-FOV DTIs differently balanced in terms of NEX/number of diffusion gradient directions: (NEX/number of diffusion gradient directions = 3/20, 5/16, 7/12, 9/9, and 12/6). Each DTI sequence lasted 4 minutes 30 seconds, an acceptable duration, to cover C1-C4 in the axial plane. Fractional anisotropy maps and tractograms were reconstructed. Qualitatively, 2 radiologists rated the DTI sets blinded to the sequence. Quantitatively, we compared distortions, SNR, variance of fractional anisotropy values, and numbers of detected fibers.

Results: Qualitatively, reduced-FOV DTI sequences with a NEX of ≥5 were significantly better rated than the full-FOV DTI and the reduced-FOV DTI with low NEX (N = 3) and a high number of diffusion gradient directions (D = 20). Quantitatively, the best trade-off was reached by the reduced-FOV DTI with a NEX of 9 and 9 diffusion gradient directions, which provided significantly fewer artifacts, higher SNR on trace at b = 750 s/mm² and an increased number of fibers tracked while maintaining similar fractional anisotropy values and dispersion.

Conclusions: Optimized reduced-FOV DTI improves spinal cord imaging. The best compromise was obtained with a NEX of 9 and 9 diffusion gradient directions, which emphasizes the need for increasing the NEX at the expense of the number of diffusion gradient directions for spinal cord DTI contrary to brain DTI.

Fig 1.

Postprocessing pipeline and analyses. A, Four sections were analyzed in detail: C1, C2, C3, and C4. Raw data diffusion-weighted images were treated for eddy current correction before generation of DTI parameters. We focused on FA because this parameter is the most commonly used. Fusions of FA and 3D-MERGE were created at these 4 levels as well as reconstructed on the sagittal orientation to facilitate the identification of distortions and pixel misregistration. B, ROI positioning: ROIs (B1) were manually delineated on 3D-MERGE, on the right and left anterior horns of the cord (red area) for gray matter, and on the right and left corticospinal tract (green area) for white matter, and then propagated on the coregistered FA map. B2, If needed, ROIs were manually adjusted to account for FA map distortion. Furthermore, because of the partial volume effect at the interface between CSF and the FA map, ROIs of the full section (blue dotted line, whose surface corresponded to S_{[Full Section − Merge]}) were adjusted to remove the pixels subject to artifacts at the periphery of the ROI (black dashed line, whose area corresponded to S_{[Full Section − FA]}).

Fig 2.

Examples of MR images available for qualitative analysis. All the images came from the same subject. Cervical levels are located on 3D T2-MERGE and sagittal T2-spin echo. Fusion of FA − 3D MERGE clearly shows that f-FOV DTI and r-FOV 3N/20D are more distorted and more blurred with less anatomic precision than the other r-FOV images.

Fig 3.

Qualitative analysis. Radiologists determined a rate for each sequence, for each subject, from 1 (nondiagnostic) to 4 (good). Mean rates ± SDs for the sequence are represented. Superimposed black lines indicate which sequences are statistically different with P < .05 (asterisk).

Fig 4.

Percentages of sections with artifacts unusable for DTI analysis due to susceptibility artifacts or poor SNR. Superimposed black lines indicate which sequences are statistically different with P < .05 (asterisk) and P < .005 (double asterisks).

Fig 5.

Quantitative comparisons on ROI-based analyses. A, The distortion ratio. B, The SNR on the trace image at b=750 s/mm². C, Representation of the dispersion of FA values, depending on the DTI sequence and, successively, a full section of the spinal cord (FS), WM, and GM. Mean rates ± SDs for the sequence are represented. Superimposed black lines indicate which sequences are statistically different with P < .05 (asterisk).

Fig 6.

Tractography-based analyses. A, The reconstructed tractograms for the whole DTI dataset. For each DTI sequence, 2 similar seed ROIs were placed on the anatomic sequence, at the C1 and C3 levels, and then propagated on diffusion data. Care was taken to exclude abnormal fiber detection (ie, in the CSF). Qualitatively, r-FOV sequences clearly exhibit better tractogram definitions. B, The number of detected fibers between the 2 seeds. P < .05 (asterisk), P < .005 (double asterisks), P < .0005 (triple asterisks).