Effects of number of diffusion gradient directions on derived diffusion tensor imaging indices in human brain

Abstract

Background and purpose: The effects of a number of diffusion-encoding gradient directions (NDGD) on diffusion tensor imaging (DTI) indices have been studied previously with theoretic analysis and numeric simulations. In this study, we made in vivo measurements in the human brain to compare different clinical scan protocols and to evaluate their effects on the calculated DTI indices.

Methods: Fifteen healthy volunteers were scanned with a 1.5T MR scanner. Single-shot DTI images were acquired using 3 protocols different in NDGD and number of excitations (NEX) for each direction (NDGD/NEX = 6/10, 21/3, 31/2). Means and standard error of mean (SEM) were calculated and compared in 6 regions of interest (ROIs) for mean diffusivity (D), fractional anisotropy (FA), diffusion tensor eigenvalues (lambda(1), lambda(2), and lambda(3)), and correlation coefficients (r) of these indices among the 3 DTI protocols.

Results: At the ROI level, no significant differences were found for the mean and SEM of D and FA among protocols (P > .05). The 6-NDGD protocol, however, yielded higher values for lambda(1) and lambda(2) and lower values for lambda(3) in most ROIs (P < .05) compared with the other protocols. At the voxel level, the correlation between the protocols r(21-31) were higher than r(6-21) and r(6-31) in most ROIs. The correlation of FA among 3 protocols also increased with increasing anisotropy.

Conclusion: For ROI analyses, different NDGDs lead to similar values of FA and D but different eigenvalues. However, different NDGDs at the voxel level provide varying values. The selection of the NDGD, therefore, should depend on the focus of different DTI applications.

Fig 1.

A, An example of coregistration across 3 protocols. Images on the left, middle, and right are obtained with the 6-NDGD, 21-NDGD, and 31-NDGD protocols, respectively. A region of interest (ROI) of the splenium of CC is initially drawn on only 1 image from 1 of the 3 protocols and then translated to the corresponding images from the other 2 protocols. B, Spatial definition of ROIs: all ROIs are positioned relative to CC in the anterior (A, CCA), middle [B, CCM, cingulum (D), SLF (E), and IC (F)], and posterior [C, CCP, cingulum (D), and SLF (E)] locations.

Fig 2.

Mean value in 6 ROIs for the 3 protocols; A for FA, B for 〈D〉, C for λ₁, D for λ₂, and E for λ₃. 〈D〉, λ₁, λ₂, and λ₃ are measured in 10⁻³ mm²/s. White bars for 6-NDGD, gray bars for 21-NDGD, and black bars for 31-NDGD protocols. *, P < .05. Error bars are for 1 SEM.

Fig 3.

Correlation coefficient (r) in 6 ROIs between 2 of the 3 protocols. A for FA, B for 〈D〉, C for λ₁, D for λ₂, and E for λ₃. White bars are for r_6–21, gray bars for r_6–31, and black bars for r_21–31. *, P < .05. Error bars are for 1 SEM.

Fig 4.

The linear fitting between correlation coefficient of FA versus FA value of different ROIs.