Effects of signal averaging, gradient encoding scheme, and spatial resolution on diffusion kurtosis imaging: An empirical study using 7T MRI

Abstract

Background Although diffusion gradient directions and b-values have been optimized for diffusion kurtosis imaging (DKI), little is known about the effect of signal averaging on DKI reliability. Purpose To evaluate how signal averaging influences the reliability of DKI indices using two gradient encoding schemes with three spatial resolutions. Study Type Prospective. Animal Model Fifteen naive Sprague-Dawley rats. Field Strength/Sequence DKI was performed at 7T using two schemes (30 directions with three b-values [30d-3b] and six directions with 15 b-values [6d-15b]), three resolutions, and eight repetitions. Assessment DKI reliability was assessed using voxelwise relative error (sigma) and test-retest error of fractional anisotropy (FA), mean diffusivity (MD), and mean kurtosis (MK) within gray matter (GM) and white matter (WM). The number of excitations (NEX) was optimized by considering DKI reliability. The influence of the partial volume effect (PVE) was also assessed. Statistical Test One-way analysis of variance. Results The 30d-3b scheme, compared with the 6d-15b scheme, exhibited apparently smaller sigma(FA) and sigma(MK) (eg, at NEX 1, in GM, for three resolutions, sigma(FA): 19.9-38.2% vs. 34.2-61.4%, sigma(MK): 6.9-11.4% vs. 14.1-15.4%) and similar sigma(MD) (all differences between two schemes <1.6%). The optimal NEX was determined as 2 for enabling a reliable measurement of DKI-derived indices. The PVE at the lowest resolution apparently increased sigma(FA) for both schemes (19.9% for 30d-3b and 34.2% for 6d-15b) and sigma(MK) for the 6d-15b scheme (14.7%) in GM, and exerted lower effects on MK values for the 30d-3b scheme (P > 0.05). Data Conclusion A higher number of diffusion directions would benefit FA and MK estimation. A higher spatial resolution helps to reduce PVE. By using the 30d-3b scheme, MK is considered a robust index to reflect microstructural changes in GM and WM. We propose a systematic approach to determine the optimal DKI protocols for appropriate preclinical settings. Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1593-1603.