Purpose: The dependence of diffusion tensor imaging (DTI) eigenvalues and fractional anisotropy (FA) on short diffusion times was investigated using oscillating gradient spin echo (OGSE) and pulsed gradient spin echo (PGSE) DTI in the human brain in vivo.Theory and Methods: DTI was performed in seven healthy volunteers at 4.7 Tesla (T) with b = 300 s/mm² and diffusion times of 4.1 ms (OGSE 50 Hz), 7.4 ms (OGSE 25 Hz), 20 ms (PGSE), and 40 ms (PGSE). Eigenvalues and FA were compared in the corpus callosum body, splenium and genu, and the corticospinal, cingulum, inferior fronto‐occipital, superior and inferior longitudinal fasciculi using tractography, and the thalamus and putamen using region‐of‐interest.Results: Relative to 40 ms, the 4.1 ms diffusion time led to significant increases in DTI eigenvalues in seven white matter tracts (6% to 20% parallel, 13% to 40% perpendicular) and both deep gray matter regions (16% parallel, 18% to 26% perpendicular), and reductions of FA (−9% to −12%) in four tracts.Conclusion: DTI eigenvalues and FA depend on diffusion time in both white and gray matter in the human brain. The ability to target different length scales by means of the diffusion time may improve sensitivity to changes in tissue microstructure associated with pathology. Magn Reson Med 72:726–736, 2014.

Background and Purpose—: Diffusion-weighted imaging (DWI) of tissue water is a sensitive and specific indicator of acute brain ischemia, where reductions of the diffusion of tissue water are observed acutely in the stroke lesion core. Although these diffusion changes have been long attributed to cell swelling, the precise nature of the biophysical mechanisms remains uncertain. Methods—: The potential cause of diffusion reductions after stroke was investigated using an advanced DWI technique, oscillating gradient spin-echo DWI, that enables much shorter diffusion times and can improve specificity for alterations of structure at the micron level. Results—: Diffusion measurements in the white matter lesions of patients with acute ischemic stroke were reduced by only 8% using oscillating gradient spin-echo DWI, in contrast to a 37% decrease using standard DWI. Neurite beading has recently been proposed as a mechanism for the diffusion changes after ischemic stroke with some ex vivo evidence. To explore whether beading could cause such differential results, simulations of beaded cylinders and axonal swelling were performed, yielding good agreement with experiment. Conclusions—: Short diffusion times result in dramatically reduced diffusion contrast of human stroke. Simulations implicate a combination of neuronal beading and axonal swelling as the key structural changes leading to the reduced apparent diffusion coefficient after stroke.

Concomitant gradient fields are transverse magnetic field components that are necessarily present to satisfy Maxwell's equations when magnetic field gradients are utilized in magnetic resonance imaging. They can have deleterious effects that are more prominent at lower static fields and/or higher gradient strengths. In diffusion tensor imaging schemes that employ large gradients that are not symmetric about a refocusing radiofrequency pulse (unlike Stejskal–Tanner, which is symmetric), concomitant fields may cause phase accrual that could corrupt the diffusion measurement. Theory predicting the error from this dephasing is described and experimentally validated for both Reese twice‐refocused and split gradient single spin‐echo diffusion gradient schemes. Bias in apparent diffusion coefficient values was experimentally found to worsen with distance from isocenter and with increasing duration of gradient asymmetry in both a phantom and in the brain. The amount of error from concomitant gradient fields depends on many variables, including the diffusion gradient pattern, pulse sequence timing, maximum effective gradient amplitude, static magnetic field strength, voxel size, slice distance from isocenter, and partial Fourier fraction. A prospective correction scheme that can reduce concomitant gradient errors is proposed and verified for diffusion imaging. Magn Reson Med, 2012.