Optimal strategies for measuring diffusion in anisotropic systems by magnetic resonance imaging

Abstract

The optimization of acquisition parameters for precise measurement of diffusion in anisotropic systems is described. First, an algorithm is presented that minimizes the bias inherent in making measurements with a fixed set of gradient vector directions by spreading out measurements in 3-dimensional gradient vector space. Next, it is shown how the set of b-matrices and echo time can be optimized for estimating the diffusion tensor and its scalar invariants. The standard deviation in the estimate of the tensor trace in a water phantom was reduced by more than 40% and the artefactual anisotropy was reduced by more than 60% when using the optimized scheme compared with a more conventional scheme for the same scan time, and marked improvements are demonstrated in the human brain with the optimized sequences. Use of these optimal schemes results in reduced scan times, increased precision, or improved resolution in diffusion tensor images. Magn Reson Med 42:515-

Reproducibility of quantitative structural and physiological MRI measurements

“…The highest b‐value ( b  = 3,800 s/mm 2 ) was chosen because the SNR for the corpus callosum in the average diffusion image (SNR = 6.1 ± 0.7) measured in five healthy volunteers (ages 25–50 years) during protocol development approached the empirically selected lower limit of SNR = 5.0. The b ‐values and the number of directions per shell were chosen for improved fit of the bi‐exponential model and SNR (Jones, Horsfield, & Simmons, 1999). The imaging data were collected using a single‐shot, echo‐planar, single refocusing spin‐echo, T2‐weighted sequence with a spatial resolution of 1.7 × 1.7 × 4.6 mm and seven slices prescribed in sagittal orientation to sample the midsagittal band of the corpus callosum.…”

Topographic separation of fornical fibers associated with the anterior and posterior hippocampus in the human brain: An MRI‐diffusion study

“…The MRI protocol consisted of the following imaging sequence: A high‐resolution T 1 ‐weighted anatomical scan (FSPGR) (256 × 256 acquisition matrix, TR = 7.8 ms, TE = 2.9 ms, flip angle = 20, 172 slices, 1 mm slice thickness, FOV = 23 cm). Diffusion data were acquired employing a spin‐echo echo‐planar HARDI sequence with diffusion encoded along 60 isotropically distributed orientations and 6 nondiffusion‐weighted scans according to an optimized gradient vector scheme (Jones, Horsfield, & Simmons, 1999) (Field of view 230 × 230 mm, 96 × 96 acquisition matrix, TR/TE = 87 ms, b‐value = 1200 s/mm 2 , 60 slices, 2.4 mm slice thickness, reconstructed spatial resolution 1.8 × 1.8 × 2.4 mm). Data acquisition was peripherally gated to the cardiac cycle with a total acquisition time of ~30 min depending on the heart rate.…”

Importance of exactb-tensor calculation for quantitative diffusion tensor imaging and tracking of neuronal fiber bundles

“…Many other DE schemas are possible, but for most clinical applications, measurements along more directions are preferable since that provides more accurate DT estimation rather than the same number of measurements along the minimum six directions with corresponding averages [16].…”