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Diffusion tensor MRI (DT-MRI) can provide estimates of fiber orientation derived from the orientational dependence of the diffusivity of water molecules, enabling the reconstruction of white matter fiber pathways using tractography methods. However, noise arising from various sources can introduce uncertainty into the estimates of the elements of the diffusion tensor, resulting in errors in fiber orientation estimates such that tractography reconstructions of fiber pathways potentially can be imprecise and inaccurate. Recently, attempts have been made to characterize the uncertainty in DT-MRI-derived parameters using the bootstrap method; however, several questions remain open regarding the number of repeat measurements and bootstraps required to accurately and precisely reconstruct the probability distributions of the DT-MRI parameters. This study investigates the accuracy and precision of the bootstrap method for characterizing distributions of DT-MRI parameters. A number of experimental bootstrap designs and sampling schemes containing different numbers of isotropically distributed gradient vectors are considered, using an idealized system where the true variability in each parameter is known. This study demonstrates that for most DT-MRI experiments, robust results will be obtained if the minimum number of bootstraps is approximately 500, and that at least five repeat samples of each diffusion-weighted intensity should be used for bootstrapping.

The accurate quantification of perfusion with arterial spin labeling (ASL) requires consideration of a number of factors, including the efficiency of the inversion and control pulses used for spin labeling. In this study the effects of spin velocity on continuous ASL efficiency when using the amplitude modulated control strategy were investigated using simulations of the Bloch equations. The inversion efficiency was determined in vivo by combining the simulations with phase-contrast velocity mapping data acquired at the level of the tagging plane. Using this novel method, an average inversion efficiency of 69% was calculated for a group of 28 subjects, in good agreement with experimental data reported previously. There was, however, a large range in inversion efficiency measured across the subject group (50-76%), indicating that the velocity dependence of the amplitude modulated control efficiency may introduce additional variability into the perfusion calculations if not properly taken into account., (Copyright 2006 Wiley-Liss, Inc.)