Diffusion-weighted images acquired with the echo-planar imaging technique are highly sensitive to eddy current induced geometric distortions that vary with the magnitude and direction of the diffusion sensitizing gradients. Such distortions cause misalignment of images acquired with different diffusion strengths and orientations. This in turn can result in errors when calculating maps of the apparent diffusion coefficient and diffusion tensor. Previous correction methods either require separate calibration data or only deal with low-order errors. In this study, we demonstrate a method that can correct for higher-order errors. The method relies on collecting pairs of images with diffusion sensitizing gradients reversed. This paired data are first corrected for shifts and linear distortion and then combined to cancel higher-order errors. All acquired data contribute to the final results. The method has been tested by simulation, on phantoms, on adult volunteers, and on neonatal brain Key words: echo-planar imaging; diffusion-weighted image; diffusion tensor imaging; eddy current induced distortion Diffusion-weighted (DW) imaging provides an important means of early detection of acute stroke (1-3) and is a key tool for probing white matter fiber tracts and tissue microstructures (4 -6). In vivo measurement of the diffusion tensor by diffusion tensor imaging (DTI) requires multiple image acquisitions with diffusion sensitivities in at least six noncollinear directions. Recently it was recognized that a simple diffusion ellipse representation may not be sufficient where there are mixed fiber directions within a voxel and so it is becoming more common to acquire data sensitized in a large number of directions. This approach requires rapid imaging and diffusion-weighted echo-planar imaging (DW-EPI) using pulsed gradient (StejskalTanner) spin echo sequences is the current method of choice. To improve signal-to-noise ratio (SNR), it is common to acquire several images for each sensitization direction.DW-EPI is highly sensitive to eddy currents, which cause image shifts and spatial distortions, particularly in the phase encoding direction. Eddy currents produced by the diffusion sensitizing gradient pulses vary with direction of diffusion sensitization, so that image distortions vary from image to image within data sets acquired for DTI. To avoid substantial errors in the calculated diffusion properties, these distortions must be fully corrected.Previous methods for dealing with this problem involved sequence modifications, acquisition of extra calibration data, and/or postprocessing. Sequence modifications can reduce the effects of eddy currents during the EPI readout period. Examples include the use of bipolar gradient pulses (7,8) or two opposite polarity gradient pulses, separated by a 180°radiofrequency refocusing pulse, to replace each Stejskal-Tanner diffusion pulse (9). These methods usually reduce diffusion sensitivity compared to the original Stejskal-Tanner diffusion pulses. A more efficient, twice-refocused me...

Hypercapnia and hyperoxia give rise to vasodilation and vasoconstriction, respectively. This study investigates the influence of hypercapnia and hyperoxia on venous vessel size in the human brain. Venous vessel radii were measured in response to hypercapnia and hyperoxia. The venous vessel radii were determined by calculation of the changes in R2* and R2 that are induced by breathing 6% CO2 or pure oxygen. The experimental paradigm consisted of two 3‐min intervals of inhaling 6% CO2 or 100% O2 interleaved with three 2‐min intervals of breathing air. Hypercapnic and hyperoxic experiments were performed on eight subjects on a 3T scanner. Parametric maps of mean venous vessel radius were calculated from the changes in R2* and R2, which were measured by simultaneous acquisition of gradient‐echo and spin‐echo signals. The mean venous vessel radii in hypercapnia were 7.3 ± 0.3 μm in gray matter and 6.6 ± 0.5 μm in white matter. The corresponding vessel radii in hyperoxia were 5.6 ± 0.2 μm in gray matter and 5.4 ± 0.2 μm in white matter. These results show that the venous vessel radius was larger in hypercapnia than that in hyperoxia in both gray matter and white matter (P < 0.005), which agrees with the hypothesis that hypercapnia causes vasodilation and hyperoxia induces vasoconstriction. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.

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