High‐resolution diffusion tensor imaging of the human pons with a reduced field‐of‐view,multishot, variable‐density,spiral acquisition at 3 T

Diffusion tensor imaging of localized anatomic regions, such as brainstem, cervical spinal cord, and optic nerve, is challenging because of the existence of significant susceptibility differences, severe physiologic motion in the surrounding tissues, and the need for high spatial resolution to resolve the underlying complex neuroarchitecture. The aim of the methodology presented here is to achieve high‐resolution diffusion tensor imaging in localized regions of the central nervous system that is motion insensitive and immune to susceptibility while acquiring a set of two‐dimensional images with more than six diffusion encoding directions within a reasonable total scan time. We accomplish this aim by implementing self‐navigated, multishot, variable‐density, spiral encoding with outer volume suppression. We establish scan protocols for achieving equal signal‐to‐noise ratio at 1.2 mm and 0.8 mm in‐plane resolution for reduced field‐of‐view diffusion tensor imaging of the brainstem. In vivo application of the technique on the human pons of three subjects shows a clear delineation of the multiple local neural tracts. By comparing scans acquired with varying in‐plane resolution but with constant signal‐to‐noise ratio, we demonstrate that increasing the resolution and reducing the partial volume effect result in higher fractional anisotropy values for the corticospinal tracts. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

High‐resolution diffusion tensor imaging of human patellar cartilage: Feasibility and preliminary findings

MR diffusion tensor imaging (DTI) was used to analyze the microstructural properties of articular cartilage. Human patellar cartilage‐on‐bone samples were imaged at 9.4T using a diffusion‐weighted SE sequence (12 gradient directions, resolution = 39 × 78 × 1500 μm³). Voxel‐based maps of the mean diffusivity, fractional anisotropy (FA), and eigenvectors were calculated. The mean diffusivity decreased from the surface (1.45 × 10^?3 mm²/s) to the tide mark (0.68 × 10^?3 mm²/s). The FA was low (0.04–0.28) and had local maxima near the surface and in the portion of the cartilage corresponding to the radial layer. The eigenvector corresponding to the largest eigenvalue showed a distinct zonal pattern, being oriented tangentially and radially in the upper and lower portions of the cartilage, respectively. The findings correspond to current scanning electron microscopy (SEM) data on the zonal architecture of cartilage. The eigenvector maps appear to reflect the alignment of the collagenous fibers in cartilage. In view of current efforts to develop and evaluate structure‐modifying therapeutic approaches in osteoarthritis (OA), DTI may offer a tool to assess the structural properties of cartilage. Magn Reson Med 53:993–998, 2005. © 2005 Wiley‐Liss, Inc.     

Super‐resolution for multislice diffusion tensor imaging

Diffusion weighted magnetic resonance images are often acquired with single shot multislice imaging sequences, because of their short scanning times and robustness to motion. To minimize noise and acquisition time, images are generally acquired with either anisotropic or isotropic low resolution voxels, which impedes subsequent posterior image processing and visualization. In this article, we propose a super‐resolution method for diffusion weighted imaging that combines anisotropic multislice images to enhance the spatial resolution of diffusion tensor data. Each diffusion weighted image is reconstructed from a set of arbitrarily oriented images with a low through‐plane resolution. The quality of the reconstructed diffusion weighted images was evaluated by diffusion tensor metrics and tractography. Experiments with simulated data, a hardware DTI phantom, as well as in vivo human brain data were conducted. Our results show a significant increase in spatial resolution of the diffusion tensor data while preserving high signal to noise ratio. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Readout-segmented EPI for rapid high resolution diffusion imaging at 3 T

Readout mosaic segmentation has been suggested as an alternative approach to EPI for high resolution diffusion-weighted imaging (DWI). In the readout-segmented EPI (RS-EPI) scheme, segments of k-space are acquired along the readout direction. This reduces geometric distortions due to the decrease in readout time. In this work, further distortion reduction is achieved by combining RS-EPI with parallel imaging (PI). The performance of the PI-accelerated RS-EPI scheme is assessed in volunteers and patients at 3T with respect to both standard EPI and PI-accelerated EPI. Peripherally cardiac gated and non-gated RS-EPI images are acquired to assess whether motion due to brain pulsation significantly degrades the image quality. Due to the low off-resonance of PI-driven RS-EPI, we also investigate if the eddy currents induced by the diffusion gradients are low enough to use the Stejskal-Tanner diffusion preparation instead of the twice-refocused eddy-current compensated diffusion preparation to reduce TE. It is shown that non-gated phase corrected DWI performs equally as well as gated acquisitions. PI-driven DW RS-EPI images with substantially less distortion compared with single-shot EPI are shown in patients-allowing the delineation of structures in the lower parts of the brain. A twice-refocused diffusion preparation was found necessary to avoid blurring in the DWI data. This paper shows that the RS-EPI scheme may be an important alternative sampling strategy to EPI to achieve high resolution T2-weighted and diffusion-weighted images.     

3D diffusion tensor MRI with isotropic resolution using a steady‐state radial acquisition

Purpose

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

Materials and Methods

A diffusion gradient was inserted in a SS 3DPR pulse sequence. The acquisition was synchronized to the cardiac cycle, linear phase errors were corrected along the readout direction, and each projection was weighted by measures of consistency with other data. A new iterative parallel imaging reconstruction method was also implemented for removing off‐resonance and undersampling artifacts simultaneously.

Results

The contrast and appearance of both the fractional anisotropy and eigenvector color maps were substantially improved after all correction techniques were applied. True 3D DTI datasets were obtained in vivo over the whole brain (240 mm field of view in all directions) with 1.87 mm isotropic spatial resolution, six diffusion encoding directions in under 19 minutes.

Conclusion

A true 3D DTI pulse sequence with high isotropic spatial resolution was developed for whole brain imaging in under 20 minutes. To minimize the effects of brain motion, a cardiac synchronized, multiecho, DW‐SSFP pulse sequence was implemented. Motion artifacts were further reduced by a combination of linear phase correction, corrupt projection detection and rejection, sampling density reweighting, and parallel imaging reconstruction. The combination of these methods greatly improved the quality of 3D DTI in the brain. J. Magn. Reson. Imaging 2009;29:1175–1184. © 2009 Wiley‐Liss, Inc.