gms | German Medical Science

Introduction: Anisotropic tissue such as white brain matter can be analysed with DTI [Ref. 1],[Ref. 2]. In principle, higher B0 fields are expected to increase the signal-to-noise ratio, spatial resolution, and to optimize fiber tracking. However, high fields exhibit strong B0 and B1 inhomogeneities leading to strong distortions. We applied parallel imaging [Ref. 3],[Ref. 4] to acquire DTI at 7T with a resolution of 1.5 x 1.5 x 3 mm.

Methods: Measurements were performed on a 7T MR scanner equipped with 30mT/m gradients and an 8 element phase array (PA) coil (RAPID, Würzburg, Germany). Optimization of parameters led to TR= 10.000 ms, TE= 96 ms, bandwidth = 868 Hz/Pixel, 12 slices, resolution 192x144 interpolated to 192x192, slice thickness 3mm, oversampling 25%, parallel imaging (GRAPPA, SENSE) with factor 3 and 92 reference lines, b-factors=0, 800 s/mm2, 12 different combinations of gradient directions. Data were acquired as single acquisitions, and transferred to an external PC, and registered with an home made software. Calculation of diffusion tensors and fiber tracking were performed using the public domain tool FSL [Ref. 5].

Results: Acquiring data with parallel imaging led to a significant improvement on image quality due to reduced distortions and higher signal-to-noise ratio. Image quality was good for the parietal brain parts while on the skull base susceptibility artefacts were still quite strong. Fiber tracking was feasible for the larger fiber bundles such as e.g. corpus callosum.

Conclusion: Remaining signal voids were due to ill-defined flip angles. However, DTI is feasible at 7T but requires parallel imaging similar to the results at 3T [Ref. 3],[Ref. 4]. No significant differences between GRAPPA and SENSE reconstruction were detected. The results allowed reliable fiber tracking for the larger white matter tracts. To improve DTI phase array coils with more elements, sequence optimisation with respect to shorter TE, and stronger gradients will be advantageous.